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, newtotlen, oldval, newmss;
  46         unsigned int i;
  47         u_int8_t *opt;
  48
  49         if (!skb_make_writable(pskb, (*pskb)->len))
  50                 return NF_DROP;
  51
  52         iph = (*pskb)->nh.iph;
  53         tcplen = (*pskb)->len - iph->ihl*4;
  54         tcph = (void *)iph + iph->ihl*4;
  55
  56         /* Since it passed flags test in tcp match, we know it is is
  57            not a fragment, and has data >= tcp header length.  SYN
  58            packets should not contain data: if they did, then we risk
  59            running over MTU, sending Frag Needed and breaking things
  60            badly. --RR */
  61         if (tcplen != tcph->doff*4) {
  62                 if (net_ratelimit())
  63                         printk(KERN_ERR
  64                                "ipt_tcpmss_target: bad length (%d bytes)\n",
  65                                (*pskb)->len);
  66                 return NF_DROP;
  67         }
  68
  69         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
  70                 if (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) +
  71                                              sizeof(struct tcphdr)) {
  72                         if (net_ratelimit())
  73                                 printk(KERN_ERR "ipt_tcpmss_target: "
  74                                        "unknown or invalid path-MTU (%d)\n",
  75                                        dst_mtu((*pskb)->dst));
  76                         return NF_DROP; /* or IPT_CONTINUE ?? */
  77                 }
  78
  79                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) -
  80                                                  sizeof(struct tcphdr);
  81         } else
  82                 newmss = tcpmssinfo->mss;
  83
  84         opt = (u_int8_t *)tcph;
  85         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
  86                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
  87                     opt[i+1] == TCPOLEN_MSS) {
  88                         u_int16_t oldmss;
  89
  90                         oldmss = (opt[i+2] << 8) | opt[i+3];
  91
  92                         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
  93                             oldmss <= newmss)
  94                                 return IPT_CONTINUE;
  95
  96                         opt[i+2] = (newmss & 0xff00) >> 8;
  97                         opt[i+3] = (newmss & 0x00ff);
  98
  99                         tcph->check = nf_proto_csum_update(*pskb,
 100                                                            htons(oldmss)^0xFFFF,
 101                                                            htons(newmss),
 102                                                            tcph->check, 0);
 103                         return IPT_CONTINUE;
 104                 }
 105         }
 106
 107         /*
 108          * MSS Option not found ?! add it..
 109          */
 110         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 111                 struct sk_buff *newskb;
 112
 113                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 114                                          TCPOLEN_MSS, GFP_ATOMIC);
 115                 if (!newskb)
 116                         return NF_DROP;
 117                 kfree_skb(*pskb);
 118                 *pskb = newskb;
 119                 iph = (*pskb)->nh.iph;
 120                 tcph = (void *)iph + iph->ihl*4;
 121         }
 122
 123         skb_put((*pskb), TCPOLEN_MSS);
 124
 125         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 126         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 127
 128         tcph->check = nf_proto_csum_update(*pskb,
 129                                            htons(tcplen) ^ 0xFFFF,
 130                                            htons(tcplen + TCPOLEN_MSS),
 131                                            tcph->check, 1);
 132         opt[0] = TCPOPT_MSS;
 133         opt[1] = TCPOLEN_MSS;
 134         opt[2] = (newmss & 0xff00) >> 8;
 135         opt[3] = (newmss & 0x00ff);
 136
 137         tcph->check = nf_proto_csum_update(*pskb, ~0, *((u_int32_t *)opt),
 138                                            tcph->check, 0);
 139
 140         oldval = ((u_int16_t *)tcph)[6];
 141         tcph->doff += TCPOLEN_MSS/4;
 142         tcph->check = nf_proto_csum_update(*pskb,
 143                                            oldval ^ 0xFFFF,
 144                                            ((u_int16_t *)tcph)[6],
 145                                            tcph->check, 0);
 146
 147         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 148         iph->check = nf_csum_update(iph->tot_len ^ 0xFFFF,
 149                                     newtotlen, iph->check);
 150         iph->tot_len = newtotlen;
 151         return IPT_CONTINUE;
 152 }
 153
 154 #define TH_SYN 0x02
 155
 156 static inline int find_syn_match(const struct ipt_entry_match *m)
 157 {
 158         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 159
 160         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 161             tcpinfo->flg_cmp & TH_SYN &&
 162             !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 163                 return 1;
 164
 165         return 0;
 166 }
 167
 168 /* Must specify -p tcp --syn/--tcp-flags SYN */
 169 static int
 170 ipt_tcpmss_checkentry(const char *tablename,
 171                       const void *e_void,
 172                       const struct xt_target *target,
 173                       void *targinfo,
 174                       unsigned int hook_mask)
 175 {
 176         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 177         const struct ipt_entry *e = e_void;
 178
 179         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 180             (hook_mask & ~((1 << NF_IP_FORWARD) |
 181                            (1 << NF_IP_LOCAL_OUT) |
 182                            (1 << NF_IP_POST_ROUTING))) != 0) {
 183                 printk("TCPMSS: path-MTU clamping only supported in "
 184                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 185                 return 0;
 186         }
 187
 188         if (IPT_MATCH_ITERATE(e, find_syn_match))
 189                 return 1;
 190         printk("TCPMSS: Only works on TCP SYN packets\n");
 191         return 0;
 192 }
 193
 194 static struct ipt_target ipt_tcpmss_reg = {
 195         .name           = "TCPMSS",
 196         .target         = ipt_tcpmss_target,
 197         .targetsize     = sizeof(struct ipt_tcpmss_info),
 198         .proto          = IPPROTO_TCP,
 199         .checkentry     = ipt_tcpmss_checkentry,
 200         .me             = THIS_MODULE,
 201 };
 202
 203 static int __init ipt_tcpmss_init(void)
 204 {
 205         return ipt_register_target(&ipt_tcpmss_reg);
 206 }
 207
 208 static void __exit ipt_tcpmss_fini(void)
 209 {
 210         ipt_unregister_target(&ipt_tcpmss_reg);
 211 }
 212
 213 module_init(ipt_tcpmss_init);
 214 module_exit(ipt_tcpmss_fini);