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 static inline unsigned int
  25 optlen(const u_int8_t *opt, unsigned int offset)
  26 {
  27         /* Beware zero-length options: make finite progress */
  28         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  29                 return 1;
  30         else
  31                 return opt[offset+1];
  32 }
  33
  34 static unsigned int
  35 ipt_tcpmss_target(struct sk_buff **pskb,
  36                   const struct net_device *in,
  37                   const struct net_device *out,
  38                   unsigned int hooknum,
  39                   const struct xt_target *target,
  40                   const void *targinfo)
  41 {
  42         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
  43         struct tcphdr *tcph;
  44         struct iphdr *iph;
  45         u_int16_t tcplen, newmss;
  46         __be16 newtotlen, oldval;
  47         unsigned int i;
  48         u_int8_t *opt;
  49
  50         if (!skb_make_writable(pskb, (*pskb)->len))
  51                 return NF_DROP;
  52
  53         iph = (*pskb)->nh.iph;
  54         tcplen = (*pskb)->len - iph->ihl*4;
  55         tcph = (void *)iph + iph->ihl*4;
  56
  57         /* Since it passed flags test in tcp match, we know it is is
  58            not a fragment, and has data >= tcp header length.  SYN
  59            packets should not contain data: if they did, then we risk
  60            running over MTU, sending Frag Needed and breaking things
  61            badly. --RR */
  62         if (tcplen != tcph->doff*4) {
  63                 if (net_ratelimit())
  64                         printk(KERN_ERR
  65                                "ipt_tcpmss_target: bad length (%d bytes)\n",
  66                                (*pskb)->len);
  67                 return NF_DROP;
  68         }
  69
  70         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
  71                 if (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) +
  72                                              sizeof(struct tcphdr)) {
  73                         if (net_ratelimit())
  74                                 printk(KERN_ERR "ipt_tcpmss_target: "
  75                                        "unknown or invalid path-MTU (%d)\n",
  76                                        dst_mtu((*pskb)->dst));
  77                         return NF_DROP; /* or IPT_CONTINUE ?? */
  78                 }
  79
  80                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) -
  81                                                  sizeof(struct tcphdr);
  82         } else
  83                 newmss = tcpmssinfo->mss;
  84
  85         opt = (u_int8_t *)tcph;
  86         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
  87                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
  88                     opt[i+1] == TCPOLEN_MSS) {
  89                         u_int16_t oldmss;
  90
  91                         oldmss = (opt[i+2] << 8) | opt[i+3];
  92
  93                         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
  94                             oldmss <= newmss)
  95                                 return IPT_CONTINUE;
  96
  97                         opt[i+2] = (newmss & 0xff00) >> 8;
  98                         opt[i+3] = (newmss & 0x00ff);
  99
 100                         tcph->check = nf_proto_csum_update(*pskb,
 101                                                            htons(oldmss)^htons(0xFFFF),
 102                                                            htons(newmss),
 103                                                            tcph->check, 0);
 104                         return IPT_CONTINUE;
 105                 }
 106         }
 107
 108         /*
 109          * MSS Option not found ?! add it..
 110          */
 111         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 112                 struct sk_buff *newskb;
 113
 114                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 115                                          TCPOLEN_MSS, GFP_ATOMIC);
 116                 if (!newskb)
 117                         return NF_DROP;
 118                 kfree_skb(*pskb);
 119                 *pskb = newskb;
 120                 iph = (*pskb)->nh.iph;
 121                 tcph = (void *)iph + iph->ihl*4;
 122         }
 123
 124         skb_put((*pskb), TCPOLEN_MSS);
 125
 126         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 127         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 128
 129         tcph->check = nf_proto_csum_update(*pskb,
 130                                            htons(tcplen) ^ htons(0xFFFF),
 131                                            htons(tcplen + TCPOLEN_MSS),
 132                                            tcph->check, 1);
 133         opt[0] = TCPOPT_MSS;
 134         opt[1] = TCPOLEN_MSS;
 135         opt[2] = (newmss & 0xff00) >> 8;
 136         opt[3] = (newmss & 0x00ff);
 137
 138         tcph->check = nf_proto_csum_update(*pskb, htonl(~0), *((__be32 *)opt),
 139                                            tcph->check, 0);
 140
 141         oldval = ((__be16 *)tcph)[6];
 142         tcph->doff += TCPOLEN_MSS/4;
 143         tcph->check = nf_proto_csum_update(*pskb,
 144                                            oldval ^ htons(0xFFFF),
 145                                            ((__be16 *)tcph)[6],
 146                                            tcph->check, 0);
 147
 148         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 149         iph->check = nf_csum_update(iph->tot_len ^ htons(0xFFFF),
 150                                     newtotlen, iph->check);
 151         iph->tot_len = newtotlen;
 152         return IPT_CONTINUE;
 153 }
 154
 155 #define TH_SYN 0x02
 156
 157 static inline int find_syn_match(const struct ipt_entry_match *m)
 158 {
 159         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 160
 161         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 162             tcpinfo->flg_cmp & TH_SYN &&
 163             !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 164                 return 1;
 165
 166         return 0;
 167 }
 168
 169 /* Must specify -p tcp --syn/--tcp-flags SYN */
 170 static int
 171 ipt_tcpmss_checkentry(const char *tablename,
 172                       const void *e_void,
 173                       const struct xt_target *target,
 174                       void *targinfo,
 175                       unsigned int hook_mask)
 176 {
 177         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 178         const struct ipt_entry *e = e_void;
 179
 180         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 181             (hook_mask & ~((1 << NF_IP_FORWARD) |
 182                            (1 << NF_IP_LOCAL_OUT) |
 183                            (1 << NF_IP_POST_ROUTING))) != 0) {
 184                 printk("TCPMSS: path-MTU clamping only supported in "
 185                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 186                 return 0;
 187         }
 188
 189         if (IPT_MATCH_ITERATE(e, find_syn_match))
 190                 return 1;
 191         printk("TCPMSS: Only works on TCP SYN packets\n");
 192         return 0;
 193 }
 194
 195 static struct ipt_target ipt_tcpmss_reg = {
 196         .name           = "TCPMSS",
 197         .target         = ipt_tcpmss_target,
 198         .targetsize     = sizeof(struct ipt_tcpmss_info),
 199         .proto          = IPPROTO_TCP,
 200         .checkentry     = ipt_tcpmss_checkentry,
 201         .me             = THIS_MODULE,
 202 };
 203
 204 static int __init ipt_tcpmss_init(void)
 205 {
 206         return ipt_register_target(&ipt_tcpmss_reg);
 207 }
 208
 209 static void __exit ipt_tcpmss_fini(void)
 210 {
 211         ipt_unregister_target(&ipt_tcpmss_reg);
 212 }
 213
 214 module_init(ipt_tcpmss_init);
 215 module_exit(ipt_tcpmss_fini);