1/proc/sys/net/ipv4/* Variables:
   3ip_forward - BOOLEAN
   4        0 - disabled (default)
   5        not 0 - enabled
   7        Forward Packets between interfaces.
   9        This variable is special, its change resets all configuration
  10        parameters to their default state (RFC1122 for hosts, RFC1812
  11        for routers)
  13ip_default_ttl - INTEGER
  14        Default value of TTL field (Time To Live) for outgoing (but not
  15        forwarded) IP packets. Should be between 1 and 255 inclusive.
  16        Default: 64 (as recommended by RFC1700)
  18ip_no_pmtu_disc - INTEGER
  19        Disable Path MTU Discovery. If enabled in mode 1 and a
  20        fragmentation-required ICMP is received, the PMTU to this
  21        destination will be set to min_pmtu (see below). You will need
  22        to raise min_pmtu to the smallest interface MTU on your system
  23        manually if you want to avoid locally generated fragments.
  25        In mode 2 incoming Path MTU Discovery messages will be
  26        discarded. Outgoing frames are handled the same as in mode 1,
  27        implicitly setting IP_PMTUDISC_DONT on every created socket.
  29        Mode 3 is a hardend pmtu discover mode. The kernel will only
  30        accept fragmentation-needed errors if the underlying protocol
  31        can verify them besides a plain socket lookup. Current
  32        protocols for which pmtu events will be honored are TCP, SCTP
  33        and DCCP as they verify e.g. the sequence number or the
  34        association. This mode should not be enabled globally but is
  35        only intended to secure e.g. name servers in namespaces where
  36        TCP path mtu must still work but path MTU information of other
  37        protocols should be discarded. If enabled globally this mode
  38        could break other protocols.
  40        Possible values: 0-3
  41        Default: FALSE
  43min_pmtu - INTEGER
  44        default 552 - minimum discovered Path MTU
  46ip_forward_use_pmtu - BOOLEAN
  47        By default we don't trust protocol path MTUs while forwarding
  48        because they could be easily forged and can lead to unwanted
  49        fragmentation by the router.
  50        You only need to enable this if you have user-space software
  51        which tries to discover path mtus by itself and depends on the
  52        kernel honoring this information. This is normally not the
  53        case.
  54        Default: 0 (disabled)
  55        Possible values:
  56        0 - disabled
  57        1 - enabled
  59fwmark_reflect - BOOLEAN
  60        Controls the fwmark of kernel-generated IPv4 reply packets that are not
  61        associated with a socket for example, TCP RSTs or ICMP echo replies).
  62        If unset, these packets have a fwmark of zero. If set, they have the
  63        fwmark of the packet they are replying to.
  64        Default: 0
  66fib_multipath_use_neigh - BOOLEAN
  67        Use status of existing neighbor entry when determining nexthop for
  68        multipath routes. If disabled, neighbor information is not used and
  69        packets could be directed to a failed nexthop. Only valid for kernels
  70        built with CONFIG_IP_ROUTE_MULTIPATH enabled.
  71        Default: 0 (disabled)
  72        Possible values:
  73        0 - disabled
  74        1 - enabled
  76route/max_size - INTEGER
  77        Maximum number of routes allowed in the kernel.  Increase
  78        this when using large numbers of interfaces and/or routes.
  79        From linux kernel 3.6 onwards, this is deprecated for ipv4
  80        as route cache is no longer used.
  82neigh/default/gc_thresh1 - INTEGER
  83        Minimum number of entries to keep.  Garbage collector will not
  84        purge entries if there are fewer than this number.
  85        Default: 128
  87neigh/default/gc_thresh2 - INTEGER
  88        Threshold when garbage collector becomes more aggressive about
  89        purging entries. Entries older than 5 seconds will be cleared
  90        when over this number.
  91        Default: 512
  93neigh/default/gc_thresh3 - INTEGER
  94        Maximum number of neighbor entries allowed.  Increase this
  95        when using large numbers of interfaces and when communicating
  96        with large numbers of directly-connected peers.
  97        Default: 1024
  99neigh/default/unres_qlen_bytes - INTEGER
 100        The maximum number of bytes which may be used by packets
 101        queued for each unresolved address by other network layers.
 102        (added in linux 3.3)
 103        Setting negative value is meaningless and will return error.
 104        Default: 65536 Bytes(64KB)
 106neigh/default/unres_qlen - INTEGER
 107        The maximum number of packets which may be queued for each
 108        unresolved address by other network layers.
 109        (deprecated in linux 3.3) : use unres_qlen_bytes instead.
 110        Prior to linux 3.3, the default value is 3 which may cause
 111        unexpected packet loss. The current default value is calculated
 112        according to default value of unres_qlen_bytes and true size of
 113        packet.
 114        Default: 31
 116mtu_expires - INTEGER
 117        Time, in seconds, that cached PMTU information is kept.
 119min_adv_mss - INTEGER
 120        The advertised MSS depends on the first hop route MTU, but will
 121        never be lower than this setting.
 123IP Fragmentation:
 125ipfrag_high_thresh - INTEGER
 126        Maximum memory used to reassemble IP fragments. When
 127        ipfrag_high_thresh bytes of memory is allocated for this purpose,
 128        the fragment handler will toss packets until ipfrag_low_thresh
 129        is reached. This also serves as a maximum limit to namespaces
 130        different from the initial one.
 132ipfrag_low_thresh - INTEGER
 133        Maximum memory used to reassemble IP fragments before the kernel
 134        begins to remove incomplete fragment queues to free up resources.
 135        The kernel still accepts new fragments for defragmentation.
 137ipfrag_time - INTEGER
 138        Time in seconds to keep an IP fragment in memory.
 140ipfrag_max_dist - INTEGER
 141        ipfrag_max_dist is a non-negative integer value which defines the
 142        maximum "disorder" which is allowed among fragments which share a
 143        common IP source address. Note that reordering of packets is
 144        not unusual, but if a large number of fragments arrive from a source
 145        IP address while a particular fragment queue remains incomplete, it
 146        probably indicates that one or more fragments belonging to that queue
 147        have been lost. When ipfrag_max_dist is positive, an additional check
 148        is done on fragments before they are added to a reassembly queue - if
 149        ipfrag_max_dist (or more) fragments have arrived from a particular IP
 150        address between additions to any IP fragment queue using that source
 151        address, it's presumed that one or more fragments in the queue are
 152        lost. The existing fragment queue will be dropped, and a new one
 153        started. An ipfrag_max_dist value of zero disables this check.
 155        Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
 156        result in unnecessarily dropping fragment queues when normal
 157        reordering of packets occurs, which could lead to poor application
 158        performance. Using a very large value, e.g. 50000, increases the
 159        likelihood of incorrectly reassembling IP fragments that originate
 160        from different IP datagrams, which could result in data corruption.
 161        Default: 64
 163INET peer storage:
 165inet_peer_threshold - INTEGER
 166        The approximate size of the storage.  Starting from this threshold
 167        entries will be thrown aggressively.  This threshold also determines
 168        entries' time-to-live and time intervals between garbage collection
 169        passes.  More entries, less time-to-live, less GC interval.
 171inet_peer_minttl - INTEGER
 172        Minimum time-to-live of entries.  Should be enough to cover fragment
 173        time-to-live on the reassembling side.  This minimum time-to-live  is
 174        guaranteed if the pool size is less than inet_peer_threshold.
 175        Measured in seconds.
 177inet_peer_maxttl - INTEGER
 178        Maximum time-to-live of entries.  Unused entries will expire after
 179        this period of time if there is no memory pressure on the pool (i.e.
 180        when the number of entries in the pool is very small).
 181        Measured in seconds.
 183TCP variables:
 185somaxconn - INTEGER
 186        Limit of socket listen() backlog, known in userspace as SOMAXCONN.
 187        Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
 188        for TCP sockets.
 190tcp_abort_on_overflow - BOOLEAN
 191        If listening service is too slow to accept new connections,
 192        reset them. Default state is FALSE. It means that if overflow
 193        occurred due to a burst, connection will recover. Enable this
 194        option _only_ if you are really sure that listening daemon
 195        cannot be tuned to accept connections faster. Enabling this
 196        option can harm clients of your server.
 198tcp_adv_win_scale - INTEGER
 199        Count buffering overhead as bytes/2^tcp_adv_win_scale
 200        (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
 201        if it is <= 0.
 202        Possible values are [-31, 31], inclusive.
 203        Default: 1
 205tcp_allowed_congestion_control - STRING
 206        Show/set the congestion control choices available to non-privileged
 207        processes. The list is a subset of those listed in
 208        tcp_available_congestion_control.
 209        Default is "reno" and the default setting (tcp_congestion_control).
 211tcp_app_win - INTEGER
 212        Reserve max(window/2^tcp_app_win, mss) of window for application
 213        buffer. Value 0 is special, it means that nothing is reserved.
 214        Default: 31
 216tcp_autocorking - BOOLEAN
 217        Enable TCP auto corking :
 218        When applications do consecutive small write()/sendmsg() system calls,
 219        we try to coalesce these small writes as much as possible, to lower
 220        total amount of sent packets. This is done if at least one prior
 221        packet for the flow is waiting in Qdisc queues or device transmit
 222        queue. Applications can still use TCP_CORK for optimal behavior
 223        when they know how/when to uncork their sockets.
 224        Default : 1
 226tcp_available_congestion_control - STRING
 227        Shows the available congestion control choices that are registered.
 228        More congestion control algorithms may be available as modules,
 229        but not loaded.
 231tcp_base_mss - INTEGER
 232        The initial value of search_low to be used by the packetization layer
 233        Path MTU discovery (MTU probing).  If MTU probing is enabled,
 234        this is the initial MSS used by the connection.
 236tcp_congestion_control - STRING
 237        Set the congestion control algorithm to be used for new
 238        connections. The algorithm "reno" is always available, but
 239        additional choices may be available based on kernel configuration.
 240        Default is set as part of kernel configuration.
 241        For passive connections, the listener congestion control choice
 242        is inherited.
 243        [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
 245tcp_dsack - BOOLEAN
 246        Allows TCP to send "duplicate" SACKs.
 248tcp_early_retrans - INTEGER
 249        Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold
 250        for triggering fast retransmit when the amount of outstanding data is
 251        small and when no previously unsent data can be transmitted (such
 252        that limited transmit could be used). Also controls the use of
 253        Tail loss probe (TLP) that converts RTOs occurring due to tail
 254        losses into fast recovery (draft-dukkipati-tcpm-tcp-loss-probe-01).
 255        Possible values:
 256                0 disables ER
 257                1 enables ER
 258                2 enables ER but delays fast recovery and fast retransmit
 259                  by a fourth of RTT. This mitigates connection falsely
 260                  recovers when network has a small degree of reordering
 261                  (less than 3 packets).
 262                3 enables delayed ER and TLP.
 263                4 enables TLP only.
 264        Default: 3
 266tcp_ecn - INTEGER
 267        Control use of Explicit Congestion Notification (ECN) by TCP.
 268        ECN is used only when both ends of the TCP connection indicate
 269        support for it.  This feature is useful in avoiding losses due
 270        to congestion by allowing supporting routers to signal
 271        congestion before having to drop packets.
 272        Possible values are:
 273                0 Disable ECN.  Neither initiate nor accept ECN.
 274                1 Enable ECN when requested by incoming connections and
 275                  also request ECN on outgoing connection attempts.
 276                2 Enable ECN when requested by incoming connections
 277                  but do not request ECN on outgoing connections.
 278        Default: 2
 280tcp_ecn_fallback - BOOLEAN
 281        If the kernel detects that ECN connection misbehaves, enable fall
 282        back to non-ECN. Currently, this knob implements the fallback
 283        from RFC3168, section, but we reserve that in future,
 284        additional detection mechanisms could be implemented under this
 285        knob. The value is not used, if tcp_ecn or per route (or congestion
 286        control) ECN settings are disabled.
 287        Default: 1 (fallback enabled)
 289tcp_fack - BOOLEAN
 290        Enable FACK congestion avoidance and fast retransmission.
 291        The value is not used, if tcp_sack is not enabled.
 293tcp_fin_timeout - INTEGER
 294        The length of time an orphaned (no longer referenced by any
 295        application) connection will remain in the FIN_WAIT_2 state
 296        before it is aborted at the local end.  While a perfectly
 297        valid "receive only" state for an un-orphaned connection, an
 298        orphaned connection in FIN_WAIT_2 state could otherwise wait
 299        forever for the remote to close its end of the connection.
 300        Cf. tcp_max_orphans
 301        Default: 60 seconds
 303tcp_frto - INTEGER
 304        Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
 305        F-RTO is an enhanced recovery algorithm for TCP retransmission
 306        timeouts.  It is particularly beneficial in networks where the
 307        RTT fluctuates (e.g., wireless). F-RTO is sender-side only
 308        modification. It does not require any support from the peer.
 310        By default it's enabled with a non-zero value. 0 disables F-RTO.
 312tcp_invalid_ratelimit - INTEGER
 313        Limit the maximal rate for sending duplicate acknowledgments
 314        in response to incoming TCP packets that are for an existing
 315        connection but that are invalid due to any of these reasons:
 317          (a) out-of-window sequence number,
 318          (b) out-of-window acknowledgment number, or
 319          (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
 321        This can help mitigate simple "ack loop" DoS attacks, wherein
 322        a buggy or malicious middlebox or man-in-the-middle can
 323        rewrite TCP header fields in manner that causes each endpoint
 324        to think that the other is sending invalid TCP segments, thus
 325        causing each side to send an unterminating stream of duplicate
 326        acknowledgments for invalid segments.
 328        Using 0 disables rate-limiting of dupacks in response to
 329        invalid segments; otherwise this value specifies the minimal
 330        space between sending such dupacks, in milliseconds.
 332        Default: 500 (milliseconds).
 334tcp_keepalive_time - INTEGER
 335        How often TCP sends out keepalive messages when keepalive is enabled.
 336        Default: 2hours.
 338tcp_keepalive_probes - INTEGER
 339        How many keepalive probes TCP sends out, until it decides that the
 340        connection is broken. Default value: 9.
 342tcp_keepalive_intvl - INTEGER
 343        How frequently the probes are send out. Multiplied by
 344        tcp_keepalive_probes it is time to kill not responding connection,
 345        after probes started. Default value: 75sec i.e. connection
 346        will be aborted after ~11 minutes of retries.
 348tcp_l3mdev_accept - BOOLEAN
 349        Enables child sockets to inherit the L3 master device index.
 350        Enabling this option allows a "global" listen socket to work
 351        across L3 master domains (e.g., VRFs) with connected sockets
 352        derived from the listen socket to be bound to the L3 domain in
 353        which the packets originated. Only valid when the kernel was
 354        compiled with CONFIG_NET_L3_MASTER_DEV.
 356tcp_low_latency - BOOLEAN
 357        If set, the TCP stack makes decisions that prefer lower
 358        latency as opposed to higher throughput.  By default, this
 359        option is not set meaning that higher throughput is preferred.
 360        An example of an application where this default should be
 361        changed would be a Beowulf compute cluster.
 362        Default: 0
 364tcp_max_orphans - INTEGER
 365        Maximal number of TCP sockets not attached to any user file handle,
 366        held by system. If this number is exceeded orphaned connections are
 367        reset immediately and warning is printed. This limit exists
 368        only to prevent simple DoS attacks, you _must_ not rely on this
 369        or lower the limit artificially, but rather increase it
 370        (probably, after increasing installed memory),
 371        if network conditions require more than default value,
 372        and tune network services to linger and kill such states
 373        more aggressively. Let me to remind again: each orphan eats
 374        up to ~64K of unswappable memory.
 376tcp_max_syn_backlog - INTEGER
 377        Maximal number of remembered connection requests, which have not
 378        received an acknowledgment from connecting client.
 379        The minimal value is 128 for low memory machines, and it will
 380        increase in proportion to the memory of machine.
 381        If server suffers from overload, try increasing this number.
 383tcp_max_tw_buckets - INTEGER
 384        Maximal number of timewait sockets held by system simultaneously.
 385        If this number is exceeded time-wait socket is immediately destroyed
 386        and warning is printed. This limit exists only to prevent
 387        simple DoS attacks, you _must_ not lower the limit artificially,
 388        but rather increase it (probably, after increasing installed memory),
 389        if network conditions require more than default value.
 391tcp_mem - vector of 3 INTEGERs: min, pressure, max
 392        min: below this number of pages TCP is not bothered about its
 393        memory appetite.
 395        pressure: when amount of memory allocated by TCP exceeds this number
 396        of pages, TCP moderates its memory consumption and enters memory
 397        pressure mode, which is exited when memory consumption falls
 398        under "min".
 400        max: number of pages allowed for queueing by all TCP sockets.
 402        Defaults are calculated at boot time from amount of available
 403        memory.
 405tcp_min_rtt_wlen - INTEGER
 406        The window length of the windowed min filter to track the minimum RTT.
 407        A shorter window lets a flow more quickly pick up new (higher)
 408        minimum RTT when it is moved to a longer path (e.g., due to traffic
 409        engineering). A longer window makes the filter more resistant to RTT
 410        inflations such as transient congestion. The unit is seconds.
 411        Default: 300
 413tcp_moderate_rcvbuf - BOOLEAN
 414        If set, TCP performs receive buffer auto-tuning, attempting to
 415        automatically size the buffer (no greater than tcp_rmem[2]) to
 416        match the size required by the path for full throughput.  Enabled by
 417        default.
 419tcp_mtu_probing - INTEGER
 420        Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
 421        values:
 422          0 - Disabled
 423          1 - Disabled by default, enabled when an ICMP black hole detected
 424          2 - Always enabled, use initial MSS of tcp_base_mss.
 426tcp_probe_interval - INTEGER
 427        Controls how often to start TCP Packetization-Layer Path MTU
 428        Discovery reprobe. The default is reprobing every 10 minutes as
 429        per RFC4821.
 431tcp_probe_threshold - INTEGER
 432        Controls when TCP Packetization-Layer Path MTU Discovery probing
 433        will stop in respect to the width of search range in bytes. Default
 434        is 8 bytes.
 436tcp_no_metrics_save - BOOLEAN
 437        By default, TCP saves various connection metrics in the route cache
 438        when the connection closes, so that connections established in the
 439        near future can use these to set initial conditions.  Usually, this
 440        increases overall performance, but may sometimes cause performance
 441        degradation.  If set, TCP will not cache metrics on closing
 442        connections.
 444tcp_orphan_retries - INTEGER
 445        This value influences the timeout of a locally closed TCP connection,
 446        when RTO retransmissions remain unacknowledged.
 447        See tcp_retries2 for more details.
 449        The default value is 8.
 450        If your machine is a loaded WEB server,
 451        you should think about lowering this value, such sockets
 452        may consume significant resources. Cf. tcp_max_orphans.
 454tcp_recovery - INTEGER
 455        This value is a bitmap to enable various experimental loss recovery
 456        features.
 458        RACK: 0x1 enables the RACK loss detection for fast detection of lost
 459              retransmissions and tail drops.
 461        Default: 0x1
 463tcp_reordering - INTEGER
 464        Initial reordering level of packets in a TCP stream.
 465        TCP stack can then dynamically adjust flow reordering level
 466        between this initial value and tcp_max_reordering
 467        Default: 3
 469tcp_max_reordering - INTEGER
 470        Maximal reordering level of packets in a TCP stream.
 471        300 is a fairly conservative value, but you might increase it
 472        if paths are using per packet load balancing (like bonding rr mode)
 473        Default: 300
 475tcp_retrans_collapse - BOOLEAN
 476        Bug-to-bug compatibility with some broken printers.
 477        On retransmit try to send bigger packets to work around bugs in
 478        certain TCP stacks.
 480tcp_retries1 - INTEGER
 481        This value influences the time, after which TCP decides, that
 482        something is wrong due to unacknowledged RTO retransmissions,
 483        and reports this suspicion to the network layer.
 484        See tcp_retries2 for more details.
 486        RFC 1122 recommends at least 3 retransmissions, which is the
 487        default.
 489tcp_retries2 - INTEGER
 490        This value influences the timeout of an alive TCP connection,
 491        when RTO retransmissions remain unacknowledged.
 492        Given a value of N, a hypothetical TCP connection following
 493        exponential backoff with an initial RTO of TCP_RTO_MIN would
 494        retransmit N times before killing the connection at the (N+1)th RTO.
 496        The default value of 15 yields a hypothetical timeout of 924.6
 497        seconds and is a lower bound for the effective timeout.
 498        TCP will effectively time out at the first RTO which exceeds the
 499        hypothetical timeout.
 501        RFC 1122 recommends at least 100 seconds for the timeout,
 502        which corresponds to a value of at least 8.
 504tcp_rfc1337 - BOOLEAN
 505        If set, the TCP stack behaves conforming to RFC1337. If unset,
 506        we are not conforming to RFC, but prevent TCP TIME_WAIT
 507        assassination.
 508        Default: 0
 510tcp_rmem - vector of 3 INTEGERs: min, default, max
 511        min: Minimal size of receive buffer used by TCP sockets.
 512        It is guaranteed to each TCP socket, even under moderate memory
 513        pressure.
 514        Default: 1 page
 516        default: initial size of receive buffer used by TCP sockets.
 517        This value overrides net.core.rmem_default used by other protocols.
 518        Default: 87380 bytes. This value results in window of 65535 with
 519        default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit
 520        less for default tcp_app_win. See below about these variables.
 522        max: maximal size of receive buffer allowed for automatically
 523        selected receiver buffers for TCP socket. This value does not override
 524        net.core.rmem_max.  Calling setsockopt() with SO_RCVBUF disables
 525        automatic tuning of that socket's receive buffer size, in which
 526        case this value is ignored.
 527        Default: between 87380B and 6MB, depending on RAM size.
 529tcp_sack - BOOLEAN
 530        Enable select acknowledgments (SACKS).
 532tcp_slow_start_after_idle - BOOLEAN
 533        If set, provide RFC2861 behavior and time out the congestion
 534        window after an idle period.  An idle period is defined at
 535        the current RTO.  If unset, the congestion window will not
 536        be timed out after an idle period.
 537        Default: 1
 539tcp_stdurg - BOOLEAN
 540        Use the Host requirements interpretation of the TCP urgent pointer field.
 541        Most hosts use the older BSD interpretation, so if you turn this on
 542        Linux might not communicate correctly with them.
 543        Default: FALSE
 545tcp_synack_retries - INTEGER
 546        Number of times SYNACKs for a passive TCP connection attempt will
 547        be retransmitted. Should not be higher than 255. Default value
 548        is 5, which corresponds to 31seconds till the last retransmission
 549        with the current initial RTO of 1second. With this the final timeout
 550        for a passive TCP connection will happen after 63seconds.
 552tcp_syncookies - BOOLEAN
 553        Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
 554        Send out syncookies when the syn backlog queue of a socket
 555        overflows. This is to prevent against the common 'SYN flood attack'
 556        Default: 1
 558        Note, that syncookies is fallback facility.
 559        It MUST NOT be used to help highly loaded servers to stand
 560        against legal connection rate. If you see SYN flood warnings
 561        in your logs, but investigation shows that they occur
 562        because of overload with legal connections, you should tune
 563        another parameters until this warning disappear.
 564        See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
 566        syncookies seriously violate TCP protocol, do not allow
 567        to use TCP extensions, can result in serious degradation
 568        of some services (f.e. SMTP relaying), visible not by you,
 569        but your clients and relays, contacting you. While you see
 570        SYN flood warnings in logs not being really flooded, your server
 571        is seriously misconfigured.
 573        If you want to test which effects syncookies have to your
 574        network connections you can set this knob to 2 to enable
 575        unconditionally generation of syncookies.
 577tcp_fastopen - INTEGER
 578        Enable TCP Fast Open (RFC7413) to send and accept data in the opening
 579        SYN packet.
 581        The client support is enabled by flag 0x1 (on by default). The client
 582        then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
 583        rather than connect() to send data in SYN.
 585        The server support is enabled by flag 0x2 (off by default). Then
 586        either enable for all listeners with another flag (0x400) or
 587        enable individual listeners via TCP_FASTOPEN socket option with
 588        the option value being the length of the syn-data backlog.
 590        The values (bitmap) are
 591          0x1: (client) enables sending data in the opening SYN on the client.
 592          0x2: (server) enables the server support, i.e., allowing data in
 593                        a SYN packet to be accepted and passed to the
 594                        application before 3-way handshake finishes.
 595          0x4: (client) send data in the opening SYN regardless of cookie
 596                        availability and without a cookie option.
 597        0x200: (server) accept data-in-SYN w/o any cookie option present.
 598        0x400: (server) enable all listeners to support Fast Open by
 599                        default without explicit TCP_FASTOPEN socket option.
 601        Default: 0x1
 603        Note that that additional client or server features are only
 604        effective if the basic support (0x1 and 0x2) are enabled respectively.
 606tcp_syn_retries - INTEGER
 607        Number of times initial SYNs for an active TCP connection attempt
 608        will be retransmitted. Should not be higher than 127. Default value
 609        is 6, which corresponds to 63seconds till the last retransmission
 610        with the current initial RTO of 1second. With this the final timeout
 611        for an active TCP connection attempt will happen after 127seconds.
 613tcp_timestamps - INTEGER
 614Enable timestamps as defined in RFC1323.
 615        0: Disabled.
 616        1: Enable timestamps as defined in RFC1323 and use random offset for
 617        each connection rather than only using the current time.
 618        2: Like 1, but without random offsets.
 619        Default: 1
 621tcp_min_tso_segs - INTEGER
 622        Minimal number of segments per TSO frame.
 623        Since linux-3.12, TCP does an automatic sizing of TSO frames,
 624        depending on flow rate, instead of filling 64Kbytes packets.
 625        For specific usages, it's possible to force TCP to build big
 626        TSO frames. Note that TCP stack might split too big TSO packets
 627        if available window is too small.
 628        Default: 2
 630tcp_pacing_ss_ratio - INTEGER
 631        sk->sk_pacing_rate is set by TCP stack using a ratio applied
 632        to current rate. (current_rate = cwnd * mss / srtt)
 633        If TCP is in slow start, tcp_pacing_ss_ratio is applied
 634        to let TCP probe for bigger speeds, assuming cwnd can be
 635        doubled every other RTT.
 636        Default: 200
 638tcp_pacing_ca_ratio - INTEGER
 639        sk->sk_pacing_rate is set by TCP stack using a ratio applied
 640        to current rate. (current_rate = cwnd * mss / srtt)
 641        If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
 642        is applied to conservatively probe for bigger throughput.
 643        Default: 120
 645tcp_tso_win_divisor - INTEGER
 646        This allows control over what percentage of the congestion window
 647        can be consumed by a single TSO frame.
 648        The setting of this parameter is a choice between burstiness and
 649        building larger TSO frames.
 650        Default: 3
 652tcp_tw_recycle - BOOLEAN
 653        Enable fast recycling TIME-WAIT sockets. Default value is 0.
 654        It should not be changed without advice/request of technical
 655        experts.
 657tcp_tw_reuse - BOOLEAN
 658        Allow to reuse TIME-WAIT sockets for new connections when it is
 659        safe from protocol viewpoint. Default value is 0.
 660        It should not be changed without advice/request of technical
 661        experts.
 663tcp_window_scaling - BOOLEAN
 664        Enable window scaling as defined in RFC1323.
 666tcp_wmem - vector of 3 INTEGERs: min, default, max
 667        min: Amount of memory reserved for send buffers for TCP sockets.
 668        Each TCP socket has rights to use it due to fact of its birth.
 669        Default: 1 page
 671        default: initial size of send buffer used by TCP sockets.  This
 672        value overrides net.core.wmem_default used by other protocols.
 673        It is usually lower than net.core.wmem_default.
 674        Default: 16K
 676        max: Maximal amount of memory allowed for automatically tuned
 677        send buffers for TCP sockets. This value does not override
 678        net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
 679        automatic tuning of that socket's send buffer size, in which case
 680        this value is ignored.
 681        Default: between 64K and 4MB, depending on RAM size.
 683tcp_notsent_lowat - UNSIGNED INTEGER
 684        A TCP socket can control the amount of unsent bytes in its write queue,
 685        thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
 686        reports POLLOUT events if the amount of unsent bytes is below a per
 687        socket value, and if the write queue is not full. sendmsg() will
 688        also not add new buffers if the limit is hit.
 690        This global variable controls the amount of unsent data for
 691        sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
 692        to the global variable has immediate effect.
 694        Default: UINT_MAX (0xFFFFFFFF)
 696tcp_workaround_signed_windows - BOOLEAN
 697        If set, assume no receipt of a window scaling option means the
 698        remote TCP is broken and treats the window as a signed quantity.
 699        If unset, assume the remote TCP is not broken even if we do
 700        not receive a window scaling option from them.
 701        Default: 0
 703tcp_thin_linear_timeouts - BOOLEAN
 704        Enable dynamic triggering of linear timeouts for thin streams.
 705        If set, a check is performed upon retransmission by timeout to
 706        determine if the stream is thin (less than 4 packets in flight).
 707        As long as the stream is found to be thin, up to 6 linear
 708        timeouts may be performed before exponential backoff mode is
 709        initiated. This improves retransmission latency for
 710        non-aggressive thin streams, often found to be time-dependent.
 711        For more information on thin streams, see
 712        Documentation/networking/tcp-thin.txt
 713        Default: 0
 715tcp_thin_dupack - BOOLEAN
 716        Enable dynamic triggering of retransmissions after one dupACK
 717        for thin streams. If set, a check is performed upon reception
 718        of a dupACK to determine if the stream is thin (less than 4
 719        packets in flight). As long as the stream is found to be thin,
 720        data is retransmitted on the first received dupACK. This
 721        improves retransmission latency for non-aggressive thin
 722        streams, often found to be time-dependent.
 723        For more information on thin streams, see
 724        Documentation/networking/tcp-thin.txt
 725        Default: 0
 727tcp_limit_output_bytes - INTEGER
 728        Controls TCP Small Queue limit per tcp socket.
 729        TCP bulk sender tends to increase packets in flight until it
 730        gets losses notifications. With SNDBUF autotuning, this can
 731        result in a large amount of packets queued in qdisc/device
 732        on the local machine, hurting latency of other flows, for
 733        typical pfifo_fast qdiscs.
 734        tcp_limit_output_bytes limits the number of bytes on qdisc
 735        or device to reduce artificial RTT/cwnd and reduce bufferbloat.
 736        Default: 262144
 738tcp_challenge_ack_limit - INTEGER
 739        Limits number of Challenge ACK sent per second, as recommended
 740        in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
 741        Default: 100
 743UDP variables:
 745udp_mem - vector of 3 INTEGERs: min, pressure, max
 746        Number of pages allowed for queueing by all UDP sockets.
 748        min: Below this number of pages UDP is not bothered about its
 749        memory appetite. When amount of memory allocated by UDP exceeds
 750        this number, UDP starts to moderate memory usage.
 752        pressure: This value was introduced to follow format of tcp_mem.
 754        max: Number of pages allowed for queueing by all UDP sockets.
 756        Default is calculated at boot time from amount of available memory.
 758udp_rmem_min - INTEGER
 759        Minimal size of receive buffer used by UDP sockets in moderation.
 760        Each UDP socket is able to use the size for receiving data, even if
 761        total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
 762        Default: 1 page
 764udp_wmem_min - INTEGER
 765        Minimal size of send buffer used by UDP sockets in moderation.
 766        Each UDP socket is able to use the size for sending data, even if
 767        total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
 768        Default: 1 page
 770CIPSOv4 Variables:
 772cipso_cache_enable - BOOLEAN
 773        If set, enable additions to and lookups from the CIPSO label mapping
 774        cache.  If unset, additions are ignored and lookups always result in a
 775        miss.  However, regardless of the setting the cache is still
 776        invalidated when required when means you can safely toggle this on and
 777        off and the cache will always be "safe".
 778        Default: 1
 780cipso_cache_bucket_size - INTEGER
 781        The CIPSO label cache consists of a fixed size hash table with each
 782        hash bucket containing a number of cache entries.  This variable limits
 783        the number of entries in each hash bucket; the larger the value the
 784        more CIPSO label mappings that can be cached.  When the number of
 785        entries in a given hash bucket reaches this limit adding new entries
 786        causes the oldest entry in the bucket to be removed to make room.
 787        Default: 10
 789cipso_rbm_optfmt - BOOLEAN
 790        Enable the "Optimized Tag 1 Format" as defined in section of
 791        the CIPSO draft specification (see Documentation/netlabel for details).
 792        This means that when set the CIPSO tag will be padded with empty
 793        categories in order to make the packet data 32-bit aligned.
 794        Default: 0
 796cipso_rbm_structvalid - BOOLEAN
 797        If set, do a very strict check of the CIPSO option when
 798        ip_options_compile() is called.  If unset, relax the checks done during
 799        ip_options_compile().  Either way is "safe" as errors are caught else
 800        where in the CIPSO processing code but setting this to 0 (False) should
 801        result in less work (i.e. it should be faster) but could cause problems
 802        with other implementations that require strict checking.
 803        Default: 0
 805IP Variables:
 807ip_local_port_range - 2 INTEGERS
 808        Defines the local port range that is used by TCP and UDP to
 809        choose the local port. The first number is the first, the
 810        second the last local port number.
 811        If possible, it is better these numbers have different parity.
 812        (one even and one odd values)
 813        The default values are 32768 and 60999 respectively.
 815ip_local_reserved_ports - list of comma separated ranges
 816        Specify the ports which are reserved for known third-party
 817        applications. These ports will not be used by automatic port
 818        assignments (e.g. when calling connect() or bind() with port
 819        number 0). Explicit port allocation behavior is unchanged.
 821        The format used for both input and output is a comma separated
 822        list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
 823        10). Writing to the file will clear all previously reserved
 824        ports and update the current list with the one given in the
 825        input.
 827        Note that ip_local_port_range and ip_local_reserved_ports
 828        settings are independent and both are considered by the kernel
 829        when determining which ports are available for automatic port
 830        assignments.
 832        You can reserve ports which are not in the current
 833        ip_local_port_range, e.g.:
 835        $ cat /proc/sys/net/ipv4/ip_local_port_range
 836        32000   60999
 837        $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
 838        8080,9148
 840        although this is redundant. However such a setting is useful
 841        if later the port range is changed to a value that will
 842        include the reserved ports.
 844        Default: Empty
 846ip_nonlocal_bind - BOOLEAN
 847        If set, allows processes to bind() to non-local IP addresses,
 848        which can be quite useful - but may break some applications.
 849        Default: 0
 851ip_dynaddr - BOOLEAN
 852        If set non-zero, enables support for dynamic addresses.
 853        If set to a non-zero value larger than 1, a kernel log
 854        message will be printed when dynamic address rewriting
 855        occurs.
 856        Default: 0
 858ip_early_demux - BOOLEAN
 859        Optimize input packet processing down to one demux for
 860        certain kinds of local sockets.  Currently we only do this
 861        for established TCP sockets.
 863        It may add an additional cost for pure routing workloads that
 864        reduces overall throughput, in such case you should disable it.
 865        Default: 1
 867icmp_echo_ignore_all - BOOLEAN
 868        If set non-zero, then the kernel will ignore all ICMP ECHO
 869        requests sent to it.
 870        Default: 0
 872icmp_echo_ignore_broadcasts - BOOLEAN
 873        If set non-zero, then the kernel will ignore all ICMP ECHO and
 874        TIMESTAMP requests sent to it via broadcast/multicast.
 875        Default: 1
 877icmp_ratelimit - INTEGER
 878        Limit the maximal rates for sending ICMP packets whose type matches
 879        icmp_ratemask (see below) to specific targets.
 880        0 to disable any limiting,
 881        otherwise the minimal space between responses in milliseconds.
 882        Note that another sysctl, icmp_msgs_per_sec limits the number
 883        of ICMP packets sent on all targets.
 884        Default: 1000
 886icmp_msgs_per_sec - INTEGER
 887        Limit maximal number of ICMP packets sent per second from this host.
 888        Only messages whose type matches icmp_ratemask (see below) are
 889        controlled by this limit.
 890        Default: 1000
 892icmp_msgs_burst - INTEGER
 893        icmp_msgs_per_sec controls number of ICMP packets sent per second,
 894        while icmp_msgs_burst controls the burst size of these packets.
 895        Default: 50
 897icmp_ratemask - INTEGER
 898        Mask made of ICMP types for which rates are being limited.
 899        Significant bits: IHGFEDCBA9876543210
 900        Default mask:     0000001100000011000 (6168)
 902        Bit definitions (see include/linux/icmp.h):
 903                0 Echo Reply
 904                3 Destination Unreachable *
 905                4 Source Quench *
 906                5 Redirect
 907                8 Echo Request
 908                B Time Exceeded *
 909                C Parameter Problem *
 910                D Timestamp Request
 911                E Timestamp Reply
 912                F Info Request
 913                G Info Reply
 914                H Address Mask Request
 915                I Address Mask Reply
 917        * These are rate limited by default (see default mask above)
 919icmp_ignore_bogus_error_responses - BOOLEAN
 920        Some routers violate RFC1122 by sending bogus responses to broadcast
 921        frames.  Such violations are normally logged via a kernel warning.
 922        If this is set to TRUE, the kernel will not give such warnings, which
 923        will avoid log file clutter.
 924        Default: 1
 926icmp_errors_use_inbound_ifaddr - BOOLEAN
 928        If zero, icmp error messages are sent with the primary address of
 929        the exiting interface.
 931        If non-zero, the message will be sent with the primary address of
 932        the interface that received the packet that caused the icmp error.
 933        This is the behaviour network many administrators will expect from
 934        a router. And it can make debugging complicated network layouts
 935        much easier.
 937        Note that if no primary address exists for the interface selected,
 938        then the primary address of the first non-loopback interface that
 939        has one will be used regardless of this setting.
 941        Default: 0
 943igmp_max_memberships - INTEGER
 944        Change the maximum number of multicast groups we can subscribe to.
 945        Default: 20
 947        Theoretical maximum value is bounded by having to send a membership
 948        report in a single datagram (i.e. the report can't span multiple
 949        datagrams, or risk confusing the switch and leaving groups you don't
 950        intend to).
 952        The number of supported groups 'M' is bounded by the number of group
 953        report entries you can fit into a single datagram of 65535 bytes.
 955        M = 65536-sizeof (ip header)/(sizeof(Group record))
 957        Group records are variable length, with a minimum of 12 bytes.
 958        So net.ipv4.igmp_max_memberships should not be set higher than:
 960        (65536-24) / 12 = 5459
 962        The value 5459 assumes no IP header options, so in practice
 963        this number may be lower.
 965igmp_max_msf - INTEGER
 966        Maximum number of addresses allowed in the source filter list for a
 967        multicast group.
 968        Default: 10
 970igmp_qrv - INTEGER
 971        Controls the IGMP query robustness variable (see RFC2236 8.1).
 972        Default: 2 (as specified by RFC2236 8.1)
 973        Minimum: 1 (as specified by RFC6636 4.5)
 975force_igmp_version - INTEGER
 976        0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
 977            allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
 978            Present timer expires.
 979        1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
 980            receive IGMPv2/v3 query.
 981        2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
 982            IGMPv1 query message. Will reply report if receive IGMPv3 query.
 983        3 - Enforce to use IGMP version 3. The same react with default 0.
 985        Note: this is not the same with force_mld_version because IGMPv3 RFC3376
 986        Security Considerations does not have clear description that we could
 987        ignore other version messages completely as MLDv2 RFC3810. So make
 988        this value as default 0 is recommended.
 990conf/interface/*  changes special settings per interface (where
 991"interface" is the name of your network interface)
 993conf/all/*        is special, changes the settings for all interfaces
 995log_martians - BOOLEAN
 996        Log packets with impossible addresses to kernel log.
 997        log_martians for the interface will be enabled if at least one of
 998        conf/{all,interface}/log_martians is set to TRUE,
 999        it will be disabled otherwise
1001accept_redirects - BOOLEAN
1002        Accept ICMP redirect messages.
1003        accept_redirects for the interface will be enabled if:
1004        - both conf/{all,interface}/accept_redirects are TRUE in the case
1005          forwarding for the interface is enabled
1006        or
1007        - at least one of conf/{all,interface}/accept_redirects is TRUE in the
1008          case forwarding for the interface is disabled
1009        accept_redirects for the interface will be disabled otherwise
1010        default TRUE (host)
1011                FALSE (router)
1013forwarding - BOOLEAN
1014        Enable IP forwarding on this interface.
1016mc_forwarding - BOOLEAN
1017        Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
1018        and a multicast routing daemon is required.
1019        conf/all/mc_forwarding must also be set to TRUE to enable multicast
1020        routing for the interface
1022medium_id - INTEGER
1023        Integer value used to differentiate the devices by the medium they
1024        are attached to. Two devices can have different id values when
1025        the broadcast packets are received only on one of them.
1026        The default value 0 means that the device is the only interface
1027        to its medium, value of -1 means that medium is not known.
1029        Currently, it is used to change the proxy_arp behavior:
1030        the proxy_arp feature is enabled for packets forwarded between
1031        two devices attached to different media.
1033proxy_arp - BOOLEAN
1034        Do proxy arp.
1035        proxy_arp for the interface will be enabled if at least one of
1036        conf/{all,interface}/proxy_arp is set to TRUE,
1037        it will be disabled otherwise
1039proxy_arp_pvlan - BOOLEAN
1040        Private VLAN proxy arp.
1041        Basically allow proxy arp replies back to the same interface
1042        (from which the ARP request/solicitation was received).
1044        This is done to support (ethernet) switch features, like RFC
1045        3069, where the individual ports are NOT allowed to
1046        communicate with each other, but they are allowed to talk to
1047        the upstream router.  As described in RFC 3069, it is possible
1048        to allow these hosts to communicate through the upstream
1049        router by proxy_arp'ing. Don't need to be used together with
1050        proxy_arp.
1052        This technology is known by different names:
1053          In RFC 3069 it is called VLAN Aggregation.
1054          Cisco and Allied Telesyn call it Private VLAN.
1055          Hewlett-Packard call it Source-Port filtering or port-isolation.
1056          Ericsson call it MAC-Forced Forwarding (RFC Draft).
1058shared_media - BOOLEAN
1059        Send(router) or accept(host) RFC1620 shared media redirects.
1060        Overrides secure_redirects.
1061        shared_media for the interface will be enabled if at least one of
1062        conf/{all,interface}/shared_media is set to TRUE,
1063        it will be disabled otherwise
1064        default TRUE
1066secure_redirects - BOOLEAN
1067        Accept ICMP redirect messages only to gateways listed in the
1068        interface's current gateway list. Even if disabled, RFC1122 redirect
1069        rules still apply.
1070        Overridden by shared_media.
1071        secure_redirects for the interface will be enabled if at least one of
1072        conf/{all,interface}/secure_redirects is set to TRUE,
1073        it will be disabled otherwise
1074        default TRUE
1076send_redirects - BOOLEAN
1077        Send redirects, if router.
1078        send_redirects for the interface will be enabled if at least one of
1079        conf/{all,interface}/send_redirects is set to TRUE,
1080        it will be disabled otherwise
1081        Default: TRUE
1083bootp_relay - BOOLEAN
1084        Accept packets with source address 0.b.c.d destined
1085        not to this host as local ones. It is supposed, that
1086        BOOTP relay daemon will catch and forward such packets.
1087        conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
1088        for the interface
1089        default FALSE
1090        Not Implemented Yet.
1092accept_source_route - BOOLEAN
1093        Accept packets with SRR option.
1094        conf/all/accept_source_route must also be set to TRUE to accept packets
1095        with SRR option on the interface
1096        default TRUE (router)
1097                FALSE (host)
1099accept_local - BOOLEAN
1100        Accept packets with local source addresses. In combination with
1101        suitable routing, this can be used to direct packets between two
1102        local interfaces over the wire and have them accepted properly.
1103        default FALSE
1105route_localnet - BOOLEAN
1106        Do not consider loopback addresses as martian source or destination
1107        while routing. This enables the use of 127/8 for local routing purposes.
1108        default FALSE
1110rp_filter - INTEGER
1111        0 - No source validation.
1112        1 - Strict mode as defined in RFC3704 Strict Reverse Path
1113            Each incoming packet is tested against the FIB and if the interface
1114            is not the best reverse path the packet check will fail.
1115            By default failed packets are discarded.
1116        2 - Loose mode as defined in RFC3704 Loose Reverse Path
1117            Each incoming packet's source address is also tested against the FIB
1118            and if the source address is not reachable via any interface
1119            the packet check will fail.
1121        Current recommended practice in RFC3704 is to enable strict mode
1122        to prevent IP spoofing from DDos attacks. If using asymmetric routing
1123        or other complicated routing, then loose mode is recommended.
1125        The max value from conf/{all,interface}/rp_filter is used
1126        when doing source validation on the {interface}.
1128        Default value is 0. Note that some distributions enable it
1129        in startup scripts.
1131arp_filter - BOOLEAN
1132        1 - Allows you to have multiple network interfaces on the same
1133        subnet, and have the ARPs for each interface be answered
1134        based on whether or not the kernel would route a packet from
1135        the ARP'd IP out that interface (therefore you must use source
1136        based routing for this to work). In other words it allows control
1137        of which cards (usually 1) will respond to an arp request.
1139        0 - (default) The kernel can respond to arp requests with addresses
1140        from other interfaces. This may seem wrong but it usually makes
1141        sense, because it increases the chance of successful communication.
1142        IP addresses are owned by the complete host on Linux, not by
1143        particular interfaces. Only for more complex setups like load-
1144        balancing, does this behaviour cause problems.
1146        arp_filter for the interface will be enabled if at least one of
1147        conf/{all,interface}/arp_filter is set to TRUE,
1148        it will be disabled otherwise
1150arp_announce - INTEGER
1151        Define different restriction levels for announcing the local
1152        source IP address from IP packets in ARP requests sent on
1153        interface:
1154        0 - (default) Use any local address, configured on any interface
1155        1 - Try to avoid local addresses that are not in the target's
1156        subnet for this interface. This mode is useful when target
1157        hosts reachable via this interface require the source IP
1158        address in ARP requests to be part of their logical network
1159        configured on the receiving interface. When we generate the
1160        request we will check all our subnets that include the
1161        target IP and will preserve the source address if it is from
1162        such subnet. If there is no such subnet we select source
1163        address according to the rules for level 2.
1164        2 - Always use the best local address for this target.
1165        In this mode we ignore the source address in the IP packet
1166        and try to select local address that we prefer for talks with
1167        the target host. Such local address is selected by looking
1168        for primary IP addresses on all our subnets on the outgoing
1169        interface that include the target IP address. If no suitable
1170        local address is found we select the first local address
1171        we have on the outgoing interface or on all other interfaces,
1172        with the hope we will receive reply for our request and
1173        even sometimes no matter the source IP address we announce.
1175        The max value from conf/{all,interface}/arp_announce is used.
1177        Increasing the restriction level gives more chance for
1178        receiving answer from the resolved target while decreasing
1179        the level announces more valid sender's information.
1181arp_ignore - INTEGER
1182        Define different modes for sending replies in response to
1183        received ARP requests that resolve local target IP addresses:
1184        0 - (default): reply for any local target IP address, configured
1185        on any interface
1186        1 - reply only if the target IP address is local address
1187        configured on the incoming interface
1188        2 - reply only if the target IP address is local address
1189        configured on the incoming interface and both with the
1190        sender's IP address are part from same subnet on this interface
1191        3 - do not reply for local addresses configured with scope host,
1192        only resolutions for global and link addresses are replied
1193        4-7 - reserved
1194        8 - do not reply for all local addresses
1196        The max value from conf/{all,interface}/arp_ignore is used
1197        when ARP request is received on the {interface}
1199arp_notify - BOOLEAN
1200        Define mode for notification of address and device changes.
1201        0 - (default): do nothing
1202        1 - Generate gratuitous arp requests when device is brought up
1203            or hardware address changes.
1205arp_accept - BOOLEAN
1206        Define behavior for gratuitous ARP frames who's IP is not
1207        already present in the ARP table:
1208        0 - don't create new entries in the ARP table
1209        1 - create new entries in the ARP table
1211        Both replies and requests type gratuitous arp will trigger the
1212        ARP table to be updated, if this setting is on.
1214        If the ARP table already contains the IP address of the
1215        gratuitous arp frame, the arp table will be updated regardless
1216        if this setting is on or off.
1218mcast_solicit - INTEGER
1219        The maximum number of multicast probes in INCOMPLETE state,
1220        when the associated hardware address is unknown.  Defaults
1221        to 3.
1223ucast_solicit - INTEGER
1224        The maximum number of unicast probes in PROBE state, when
1225        the hardware address is being reconfirmed.  Defaults to 3.
1227app_solicit - INTEGER
1228        The maximum number of probes to send to the user space ARP daemon
1229        via netlink before dropping back to multicast probes (see
1230        mcast_resolicit).  Defaults to 0.
1232mcast_resolicit - INTEGER
1233        The maximum number of multicast probes after unicast and
1234        app probes in PROBE state.  Defaults to 0.
1236disable_policy - BOOLEAN
1237        Disable IPSEC policy (SPD) for this interface
1239disable_xfrm - BOOLEAN
1240        Disable IPSEC encryption on this interface, whatever the policy
1242igmpv2_unsolicited_report_interval - INTEGER
1243        The interval in milliseconds in which the next unsolicited
1244        IGMPv1 or IGMPv2 report retransmit will take place.
1245        Default: 10000 (10 seconds)
1247igmpv3_unsolicited_report_interval - INTEGER
1248        The interval in milliseconds in which the next unsolicited
1249        IGMPv3 report retransmit will take place.
1250        Default: 1000 (1 seconds)
1252promote_secondaries - BOOLEAN
1253        When a primary IP address is removed from this interface
1254        promote a corresponding secondary IP address instead of
1255        removing all the corresponding secondary IP addresses.
1257drop_unicast_in_l2_multicast - BOOLEAN
1258        Drop any unicast IP packets that are received in link-layer
1259        multicast (or broadcast) frames.
1260        This behavior (for multicast) is actually a SHOULD in RFC
1261        1122, but is disabled by default for compatibility reasons.
1262        Default: off (0)
1264drop_gratuitous_arp - BOOLEAN
1265        Drop all gratuitous ARP frames, for example if there's a known
1266        good ARP proxy on the network and such frames need not be used
1267        (or in the case of 802.11, must not be used to prevent attacks.)
1268        Default: off (0)
1271tag - INTEGER
1272        Allows you to write a number, which can be used as required.
1273        Default value is 0.
1275xfrm4_gc_thresh - INTEGER
1276        The threshold at which we will start garbage collecting for IPv4
1277        destination cache entries.  At twice this value the system will
1278        refuse new allocations. The value must be set below the flowcache
1279        limit (4096 * number of online cpus) to take effect.
1281igmp_link_local_mcast_reports - BOOLEAN
1282        Enable IGMP reports for link local multicast groups in the
1283        224.0.0.X range.
1284        Default TRUE
1286Alexey Kuznetsov.
1289Updated by:
1290Andi Kleen
1292Nicolas Delon
1298/proc/sys/net/ipv6/* Variables:
1300IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
1301apply to IPv6 [XXX?].
1303bindv6only - BOOLEAN
1304        Default value for IPV6_V6ONLY socket option,
1305        which restricts use of the IPv6 socket to IPv6 communication
1306        only.
1307                TRUE: disable IPv4-mapped address feature
1308                FALSE: enable IPv4-mapped address feature
1310        Default: FALSE (as specified in RFC3493)
1312flowlabel_consistency - BOOLEAN
1313        Protect the consistency (and unicity) of flow label.
1314        You have to disable it to use IPV6_FL_F_REFLECT flag on the
1315        flow label manager.
1316        TRUE: enabled
1317        FALSE: disabled
1318        Default: TRUE
1320auto_flowlabels - INTEGER
1321        Automatically generate flow labels based on a flow hash of the
1322        packet. This allows intermediate devices, such as routers, to
1323        identify packet flows for mechanisms like Equal Cost Multipath
1324        Routing (see RFC 6438).
1325        0: automatic flow labels are completely disabled
1326        1: automatic flow labels are enabled by default, they can be
1327           disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
1328           socket option
1329        2: automatic flow labels are allowed, they may be enabled on a
1330           per socket basis using the IPV6_AUTOFLOWLABEL socket option
1331        3: automatic flow labels are enabled and enforced, they cannot
1332           be disabled by the socket option
1333        Default: 1
1335flowlabel_state_ranges - BOOLEAN
1336        Split the flow label number space into two ranges. 0-0x7FFFF is
1337        reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
1338        is reserved for stateless flow labels as described in RFC6437.
1339        TRUE: enabled
1340        FALSE: disabled
1341        Default: true
1343anycast_src_echo_reply - BOOLEAN
1344        Controls the use of anycast addresses as source addresses for ICMPv6
1345        echo reply
1346        TRUE:  enabled
1347        FALSE: disabled
1348        Default: FALSE
1350idgen_delay - INTEGER
1351        Controls the delay in seconds after which time to retry
1352        privacy stable address generation if a DAD conflict is
1353        detected.
1354        Default: 1 (as specified in RFC7217)
1356idgen_retries - INTEGER
1357        Controls the number of retries to generate a stable privacy
1358        address if a DAD conflict is detected.
1359        Default: 3 (as specified in RFC7217)
1361mld_qrv - INTEGER
1362        Controls the MLD query robustness variable (see RFC3810 9.1).
1363        Default: 2 (as specified by RFC3810 9.1)
1364        Minimum: 1 (as specified by RFC6636 4.5)
1366IPv6 Fragmentation:
1368ip6frag_high_thresh - INTEGER
1369        Maximum memory used to reassemble IPv6 fragments. When
1370        ip6frag_high_thresh bytes of memory is allocated for this purpose,
1371        the fragment handler will toss packets until ip6frag_low_thresh
1372        is reached.
1374ip6frag_low_thresh - INTEGER
1375        See ip6frag_high_thresh
1377ip6frag_time - INTEGER
1378        Time in seconds to keep an IPv6 fragment in memory.
1381        Change the interface-specific default settings.
1385        Change all the interface-specific settings.
1387        [XXX:  Other special features than forwarding?]
1389conf/all/forwarding - BOOLEAN
1390        Enable global IPv6 forwarding between all interfaces.
1392        IPv4 and IPv6 work differently here; e.g. netfilter must be used
1393        to control which interfaces may forward packets and which not.
1395        This also sets all interfaces' Host/Router setting
1396        'forwarding' to the specified value.  See below for details.
1398        This referred to as global forwarding.
1400proxy_ndp - BOOLEAN
1401        Do proxy ndp.
1403fwmark_reflect - BOOLEAN
1404        Controls the fwmark of kernel-generated IPv6 reply packets that are not
1405        associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
1406        If unset, these packets have a fwmark of zero. If set, they have the
1407        fwmark of the packet they are replying to.
1408        Default: 0
1411        Change special settings per interface.
1413        The functional behaviour for certain settings is different
1414        depending on whether local forwarding is enabled or not.
1416accept_ra - INTEGER
1417        Accept Router Advertisements; autoconfigure using them.
1419        It also determines whether or not to transmit Router
1420        Solicitations. If and only if the functional setting is to
1421        accept Router Advertisements, Router Solicitations will be
1422        transmitted.
1424        Possible values are:
1425                0 Do not accept Router Advertisements.
1426                1 Accept Router Advertisements if forwarding is disabled.
1427                2 Overrule forwarding behaviour. Accept Router Advertisements
1428                  even if forwarding is enabled.
1430        Functional default: enabled if local forwarding is disabled.
1431                            disabled if local forwarding is enabled.
1433accept_ra_defrtr - BOOLEAN
1434        Learn default router in Router Advertisement.
1436        Functional default: enabled if accept_ra is enabled.
1437                            disabled if accept_ra is disabled.
1439accept_ra_from_local - BOOLEAN
1440        Accept RA with source-address that is found on local machine
1441        if the RA is otherwise proper and able to be accepted.
1442        Default is to NOT accept these as it may be an un-intended
1443        network loop.
1445        Functional default:
1446           enabled if accept_ra_from_local is enabled
1447               on a specific interface.
1448           disabled if accept_ra_from_local is disabled
1449               on a specific interface.
1451accept_ra_min_hop_limit - INTEGER
1452        Minimum hop limit Information in Router Advertisement.
1454        Hop limit Information in Router Advertisement less than this
1455        variable shall be ignored.
1457        Default: 1
1459accept_ra_pinfo - BOOLEAN
1460        Learn Prefix Information in Router Advertisement.
1462        Functional default: enabled if accept_ra is enabled.
1463                            disabled if accept_ra is disabled.
1465accept_ra_rt_info_max_plen - INTEGER
1466        Maximum prefix length of Route Information in RA.
1468        Route Information w/ prefix larger than or equal to this
1469        variable shall be ignored.
1471        Functional default: 0 if accept_ra_rtr_pref is enabled.
1472                            -1 if accept_ra_rtr_pref is disabled.
1474accept_ra_rtr_pref - BOOLEAN
1475        Accept Router Preference in RA.
1477        Functional default: enabled if accept_ra is enabled.
1478                            disabled if accept_ra is disabled.
1480accept_ra_mtu - BOOLEAN
1481        Apply the MTU value specified in RA option 5 (RFC4861). If
1482        disabled, the MTU specified in the RA will be ignored.
1484        Functional default: enabled if accept_ra is enabled.
1485                            disabled if accept_ra is disabled.
1487accept_redirects - BOOLEAN
1488        Accept Redirects.
1490        Functional default: enabled if local forwarding is disabled.
1491                            disabled if local forwarding is enabled.
1493accept_source_route - INTEGER
1494        Accept source routing (routing extension header).
1496        >= 0: Accept only routing header type 2.
1497        < 0: Do not accept routing header.
1499        Default: 0
1501autoconf - BOOLEAN
1502        Autoconfigure addresses using Prefix Information in Router
1503        Advertisements.
1505        Functional default: enabled if accept_ra_pinfo is enabled.
1506                            disabled if accept_ra_pinfo is disabled.
1508dad_transmits - INTEGER
1509        The amount of Duplicate Address Detection probes to send.
1510        Default: 1
1512forwarding - INTEGER
1513        Configure interface-specific Host/Router behaviour.
1515        Note: It is recommended to have the same setting on all
1516        interfaces; mixed router/host scenarios are rather uncommon.
1518        Possible values are:
1519                0 Forwarding disabled
1520                1 Forwarding enabled
1522        FALSE (0):
1524        By default, Host behaviour is assumed.  This means:
1526        1. IsRouter flag is not set in Neighbour Advertisements.
1527        2. If accept_ra is TRUE (default), transmit Router
1528           Solicitations.
1529        3. If accept_ra is TRUE (default), accept Router
1530           Advertisements (and do autoconfiguration).
1531        4. If accept_redirects is TRUE (default), accept Redirects.
1533        TRUE (1):
1535        If local forwarding is enabled, Router behaviour is assumed.
1536        This means exactly the reverse from the above:
1538        1. IsRouter flag is set in Neighbour Advertisements.
1539        2. Router Solicitations are not sent unless accept_ra is 2.
1540        3. Router Advertisements are ignored unless accept_ra is 2.
1541        4. Redirects are ignored.
1543        Default: 0 (disabled) if global forwarding is disabled (default),
1544                 otherwise 1 (enabled).
1546hop_limit - INTEGER
1547        Default Hop Limit to set.
1548        Default: 64
1550mtu - INTEGER
1551        Default Maximum Transfer Unit
1552        Default: 1280 (IPv6 required minimum)
1554ip_nonlocal_bind - BOOLEAN
1555        If set, allows processes to bind() to non-local IPv6 addresses,
1556        which can be quite useful - but may break some applications.
1557        Default: 0
1559router_probe_interval - INTEGER
1560        Minimum interval (in seconds) between Router Probing described
1561        in RFC4191.
1563        Default: 60
1565router_solicitation_delay - INTEGER
1566        Number of seconds to wait after interface is brought up
1567        before sending Router Solicitations.
1568        Default: 1
1570router_solicitation_interval - INTEGER
1571        Number of seconds to wait between Router Solicitations.
1572        Default: 4
1574router_solicitations - INTEGER
1575        Number of Router Solicitations to send until assuming no
1576        routers are present.
1577        Default: 3
1579use_oif_addrs_only - BOOLEAN
1580        When enabled, the candidate source addresses for destinations
1581        routed via this interface are restricted to the set of addresses
1582        configured on this interface (vis. RFC 6724, section 4).
1584        Default: false
1586use_tempaddr - INTEGER
1587        Preference for Privacy Extensions (RFC3041).
1588          <= 0 : disable Privacy Extensions
1589          == 1 : enable Privacy Extensions, but prefer public
1590                 addresses over temporary addresses.
1591          >  1 : enable Privacy Extensions and prefer temporary
1592                 addresses over public addresses.
1593        Default:  0 (for most devices)
1594                 -1 (for point-to-point devices and loopback devices)
1596temp_valid_lft - INTEGER
1597        valid lifetime (in seconds) for temporary addresses.
1598        Default: 604800 (7 days)
1600temp_prefered_lft - INTEGER
1601        Preferred lifetime (in seconds) for temporary addresses.
1602        Default: 86400 (1 day)
1604keep_addr_on_down - INTEGER
1605        Keep all IPv6 addresses on an interface down event. If set static
1606        global addresses with no expiration time are not flushed.
1607          >0 : enabled
1608           0 : system default
1609          <0 : disabled
1611        Default: 0 (addresses are removed)
1613max_desync_factor - INTEGER
1614        Maximum value for DESYNC_FACTOR, which is a random value
1615        that ensures that clients don't synchronize with each
1616        other and generate new addresses at exactly the same time.
1617        value is in seconds.
1618        Default: 600
1620regen_max_retry - INTEGER
1621        Number of attempts before give up attempting to generate
1622        valid temporary addresses.
1623        Default: 5
1625max_addresses - INTEGER
1626        Maximum number of autoconfigured addresses per interface.  Setting
1627        to zero disables the limitation.  It is not recommended to set this
1628        value too large (or to zero) because it would be an easy way to
1629        crash the kernel by allowing too many addresses to be created.
1630        Default: 16
1632disable_ipv6 - BOOLEAN
1633        Disable IPv6 operation.  If accept_dad is set to 2, this value
1634        will be dynamically set to TRUE if DAD fails for the link-local
1635        address.
1636        Default: FALSE (enable IPv6 operation)
1638        When this value is changed from 1 to 0 (IPv6 is being enabled),
1639        it will dynamically create a link-local address on the given
1640        interface and start Duplicate Address Detection, if necessary.
1642        When this value is changed from 0 to 1 (IPv6 is being disabled),
1643        it will dynamically delete all address on the given interface.
1645accept_dad - INTEGER
1646        Whether to accept DAD (Duplicate Address Detection).
1647        0: Disable DAD
1648        1: Enable DAD (default)
1649        2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
1650           link-local address has been found.
1652force_tllao - BOOLEAN
1653        Enable sending the target link-layer address option even when
1654        responding to a unicast neighbor solicitation.
1655        Default: FALSE
1657        Quoting from RFC 2461, section 4.4, Target link-layer address:
1659        "The option MUST be included for multicast solicitations in order to
1660        avoid infinite Neighbor Solicitation "recursion" when the peer node
1661        does not have a cache entry to return a Neighbor Advertisements
1662        message.  When responding to unicast solicitations, the option can be
1663        omitted since the sender of the solicitation has the correct link-
1664        layer address; otherwise it would not have be able to send the unicast
1665        solicitation in the first place. However, including the link-layer
1666        address in this case adds little overhead and eliminates a potential
1667        race condition where the sender deletes the cached link-layer address
1668        prior to receiving a response to a previous solicitation."
1670ndisc_notify - BOOLEAN
1671        Define mode for notification of address and device changes.
1672        0 - (default): do nothing
1673        1 - Generate unsolicited neighbour advertisements when device is brought
1674            up or hardware address changes.
1676mldv1_unsolicited_report_interval - INTEGER
1677        The interval in milliseconds in which the next unsolicited
1678        MLDv1 report retransmit will take place.
1679        Default: 10000 (10 seconds)
1681mldv2_unsolicited_report_interval - INTEGER
1682        The interval in milliseconds in which the next unsolicited
1683        MLDv2 report retransmit will take place.
1684        Default: 1000 (1 second)
1686force_mld_version - INTEGER
1687        0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
1688        1 - Enforce to use MLD version 1
1689        2 - Enforce to use MLD version 2
1691suppress_frag_ndisc - INTEGER
1692        Control RFC 6980 (Security Implications of IPv6 Fragmentation
1693        with IPv6 Neighbor Discovery) behavior:
1694        1 - (default) discard fragmented neighbor discovery packets
1695        0 - allow fragmented neighbor discovery packets
1697optimistic_dad - BOOLEAN
1698        Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
1699                0: disabled (default)
1700                1: enabled
1702use_optimistic - BOOLEAN
1703        If enabled, do not classify optimistic addresses as deprecated during
1704        source address selection.  Preferred addresses will still be chosen
1705        before optimistic addresses, subject to other ranking in the source
1706        address selection algorithm.
1707                0: disabled (default)
1708                1: enabled
1710stable_secret - IPv6 address
1711        This IPv6 address will be used as a secret to generate IPv6
1712        addresses for link-local addresses and autoconfigured
1713        ones. All addresses generated after setting this secret will
1714        be stable privacy ones by default. This can be changed via the
1715        addrgenmode ip-link. conf/default/stable_secret is used as the
1716        secret for the namespace, the interface specific ones can
1717        overwrite that. Writes to conf/all/stable_secret are refused.
1719        It is recommended to generate this secret during installation
1720        of a system and keep it stable after that.
1722        By default the stable secret is unset.
1724drop_unicast_in_l2_multicast - BOOLEAN
1725        Drop any unicast IPv6 packets that are received in link-layer
1726        multicast (or broadcast) frames.
1728        By default this is turned off.
1730drop_unsolicited_na - BOOLEAN
1731        Drop all unsolicited neighbor advertisements, for example if there's
1732        a known good NA proxy on the network and such frames need not be used
1733        (or in the case of 802.11, must not be used to prevent attacks.)
1735        By default this is turned off.
1737enhanced_dad - BOOLEAN
1738        Include a nonce option in the IPv6 neighbor solicitation messages used for
1739        duplicate address detection per RFC7527. A received DAD NS will only signal
1740        a duplicate address if the nonce is different. This avoids any false
1741        detection of duplicates due to loopback of the NS messages that we send.
1742        The nonce option will be sent on an interface unless both of
1743        conf/{all,interface}/enhanced_dad are set to FALSE.
1744        Default: TRUE
1747ratelimit - INTEGER
1748        Limit the maximal rates for sending ICMPv6 packets.
1749        0 to disable any limiting,
1750        otherwise the minimal space between responses in milliseconds.
1751        Default: 1000
1753xfrm6_gc_thresh - INTEGER
1754        The threshold at which we will start garbage collecting for IPv6
1755        destination cache entries.  At twice this value the system will
1756        refuse new allocations. The value must be set below the flowcache
1757        limit (4096 * number of online cpus) to take effect.
1760IPv6 Update by:
1761Pekka Savola <>
1762YOSHIFUJI Hideaki / USAGI Project <>
1765/proc/sys/net/bridge/* Variables:
1767bridge-nf-call-arptables - BOOLEAN
1768        1 : pass bridged ARP traffic to arptables' FORWARD chain.
1769        0 : disable this.
1770        Default: 1
1772bridge-nf-call-iptables - BOOLEAN
1773        1 : pass bridged IPv4 traffic to iptables' chains.
1774        0 : disable this.
1775        Default: 1
1777bridge-nf-call-ip6tables - BOOLEAN
1778        1 : pass bridged IPv6 traffic to ip6tables' chains.
1779        0 : disable this.
1780        Default: 1
1782bridge-nf-filter-vlan-tagged - BOOLEAN
1783        1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
1784        0 : disable this.
1785        Default: 0
1787bridge-nf-filter-pppoe-tagged - BOOLEAN
1788        1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
1789        0 : disable this.
1790        Default: 0
1792bridge-nf-pass-vlan-input-dev - BOOLEAN
1793        1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
1794        interface on the bridge and set the netfilter input device to the vlan.
1795        This allows use of e.g. "iptables -i br0.1" and makes the REDIRECT
1796        target work with vlan-on-top-of-bridge interfaces.  When no matching
1797        vlan interface is found, or this switch is off, the input device is
1798        set to the bridge interface.
1799        0: disable bridge netfilter vlan interface lookup.
1800        Default: 0
1802proc/sys/net/sctp/* Variables:
1804addip_enable - BOOLEAN
1805        Enable or disable extension of  Dynamic Address Reconfiguration
1806        (ADD-IP) functionality specified in RFC5061.  This extension provides
1807        the ability to dynamically add and remove new addresses for the SCTP
1808        associations.
1810        1: Enable extension.
1812        0: Disable extension.
1814        Default: 0
1816pf_enable - INTEGER
1817        Enable or disable pf (pf is short for potentially failed) state. A value
1818        of pf_retrans > path_max_retrans also disables pf state. That is, one of
1819        both pf_enable and pf_retrans > path_max_retrans can disable pf state.
1820        Since pf_retrans and path_max_retrans can be changed by userspace
1821        application, sometimes user expects to disable pf state by the value of
1822        pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
1823        or path_max_retrans is changed by the user application, this pf state is
1824        enabled. As such, it is necessary to add this to dynamically enable
1825        and disable pf state. See:
1826 for
1827        details.
1829        1: Enable pf.
1831        0: Disable pf.
1833        Default: 1
1835addip_noauth_enable - BOOLEAN
1836        Dynamic Address Reconfiguration (ADD-IP) requires the use of
1837        authentication to protect the operations of adding or removing new
1838        addresses.  This requirement is mandated so that unauthorized hosts
1839        would not be able to hijack associations.  However, older
1840        implementations may not have implemented this requirement while
1841        allowing the ADD-IP extension.  For reasons of interoperability,
1842        we provide this variable to control the enforcement of the
1843        authentication requirement.
1845        1: Allow ADD-IP extension to be used without authentication.  This
1846           should only be set in a closed environment for interoperability
1847           with older implementations.
1849        0: Enforce the authentication requirement
1851        Default: 0
1853auth_enable - BOOLEAN
1854        Enable or disable Authenticated Chunks extension.  This extension
1855        provides the ability to send and receive authenticated chunks and is
1856        required for secure operation of Dynamic Address Reconfiguration
1857        (ADD-IP) extension.
1859        1: Enable this extension.
1860        0: Disable this extension.
1862        Default: 0
1864prsctp_enable - BOOLEAN
1865        Enable or disable the Partial Reliability extension (RFC3758) which
1866        is used to notify peers that a given DATA should no longer be expected.
1868        1: Enable extension
1869        0: Disable
1871        Default: 1
1873max_burst - INTEGER
1874        The limit of the number of new packets that can be initially sent.  It
1875        controls how bursty the generated traffic can be.
1877        Default: 4
1879association_max_retrans - INTEGER
1880        Set the maximum number for retransmissions that an association can
1881        attempt deciding that the remote end is unreachable.  If this value
1882        is exceeded, the association is terminated.
1884        Default: 10
1886max_init_retransmits - INTEGER
1887        The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
1888        that an association will attempt before declaring the destination
1889        unreachable and terminating.
1891        Default: 8
1893path_max_retrans - INTEGER
1894        The maximum number of retransmissions that will be attempted on a given
1895        path.  Once this threshold is exceeded, the path is considered
1896        unreachable, and new traffic will use a different path when the
1897        association is multihomed.
1899        Default: 5
1901pf_retrans - INTEGER
1902        The number of retransmissions that will be attempted on a given path
1903        before traffic is redirected to an alternate transport (should one
1904        exist).  Note this is distinct from path_max_retrans, as a path that
1905        passes the pf_retrans threshold can still be used.  Its only
1906        deprioritized when a transmission path is selected by the stack.  This
1907        setting is primarily used to enable fast failover mechanisms without
1908        having to reduce path_max_retrans to a very low value.  See:
1910        for details.  Note also that a value of pf_retrans > path_max_retrans
1911        disables this feature. Since both pf_retrans and path_max_retrans can
1912        be changed by userspace application, a variable pf_enable is used to
1913        disable pf state.
1915        Default: 0
1917rto_initial - INTEGER
1918        The initial round trip timeout value in milliseconds that will be used
1919        in calculating round trip times.  This is the initial time interval
1920        for retransmissions.
1922        Default: 3000
1924rto_max - INTEGER
1925        The maximum value (in milliseconds) of the round trip timeout.  This
1926        is the largest time interval that can elapse between retransmissions.
1928        Default: 60000
1930rto_min - INTEGER
1931        The minimum value (in milliseconds) of the round trip timeout.  This
1932        is the smallest time interval the can elapse between retransmissions.
1934        Default: 1000
1936hb_interval - INTEGER
1937        The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
1938        are sent at the specified interval on idle paths to probe the state of
1939        a given path between 2 associations.
1941        Default: 30000
1943sack_timeout - INTEGER
1944        The amount of time (in milliseconds) that the implementation will wait
1945        to send a SACK.
1947        Default: 200
1949valid_cookie_life - INTEGER
1950        The default lifetime of the SCTP cookie (in milliseconds).  The cookie
1951        is used during association establishment.
1953        Default: 60000
1955cookie_preserve_enable - BOOLEAN
1956        Enable or disable the ability to extend the lifetime of the SCTP cookie
1957        that is used during the establishment phase of SCTP association
1959        1: Enable cookie lifetime extension.
1960        0: Disable
1962        Default: 1
1964cookie_hmac_alg - STRING
1965        Select the hmac algorithm used when generating the cookie value sent by
1966        a listening sctp socket to a connecting client in the INIT-ACK chunk.
1967        Valid values are:
1968        * md5
1969        * sha1
1970        * none
1971        Ability to assign md5 or sha1 as the selected alg is predicated on the
1972        configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
1973        CONFIG_CRYPTO_SHA1).
1975        Default: Dependent on configuration.  MD5 if available, else SHA1 if
1976        available, else none.
1978rcvbuf_policy - INTEGER
1979        Determines if the receive buffer is attributed to the socket or to
1980        association.   SCTP supports the capability to create multiple
1981        associations on a single socket.  When using this capability, it is
1982        possible that a single stalled association that's buffering a lot
1983        of data may block other associations from delivering their data by
1984        consuming all of the receive buffer space.  To work around this,
1985        the rcvbuf_policy could be set to attribute the receiver buffer space
1986        to each association instead of the socket.  This prevents the described
1987        blocking.
1989        1: rcvbuf space is per association
1990        0: rcvbuf space is per socket
1992        Default: 0
1994sndbuf_policy - INTEGER
1995        Similar to rcvbuf_policy above, this applies to send buffer space.
1997        1: Send buffer is tracked per association
1998        0: Send buffer is tracked per socket.
2000        Default: 0
2002sctp_mem - vector of 3 INTEGERs: min, pressure, max
2003        Number of pages allowed for queueing by all SCTP sockets.
2005        min: Below this number of pages SCTP is not bothered about its
2006        memory appetite. When amount of memory allocated by SCTP exceeds
2007        this number, SCTP starts to moderate memory usage.
2009        pressure: This value was introduced to follow format of tcp_mem.
2011        max: Number of pages allowed for queueing by all SCTP sockets.
2013        Default is calculated at boot time from amount of available memory.
2015sctp_rmem - vector of 3 INTEGERs: min, default, max
2016        Only the first value ("min") is used, "default" and "max" are
2017        ignored.
2019        min: Minimal size of receive buffer used by SCTP socket.
2020        It is guaranteed to each SCTP socket (but not association) even
2021        under moderate memory pressure.
2023        Default: 1 page
2025sctp_wmem  - vector of 3 INTEGERs: min, default, max
2026        Currently this tunable has no effect.
2028addr_scope_policy - INTEGER
2029        Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
2031        0   - Disable IPv4 address scoping
2032        1   - Enable IPv4 address scoping
2033        2   - Follow draft but allow IPv4 private addresses
2034        3   - Follow draft but allow IPv4 link local addresses
2036        Default: 1
2040        Please see: Documentation/sysctl/net.txt for descriptions of these entries.
2044max_dgram_qlen - INTEGER
2045        The maximum length of dgram socket receive queue
2047        Default: 10
2053        fast_poll_increase FIXME
2054        warn_noreply_time FIXME
2055        discovery_slots FIXME
2056        slot_timeout FIXME
2057        max_baud_rate FIXME
2058        discovery_timeout FIXME
2059        lap_keepalive_time FIXME
2060        max_noreply_time FIXME
2061        max_tx_data_size FIXME
2062        max_tx_window FIXME
2063        min_tx_turn_time FIXME
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