linux/net/netfilter/xt_TCPMSS.c
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   1/*
   2 * This is a module which is used for setting the MSS option in TCP packets.
   3 *
   4 * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
   5 * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
   6 *
   7 * This program is free software; you can redistribute it and/or modify
   8 * it under the terms of the GNU General Public License version 2 as
   9 * published by the Free Software Foundation.
  10 */
  11#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  12#include <linux/module.h>
  13#include <linux/skbuff.h>
  14#include <linux/ip.h>
  15#include <linux/gfp.h>
  16#include <linux/ipv6.h>
  17#include <linux/tcp.h>
  18#include <net/dst.h>
  19#include <net/flow.h>
  20#include <net/ipv6.h>
  21#include <net/route.h>
  22#include <net/tcp.h>
  23
  24#include <linux/netfilter_ipv4/ip_tables.h>
  25#include <linux/netfilter_ipv6/ip6_tables.h>
  26#include <linux/netfilter/x_tables.h>
  27#include <linux/netfilter/xt_tcpudp.h>
  28#include <linux/netfilter/xt_TCPMSS.h>
  29
  30MODULE_LICENSE("GPL");
  31MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  32MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
  33MODULE_ALIAS("ipt_TCPMSS");
  34MODULE_ALIAS("ip6t_TCPMSS");
  35
  36static inline unsigned int
  37optlen(const u_int8_t *opt, unsigned int offset)
  38{
  39        /* Beware zero-length options: make finite progress */
  40        if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  41                return 1;
  42        else
  43                return opt[offset+1];
  44}
  45
  46static int
  47tcpmss_mangle_packet(struct sk_buff *skb,
  48                     const struct xt_action_param *par,
  49                     unsigned int in_mtu,
  50                     unsigned int tcphoff,
  51                     unsigned int minlen)
  52{
  53        const struct xt_tcpmss_info *info = par->targinfo;
  54        struct tcphdr *tcph;
  55        int len, tcp_hdrlen;
  56        unsigned int i;
  57        __be16 oldval;
  58        u16 newmss;
  59        u8 *opt;
  60
  61        /* This is a fragment, no TCP header is available */
  62        if (par->fragoff != 0)
  63                return XT_CONTINUE;
  64
  65        if (!skb_make_writable(skb, skb->len))
  66                return -1;
  67
  68        len = skb->len - tcphoff;
  69        if (len < (int)sizeof(struct tcphdr))
  70                return -1;
  71
  72        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  73        tcp_hdrlen = tcph->doff * 4;
  74
  75        if (len < tcp_hdrlen)
  76                return -1;
  77
  78        if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
  79                if (dst_mtu(skb_dst(skb)) <= minlen) {
  80                        net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
  81                                            dst_mtu(skb_dst(skb)));
  82                        return -1;
  83                }
  84                if (in_mtu <= minlen) {
  85                        net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
  86                                            in_mtu);
  87                        return -1;
  88                }
  89                newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
  90        } else
  91                newmss = info->mss;
  92
  93        opt = (u_int8_t *)tcph;
  94        for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
  95                if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
  96                        u_int16_t oldmss;
  97
  98                        oldmss = (opt[i+2] << 8) | opt[i+3];
  99
 100                        /* Never increase MSS, even when setting it, as
 101                         * doing so results in problems for hosts that rely
 102                         * on MSS being set correctly.
 103                         */
 104                        if (oldmss <= newmss)
 105                                return 0;
 106
 107                        opt[i+2] = (newmss & 0xff00) >> 8;
 108                        opt[i+3] = newmss & 0x00ff;
 109
 110                        inet_proto_csum_replace2(&tcph->check, skb,
 111                                                 htons(oldmss), htons(newmss),
 112                                                 0);
 113                        return 0;
 114                }
 115        }
 116
 117        /* There is data after the header so the option can't be added
 118         * without moving it, and doing so may make the SYN packet
 119         * itself too large. Accept the packet unmodified instead.
 120         */
 121        if (len > tcp_hdrlen)
 122                return 0;
 123
 124        /*
 125         * MSS Option not found ?! add it..
 126         */
 127        if (skb_tailroom(skb) < TCPOLEN_MSS) {
 128                if (pskb_expand_head(skb, 0,
 129                                     TCPOLEN_MSS - skb_tailroom(skb),
 130                                     GFP_ATOMIC))
 131                        return -1;
 132                tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 133        }
 134
 135        skb_put(skb, TCPOLEN_MSS);
 136
 137        /*
 138         * IPv4: RFC 1122 states "If an MSS option is not received at
 139         * connection setup, TCP MUST assume a default send MSS of 536".
 140         * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
 141         * length IPv6 header of 60, ergo the default MSS value is 1220
 142         * Since no MSS was provided, we must use the default values
 143         */
 144        if (par->family == NFPROTO_IPV4)
 145                newmss = min(newmss, (u16)536);
 146        else
 147                newmss = min(newmss, (u16)1220);
 148
 149        opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 150        memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
 151
 152        inet_proto_csum_replace2(&tcph->check, skb,
 153                                 htons(len), htons(len + TCPOLEN_MSS), 1);
 154        opt[0] = TCPOPT_MSS;
 155        opt[1] = TCPOLEN_MSS;
 156        opt[2] = (newmss & 0xff00) >> 8;
 157        opt[3] = newmss & 0x00ff;
 158
 159        inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
 160
 161        oldval = ((__be16 *)tcph)[6];
 162        tcph->doff += TCPOLEN_MSS/4;
 163        inet_proto_csum_replace2(&tcph->check, skb,
 164                                 oldval, ((__be16 *)tcph)[6], 0);
 165        return TCPOLEN_MSS;
 166}
 167
 168static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
 169                                    unsigned int family)
 170{
 171        struct flowi fl;
 172        const struct nf_afinfo *ai;
 173        struct rtable *rt = NULL;
 174        u_int32_t mtu     = ~0U;
 175
 176        if (family == PF_INET) {
 177                struct flowi4 *fl4 = &fl.u.ip4;
 178                memset(fl4, 0, sizeof(*fl4));
 179                fl4->daddr = ip_hdr(skb)->saddr;
 180        } else {
 181                struct flowi6 *fl6 = &fl.u.ip6;
 182
 183                memset(fl6, 0, sizeof(*fl6));
 184                fl6->daddr = ipv6_hdr(skb)->saddr;
 185        }
 186        rcu_read_lock();
 187        ai = nf_get_afinfo(family);
 188        if (ai != NULL)
 189                ai->route(&init_net, (struct dst_entry **)&rt, &fl, false);
 190        rcu_read_unlock();
 191
 192        if (rt != NULL) {
 193                mtu = dst_mtu(&rt->dst);
 194                dst_release(&rt->dst);
 195        }
 196        return mtu;
 197}
 198
 199static unsigned int
 200tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
 201{
 202        struct iphdr *iph = ip_hdr(skb);
 203        __be16 newlen;
 204        int ret;
 205
 206        ret = tcpmss_mangle_packet(skb, par,
 207                                   tcpmss_reverse_mtu(skb, PF_INET),
 208                                   iph->ihl * 4,
 209                                   sizeof(*iph) + sizeof(struct tcphdr));
 210        if (ret < 0)
 211                return NF_DROP;
 212        if (ret > 0) {
 213                iph = ip_hdr(skb);
 214                newlen = htons(ntohs(iph->tot_len) + ret);
 215                csum_replace2(&iph->check, iph->tot_len, newlen);
 216                iph->tot_len = newlen;
 217        }
 218        return XT_CONTINUE;
 219}
 220
 221#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 222static unsigned int
 223tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 224{
 225        struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 226        u8 nexthdr;
 227        __be16 frag_off;
 228        int tcphoff;
 229        int ret;
 230
 231        nexthdr = ipv6h->nexthdr;
 232        tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 233        if (tcphoff < 0)
 234                return NF_DROP;
 235        ret = tcpmss_mangle_packet(skb, par,
 236                                   tcpmss_reverse_mtu(skb, PF_INET6),
 237                                   tcphoff,
 238                                   sizeof(*ipv6h) + sizeof(struct tcphdr));
 239        if (ret < 0)
 240                return NF_DROP;
 241        if (ret > 0) {
 242                ipv6h = ipv6_hdr(skb);
 243                ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
 244        }
 245        return XT_CONTINUE;
 246}
 247#endif
 248
 249/* Must specify -p tcp --syn */
 250static inline bool find_syn_match(const struct xt_entry_match *m)
 251{
 252        const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 253
 254        if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 255            tcpinfo->flg_cmp & TCPHDR_SYN &&
 256            !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 257                return true;
 258
 259        return false;
 260}
 261
 262static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
 263{
 264        const struct xt_tcpmss_info *info = par->targinfo;
 265        const struct ipt_entry *e = par->entryinfo;
 266        const struct xt_entry_match *ematch;
 267
 268        if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 269            (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 270                           (1 << NF_INET_LOCAL_OUT) |
 271                           (1 << NF_INET_POST_ROUTING))) != 0) {
 272                pr_info("path-MTU clamping only supported in "
 273                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 274                return -EINVAL;
 275        }
 276        xt_ematch_foreach(ematch, e)
 277                if (find_syn_match(ematch))
 278                        return 0;
 279        pr_info("Only works on TCP SYN packets\n");
 280        return -EINVAL;
 281}
 282
 283#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 284static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
 285{
 286        const struct xt_tcpmss_info *info = par->targinfo;
 287        const struct ip6t_entry *e = par->entryinfo;
 288        const struct xt_entry_match *ematch;
 289
 290        if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 291            (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 292                           (1 << NF_INET_LOCAL_OUT) |
 293                           (1 << NF_INET_POST_ROUTING))) != 0) {
 294                pr_info("path-MTU clamping only supported in "
 295                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 296                return -EINVAL;
 297        }
 298        xt_ematch_foreach(ematch, e)
 299                if (find_syn_match(ematch))
 300                        return 0;
 301        pr_info("Only works on TCP SYN packets\n");
 302        return -EINVAL;
 303}
 304#endif
 305
 306static struct xt_target tcpmss_tg_reg[] __read_mostly = {
 307        {
 308                .family         = NFPROTO_IPV4,
 309                .name           = "TCPMSS",
 310                .checkentry     = tcpmss_tg4_check,
 311                .target         = tcpmss_tg4,
 312                .targetsize     = sizeof(struct xt_tcpmss_info),
 313                .proto          = IPPROTO_TCP,
 314                .me             = THIS_MODULE,
 315        },
 316#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 317        {
 318                .family         = NFPROTO_IPV6,
 319                .name           = "TCPMSS",
 320                .checkentry     = tcpmss_tg6_check,
 321                .target         = tcpmss_tg6,
 322                .targetsize     = sizeof(struct xt_tcpmss_info),
 323                .proto          = IPPROTO_TCP,
 324                .me             = THIS_MODULE,
 325        },
 326#endif
 327};
 328
 329static int __init tcpmss_tg_init(void)
 330{
 331        return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 332}
 333
 334static void __exit tcpmss_tg_exit(void)
 335{
 336        xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 337}
 338
 339module_init(tcpmss_tg_init);
 340module_exit(tcpmss_tg_exit);
 341
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