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) return 1;
  35         else return opt[offset+1];
  36 }
  37
  38 static unsigned int
  39 ipt_tcpmss_target(struct sk_buff **pskb,
  40                   const struct net_device *in,
  41                   const struct net_device *out,
  42                   unsigned int hooknum,
  43                   const struct xt_target *target,
  44                   const void *targinfo,
  45                   void *userinfo)
  46 {
  47         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
  48         struct tcphdr *tcph;
  49         struct iphdr *iph;
  50         u_int16_t tcplen, newtotlen, oldval, newmss;
  51         unsigned int i;
  52         u_int8_t *opt;
  53
  54         if (!skb_make_writable(pskb, (*pskb)->len))
  55                 return NF_DROP;
  56
  57         iph = (*pskb)->nh.iph;
  58         tcplen = (*pskb)->len - iph->ihl*4;
  59
  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
  79                                         "ipt_tcpmss_target: 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) + sizeof(struct tcphdr))) {
  84                         if (net_ratelimit())
  85                                 printk(KERN_ERR
  86                                         "ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", dst_mtu((*pskb)->dst));
  87                         return NF_DROP; /* or IPT_CONTINUE ?? */
  88                 }
  89
  90                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) - sizeof(struct tcphdr);
  91         } else
  92                 newmss = tcpmssinfo->mss;
  93
  94         opt = (u_int8_t *)tcph;
  95         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)){
  96                 if ((opt[i] == TCPOPT_MSS) &&
  97                     ((tcph->doff*4 - i) >= TCPOLEN_MSS) &&
  98                     (opt[i+1] == TCPOLEN_MSS)) {
  99                         u_int16_t oldmss;
 100
 101                         oldmss = (opt[i+2] << 8) | opt[i+3];
 102
 103                         if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
 104                                 (oldmss <= newmss))
 105                                         return IPT_CONTINUE;
 106
 107                         opt[i+2] = (newmss & 0xff00) >> 8;
 108                         opt[i+3] = (newmss & 0x00ff);
 109
 110                         tcph->check = nf_proto_csum_update(*pskb,
 111                                                            htons(oldmss)^0xFFFF,
 112                                                            htons(newmss),
 113                                                            tcph->check, 0);
 114
 115                         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 116                                "->%u.%u.%u.%u:%hu changed TCP MSS option"
 117                                " (from %u to %u)\n",
 118                                NIPQUAD((*pskb)->nh.iph->saddr),
 119                                ntohs(tcph->source),
 120                                NIPQUAD((*pskb)->nh.iph->daddr),
 121                                ntohs(tcph->dest),
 122                                oldmss, newmss);
 123                         goto retmodified;
 124                 }
 125         }
 126
 127         /*
 128          * MSS Option not found ?! add it..
 129          */
 130         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 131                 struct sk_buff *newskb;
 132
 133                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 134                                          TCPOLEN_MSS, GFP_ATOMIC);
 135                 if (!newskb) {
 136                         if (net_ratelimit())
 137                                 printk(KERN_ERR "ipt_tcpmss_target:"
 138                                        " unable to allocate larger skb\n");
 139                         return NF_DROP;
 140                 }
 141
 142                 kfree_skb(*pskb);
 143                 *pskb = newskb;
 144                 iph = (*pskb)->nh.iph;
 145                 tcph = (void *)iph + iph->ihl*4;
 146         }
 147
 148         skb_put((*pskb), TCPOLEN_MSS);
 149
 150         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 151         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 152
 153         tcph->check = nf_proto_csum_update(*pskb,
 154                                            htons(tcplen) ^ 0xFFFF,
 155                                            htons(tcplen + TCPOLEN_MSS),
 156                                            tcph->check, 1);
 157         tcplen += TCPOLEN_MSS;
 158
 159         opt[0] = TCPOPT_MSS;
 160         opt[1] = TCPOLEN_MSS;
 161         opt[2] = (newmss & 0xff00) >> 8;
 162         opt[3] = (newmss & 0x00ff);
 163
 164         tcph->check = nf_proto_csum_update(*pskb, ~0, *((u_int32_t *)opt),
 165                                            tcph->check, 0);
 166
 167         oldval = ((u_int16_t *)tcph)[6];
 168         tcph->doff += TCPOLEN_MSS/4;
 169         tcph->check = nf_proto_csum_update(*pskb,
 170                                            oldval ^ 0xFFFF,
 171                                            ((u_int16_t *)tcph)[6],
 172                                            tcph->check, 0);
 173
 174         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 175         iph->check = nf_csum_update(iph->tot_len ^ 0xFFFF,
 176                                     newtotlen, iph->check);
 177         iph->tot_len = newtotlen;
 178
 179         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 180                "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
 181                NIPQUAD((*pskb)->nh.iph->saddr),
 182                ntohs(tcph->source),
 183                NIPQUAD((*pskb)->nh.iph->daddr),
 184                ntohs(tcph->dest),
 185                newmss);
 186
 187  retmodified:
 188         return IPT_CONTINUE;
 189 }
 190
 191 #define TH_SYN 0x02
 192
 193 static inline int find_syn_match(const struct ipt_entry_match *m)
 194 {
 195         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 196
 197         if (strcmp(m->u.kernel.match->name, "tcp") == 0
 198             && (tcpinfo->flg_cmp & TH_SYN)
 199             && !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 200                 return 1;
 201
 202         return 0;
 203 }
 204
 205 /* Must specify -p tcp --syn/--tcp-flags SYN */
 206 static int
 207 ipt_tcpmss_checkentry(const char *tablename,
 208                       const void *e_void,
 209                       const struct xt_target *target,
 210                       void *targinfo,
 211                       unsigned int targinfosize,
 212                       unsigned int hook_mask)
 213 {
 214         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 215         const struct ipt_entry *e = e_void;
 216
 217         if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
 218                         ((hook_mask & ~((1 << NF_IP_FORWARD)
 219                                 | (1 << NF_IP_LOCAL_OUT)
 220                                 | (1 << NF_IP_POST_ROUTING))) != 0)) {
 221                 printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
 222                 return 0;
 223         }
 224
 225         if (IPT_MATCH_ITERATE(e, find_syn_match))
 226                 return 1;
 227         printk("TCPMSS: Only works on TCP SYN packets\n");
 228         return 0;
 229 }
 230
 231 static struct ipt_target ipt_tcpmss_reg = {
 232         .name           = "TCPMSS",
 233         .target         = ipt_tcpmss_target,
 234         .targetsize     = sizeof(struct ipt_tcpmss_info),
 235         .proto          = IPPROTO_TCP,
 236         .checkentry     = ipt_tcpmss_checkentry,
 237         .me             = THIS_MODULE,
 238 };
 239
 240 static int __init ipt_tcpmss_init(void)
 241 {
 242         return ipt_register_target(&ipt_tcpmss_reg);
 243 }
 244
 245 static void __exit ipt_tcpmss_fini(void)
 246 {
 247         ipt_unregister_target(&ipt_tcpmss_reg);
 248 }
 249
 250 module_init(ipt_tcpmss_init);
 251 module_exit(ipt_tcpmss_fini);