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