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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 **pskb,
  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(pskb, (*pskb)->len))
  54                 return -1;
  55
  56         tcplen = (*pskb)->len - tcphoff;
  57         tcph = (struct tcphdr *)(skb_network_header(*pskb) + 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                                (*pskb)->len);
  68                 return -1;
  69         }
  70
  71         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
  72                 if (dst_mtu((*pskb)->dst) <= minlen) {
  73                         if (net_ratelimit())
  74                                 printk(KERN_ERR "xt_TCPMSS: "
  75                                        "unknown or invalid path-MTU (%u)\n",
  76                                        dst_mtu((*pskb)->dst));
  77                         return -1;
  78                 }
  79                 newmss = dst_mtu((*pskb)->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, *pskb,
  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((*pskb)) < TCPOLEN_MSS) {
 108                 struct sk_buff *newskb;
 109
 110                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 111                                          TCPOLEN_MSS, GFP_ATOMIC);
 112                 if (!newskb)
 113                         return -1;
 114                 kfree_skb(*pskb);
 115                 *pskb = newskb;
 116                 tcph = (struct tcphdr *)(skb_network_header(*pskb) + tcphoff);
 117         }
 118
 119         skb_put((*pskb), TCPOLEN_MSS);
 120
 121         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 122         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 123
 124         nf_proto_csum_replace2(&tcph->check, *pskb,
 125                                htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
 126         opt[0] = TCPOPT_MSS;
 127         opt[1] = TCPOLEN_MSS;
 128         opt[2] = (newmss & 0xff00) >> 8;
 129         opt[3] = newmss & 0x00ff;
 130
 131         nf_proto_csum_replace4(&tcph->check, *pskb, 0, *((__be32 *)opt), 0);
 132
 133         oldval = ((__be16 *)tcph)[6];
 134         tcph->doff += TCPOLEN_MSS/4;
 135         nf_proto_csum_replace2(&tcph->check, *pskb,
 136                                 oldval, ((__be16 *)tcph)[6], 0);
 137         return TCPOLEN_MSS;
 138 }
 139
 140 static unsigned int
 141 xt_tcpmss_target4(struct sk_buff **pskb,
 142                   const struct net_device *in,
 143                   const struct net_device *out,
 144                   unsigned int hooknum,
 145                   const struct xt_target *target,
 146                   const void *targinfo)
 147 {
 148         struct iphdr *iph = ip_hdr(*pskb);
 149         __be16 newlen;
 150         int ret;
 151
 152         ret = tcpmss_mangle_packet(pskb, targinfo, iph->ihl * 4,
 153                                    sizeof(*iph) + sizeof(struct tcphdr));
 154         if (ret < 0)
 155                 return NF_DROP;
 156         if (ret > 0) {
 157                 iph = ip_hdr(*pskb);
 158                 newlen = htons(ntohs(iph->tot_len) + ret);
 159                 nf_csum_replace2(&iph->check, iph->tot_len, newlen);
 160                 iph->tot_len = newlen;
 161         }
 162         return XT_CONTINUE;
 163 }
 164
 165 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 166 static unsigned int
 167 xt_tcpmss_target6(struct sk_buff **pskb,
 168                   const struct net_device *in,
 169                   const struct net_device *out,
 170                   unsigned int hooknum,
 171                   const struct xt_target *target,
 172                   const void *targinfo)
 173 {
 174         struct ipv6hdr *ipv6h = ipv6_hdr(*pskb);
 175         u8 nexthdr;
 176         int tcphoff;
 177         int ret;
 178
 179         nexthdr = ipv6h->nexthdr;
 180         tcphoff = ipv6_skip_exthdr(*pskb, sizeof(*ipv6h), &nexthdr);
 181         if (tcphoff < 0) {
 182                 WARN_ON(1);
 183                 return NF_DROP;
 184         }
 185         ret = tcpmss_mangle_packet(pskb, targinfo, tcphoff,
 186                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
 187         if (ret < 0)
 188                 return NF_DROP;
 189         if (ret > 0) {
 190                 ipv6h = ipv6_hdr(*pskb);
 191                 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
 192         }
 193         return XT_CONTINUE;
 194 }
 195 #endif
 196
 197 #define TH_SYN 0x02
 198
 199 /* Must specify -p tcp --syn */
 200 static inline bool find_syn_match(const struct xt_entry_match *m)
 201 {
 202         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 203
 204         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 205             tcpinfo->flg_cmp & TH_SYN &&
 206             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 207                 return true;
 208
 209         return false;
 210 }
 211
 212 static bool
 213 xt_tcpmss_checkentry4(const char *tablename,
 214                       const void *entry,
 215                       const struct xt_target *target,
 216                       void *targinfo,
 217                       unsigned int hook_mask)
 218 {
 219         const struct xt_tcpmss_info *info = targinfo;
 220         const struct ipt_entry *e = entry;
 221
 222         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 223             (hook_mask & ~((1 << NF_IP_FORWARD) |
 224                            (1 << NF_IP_LOCAL_OUT) |
 225                            (1 << NF_IP_POST_ROUTING))) != 0) {
 226                 printk("xt_TCPMSS: path-MTU clamping only supported in "
 227                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 228                 return false;
 229         }
 230         if (IPT_MATCH_ITERATE(e, find_syn_match))
 231                 return true;
 232         printk("xt_TCPMSS: Only works on TCP SYN packets\n");
 233         return false;
 234 }
 235
 236 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 237 static bool
 238 xt_tcpmss_checkentry6(const char *tablename,
 239                       const void *entry,
 240                       const struct xt_target *target,
 241                       void *targinfo,
 242                       unsigned int hook_mask)
 243 {
 244         const struct xt_tcpmss_info *info = targinfo;
 245         const struct ip6t_entry *e = entry;
 246
 247         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 248             (hook_mask & ~((1 << NF_IP6_FORWARD) |
 249                            (1 << NF_IP6_LOCAL_OUT) |
 250                            (1 << NF_IP6_POST_ROUTING))) != 0) {
 251                 printk("xt_TCPMSS: path-MTU clamping only supported in "
 252                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 253                 return false;
 254         }
 255         if (IP6T_MATCH_ITERATE(e, find_syn_match))
 256                 return true;
 257         printk("xt_TCPMSS: Only works on TCP SYN packets\n");
 258         return false;
 259 }
 260 #endif
 261
 262 static struct xt_target xt_tcpmss_reg[] __read_mostly = {
 263         {
 264                 .family         = AF_INET,
 265                 .name           = "TCPMSS",
 266                 .checkentry     = xt_tcpmss_checkentry4,
 267                 .target         = xt_tcpmss_target4,
 268                 .targetsize     = sizeof(struct xt_tcpmss_info),
 269                 .proto          = IPPROTO_TCP,
 270                 .me             = THIS_MODULE,
 271         },
 272 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 273         {
 274                 .family         = AF_INET6,
 275                 .name           = "TCPMSS",
 276                 .checkentry     = xt_tcpmss_checkentry6,
 277                 .target         = xt_tcpmss_target6,
 278                 .targetsize     = sizeof(struct xt_tcpmss_info),
 279                 .proto          = IPPROTO_TCP,
 280                 .me             = THIS_MODULE,
 281         },
 282 #endif
 283 };
 284
 285 static int __init xt_tcpmss_init(void)
 286 {
 287         return xt_register_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
 288 }
 289
 290 static void __exit xt_tcpmss_fini(void)
 291 {
 292         xt_unregister_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
 293 }
 294
 295 module_init(xt_tcpmss_init);
 296 module_exit(xt_tcpmss_fini);