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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  *
   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/ipv6.h>
  17 #include <net/tcp.h>
  18
  19 #include <linux/netfilter_ipv4/ip_tables.h>
  20 #include <linux/netfilter_ipv6/ip6_tables.h>
  21 #include <linux/netfilter/x_tables.h>
  22 #include <linux/netfilter/xt_tcpudp.h>
  23 #include <linux/netfilter/xt_TCPMSS.h>
  24
  25 MODULE_LICENSE("GPL");
  26 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  27 MODULE_DESCRIPTION("x_tables TCP MSS modification module");
  28 MODULE_ALIAS("ipt_TCPMSS");
  29 MODULE_ALIAS("ip6t_TCPMSS");
  30
  31 static inline unsigned int
  32 optlen(const u_int8_t *opt, unsigned int offset)
  33 {
  34         /* Beware zero-length options: make finite progress */
  35         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  36                 return 1;
  37         else
  38                 return opt[offset+1];
  39 }
  40
  41 static int
  42 tcpmss_mangle_packet(struct sk_buff *skb,
  43                      const struct xt_tcpmss_info *info,
  44                      unsigned int tcphoff,
  45                      unsigned int minlen)
  46 {
  47         struct tcphdr *tcph;
  48         unsigned int tcplen, i;
  49         __be16 oldval;
  50         u16 newmss;
  51         u8 *opt;
  52
  53         if (!skb_make_writable(skb, skb->len))
  54                 return -1;
  55
  56         tcplen = skb->len - tcphoff;
  57         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  58
  59         /* Since it passed flags test in tcp match, we know it is is
  60            not a fragment, and has data >= tcp header length.  SYN
  61            packets should not contain data: if they did, then we risk
  62            running over MTU, sending Frag Needed and breaking things
  63            badly. --RR */
  64         if (tcplen != tcph->doff*4) {
  65                 if (net_ratelimit())
  66                         printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
  67                                skb->len);
  68                 return -1;
  69         }
  70
  71         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
  72                 if (dst_mtu(skb->dst) <= minlen) {
  73                         if (net_ratelimit())
  74                                 printk(KERN_ERR "xt_TCPMSS: "
  75                                        "unknown or invalid path-MTU (%u)\n",
  76                                        dst_mtu(skb->dst));
  77                         return -1;
  78                 }
  79                 newmss = dst_mtu(skb->dst) - minlen;
  80         } else
  81                 newmss = info->mss;
  82
  83         opt = (u_int8_t *)tcph;
  84         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
  85                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
  86                     opt[i+1] == TCPOLEN_MSS) {
  87                         u_int16_t oldmss;
  88
  89                         oldmss = (opt[i+2] << 8) | opt[i+3];
  90
  91                         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
  92                             oldmss <= newmss)
  93                                 return 0;
  94
  95                         opt[i+2] = (newmss & 0xff00) >> 8;
  96                         opt[i+3] = newmss & 0x00ff;
  97
  98                         nf_proto_csum_replace2(&tcph->check, skb,
  99                                                htons(oldmss), htons(newmss), 0);
 100                         return 0;
 101                 }
 102         }
 103
 104         /*
 105          * MSS Option not found ?! add it..
 106          */
 107         if (skb_tailroom(skb) < TCPOLEN_MSS) {
 108                 if (pskb_expand_head(skb, 0,
 109                                      TCPOLEN_MSS - skb_tailroom(skb),
 110                                      GFP_ATOMIC))
 111                         return -1;
 112                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 113         }
 114
 115         skb_put(skb, TCPOLEN_MSS);
 116
 117         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 118         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 119
 120         nf_proto_csum_replace2(&tcph->check, skb,
 121                                htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
 122         opt[0] = TCPOPT_MSS;
 123         opt[1] = TCPOLEN_MSS;
 124         opt[2] = (newmss & 0xff00) >> 8;
 125         opt[3] = newmss & 0x00ff;
 126
 127         nf_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
 128
 129         oldval = ((__be16 *)tcph)[6];
 130         tcph->doff += TCPOLEN_MSS/4;
 131         nf_proto_csum_replace2(&tcph->check, skb,
 132                                 oldval, ((__be16 *)tcph)[6], 0);
 133         return TCPOLEN_MSS;
 134 }
 135
 136 static unsigned int
 137 xt_tcpmss_target4(struct sk_buff *skb,
 138                   const struct net_device *in,
 139                   const struct net_device *out,
 140                   unsigned int hooknum,
 141                   const struct xt_target *target,
 142                   const void *targinfo)
 143 {
 144         struct iphdr *iph = ip_hdr(skb);
 145         __be16 newlen;
 146         int ret;
 147
 148         ret = tcpmss_mangle_packet(skb, targinfo, iph->ihl * 4,
 149                                    sizeof(*iph) + sizeof(struct tcphdr));
 150         if (ret < 0)
 151                 return NF_DROP;
 152         if (ret > 0) {
 153                 iph = ip_hdr(skb);
 154                 newlen = htons(ntohs(iph->tot_len) + ret);
 155                 nf_csum_replace2(&iph->check, iph->tot_len, newlen);
 156                 iph->tot_len = newlen;
 157         }
 158         return XT_CONTINUE;
 159 }
 160
 161 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 162 static unsigned int
 163 xt_tcpmss_target6(struct sk_buff *skb,
 164                   const struct net_device *in,
 165                   const struct net_device *out,
 166                   unsigned int hooknum,
 167                   const struct xt_target *target,
 168                   const void *targinfo)
 169 {
 170         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 171         u8 nexthdr;
 172         int tcphoff;
 173         int ret;
 174
 175         nexthdr = ipv6h->nexthdr;
 176         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
 177         if (tcphoff < 0)
 178                 return NF_DROP;
 179         ret = tcpmss_mangle_packet(skb, targinfo, tcphoff,
 180                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
 181         if (ret < 0)
 182                 return NF_DROP;
 183         if (ret > 0) {
 184                 ipv6h = ipv6_hdr(skb);
 185                 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
 186         }
 187         return XT_CONTINUE;
 188 }
 189 #endif
 190
 191 #define TH_SYN 0x02
 192
 193 /* Must specify -p tcp --syn */
 194 static inline bool find_syn_match(const struct xt_entry_match *m)
 195 {
 196         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 197
 198         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 199             tcpinfo->flg_cmp & TH_SYN &&
 200             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 201                 return true;
 202
 203         return false;
 204 }
 205
 206 static bool
 207 xt_tcpmss_checkentry4(const char *tablename,
 208                       const void *entry,
 209                       const struct xt_target *target,
 210                       void *targinfo,
 211                       unsigned int hook_mask)
 212 {
 213         const struct xt_tcpmss_info *info = targinfo;
 214         const struct ipt_entry *e = entry;
 215
 216         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 217             (hook_mask & ~((1 << NF_INET_FORWARD) |
 218                            (1 << NF_INET_LOCAL_OUT) |
 219                            (1 << NF_INET_POST_ROUTING))) != 0) {
 220                 printk("xt_TCPMSS: path-MTU clamping only supported in "
 221                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 222                 return false;
 223         }
 224         if (IPT_MATCH_ITERATE(e, find_syn_match))
 225                 return true;
 226         printk("xt_TCPMSS: Only works on TCP SYN packets\n");
 227         return false;
 228 }
 229
 230 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 231 static bool
 232 xt_tcpmss_checkentry6(const char *tablename,
 233                       const void *entry,
 234                       const struct xt_target *target,
 235                       void *targinfo,
 236                       unsigned int hook_mask)
 237 {
 238         const struct xt_tcpmss_info *info = targinfo;
 239         const struct ip6t_entry *e = entry;
 240
 241         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 242             (hook_mask & ~((1 << NF_INET_FORWARD) |
 243                            (1 << NF_INET_LOCAL_OUT) |
 244                            (1 << NF_INET_POST_ROUTING))) != 0) {
 245                 printk("xt_TCPMSS: path-MTU clamping only supported in "
 246                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 247                 return false;
 248         }
 249         if (IP6T_MATCH_ITERATE(e, find_syn_match))
 250                 return true;
 251         printk("xt_TCPMSS: Only works on TCP SYN packets\n");
 252         return false;
 253 }
 254 #endif
 255
 256 static struct xt_target xt_tcpmss_reg[] __read_mostly = {
 257         {
 258                 .family         = AF_INET,
 259                 .name           = "TCPMSS",
 260                 .checkentry     = xt_tcpmss_checkentry4,
 261                 .target         = xt_tcpmss_target4,
 262                 .targetsize     = sizeof(struct xt_tcpmss_info),
 263                 .proto          = IPPROTO_TCP,
 264                 .me             = THIS_MODULE,
 265         },
 266 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 267         {
 268                 .family         = AF_INET6,
 269                 .name           = "TCPMSS",
 270                 .checkentry     = xt_tcpmss_checkentry6,
 271                 .target         = xt_tcpmss_target6,
 272                 .targetsize     = sizeof(struct xt_tcpmss_info),
 273                 .proto          = IPPROTO_TCP,
 274                 .me             = THIS_MODULE,
 275         },
 276 #endif
 277 };
 278
 279 static int __init xt_tcpmss_init(void)
 280 {
 281         return xt_register_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
 282 }
 283
 284 static void __exit xt_tcpmss_fini(void)
 285 {
 286         xt_unregister_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
 287 }
 288
 289 module_init(xt_tcpmss_init);
 290 module_exit(xt_tcpmss_fini);