err.no Git - linux-2.6/blob - net/ipv4/netfilter/ipt_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
  14 #include <linux/ip.h>
  15 #include <net/tcp.h>
  16
  17 #include <linux/netfilter_ipv4/ip_tables.h>
  18 #include <linux/netfilter_ipv4/ipt_TCPMSS.h>
  19
  20 MODULE_LICENSE("GPL");
  21 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  22 MODULE_DESCRIPTION("iptables TCP MSS modification module");
  23
  24 #if 0
  25 #define DEBUGP printk
  26 #else
  27 #define DEBUGP(format, args...)
  28 #endif
  29
  30 static inline unsigned int
  31 optlen(const u_int8_t *opt, unsigned int offset)
  32 {
  33         /* Beware zero-length options: make finite progress */
  34         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  35                 return 1;
  36         else
  37                 return opt[offset+1];
  38 }
  39
  40 static unsigned int
  41 ipt_tcpmss_target(struct sk_buff **pskb,
  42                   const struct net_device *in,
  43                   const struct net_device *out,
  44                   unsigned int hooknum,
  45                   const struct xt_target *target,
  46                   const void *targinfo)
  47 {
  48         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
  49         struct tcphdr *tcph;
  50         struct iphdr *iph;
  51         u_int16_t tcplen, newtotlen, oldval, newmss;
  52         unsigned int i;
  53         u_int8_t *opt;
  54
  55         if (!skb_make_writable(pskb, (*pskb)->len))
  56                 return NF_DROP;
  57
  58         iph = (*pskb)->nh.iph;
  59         tcplen = (*pskb)->len - iph->ihl*4;
  60         tcph = (void *)iph + iph->ihl*4;
  61
  62         /* Since it passed flags test in tcp match, we know it is is
  63            not a fragment, and has data >= tcp header length.  SYN
  64            packets should not contain data: if they did, then we risk
  65            running over MTU, sending Frag Needed and breaking things
  66            badly. --RR */
  67         if (tcplen != tcph->doff*4) {
  68                 if (net_ratelimit())
  69                         printk(KERN_ERR
  70                                "ipt_tcpmss_target: bad length (%d bytes)\n",
  71                                (*pskb)->len);
  72                 return NF_DROP;
  73         }
  74
  75         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
  76                 if (!(*pskb)->dst) {
  77                         if (net_ratelimit())
  78                                 printk(KERN_ERR "ipt_tcpmss_target: "
  79                                        "no dst?! can't determine path-MTU\n");
  80                         return NF_DROP; /* or IPT_CONTINUE ?? */
  81                 }
  82
  83                 if (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) +
  84                                              sizeof(struct tcphdr)) {
  85                         if (net_ratelimit())
  86                                 printk(KERN_ERR "ipt_tcpmss_target: "
  87                                        "unknown or invalid path-MTU (%d)\n",
  88                                        dst_mtu((*pskb)->dst));
  89                         return NF_DROP; /* or IPT_CONTINUE ?? */
  90                 }
  91
  92                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) -
  93                                                  sizeof(struct tcphdr);
  94         } else
  95                 newmss = tcpmssinfo->mss;
  96
  97         opt = (u_int8_t *)tcph;
  98         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
  99                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
 100                     opt[i+1] == TCPOLEN_MSS) {
 101                         u_int16_t oldmss;
 102
 103                         oldmss = (opt[i+2] << 8) | opt[i+3];
 104
 105                         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 106                             oldmss <= newmss)
 107                                 return IPT_CONTINUE;
 108
 109                         opt[i+2] = (newmss & 0xff00) >> 8;
 110                         opt[i+3] = (newmss & 0x00ff);
 111
 112                         tcph->check = nf_proto_csum_update(*pskb,
 113                                                            htons(oldmss)^0xFFFF,
 114                                                            htons(newmss),
 115                                                            tcph->check, 0);
 116
 117                         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 118                                "->%u.%u.%u.%u:%hu changed TCP MSS option"
 119                                " (from %u to %u)\n",
 120                                NIPQUAD((*pskb)->nh.iph->saddr),
 121                                ntohs(tcph->source),
 122                                NIPQUAD((*pskb)->nh.iph->daddr),
 123                                ntohs(tcph->dest),
 124                                oldmss, newmss);
 125                         goto retmodified;
 126                 }
 127         }
 128
 129         /*
 130          * MSS Option not found ?! add it..
 131          */
 132         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 133                 struct sk_buff *newskb;
 134
 135                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 136                                          TCPOLEN_MSS, GFP_ATOMIC);
 137                 if (!newskb) {
 138                         if (net_ratelimit())
 139                                 printk(KERN_ERR "ipt_tcpmss_target:"
 140                                        " unable to allocate larger skb\n");
 141                         return NF_DROP;
 142                 }
 143
 144                 kfree_skb(*pskb);
 145                 *pskb = newskb;
 146                 iph = (*pskb)->nh.iph;
 147                 tcph = (void *)iph + iph->ihl*4;
 148         }
 149
 150         skb_put((*pskb), TCPOLEN_MSS);
 151
 152         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 153         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 154
 155         tcph->check = nf_proto_csum_update(*pskb,
 156                                            htons(tcplen) ^ 0xFFFF,
 157                                            htons(tcplen + TCPOLEN_MSS),
 158                                            tcph->check, 1);
 159         tcplen += TCPOLEN_MSS;
 160
 161         opt[0] = TCPOPT_MSS;
 162         opt[1] = TCPOLEN_MSS;
 163         opt[2] = (newmss & 0xff00) >> 8;
 164         opt[3] = (newmss & 0x00ff);
 165
 166         tcph->check = nf_proto_csum_update(*pskb, ~0, *((u_int32_t *)opt),
 167                                            tcph->check, 0);
 168
 169         oldval = ((u_int16_t *)tcph)[6];
 170         tcph->doff += TCPOLEN_MSS/4;
 171         tcph->check = nf_proto_csum_update(*pskb,
 172                                            oldval ^ 0xFFFF,
 173                                            ((u_int16_t *)tcph)[6],
 174                                            tcph->check, 0);
 175
 176         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 177         iph->check = nf_csum_update(iph->tot_len ^ 0xFFFF,
 178                                     newtotlen, iph->check);
 179         iph->tot_len = newtotlen;
 180
 181         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 182                "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
 183                NIPQUAD((*pskb)->nh.iph->saddr),
 184                ntohs(tcph->source),
 185                NIPQUAD((*pskb)->nh.iph->daddr),
 186                ntohs(tcph->dest),
 187                newmss);
 188
 189  retmodified:
 190         return IPT_CONTINUE;
 191 }
 192
 193 #define TH_SYN 0x02
 194
 195 static inline int find_syn_match(const struct ipt_entry_match *m)
 196 {
 197         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 198
 199         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 200             tcpinfo->flg_cmp & TH_SYN &&
 201             !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 202                 return 1;
 203
 204         return 0;
 205 }
 206
 207 /* Must specify -p tcp --syn/--tcp-flags SYN */
 208 static int
 209 ipt_tcpmss_checkentry(const char *tablename,
 210                       const void *e_void,
 211                       const struct xt_target *target,
 212                       void *targinfo,
 213                       unsigned int hook_mask)
 214 {
 215         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 216         const struct ipt_entry *e = e_void;
 217
 218         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 219             (hook_mask & ~((1 << NF_IP_FORWARD) |
 220                            (1 << NF_IP_LOCAL_OUT) |
 221                            (1 << NF_IP_POST_ROUTING))) != 0) {
 222                 printk("TCPMSS: path-MTU clamping only supported in "
 223                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 224                 return 0;
 225         }
 226
 227         if (IPT_MATCH_ITERATE(e, find_syn_match))
 228                 return 1;
 229         printk("TCPMSS: Only works on TCP SYN packets\n");
 230         return 0;
 231 }
 232
 233 static struct ipt_target ipt_tcpmss_reg = {
 234         .name           = "TCPMSS",
 235         .target         = ipt_tcpmss_target,
 236         .targetsize     = sizeof(struct ipt_tcpmss_info),
 237         .proto          = IPPROTO_TCP,
 238         .checkentry     = ipt_tcpmss_checkentry,
 239         .me             = THIS_MODULE,
 240 };
 241
 242 static int __init ipt_tcpmss_init(void)
 243 {
 244         return ipt_register_target(&ipt_tcpmss_reg);
 245 }
 246
 247 static void __exit ipt_tcpmss_fini(void)
 248 {
 249         ipt_unregister_target(&ipt_tcpmss_reg);
 250 }
 251
 252 module_init(ipt_tcpmss_init);
 253 module_exit(ipt_tcpmss_fini);