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 (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) +
  77                                              sizeof(struct tcphdr)) {
  78                         if (net_ratelimit())
  79                                 printk(KERN_ERR "ipt_tcpmss_target: "
  80                                        "unknown or invalid path-MTU (%d)\n",
  81                                        dst_mtu((*pskb)->dst));
  82                         return NF_DROP; /* or IPT_CONTINUE ?? */
  83                 }
  84
  85                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) -
  86                                                  sizeof(struct tcphdr);
  87         } else
  88                 newmss = tcpmssinfo->mss;
  89
  90         opt = (u_int8_t *)tcph;
  91         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
  92                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
  93                     opt[i+1] == TCPOLEN_MSS) {
  94                         u_int16_t oldmss;
  95
  96                         oldmss = (opt[i+2] << 8) | opt[i+3];
  97
  98                         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
  99                             oldmss <= newmss)
 100                                 return IPT_CONTINUE;
 101
 102                         opt[i+2] = (newmss & 0xff00) >> 8;
 103                         opt[i+3] = (newmss & 0x00ff);
 104
 105                         tcph->check = nf_proto_csum_update(*pskb,
 106                                                            htons(oldmss)^0xFFFF,
 107                                                            htons(newmss),
 108                                                            tcph->check, 0);
 109
 110                         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 111                                "->%u.%u.%u.%u:%hu changed TCP MSS option"
 112                                " (from %u to %u)\n",
 113                                NIPQUAD((*pskb)->nh.iph->saddr),
 114                                ntohs(tcph->source),
 115                                NIPQUAD((*pskb)->nh.iph->daddr),
 116                                ntohs(tcph->dest),
 117                                oldmss, newmss);
 118                         goto retmodified;
 119                 }
 120         }
 121
 122         /*
 123          * MSS Option not found ?! add it..
 124          */
 125         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 126                 struct sk_buff *newskb;
 127
 128                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 129                                          TCPOLEN_MSS, GFP_ATOMIC);
 130                 if (!newskb) {
 131                         if (net_ratelimit())
 132                                 printk(KERN_ERR "ipt_tcpmss_target:"
 133                                        " unable to allocate larger skb\n");
 134                         return NF_DROP;
 135                 }
 136
 137                 kfree_skb(*pskb);
 138                 *pskb = newskb;
 139                 iph = (*pskb)->nh.iph;
 140                 tcph = (void *)iph + iph->ihl*4;
 141         }
 142
 143         skb_put((*pskb), TCPOLEN_MSS);
 144
 145         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 146         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 147
 148         tcph->check = nf_proto_csum_update(*pskb,
 149                                            htons(tcplen) ^ 0xFFFF,
 150                                            htons(tcplen + TCPOLEN_MSS),
 151                                            tcph->check, 1);
 152         tcplen += TCPOLEN_MSS;
 153
 154         opt[0] = TCPOPT_MSS;
 155         opt[1] = TCPOLEN_MSS;
 156         opt[2] = (newmss & 0xff00) >> 8;
 157         opt[3] = (newmss & 0x00ff);
 158
 159         tcph->check = nf_proto_csum_update(*pskb, ~0, *((u_int32_t *)opt),
 160                                            tcph->check, 0);
 161
 162         oldval = ((u_int16_t *)tcph)[6];
 163         tcph->doff += TCPOLEN_MSS/4;
 164         tcph->check = nf_proto_csum_update(*pskb,
 165                                            oldval ^ 0xFFFF,
 166                                            ((u_int16_t *)tcph)[6],
 167                                            tcph->check, 0);
 168
 169         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 170         iph->check = nf_csum_update(iph->tot_len ^ 0xFFFF,
 171                                     newtotlen, iph->check);
 172         iph->tot_len = newtotlen;
 173
 174         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 175                "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
 176                NIPQUAD((*pskb)->nh.iph->saddr),
 177                ntohs(tcph->source),
 178                NIPQUAD((*pskb)->nh.iph->daddr),
 179                ntohs(tcph->dest),
 180                newmss);
 181
 182  retmodified:
 183         return IPT_CONTINUE;
 184 }
 185
 186 #define TH_SYN 0x02
 187
 188 static inline int find_syn_match(const struct ipt_entry_match *m)
 189 {
 190         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 191
 192         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 193             tcpinfo->flg_cmp & TH_SYN &&
 194             !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 195                 return 1;
 196
 197         return 0;
 198 }
 199
 200 /* Must specify -p tcp --syn/--tcp-flags SYN */
 201 static int
 202 ipt_tcpmss_checkentry(const char *tablename,
 203                       const void *e_void,
 204                       const struct xt_target *target,
 205                       void *targinfo,
 206                       unsigned int hook_mask)
 207 {
 208         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 209         const struct ipt_entry *e = e_void;
 210
 211         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 212             (hook_mask & ~((1 << NF_IP_FORWARD) |
 213                            (1 << NF_IP_LOCAL_OUT) |
 214                            (1 << NF_IP_POST_ROUTING))) != 0) {
 215                 printk("TCPMSS: path-MTU clamping only supported in "
 216                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 217                 return 0;
 218         }
 219
 220         if (IPT_MATCH_ITERATE(e, find_syn_match))
 221                 return 1;
 222         printk("TCPMSS: Only works on TCP SYN packets\n");
 223         return 0;
 224 }
 225
 226 static struct ipt_target ipt_tcpmss_reg = {
 227         .name           = "TCPMSS",
 228         .target         = ipt_tcpmss_target,
 229         .targetsize     = sizeof(struct ipt_tcpmss_info),
 230         .proto          = IPPROTO_TCP,
 231         .checkentry     = ipt_tcpmss_checkentry,
 232         .me             = THIS_MODULE,
 233 };
 234
 235 static int __init ipt_tcpmss_init(void)
 236 {
 237         return ipt_register_target(&ipt_tcpmss_reg);
 238 }
 239
 240 static void __exit ipt_tcpmss_fini(void)
 241 {
 242         ipt_unregister_target(&ipt_tcpmss_reg);
 243 }
 244
 245 module_init(ipt_tcpmss_init);
 246 module_exit(ipt_tcpmss_fini);