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 64
  16ip_no_pmtu_disc - BOOLEAN
  17        Disable Path MTU Discovery.
  18        default FALSE
  20min_pmtu - INTEGER
  21        default 562 - minimum discovered Path MTU
  23mtu_expires - INTEGER
  24        Time, in seconds, that cached PMTU information is kept.
  26min_adv_mss - INTEGER
  27        The advertised MSS depends on the first hop route MTU, but will
  28        never be lower than this setting.
  30rt_cache_rebuild_count - INTEGER
  31        The per net-namespace route cache emergency rebuild threshold.
  32        Any net-namespace having its route cache rebuilt due to
  33        a hash bucket chain being too long more than this many times
  34        will have its route caching disabled
  36IP Fragmentation:
  38ipfrag_high_thresh - INTEGER
  39        Maximum memory used to reassemble IP fragments. When
  40        ipfrag_high_thresh bytes of memory is allocated for this purpose,
  41        the fragment handler will toss packets until ipfrag_low_thresh
  42        is reached.
  44ipfrag_low_thresh - INTEGER
  45        See ipfrag_high_thresh
  47ipfrag_time - INTEGER
  48        Time in seconds to keep an IP fragment in memory.
  50ipfrag_secret_interval - INTEGER
  51        Regeneration interval (in seconds) of the hash secret (or lifetime
  52        for the hash secret) for IP fragments.
  53        Default: 600
  55ipfrag_max_dist - INTEGER
  56        ipfrag_max_dist is a non-negative integer value which defines the
  57        maximum "disorder" which is allowed among fragments which share a
  58        common IP source address. Note that reordering of packets is
  59        not unusual, but if a large number of fragments arrive from a source
  60        IP address while a particular fragment queue remains incomplete, it
  61        probably indicates that one or more fragments belonging to that queue
  62        have been lost. When ipfrag_max_dist is positive, an additional check
  63        is done on fragments before they are added to a reassembly queue - if
  64        ipfrag_max_dist (or more) fragments have arrived from a particular IP
  65        address between additions to any IP fragment queue using that source
  66        address, it's presumed that one or more fragments in the queue are
  67        lost. The existing fragment queue will be dropped, and a new one
  68        started. An ipfrag_max_dist value of zero disables this check.
  70        Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
  71        result in unnecessarily dropping fragment queues when normal
  72        reordering of packets occurs, which could lead to poor application
  73        performance. Using a very large value, e.g. 50000, increases the
  74        likelihood of incorrectly reassembling IP fragments that originate
  75        from different IP datagrams, which could result in data corruption.
  76        Default: 64
  78INET peer storage:
  80inet_peer_threshold - INTEGER
  81        The approximate size of the storage.  Starting from this threshold
  82        entries will be thrown aggressively.  This threshold also determines
  83        entries' time-to-live and time intervals between garbage collection
  84        passes.  More entries, less time-to-live, less GC interval.
  86inet_peer_minttl - INTEGER
  87        Minimum time-to-live of entries.  Should be enough to cover fragment
  88        time-to-live on the reassembling side.  This minimum time-to-live  is
  89        guaranteed if the pool size is less than inet_peer_threshold.
  90        Measured in seconds.
  92inet_peer_maxttl - INTEGER
  93        Maximum time-to-live of entries.  Unused entries will expire after
  94        this period of time if there is no memory pressure on the pool (i.e.
  95        when the number of entries in the pool is very small).
  96        Measured in seconds.
  98inet_peer_gc_mintime - INTEGER
  99        Minimum interval between garbage collection passes.  This interval is
 100        in effect under high memory pressure on the pool.
 101        Measured in seconds.
 103inet_peer_gc_maxtime - INTEGER
 104        Minimum interval between garbage collection passes.  This interval is
 105        in effect under low (or absent) memory pressure on the pool.
 106        Measured in seconds.
 108TCP variables:
 110somaxconn - INTEGER
 111        Limit of socket listen() backlog, known in userspace as SOMAXCONN.
 112        Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
 113        for TCP sockets.
 115tcp_abc - INTEGER
 116        Controls Appropriate Byte Count (ABC) defined in RFC3465.
 117        ABC is a way of increasing congestion window (cwnd) more slowly
 118        in response to partial acknowledgments.
 119        Possible values are:
 120                0 increase cwnd once per acknowledgment (no ABC)
 121                1 increase cwnd once per acknowledgment of full sized segment
 122                2 allow increase cwnd by two if acknowledgment is
 123                  of two segments to compensate for delayed acknowledgments.
 124        Default: 0 (off)
 126tcp_abort_on_overflow - BOOLEAN
 127        If listening service is too slow to accept new connections,
 128        reset them. Default state is FALSE. It means that if overflow
 129        occurred due to a burst, connection will recover. Enable this
 130        option _only_ if you are really sure that listening daemon
 131        cannot be tuned to accept connections faster. Enabling this
 132        option can harm clients of your server.
 134tcp_adv_win_scale - INTEGER
 135        Count buffering overhead as bytes/2^tcp_adv_win_scale
 136        (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
 137        if it is <= 0.
 138        Default: 2
 140tcp_allowed_congestion_control - STRING
 141        Show/set the congestion control choices available to non-privileged
 142        processes. The list is a subset of those listed in
 143        tcp_available_congestion_control.
 144        Default is "reno" and the default setting (tcp_congestion_control).
 146tcp_app_win - INTEGER
 147        Reserve max(window/2^tcp_app_win, mss) of window for application
 148        buffer. Value 0 is special, it means that nothing is reserved.
 149        Default: 31
 151tcp_available_congestion_control - STRING
 152        Shows the available congestion control choices that are registered.
 153        More congestion control algorithms may be available as modules,
 154        but not loaded.
 156tcp_base_mss - INTEGER
 157        The initial value of search_low to be used by the packetization layer
 158        Path MTU discovery (MTU probing).  If MTU probing is enabled,
 159        this is the initial MSS used by the connection.
 161tcp_congestion_control - STRING
 162        Set the congestion control algorithm to be used for new
 163        connections. The algorithm "reno" is always available, but
 164        additional choices may be available based on kernel configuration.
 165        Default is set as part of kernel configuration.
 167tcp_dsack - BOOLEAN
 168        Allows TCP to send "duplicate" SACKs.
 170tcp_ecn - BOOLEAN
 171        Enable Explicit Congestion Notification in TCP.
 173tcp_fack - BOOLEAN
 174        Enable FACK congestion avoidance and fast retransmission.
 175        The value is not used, if tcp_sack is not enabled.
 177tcp_fin_timeout - INTEGER
 178        Time to hold socket in state FIN-WAIT-2, if it was closed
 179        by our side. Peer can be broken and never close its side,
 180        or even died unexpectedly. Default value is 60sec.
 181        Usual value used in 2.2 was 180 seconds, you may restore
 182        it, but remember that if your machine is even underloaded WEB server,
 183        you risk to overflow memory with kilotons of dead sockets,
 184        FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1,
 185        because they eat maximum 1.5K of memory, but they tend
 186        to live longer. Cf. tcp_max_orphans.
 188tcp_frto - INTEGER
 189        Enables Forward RTO-Recovery (F-RTO) defined in RFC4138.
 190        F-RTO is an enhanced recovery algorithm for TCP retransmission
 191        timeouts.  It is particularly beneficial in wireless environments
 192        where packet loss is typically due to random radio interference
 193        rather than intermediate router congestion.  F-RTO is sender-side
 194        only modification. Therefore it does not require any support from
 195        the peer.
 197        If set to 1, basic version is enabled.  2 enables SACK enhanced
 198        F-RTO if flow uses SACK.  The basic version can be used also when
 199        SACK is in use though scenario(s) with it exists where F-RTO
 200        interacts badly with the packet counting of the SACK enabled TCP
 201        flow.
 203tcp_frto_response - INTEGER
 204        When F-RTO has detected that a TCP retransmission timeout was
 205        spurious (i.e, the timeout would have been avoided had TCP set a
 206        longer retransmission timeout), TCP has several options what to do
 207        next. Possible values are:
 208                0 Rate halving based; a smooth and conservative response,
 209                  results in halved cwnd and ssthresh after one RTT
 210                1 Very conservative response; not recommended because even
 211                  though being valid, it interacts poorly with the rest of
 212                  Linux TCP, halves cwnd and ssthresh immediately
 213                2 Aggressive response; undoes congestion control measures
 214                  that are now known to be unnecessary (ignoring the
 215                  possibility of a lost retransmission that would require
 216                  TCP to be more cautious), cwnd and ssthresh are restored
 217                  to the values prior timeout
 218        Default: 0 (rate halving based)
 220tcp_keepalive_time - INTEGER
 221        How often TCP sends out keepalive messages when keepalive is enabled.
 222        Default: 2hours.
 224tcp_keepalive_probes - INTEGER
 225        How many keepalive probes TCP sends out, until it decides that the
 226        connection is broken. Default value: 9.
 228tcp_keepalive_intvl - INTEGER
 229        How frequently the probes are send out. Multiplied by
 230        tcp_keepalive_probes it is time to kill not responding connection,
 231        after probes started. Default value: 75sec i.e. connection
 232        will be aborted after ~11 minutes of retries.
 234tcp_low_latency - BOOLEAN
 235        If set, the TCP stack makes decisions that prefer lower
 236        latency as opposed to higher throughput.  By default, this
 237        option is not set meaning that higher throughput is preferred.
 238        An example of an application where this default should be
 239        changed would be a Beowulf compute cluster.
 240        Default: 0
 242tcp_max_orphans - INTEGER
 243        Maximal number of TCP sockets not attached to any user file handle,
 244        held by system. If this number is exceeded orphaned connections are
 245        reset immediately and warning is printed. This limit exists
 246        only to prevent simple DoS attacks, you _must_ not rely on this
 247        or lower the limit artificially, but rather increase it
 248        (probably, after increasing installed memory),
 249        if network conditions require more than default value,
 250        and tune network services to linger and kill such states
 251        more aggressively. Let me to remind again: each orphan eats
 252        up to ~64K of unswappable memory.
 254tcp_max_syn_backlog - INTEGER
 255        Maximal number of remembered connection requests, which are
 256        still did not receive an acknowledgment from connecting client.
 257        Default value is 1024 for systems with more than 128Mb of memory,
 258        and 128 for low memory machines. If server suffers of overload,
 259        try to increase this number.
 261tcp_max_tw_buckets - INTEGER
 262        Maximal number of timewait sockets held by system simultaneously.
 263        If this number is exceeded time-wait socket is immediately destroyed
 264        and warning is printed. This limit exists only to prevent
 265        simple DoS attacks, you _must_ not lower the limit artificially,
 266        but rather increase it (probably, after increasing installed memory),
 267        if network conditions require more than default value.
 269tcp_mem - vector of 3 INTEGERs: min, pressure, max
 270        min: below this number of pages TCP is not bothered about its
 271        memory appetite.
 273        pressure: when amount of memory allocated by TCP exceeds this number
 274        of pages, TCP moderates its memory consumption and enters memory
 275        pressure mode, which is exited when memory consumption falls
 276        under "min".
 278        max: number of pages allowed for queueing by all TCP sockets.
 280        Defaults are calculated at boot time from amount of available
 281        memory.
 283tcp_moderate_rcvbuf - BOOLEAN
 284        If set, TCP performs receive buffer auto-tuning, attempting to
 285        automatically size the buffer (no greater than tcp_rmem[2]) to
 286        match the size required by the path for full throughput.  Enabled by
 287        default.
 289tcp_mtu_probing - INTEGER
 290        Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
 291        values:
 292          0 - Disabled
 293          1 - Disabled by default, enabled when an ICMP black hole detected
 294          2 - Always enabled, use initial MSS of tcp_base_mss.
 296tcp_no_metrics_save - BOOLEAN
 297        By default, TCP saves various connection metrics in the route cache
 298        when the connection closes, so that connections established in the
 299        near future can use these to set initial conditions.  Usually, this
 300        increases overall performance, but may sometimes cause performance
 301        degradation.  If set, TCP will not cache metrics on closing
 302        connections.
 304tcp_orphan_retries - INTEGER
 305        How may times to retry before killing TCP connection, closed
 306        by our side. Default value 7 corresponds to ~50sec-16min
 307        depending on RTO. If you machine is loaded WEB server,
 308        you should think about lowering this value, such sockets
 309        may consume significant resources. Cf. tcp_max_orphans.
 311tcp_reordering - INTEGER
 312        Maximal reordering of packets in a TCP stream.
 313        Default: 3
 315tcp_retrans_collapse - BOOLEAN
 316        Bug-to-bug compatibility with some broken printers.
 317        On retransmit try to send bigger packets to work around bugs in
 318        certain TCP stacks.
 320tcp_retries1 - INTEGER
 321        How many times to retry before deciding that something is wrong
 322        and it is necessary to report this suspicion to network layer.
 323        Minimal RFC value is 3, it is default, which corresponds
 324        to ~3sec-8min depending on RTO.
 326tcp_retries2 - INTEGER
 327        How may times to retry before killing alive TCP connection.
 328        RFC1122 says that the limit should be longer than 100 sec.
 329        It is too small number. Default value 15 corresponds to ~13-30min
 330        depending on RTO.
 332tcp_rfc1337 - BOOLEAN
 333        If set, the TCP stack behaves conforming to RFC1337. If unset,
 334        we are not conforming to RFC, but prevent TCP TIME_WAIT
 335        assassination.
 336        Default: 0
 338tcp_rmem - vector of 3 INTEGERs: min, default, max
 339        min: Minimal size of receive buffer used by TCP sockets.
 340        It is guaranteed to each TCP socket, even under moderate memory
 341        pressure.
 342        Default: 8K
 344        default: initial size of receive buffer used by TCP sockets.
 345        This value overrides net.core.rmem_default used by other protocols.
 346        Default: 87380 bytes. This value results in window of 65535 with
 347        default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit
 348        less for default tcp_app_win. See below about these variables.
 350        max: maximal size of receive buffer allowed for automatically
 351        selected receiver buffers for TCP socket. This value does not override
 352        net.core.rmem_max.  Calling setsockopt() with SO_RCVBUF disables
 353        automatic tuning of that socket's receive buffer size, in which
 354        case this value is ignored.
 355        Default: between 87380B and 4MB, depending on RAM size.
 357tcp_sack - BOOLEAN
 358        Enable select acknowledgments (SACKS).
 360tcp_slow_start_after_idle - BOOLEAN
 361        If set, provide RFC2861 behavior and time out the congestion
 362        window after an idle period.  An idle period is defined at
 363        the current RTO.  If unset, the congestion window will not
 364        be timed out after an idle period.
 365        Default: 1
 367tcp_stdurg - BOOLEAN
 368        Use the Host requirements interpretation of the TCP urgent pointer field.
 369        Most hosts use the older BSD interpretation, so if you turn this on
 370        Linux might not communicate correctly with them.
 371        Default: FALSE
 373tcp_synack_retries - INTEGER
 374        Number of times SYNACKs for a passive TCP connection attempt will
 375        be retransmitted. Should not be higher than 255. Default value
 376        is 5, which corresponds to ~180seconds.
 378tcp_syncookies - BOOLEAN
 379        Only valid when the kernel was compiled with CONFIG_SYNCOOKIES
 380        Send out syncookies when the syn backlog queue of a socket
 381        overflows. This is to prevent against the common 'SYN flood attack'
 382        Default: FALSE
 384        Note, that syncookies is fallback facility.
 385        It MUST NOT be used to help highly loaded servers to stand
 386        against legal connection rate. If you see SYN flood warnings
 387        in your logs, but investigation shows that they occur
 388        because of overload with legal connections, you should tune
 389        another parameters until this warning disappear.
 390        See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
 392        syncookies seriously violate TCP protocol, do not allow
 393        to use TCP extensions, can result in serious degradation
 394        of some services (f.e. SMTP relaying), visible not by you,
 395        but your clients and relays, contacting you. While you see
 396        SYN flood warnings in logs not being really flooded, your server
 397        is seriously misconfigured.
 399tcp_syn_retries - INTEGER
 400        Number of times initial SYNs for an active TCP connection attempt
 401        will be retransmitted. Should not be higher than 255. Default value
 402        is 5, which corresponds to ~180seconds.
 404tcp_timestamps - BOOLEAN
 405        Enable timestamps as defined in RFC1323.
 407tcp_tso_win_divisor - INTEGER
 408        This allows control over what percentage of the congestion window
 409        can be consumed by a single TSO frame.
 410        The setting of this parameter is a choice between burstiness and
 411        building larger TSO frames.
 412        Default: 3
 414tcp_tw_recycle - BOOLEAN
 415        Enable fast recycling TIME-WAIT sockets. Default value is 0.
 416        It should not be changed without advice/request of technical
 417        experts.
 419tcp_tw_reuse - BOOLEAN
 420        Allow to reuse TIME-WAIT sockets for new connections when it is
 421        safe from protocol viewpoint. Default value is 0.
 422        It should not be changed without advice/request of technical
 423        experts.
 425tcp_window_scaling - BOOLEAN
 426        Enable window scaling as defined in RFC1323.
 428tcp_wmem - vector of 3 INTEGERs: min, default, max
 429        min: Amount of memory reserved for send buffers for TCP sockets.
 430        Each TCP socket has rights to use it due to fact of its birth.
 431        Default: 4K
 433        default: initial size of send buffer used by TCP sockets.  This
 434        value overrides net.core.wmem_default used by other protocols.
 435        It is usually lower than net.core.wmem_default.
 436        Default: 16K
 438        max: Maximal amount of memory allowed for automatically tuned
 439        send buffers for TCP sockets. This value does not override
 440        net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
 441        automatic tuning of that socket's send buffer size, in which case
 442        this value is ignored.
 443        Default: between 64K and 4MB, depending on RAM size.
 445tcp_workaround_signed_windows - BOOLEAN
 446        If set, assume no receipt of a window scaling option means the
 447        remote TCP is broken and treats the window as a signed quantity.
 448        If unset, assume the remote TCP is not broken even if we do
 449        not receive a window scaling option from them.
 450        Default: 0
 452tcp_dma_copybreak - INTEGER
 453        Lower limit, in bytes, of the size of socket reads that will be
 454        offloaded to a DMA copy engine, if one is present in the system
 455        and CONFIG_NET_DMA is enabled.
 456        Default: 4096
 458UDP variables:
 460udp_mem - vector of 3 INTEGERs: min, pressure, max
 461        Number of pages allowed for queueing by all UDP sockets.
 463        min: Below this number of pages UDP is not bothered about its
 464        memory appetite. When amount of memory allocated by UDP exceeds
 465        this number, UDP starts to moderate memory usage.
 467        pressure: This value was introduced to follow format of tcp_mem.
 469        max: Number of pages allowed for queueing by all UDP sockets.
 471        Default is calculated at boot time from amount of available memory.
 473udp_rmem_min - INTEGER
 474        Minimal size of receive buffer used by UDP sockets in moderation.
 475        Each UDP socket is able to use the size for receiving data, even if
 476        total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
 477        Default: 4096
 479udp_wmem_min - INTEGER
 480        Minimal size of send buffer used by UDP sockets in moderation.
 481        Each UDP socket is able to use the size for sending data, even if
 482        total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
 483        Default: 4096
 485CIPSOv4 Variables:
 487cipso_cache_enable - BOOLEAN
 488        If set, enable additions to and lookups from the CIPSO label mapping
 489        cache.  If unset, additions are ignored and lookups always result in a
 490        miss.  However, regardless of the setting the cache is still
 491        invalidated when required when means you can safely toggle this on and
 492        off and the cache will always be "safe".
 493        Default: 1
 495cipso_cache_bucket_size - INTEGER
 496        The CIPSO label cache consists of a fixed size hash table with each
 497        hash bucket containing a number of cache entries.  This variable limits
 498        the number of entries in each hash bucket; the larger the value the
 499        more CIPSO label mappings that can be cached.  When the number of
 500        entries in a given hash bucket reaches this limit adding new entries
 501        causes the oldest entry in the bucket to be removed to make room.
 502        Default: 10
 504cipso_rbm_optfmt - BOOLEAN
 505        Enable the "Optimized Tag 1 Format" as defined in section of
 506        the CIPSO draft specification (see Documentation/netlabel for details).
 507        This means that when set the CIPSO tag will be padded with empty
 508        categories in order to make the packet data 32-bit aligned.
 509        Default: 0
 511cipso_rbm_structvalid - BOOLEAN
 512        If set, do a very strict check of the CIPSO option when
 513        ip_options_compile() is called.  If unset, relax the checks done during
 514        ip_options_compile().  Either way is "safe" as errors are caught else
 515        where in the CIPSO processing code but setting this to 0 (False) should
 516        result in less work (i.e. it should be faster) but could cause problems
 517        with other implementations that require strict checking.
 518        Default: 0
 520IP Variables:
 522ip_local_port_range - 2 INTEGERS
 523        Defines the local port range that is used by TCP and UDP to
 524        choose the local port. The first number is the first, the
 525        second the last local port number. Default value depends on
 526        amount of memory available on the system:
 527        > 128Mb 32768-61000
 528        < 128Mb 1024-4999 or even less.
 529        This number defines number of active connections, which this
 530        system can issue simultaneously to systems not supporting
 531        TCP extensions (timestamps). With tcp_tw_recycle enabled
 532        (i.e. by default) range 1024-4999 is enough to issue up to
 533        2000 connections per second to systems supporting timestamps.
 535ip_nonlocal_bind - BOOLEAN
 536        If set, allows processes to bind() to non-local IP addresses,
 537        which can be quite useful - but may break some applications.
 538        Default: 0
 540ip_dynaddr - BOOLEAN
 541        If set non-zero, enables support for dynamic addresses.
 542        If set to a non-zero value larger than 1, a kernel log
 543        message will be printed when dynamic address rewriting
 544        occurs.
 545        Default: 0
 547icmp_echo_ignore_all - BOOLEAN
 548        If set non-zero, then the kernel will ignore all ICMP ECHO
 549        requests sent to it.
 550        Default: 0
 552icmp_echo_ignore_broadcasts - BOOLEAN
 553        If set non-zero, then the kernel will ignore all ICMP ECHO and
 554        TIMESTAMP requests sent to it via broadcast/multicast.
 555        Default: 1
 557icmp_ratelimit - INTEGER
 558        Limit the maximal rates for sending ICMP packets whose type matches
 559        icmp_ratemask (see below) to specific targets.
 560        0 to disable any limiting,
 561        otherwise the minimal space between responses in milliseconds.
 562        Default: 1000
 564icmp_ratemask - INTEGER
 565        Mask made of ICMP types for which rates are being limited.
 566        Significant bits: IHGFEDCBA9876543210
 567        Default mask:     0000001100000011000 (6168)
 569        Bit definitions (see include/linux/icmp.h):
 570                0 Echo Reply
 571                3 Destination Unreachable *
 572                4 Source Quench *
 573                5 Redirect
 574                8 Echo Request
 575                B Time Exceeded *
 576                C Parameter Problem *
 577                D Timestamp Request
 578                E Timestamp Reply
 579                F Info Request
 580                G Info Reply
 581                H Address Mask Request
 582                I Address Mask Reply
 584        * These are rate limited by default (see default mask above)
 586icmp_ignore_bogus_error_responses - BOOLEAN
 587        Some routers violate RFC1122 by sending bogus responses to broadcast
 588        frames.  Such violations are normally logged via a kernel warning.
 589        If this is set to TRUE, the kernel will not give such warnings, which
 590        will avoid log file clutter.
 591        Default: FALSE
 593icmp_errors_use_inbound_ifaddr - BOOLEAN
 595        If zero, icmp error messages are sent with the primary address of
 596        the exiting interface.
 598        If non-zero, the message will be sent with the primary address of
 599        the interface that received the packet that caused the icmp error.
 600        This is the behaviour network many administrators will expect from
 601        a router. And it can make debugging complicated network layouts
 602        much easier.
 604        Note that if no primary address exists for the interface selected,
 605        then the primary address of the first non-loopback interface that
 606        has one will be used regardless of this setting.
 608        Default: 0
 610igmp_max_memberships - INTEGER
 611        Change the maximum number of multicast groups we can subscribe to.
 612        Default: 20
 614conf/interface/*  changes special settings per interface (where "interface" is
 615                  the name of your network interface)
 616conf/all/*        is special, changes the settings for all interfaces
 619log_martians - BOOLEAN
 620        Log packets with impossible addresses to kernel log.
 621        log_martians for the interface will be enabled if at least one of
 622        conf/{all,interface}/log_martians is set to TRUE,
 623        it will be disabled otherwise
 625accept_redirects - BOOLEAN
 626        Accept ICMP redirect messages.
 627        accept_redirects for the interface will be enabled if:
 628        - both conf/{all,interface}/accept_redirects are TRUE in the case
 629          forwarding for the interface is enabled
 630        or
 631        - at least one of conf/{all,interface}/accept_redirects is TRUE in the
 632          case forwarding for the interface is disabled
 633        accept_redirects for the interface will be disabled otherwise
 634        default TRUE (host)
 635                FALSE (router)
 637forwarding - BOOLEAN
 638        Enable IP forwarding on this interface.
 640mc_forwarding - BOOLEAN
 641        Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
 642        and a multicast routing daemon is required.
 643        conf/all/mc_forwarding must also be set to TRUE to enable multicast
 644        routing for the interface
 646medium_id - INTEGER
 647        Integer value used to differentiate the devices by the medium they
 648        are attached to. Two devices can have different id values when
 649        the broadcast packets are received only on one of them.
 650        The default value 0 means that the device is the only interface
 651        to its medium, value of -1 means that medium is not known.
 653        Currently, it is used to change the proxy_arp behavior:
 654        the proxy_arp feature is enabled for packets forwarded between
 655        two devices attached to different media.
 657proxy_arp - BOOLEAN
 658        Do proxy arp.
 659        proxy_arp for the interface will be enabled if at least one of
 660        conf/{all,interface}/proxy_arp is set to TRUE,
 661        it will be disabled otherwise
 663shared_media - BOOLEAN
 664        Send(router) or accept(host) RFC1620 shared media redirects.
 665        Overrides ip_secure_redirects.
 666        shared_media for the interface will be enabled if at least one of
 667        conf/{all,interface}/shared_media is set to TRUE,
 668        it will be disabled otherwise
 669        default TRUE
 671secure_redirects - BOOLEAN
 672        Accept ICMP redirect messages only for gateways,
 673        listed in default gateway list.
 674        secure_redirects for the interface will be enabled if at least one of
 675        conf/{all,interface}/secure_redirects is set to TRUE,
 676        it will be disabled otherwise
 677        default TRUE
 679send_redirects - BOOLEAN
 680        Send redirects, if router.
 681        send_redirects for the interface will be enabled if at least one of
 682        conf/{all,interface}/send_redirects is set to TRUE,
 683        it will be disabled otherwise
 684        Default: TRUE
 686bootp_relay - BOOLEAN
 687        Accept packets with source address 0.b.c.d destined
 688        not to this host as local ones. It is supposed, that
 689        BOOTP relay daemon will catch and forward such packets.
 690        conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
 691        for the interface
 692        default FALSE
 693        Not Implemented Yet.
 695accept_source_route - BOOLEAN
 696        Accept packets with SRR option.
 697        conf/all/accept_source_route must also be set to TRUE to accept packets
 698        with SRR option on the interface
 699        default TRUE (router)
 700                FALSE (host)
 702rp_filter - INTEGER
 703        0 - No source validation.
 704        1 - Strict mode as defined in RFC3704 Strict Reverse Path
 705            Each incoming packet is tested against the FIB and if the interface
 706            is not the best reverse path the packet check will fail.
 707            By default failed packets are discarded.
 708        2 - Loose mode as defined in RFC3704 Loose Reverse Path
 709            Each incoming packet's source address is also tested against the FIB
 710            and if the source address is not reachable via any interface
 711            the packet check will fail.
 713        Current recommended practice in RFC3704 is to enable strict mode
 714        to prevent IP spoofing from DDos attacks. If using asymmetric routing
 715        or other complicated routing, then loose mode is recommended.
 717        conf/all/rp_filter must also be set to non-zero to do source validation
 718        on the interface
 720        Default value is 0. Note that some distributions enable it
 721        in startup scripts.
 723arp_filter - BOOLEAN
 724        1 - Allows you to have multiple network interfaces on the same
 725        subnet, and have the ARPs for each interface be answered
 726        based on whether or not the kernel would route a packet from
 727        the ARP'd IP out that interface (therefore you must use source
 728        based routing for this to work). In other words it allows control
 729        of which cards (usually 1) will respond to an arp request.
 731        0 - (default) The kernel can respond to arp requests with addresses
 732        from other interfaces. This may seem wrong but it usually makes
 733        sense, because it increases the chance of successful communication.
 734        IP addresses are owned by the complete host on Linux, not by
 735        particular interfaces. Only for more complex setups like load-
 736        balancing, does this behaviour cause problems.
 738        arp_filter for the interface will be enabled if at least one of
 739        conf/{all,interface}/arp_filter is set to TRUE,
 740        it will be disabled otherwise
 742arp_announce - INTEGER
 743        Define different restriction levels for announcing the local
 744        source IP address from IP packets in ARP requests sent on
 745        interface:
 746        0 - (default) Use any local address, configured on any interface
 747        1 - Try to avoid local addresses that are not in the target's
 748        subnet for this interface. This mode is useful when target
 749        hosts reachable via this interface require the source IP
 750        address in ARP requests to be part of their logical network
 751        configured on the receiving interface. When we generate the
 752        request we will check all our subnets that include the
 753        target IP and will preserve the source address if it is from
 754        such subnet. If there is no such subnet we select source
 755        address according to the rules for level 2.
 756        2 - Always use the best local address for this target.
 757        In this mode we ignore the source address in the IP packet
 758        and try to select local address that we prefer for talks with
 759        the target host. Such local address is selected by looking
 760        for primary IP addresses on all our subnets on the outgoing
 761        interface that include the target IP address. If no suitable
 762        local address is found we select the first local address
 763        we have on the outgoing interface or on all other interfaces,
 764        with the hope we will receive reply for our request and
 765        even sometimes no matter the source IP address we announce.
 767        The max value from conf/{all,interface}/arp_announce is used.
 769        Increasing the restriction level gives more chance for
 770        receiving answer from the resolved target while decreasing
 771        the level announces more valid sender's information.
 773arp_ignore - INTEGER
 774        Define different modes for sending replies in response to
 775        received ARP requests that resolve local target IP addresses:
 776        0 - (default): reply for any local target IP address, configured
 777        on any interface
 778        1 - reply only if the target IP address is local address
 779        configured on the incoming interface
 780        2 - reply only if the target IP address is local address
 781        configured on the incoming interface and both with the
 782        sender's IP address are part from same subnet on this interface
 783        3 - do not reply for local addresses configured with scope host,
 784        only resolutions for global and link addresses are replied
 785        4-7 - reserved
 786        8 - do not reply for all local addresses
 788        The max value from conf/{all,interface}/arp_ignore is used
 789        when ARP request is received on the {interface}
 791arp_notify - BOOLEAN
 792        Define mode for notification of address and device changes.
 793        0 - (default): do nothing
 794        1 - Generate gratuitous arp replies when device is brought up
 795            or hardware address changes.
 797arp_accept - BOOLEAN
 798        Define behavior when gratuitous arp replies are received:
 799        0 - drop gratuitous arp frames
 800        1 - accept gratuitous arp frames
 802app_solicit - INTEGER
 803        The maximum number of probes to send to the user space ARP daemon
 804        via netlink before dropping back to multicast probes (see
 805        mcast_solicit).  Defaults to 0.
 807disable_policy - BOOLEAN
 808        Disable IPSEC policy (SPD) for this interface
 810disable_xfrm - BOOLEAN
 811        Disable IPSEC encryption on this interface, whatever the policy
 815tag - INTEGER
 816        Allows you to write a number, which can be used as required.
 817        Default value is 0.
 819Alexey Kuznetsov.
 822Updated by:
 823Andi Kleen
 825Nicolas Delon
 831/proc/sys/net/ipv6/* Variables:
 833IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
 834apply to IPv6 [XXX?].
 836bindv6only - BOOLEAN
 837        Default value for IPV6_V6ONLY socket option,
 838        which restricts use of the IPv6 socket to IPv6 communication
 839        only.
 840                TRUE: disable IPv4-mapped address feature
 841                FALSE: enable IPv4-mapped address feature
 843        Default: FALSE (as specified in RFC2553bis)
 845IPv6 Fragmentation:
 847ip6frag_high_thresh - INTEGER
 848        Maximum memory used to reassemble IPv6 fragments. When
 849        ip6frag_high_thresh bytes of memory is allocated for this purpose,
 850        the fragment handler will toss packets until ip6frag_low_thresh
 851        is reached.
 853ip6frag_low_thresh - INTEGER
 854        See ip6frag_high_thresh
 856ip6frag_time - INTEGER
 857        Time in seconds to keep an IPv6 fragment in memory.
 859ip6frag_secret_interval - INTEGER
 860        Regeneration interval (in seconds) of the hash secret (or lifetime
 861        for the hash secret) for IPv6 fragments.
 862        Default: 600
 865        Change the interface-specific default settings.
 869        Change all the interface-specific settings.
 871        [XXX:  Other special features than forwarding?]
 873conf/all/forwarding - BOOLEAN
 874        Enable global IPv6 forwarding between all interfaces.
 876        IPv4 and IPv6 work differently here; e.g. netfilter must be used
 877        to control which interfaces may forward packets and which not.
 879        This also sets all interfaces' Host/Router setting
 880        'forwarding' to the specified value.  See below for details.
 882        This referred to as global forwarding.
 884proxy_ndp - BOOLEAN
 885        Do proxy ndp.
 888        Change special settings per interface.
 890        The functional behaviour for certain settings is different
 891        depending on whether local forwarding is enabled or not.
 893accept_ra - BOOLEAN
 894        Accept Router Advertisements; autoconfigure using them.
 896        Functional default: enabled if local forwarding is disabled.
 897                            disabled if local forwarding is enabled.
 899accept_ra_defrtr - BOOLEAN
 900        Learn default router in Router Advertisement.
 902        Functional default: enabled if accept_ra is enabled.
 903                            disabled if accept_ra is disabled.
 905accept_ra_pinfo - BOOLEAN
 906        Learn Prefix Information in Router Advertisement.
 908        Functional default: enabled if accept_ra is enabled.
 909                            disabled if accept_ra is disabled.
 911accept_ra_rt_info_max_plen - INTEGER
 912        Maximum prefix length of Route Information in RA.
 914        Route Information w/ prefix larger than or equal to this
 915        variable shall be ignored.
 917        Functional default: 0 if accept_ra_rtr_pref is enabled.
 918                            -1 if accept_ra_rtr_pref is disabled.
 920accept_ra_rtr_pref - BOOLEAN
 921        Accept Router Preference in RA.
 923        Functional default: enabled if accept_ra is enabled.
 924                            disabled if accept_ra is disabled.
 926accept_redirects - BOOLEAN
 927        Accept Redirects.
 929        Functional default: enabled if local forwarding is disabled.
 930                            disabled if local forwarding is enabled.
 932accept_source_route - INTEGER
 933        Accept source routing (routing extension header).
 935        >= 0: Accept only routing header type 2.
 936        < 0: Do not accept routing header.
 938        Default: 0
 940autoconf - BOOLEAN
 941        Autoconfigure addresses using Prefix Information in Router
 942        Advertisements.
 944        Functional default: enabled if accept_ra_pinfo is enabled.
 945                            disabled if accept_ra_pinfo is disabled.
 947dad_transmits - INTEGER
 948        The amount of Duplicate Address Detection probes to send.
 949        Default: 1
 951forwarding - BOOLEAN
 952        Configure interface-specific Host/Router behaviour.
 954        Note: It is recommended to have the same setting on all
 955        interfaces; mixed router/host scenarios are rather uncommon.
 957        FALSE:
 959        By default, Host behaviour is assumed.  This means:
 961        1. IsRouter flag is not set in Neighbour Advertisements.
 962        2. Router Solicitations are being sent when necessary.
 963        3. If accept_ra is TRUE (default), accept Router
 964           Advertisements (and do autoconfiguration).
 965        4. If accept_redirects is TRUE (default), accept Redirects.
 967        TRUE:
 969        If local forwarding is enabled, Router behaviour is assumed.
 970        This means exactly the reverse from the above:
 972        1. IsRouter flag is set in Neighbour Advertisements.
 973        2. Router Solicitations are not sent.
 974        3. Router Advertisements are ignored.
 975        4. Redirects are ignored.
 977        Default: FALSE if global forwarding is disabled (default),
 978                 otherwise TRUE.
 980hop_limit - INTEGER
 981        Default Hop Limit to set.
 982        Default: 64
 984mtu - INTEGER
 985        Default Maximum Transfer Unit
 986        Default: 1280 (IPv6 required minimum)
 988router_probe_interval - INTEGER
 989        Minimum interval (in seconds) between Router Probing described
 990        in RFC4191.
 992        Default: 60
 994router_solicitation_delay - INTEGER
 995        Number of seconds to wait after interface is brought up
 996        before sending Router Solicitations.
 997        Default: 1
 999router_solicitation_interval - INTEGER
1000        Number of seconds to wait between Router Solicitations.
1001        Default: 4
1003router_solicitations - INTEGER
1004        Number of Router Solicitations to send until assuming no
1005        routers are present.
1006        Default: 3
1008use_tempaddr - INTEGER
1009        Preference for Privacy Extensions (RFC3041).
1010          <= 0 : disable Privacy Extensions
1011          == 1 : enable Privacy Extensions, but prefer public
1012                 addresses over temporary addresses.
1013          >  1 : enable Privacy Extensions and prefer temporary
1014                 addresses over public addresses.
1015        Default:  0 (for most devices)
1016                 -1 (for point-to-point devices and loopback devices)
1018temp_valid_lft - INTEGER
1019        valid lifetime (in seconds) for temporary addresses.
1020        Default: 604800 (7 days)
1022temp_prefered_lft - INTEGER
1023        Preferred lifetime (in seconds) for temporary addresses.
1024        Default: 86400 (1 day)
1026max_desync_factor - INTEGER
1027        Maximum value for DESYNC_FACTOR, which is a random value
1028        that ensures that clients don't synchronize with each
1029        other and generate new addresses at exactly the same time.
1030        value is in seconds.
1031        Default: 600
1033regen_max_retry - INTEGER
1034        Number of attempts before give up attempting to generate
1035        valid temporary addresses.
1036        Default: 5
1038max_addresses - INTEGER
1039        Number of maximum addresses per interface.  0 disables limitation.
1040        It is recommended not set too large value (or 0) because it would
1041        be too easy way to crash kernel to allow to create too much of
1042        autoconfigured addresses.
1043        Default: 16
1045disable_ipv6 - BOOLEAN
1046        Disable IPv6 operation.  If accept_dad is set to 2, this value
1047        will be dynamically set to TRUE if DAD fails for the link-local
1048        address.
1049        Default: FALSE (enable IPv6 operation)
1051accept_dad - INTEGER
1052        Whether to accept DAD (Duplicate Address Detection).
1053        0: Disable DAD
1054        1: Enable DAD (default)
1055        2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
1056           link-local address has been found.
1059ratelimit - INTEGER
1060        Limit the maximal rates for sending ICMPv6 packets.
1061        0 to disable any limiting,
1062        otherwise the minimal space between responses in milliseconds.
1063        Default: 1000
1066IPv6 Update by:
1067Pekka Savola <>
1068YOSHIFUJI Hideaki / USAGI Project <>
1071/proc/sys/net/bridge/* Variables:
1073bridge-nf-call-arptables - BOOLEAN
1074        1 : pass bridged ARP traffic to arptables' FORWARD chain.
1075        0 : disable this.
1076        Default: 1
1078bridge-nf-call-iptables - BOOLEAN
1079        1 : pass bridged IPv4 traffic to iptables' chains.
1080        0 : disable this.
1081        Default: 1
1083bridge-nf-call-ip6tables - BOOLEAN
1084        1 : pass bridged IPv6 traffic to ip6tables' chains.
1085        0 : disable this.
1086        Default: 1
1088bridge-nf-filter-vlan-tagged - BOOLEAN
1089        1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
1090        0 : disable this.
1091        Default: 1
1093bridge-nf-filter-pppoe-tagged - BOOLEAN
1094        1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
1095        0 : disable this.
1096        Default: 1
1099proc/sys/net/sctp/* Variables:
1101addip_enable - BOOLEAN
1102        Enable or disable extension of  Dynamic Address Reconfiguration
1103        (ADD-IP) functionality specified in RFC5061.  This extension provides
1104        the ability to dynamically add and remove new addresses for the SCTP
1105        associations.
1107        1: Enable extension.
1109        0: Disable extension.
1111        Default: 0
1113addip_noauth_enable - BOOLEAN
1114        Dynamic Address Reconfiguration (ADD-IP) requires the use of
1115        authentication to protect the operations of adding or removing new
1116        addresses.  This requirement is mandated so that unauthorized hosts
1117        would not be able to hijack associations.  However, older
1118        implementations may not have implemented this requirement while
1119        allowing the ADD-IP extension.  For reasons of interoperability,
1120        we provide this variable to control the enforcement of the
1121        authentication requirement.
1123        1: Allow ADD-IP extension to be used without authentication.  This
1124           should only be set in a closed environment for interoperability
1125           with older implementations.
1127        0: Enforce the authentication requirement
1129        Default: 0
1131auth_enable - BOOLEAN
1132        Enable or disable Authenticated Chunks extension.  This extension
1133        provides the ability to send and receive authenticated chunks and is
1134        required for secure operation of Dynamic Address Reconfiguration
1135        (ADD-IP) extension.
1137        1: Enable this extension.
1138        0: Disable this extension.
1140        Default: 0
1142prsctp_enable - BOOLEAN
1143        Enable or disable the Partial Reliability extension (RFC3758) which
1144        is used to notify peers that a given DATA should no longer be expected.
1146        1: Enable extension
1147        0: Disable
1149        Default: 1
1151max_burst - INTEGER
1152        The limit of the number of new packets that can be initially sent.  It
1153        controls how bursty the generated traffic can be.
1155        Default: 4
1157association_max_retrans - INTEGER
1158        Set the maximum number for retransmissions that an association can
1159        attempt deciding that the remote end is unreachable.  If this value
1160        is exceeded, the association is terminated.
1162        Default: 10
1164max_init_retransmits - INTEGER
1165        The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
1166        that an association will attempt before declaring the destination
1167        unreachable and terminating.
1169        Default: 8
1171path_max_retrans - INTEGER
1172        The maximum number of retransmissions that will be attempted on a given
1173        path.  Once this threshold is exceeded, the path is considered
1174        unreachable, and new traffic will use a different path when the
1175        association is multihomed.
1177        Default: 5
1179rto_initial - INTEGER
1180        The initial round trip timeout value in milliseconds that will be used
1181        in calculating round trip times.  This is the initial time interval
1182        for retransmissions.
1184        Default: 3000
1186rto_max - INTEGER
1187        The maximum value (in milliseconds) of the round trip timeout.  This
1188        is the largest time interval that can elapse between retransmissions.
1190        Default: 60000
1192rto_min - INTEGER
1193        The minimum value (in milliseconds) of the round trip timeout.  This
1194        is the smallest time interval the can elapse between retransmissions.
1196        Default: 1000
1198hb_interval - INTEGER
1199        The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
1200        are sent at the specified interval on idle paths to probe the state of
1201        a given path between 2 associations.
1203        Default: 30000
1205sack_timeout - INTEGER
1206        The amount of time (in milliseconds) that the implementation will wait
1207        to send a SACK.
1209        Default: 200
1211valid_cookie_life - INTEGER
1212        The default lifetime of the SCTP cookie (in milliseconds).  The cookie
1213        is used during association establishment.
1215        Default: 60000
1217cookie_preserve_enable - BOOLEAN
1218        Enable or disable the ability to extend the lifetime of the SCTP cookie
1219        that is used during the establishment phase of SCTP association
1221        1: Enable cookie lifetime extension.
1222        0: Disable
1224        Default: 1
1226rcvbuf_policy - INTEGER
1227        Determines if the receive buffer is attributed to the socket or to
1228        association.   SCTP supports the capability to create multiple
1229        associations on a single socket.  When using this capability, it is
1230        possible that a single stalled association that's buffering a lot
1231        of data may block other associations from delivering their data by
1232        consuming all of the receive buffer space.  To work around this,
1233        the rcvbuf_policy could be set to attribute the receiver buffer space
1234        to each association instead of the socket.  This prevents the described
1235        blocking.
1237        1: rcvbuf space is per association
1238        0: recbuf space is per socket
1240        Default: 0
1242sndbuf_policy - INTEGER
1243        Similar to rcvbuf_policy above, this applies to send buffer space.
1245        1: Send buffer is tracked per association
1246        0: Send buffer is tracked per socket.
1248        Default: 0
1250sctp_mem - vector of 3 INTEGERs: min, pressure, max
1251        Number of pages allowed for queueing by all SCTP sockets.
1253        min: Below this number of pages SCTP is not bothered about its
1254        memory appetite. When amount of memory allocated by SCTP exceeds
1255        this number, SCTP starts to moderate memory usage.
1257        pressure: This value was introduced to follow format of tcp_mem.
1259        max: Number of pages allowed for queueing by all SCTP sockets.
1261        Default is calculated at boot time from amount of available memory.
1263sctp_rmem - vector of 3 INTEGERs: min, default, max
1264        See tcp_rmem for a description.
1266sctp_wmem  - vector of 3 INTEGERs: min, default, max
1267        See tcp_wmem for a description.
1271dev_weight - INTEGER
1272        The maximum number of packets that kernel can handle on a NAPI
1273        interrupt, it's a Per-CPU variable.
1275        Default: 64
1278max_dgram_qlen - INTEGER
1279        The maximum length of dgram socket receive queue
1281        Default: 10
1287        fast_poll_increase FIXME
1288        warn_noreply_time FIXME
1289        discovery_slots FIXME
1290        slot_timeout FIXME
1291        max_baud_rate FIXME
1292        discovery_timeout FIXME
1293        lap_keepalive_time FIXME
1294        max_noreply_time FIXME
1295        max_tx_data_size FIXME
1296        max_tx_window FIXME
1297        min_tx_turn_time FIXME
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