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 #define HDRSIZE (sizeof(struct iphdr) + sizeof(struct tcphdr))
  31
  32 static u_int16_t
  33 cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck)
  34 {
  35         u_int32_t diffs[] = { oldvalinv, newval };
  36         return csum_fold(csum_partial((char *)diffs, sizeof(diffs),
  37                                       oldcheck^0xFFFF));
  38 }
  39
  40 static inline unsigned int
  41 optlen(const u_int8_t *opt, unsigned int offset)
  42 {
  43         /* Beware zero-length options: make finite progress */
  44         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  45                 return 1;
  46         else
  47                 return opt[offset+1];
  48 }
  49
  50 static unsigned int
  51 ipt_tcpmss_target(struct sk_buff **pskb,
  52                   const struct net_device *in,
  53                   const struct net_device *out,
  54                   unsigned int hooknum,
  55                   const struct xt_target *target,
  56                   const void *targinfo)
  57 {
  58         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
  59         struct tcphdr *tcph;
  60         struct iphdr *iph;
  61         u_int16_t tcplen, newmss;
  62         __be16 newtotlen, oldval;
  63         unsigned int i;
  64         u_int8_t *opt;
  65
  66         if (!skb_make_writable(pskb, (*pskb)->len))
  67                 return NF_DROP;
  68
  69         iph = (*pskb)->nh.iph;
  70         tcplen = (*pskb)->len - iph->ihl*4;
  71         tcph = (void *)iph + iph->ihl*4;
  72
  73         /* Since it passed flags test in tcp match, we know it is is
  74            not a fragment, and has data >= tcp header length.  SYN
  75            packets should not contain data: if they did, then we risk
  76            running over MTU, sending Frag Needed and breaking things
  77            badly. --RR */
  78         if (tcplen != tcph->doff*4) {
  79                 if (net_ratelimit())
  80                         printk(KERN_ERR
  81                                "ipt_tcpmss_target: bad length (%d bytes)\n",
  82                                (*pskb)->len);
  83                 return IPT_CONTINUE;
  84         }
  85
  86         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
  87                 if (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) +
  88                                              sizeof(struct tcphdr)) {
  89                         if (net_ratelimit())
  90                                 printk(KERN_ERR "ipt_tcpmss_target: "
  91                                        "unknown or invalid path-MTU (%d)\n",
  92                                        dst_mtu((*pskb)->dst));
  93                         return NF_DROP; /* or IPT_CONTINUE ?? */
  94                 }
  95
  96                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) -
  97                                                  sizeof(struct tcphdr);
  98         } else
  99                 newmss = tcpmssinfo->mss;
 100
 101         opt = (u_int8_t *)tcph;
 102         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
 103                 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
 104                     opt[i+1] == TCPOLEN_MSS) {
 105                         u_int16_t oldmss;
 106
 107                         oldmss = (opt[i+2] << 8) | opt[i+3];
 108
 109                         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 110                             oldmss <= newmss)
 111                                 return IPT_CONTINUE;
 112
 113                         opt[i+2] = (newmss & 0xff00) >> 8;
 114                         opt[i+3] = (newmss & 0x00ff);
 115
 116                         tcph->check = nf_proto_csum_update(*pskb,
 117                                                            htons(oldmss)^htons(0xFFFF),
 118                                                            htons(newmss),
 119                                                            tcph->check, 0);
 120                         return IPT_CONTINUE;
 121                 }
 122         }
 123
 124         /*
 125          * MSS Option not found ?! add it..
 126          */
 127         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 128                 struct sk_buff *newskb;
 129
 130                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 131                                          TCPOLEN_MSS, GFP_ATOMIC);
 132                 if (!newskb)
 133                         return NF_DROP;
 134                 kfree_skb(*pskb);
 135                 *pskb = newskb;
 136                 iph = (*pskb)->nh.iph;
 137                 tcph = (void *)iph + iph->ihl*4;
 138         }
 139
 140         skb_put((*pskb), TCPOLEN_MSS);
 141
 142         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 143         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 144
 145         tcph->check = nf_proto_csum_update(*pskb,
 146                                            htons(tcplen) ^ htons(0xFFFF),
 147                                            htons(tcplen + TCPOLEN_MSS),
 148                                            tcph->check, 1);
 149         opt[0] = TCPOPT_MSS;
 150         opt[1] = TCPOLEN_MSS;
 151         opt[2] = (newmss & 0xff00) >> 8;
 152         opt[3] = (newmss & 0x00ff);
 153
 154         tcph->check = nf_proto_csum_update(*pskb, htonl(~0), *((__be32 *)opt),
 155                                            tcph->check, 0);
 156
 157         oldval = ((__be16 *)tcph)[6];
 158         tcph->doff += TCPOLEN_MSS/4;
 159         tcph->check = nf_proto_csum_update(*pskb,
 160                                            oldval ^ htons(0xFFFF),
 161                                            ((__be16 *)tcph)[6],
 162                                            tcph->check, 0);
 163
 164         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 165         iph->check = nf_csum_update(iph->tot_len ^ htons(0xFFFF),
 166                                     newtotlen, iph->check);
 167         iph->tot_len = newtotlen;
 168         return IPT_CONTINUE;
 169 }
 170
 171 #define TH_SYN 0x02
 172
 173 static inline int find_syn_match(const struct ipt_entry_match *m)
 174 {
 175         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 176
 177         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 178             tcpinfo->flg_cmp & TH_SYN &&
 179             !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 180                 return 1;
 181
 182         return 0;
 183 }
 184
 185 /* Must specify -p tcp --syn/--tcp-flags SYN */
 186 static int
 187 ipt_tcpmss_checkentry(const char *tablename,
 188                       const void *e_void,
 189                       const struct xt_target *target,
 190                       void *targinfo,
 191                       unsigned int hook_mask)
 192 {
 193         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 194         const struct ipt_entry *e = e_void;
 195
 196         if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
 197             (hook_mask & ~((1 << NF_IP_FORWARD) |
 198                            (1 << NF_IP_LOCAL_OUT) |
 199                            (1 << NF_IP_POST_ROUTING))) != 0) {
 200                 printk("TCPMSS: path-MTU clamping only supported in "
 201                        "FORWARD, OUTPUT and POSTROUTING hooks\n");
 202                 return 0;
 203         }
 204
 205         if (IPT_MATCH_ITERATE(e, find_syn_match))
 206                 return 1;
 207         printk("TCPMSS: Only works on TCP SYN packets\n");
 208         return 0;
 209 }
 210
 211 static struct ipt_target ipt_tcpmss_reg = {
 212         .name           = "TCPMSS",
 213         .target         = ipt_tcpmss_target,
 214         .targetsize     = sizeof(struct ipt_tcpmss_info),
 215         .proto          = IPPROTO_TCP,
 216         .checkentry     = ipt_tcpmss_checkentry,
 217         .me             = THIS_MODULE,
 218 };
 219
 220 static int __init ipt_tcpmss_init(void)
 221 {
 222         return ipt_register_target(&ipt_tcpmss_reg);
 223 }
 224
 225 static void __exit ipt_tcpmss_fini(void)
 226 {
 227         ipt_unregister_target(&ipt_tcpmss_reg);
 228 }
 229
 230 module_init(ipt_tcpmss_init);
 231 module_exit(ipt_tcpmss_fini);