net/netfilter/xt_TCPMSS.c

   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  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License version 2 as
   8  * published by the Free Software Foundation.
   9  */
  10
  11 #include <linux/module.h>
  12 #include <linux/skbuff.h>
  13 #include <linux/ip.h>
  14 #include <linux/ipv6.h>
  15 #include <linux/tcp.h>
  16 #include <net/dst.h>
  17 #include <net/flow.h>
  18 #include <net/ipv6.h>
  19 #include <net/route.h>
  20 #include <net/tcp.h>
  21
  22 #include <linux/netfilter_ipv4/ip_tables.h>
  23 #include <linux/netfilter_ipv6/ip6_tables.h>
  24 #include <linux/netfilter/x_tables.h>
  25 #include <linux/netfilter/xt_tcpudp.h>
  26 #include <linux/netfilter/xt_TCPMSS.h>
  27
  28 MODULE_LICENSE("GPL");
  29 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  30 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
  31 MODULE_ALIAS("ipt_TCPMSS");
  32 MODULE_ALIAS("ip6t_TCPMSS");
  33
  34 static inline unsigned int
  35 optlen(const u_int8_t *opt, unsigned int offset)
  36 {
  37         /* Beware zero-length options: make finite progress */
  38         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  39                 return 1;
  40         else
  41                 return opt[offset+1];
  42 }
  43
  44 static int
  45 tcpmss_mangle_packet(struct sk_buff *skb,
  46                      const struct xt_tcpmss_info *info,
  47                      unsigned int in_mtu,
  48                      unsigned int tcphoff,
  49                      unsigned int minlen)
  50 {
  51         struct tcphdr *tcph;
  52         unsigned int tcplen, i;
  53         __be16 oldval;
  54         u16 newmss;
  55         u8 *opt;
  56
  57         if (!skb_make_writable(skb, skb->len))
  58                 return -1;
  59
  60         tcplen = skb->len - tcphoff;
  61         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  62
  63         /* Since it passed flags test in tcp match, we know it is is
  64            not a fragment, and has data >= tcp header length.  SYN
  65            packets should not contain data: if they did, then we risk
  66            running over MTU, sending Frag Needed and breaking things
  67            badly. --RR */
  68         if (tcplen != tcph->doff*4) {
  69                 if (net_ratelimit())
  70                         printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
  71                                skb->len);
  72                 return -1;
  73         }
  74
  75         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
  76                 if (dst_mtu(skb->dst) <= minlen) {
  77                         if (net_ratelimit())
  78                                 printk(KERN_ERR "xt_TCPMSS: "
  79                                        "unknown or invalid path-MTU (%u)\n",
  80                                        dst_mtu(skb->dst));
  81                         return -1;
  82                 }
  83                 if (in_mtu <= minlen) {
  84                         if (net_ratelimit())
  85                                 printk(KERN_ERR "xt_TCPMSS: unknown or "
  86                                        "invalid path-MTU (%u)\n", in_mtu);
  87                         return -1;
  88                 }
  89                 newmss = min(dst_mtu(skb->dst), in_mtu) - minlen;
  90         } else
  91                 newmss = info->mss;
  92
  93         opt = (u_int8_t *)tcph;
  94         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
  95                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
  96                     opt[i+1] == TCPOLEN_MSS) {
  97                         u_int16_t oldmss;
  98
  99                         oldmss = (opt[i+2] << 8) | opt[i+3];
 100
 101                         /* Never increase MSS, even when setting it, as
 102                          * doing so results in problems for hosts that rely
 103                          * on MSS being set correctly.
 104                          */
 105                         if (oldmss <= newmss)
 106                                 return 0;
 107
 108                         opt[i+2] = (newmss & 0xff00) >> 8;
 109                         opt[i+3] = newmss & 0x00ff;
 110
 111                         inet_proto_csum_replace2(&tcph->check, skb,
 112                                                  htons(oldmss), htons(newmss),
 113                                                  0);
 114                         return 0;
 115                 }
 116         }
 117
 118         /*
 119          * MSS Option not found ?! add it..
 120          */
 121         if (skb_tailroom(skb) < TCPOLEN_MSS) {
 122                 if (pskb_expand_head(skb, 0,
 123                                      TCPOLEN_MSS - skb_tailroom(skb),
 124                                      GFP_ATOMIC))
 125                         return -1;
 126                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 127         }
 128
 129         skb_put(skb, TCPOLEN_MSS);
 130
 131         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 132         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 133
 134         inet_proto_csum_replace2(&tcph->check, skb,
 135                                  htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
 136         opt[0] = TCPOPT_MSS;
 137         opt[1] = TCPOLEN_MSS;
 138         opt[2] = (newmss & 0xff00) >> 8;
 139         opt[3] = newmss & 0x00ff;
 140
 141         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
 142
 143         oldval = ((__be16 *)tcph)[6];
 144         tcph->doff += TCPOLEN_MSS/4;
 145         inet_proto_csum_replace2(&tcph->check, skb,
 146                                  oldval, ((__be16 *)tcph)[6], 0);
 147         return TCPOLEN_MSS;
 148 }
 149
 150 static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
 151                                     unsigned int family)
 152 {
 153         struct flowi fl = {};
 154         const struct nf_afinfo *ai;
 155         struct rtable *rt = NULL;
 156         u_int32_t mtu     = ~0U;
 157
 158         if (family == PF_INET)
 159                 fl.fl4_dst = ip_hdr(skb)->saddr;
 160         else
 161                 fl.fl6_dst = ipv6_hdr(skb)->saddr;
 162
 163         rcu_read_lock();
 164         ai = nf_get_afinfo(family);
 165         if (ai != NULL)
 166                 ai->route((struct dst_entry **)&rt, &fl);
 167         rcu_read_unlock();
 168
 169         if (rt != NULL) {
 170                 mtu = dst_mtu(&rt->u.dst);
 171                 dst_release(&rt->u.dst);
 172         }
 173         return mtu;
 174 }
 175
 176 static unsigned int
 177 tcpmss_tg4(struct sk_buff *skb, const struct xt_target_param *par)
 178 {
 179         struct iphdr *iph = ip_hdr(skb);
 180         __be16 newlen;
 181         int ret;
 182
 183         ret = tcpmss_mangle_packet(skb, par->targinfo,
 184                                    tcpmss_reverse_mtu(skb, PF_INET),
 185                                    iph->ihl * 4,
 186                                    sizeof(*iph) + sizeof(struct tcphdr));
 187         if (ret < 0)
 188                 return NF_DROP;
 189         if (ret > 0) {
 190                 iph = ip_hdr(skb);
 191                 newlen = htons(ntohs(iph->tot_len) + ret);
 192                 csum_replace2(&iph->check, iph->tot_len, newlen);
 193                 iph->tot_len = newlen;
 194         }
 195         return XT_CONTINUE;
 196 }
 197
 198 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 199 static unsigned int
 200 tcpmss_tg6(struct sk_buff *skb, const struct xt_target_param *par)
 201 {
 202         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 203         u8 nexthdr;
 204         int tcphoff;
 205         int ret;
 206
 207         nexthdr = ipv6h->nexthdr;
 208         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
 209         if (tcphoff < 0)
 210                 return NF_DROP;
 211         ret = tcpmss_mangle_packet(skb, par->targinfo,
 212                                    tcpmss_reverse_mtu(skb, PF_INET6),
 213                                    tcphoff,
 214                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
 215         if (ret < 0)
 216                 return NF_DROP;
 217         if (ret > 0) {
 218                 ipv6h = ipv6_hdr(skb);
 219                 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
 220         }
 221         return XT_CONTINUE;
 222 }
 223 #endif
 224
 225 #define TH_SYN 0x02
 226
 227 /* Must specify -p tcp --syn */
 228 static inline bool find_syn_match(const struct xt_entry_match *m)
 229 {
 230         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 231
 232         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 233             tcpinfo->flg_cmp & TH_SYN &&
 234             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 235                 return true;
 236
 237         return false;
 238 }
 239
 240 static bool tcpmss_tg4_check(const struct xt_tgchk_param *par)
 241 {
 242         const struct xt_tcpmss_info *info = par->targinfo;
 243         const struct ipt_entry *e = par->entryinfo;
 244
 245         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 246             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 247                            (1 << NF_INET_LOCAL_OUT) |
 248                            (1 << NF_INET_POST_ROUTING))) != 0) {
 249                 printk("xt_TCPMSS: path-MTU clamping only supported in "
 250                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 251                 return false;
 252         }
 253         if (IPT_MATCH_ITERATE(e, find_syn_match))
 254                 return true;
 255         printk("xt_TCPMSS: Only works on TCP SYN packets\n");
 256         return false;
 257 }
 258
 259 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 260 static bool tcpmss_tg6_check(const struct xt_tgchk_param *par)
 261 {
 262         const struct xt_tcpmss_info *info = par->targinfo;
 263         const struct ip6t_entry *e = par->entryinfo;
 264
 265         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 266             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 267                            (1 << NF_INET_LOCAL_OUT) |
 268                            (1 << NF_INET_POST_ROUTING))) != 0) {
 269                 printk("xt_TCPMSS: path-MTU clamping only supported in "
 270                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 271                 return false;
 272         }
 273         if (IP6T_MATCH_ITERATE(e, find_syn_match))
 274                 return true;
 275         printk("xt_TCPMSS: Only works on TCP SYN packets\n");
 276         return false;
 277 }
 278 #endif
 279
 280 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
 281         {
 282                 .family         = NFPROTO_IPV4,
 283                 .name           = "TCPMSS",
 284                 .checkentry     = tcpmss_tg4_check,
 285                 .target         = tcpmss_tg4,
 286                 .targetsize     = sizeof(struct xt_tcpmss_info),
 287                 .proto          = IPPROTO_TCP,
 288                 .me             = THIS_MODULE,
 289         },
 290 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 291         {
 292                 .family         = NFPROTO_IPV6,
 293                 .name           = "TCPMSS",
 294                 .checkentry     = tcpmss_tg6_check,
 295                 .target         = tcpmss_tg6,
 296                 .targetsize     = sizeof(struct xt_tcpmss_info),
 297                 .proto          = IPPROTO_TCP,
 298                 .me             = THIS_MODULE,
 299         },
 300 #endif
 301 };
 302
 303 static int __init tcpmss_tg_init(void)
 304 {
 305         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 306 }
 307
 308 static void __exit tcpmss_tg_exit(void)
 309 {
 310         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 311 }
 312
 313 module_init(tcpmss_tg_init);
 314 module_exit(tcpmss_tg_exit);