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 u_int16_t
  31 cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck)
  32 {
  33         u_int32_t diffs[] = { oldvalinv, newval };
  34         return csum_fold(csum_partial((char *)diffs, sizeof(diffs),
  35                                       oldcheck^0xFFFF));
  36 }
  37
  38 static inline unsigned int
  39 optlen(const u_int8_t *opt, unsigned int offset)
  40 {
  41         /* Beware zero-length options: make finite progress */
  42         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) return 1;
  43         else return opt[offset+1];
  44 }
  45
  46 static unsigned int
  47 ipt_tcpmss_target(struct sk_buff **pskb,
  48                   const struct net_device *in,
  49                   const struct net_device *out,
  50                   unsigned int hooknum,
  51                   const void *targinfo,
  52                   void *userinfo)
  53 {
  54         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
  55         struct tcphdr *tcph;
  56         struct iphdr *iph;
  57         u_int16_t tcplen, newtotlen, oldval, newmss;
  58         unsigned int i;
  59         u_int8_t *opt;
  60
  61         if (!skb_ip_make_writable(pskb, (*pskb)->len))
  62                 return NF_DROP;
  63
  64         iph = (*pskb)->nh.iph;
  65         tcplen = (*pskb)->len - iph->ihl*4;
  66
  67         tcph = (void *)iph + iph->ihl*4;
  68
  69         /* Since it passed flags test in tcp match, we know it is is
  70            not a fragment, and has data >= tcp header length.  SYN
  71            packets should not contain data: if they did, then we risk
  72            running over MTU, sending Frag Needed and breaking things
  73            badly. --RR */
  74         if (tcplen != tcph->doff*4) {
  75                 if (net_ratelimit())
  76                         printk(KERN_ERR
  77                                "ipt_tcpmss_target: bad length (%d bytes)\n",
  78                                (*pskb)->len);
  79                 return NF_DROP;
  80         }
  81
  82         if(tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
  83                 if(!(*pskb)->dst) {
  84                         if (net_ratelimit())
  85                                 printk(KERN_ERR
  86                                         "ipt_tcpmss_target: no dst?! can't determine path-MTU\n");
  87                         return NF_DROP; /* or IPT_CONTINUE ?? */
  88                 }
  89
  90                 if(dst_mtu((*pskb)->dst) <= (sizeof(struct iphdr) + sizeof(struct tcphdr))) {
  91                         if (net_ratelimit())
  92                                 printk(KERN_ERR
  93                                         "ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", dst_mtu((*pskb)->dst));
  94                         return NF_DROP; /* or IPT_CONTINUE ?? */
  95                 }
  96
  97                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) - 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) &&
 104                     ((tcph->doff*4 - i) >= TCPOLEN_MSS) &&
 105                     (opt[i+1] == TCPOLEN_MSS)) {
 106                         u_int16_t oldmss;
 107
 108                         oldmss = (opt[i+2] << 8) | opt[i+3];
 109
 110                         if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
 111                                 (oldmss <= newmss))
 112                                         return IPT_CONTINUE;
 113
 114                         opt[i+2] = (newmss & 0xff00) >> 8;
 115                         opt[i+3] = (newmss & 0x00ff);
 116
 117                         tcph->check = cheat_check(htons(oldmss)^0xFFFF,
 118                                                   htons(newmss),
 119                                                   tcph->check);
 120
 121                         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 122                                "->%u.%u.%u.%u:%hu changed TCP MSS option"
 123                                " (from %u to %u)\n",
 124                                NIPQUAD((*pskb)->nh.iph->saddr),
 125                                ntohs(tcph->source),
 126                                NIPQUAD((*pskb)->nh.iph->daddr),
 127                                ntohs(tcph->dest),
 128                                oldmss, newmss);
 129                         goto retmodified;
 130                 }
 131         }
 132
 133         /*
 134          * MSS Option not found ?! add it..
 135          */
 136         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
 137                 struct sk_buff *newskb;
 138
 139                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
 140                                          TCPOLEN_MSS, GFP_ATOMIC);
 141                 if (!newskb) {
 142                         if (net_ratelimit())
 143                                 printk(KERN_ERR "ipt_tcpmss_target:"
 144                                        " unable to allocate larger skb\n");
 145                         return NF_DROP;
 146                 }
 147
 148                 kfree_skb(*pskb);
 149                 *pskb = newskb;
 150                 iph = (*pskb)->nh.iph;
 151                 tcph = (void *)iph + iph->ihl*4;
 152         }
 153
 154         skb_put((*pskb), TCPOLEN_MSS);
 155
 156         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 157         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
 158
 159         tcph->check = cheat_check(htons(tcplen) ^ 0xFFFF,
 160                                   htons(tcplen + TCPOLEN_MSS), tcph->check);
 161         tcplen += TCPOLEN_MSS;
 162
 163         opt[0] = TCPOPT_MSS;
 164         opt[1] = TCPOLEN_MSS;
 165         opt[2] = (newmss & 0xff00) >> 8;
 166         opt[3] = (newmss & 0x00ff);
 167
 168         tcph->check = cheat_check(~0, *((u_int32_t *)opt), tcph->check);
 169
 170         oldval = ((u_int16_t *)tcph)[6];
 171         tcph->doff += TCPOLEN_MSS/4;
 172         tcph->check = cheat_check(oldval ^ 0xFFFF,
 173                                   ((u_int16_t *)tcph)[6], tcph->check);
 174
 175         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
 176         iph->check = cheat_check(iph->tot_len ^ 0xFFFF,
 177                                  newtotlen, iph->check);
 178         iph->tot_len = newtotlen;
 179
 180         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
 181                "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
 182                NIPQUAD((*pskb)->nh.iph->saddr),
 183                ntohs(tcph->source),
 184                NIPQUAD((*pskb)->nh.iph->daddr),
 185                ntohs(tcph->dest),
 186                newmss);
 187
 188  retmodified:
 189         /* We never hw checksum SYN packets.  */
 190         BUG_ON((*pskb)->ip_summed == CHECKSUM_HW);
 191
 192         (*pskb)->nfcache |= NFC_UNKNOWN | NFC_ALTERED;
 193         return IPT_CONTINUE;
 194 }
 195
 196 #define TH_SYN 0x02
 197
 198 static inline int find_syn_match(const struct ipt_entry_match *m)
 199 {
 200         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
 201
 202         if (strcmp(m->u.kernel.match->name, "tcp") == 0
 203             && (tcpinfo->flg_cmp & TH_SYN)
 204             && !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
 205                 return 1;
 206
 207         return 0;
 208 }
 209
 210 /* Must specify -p tcp --syn/--tcp-flags SYN */
 211 static int
 212 ipt_tcpmss_checkentry(const char *tablename,
 213                       const struct ipt_entry *e,
 214                       void *targinfo,
 215                       unsigned int targinfosize,
 216                       unsigned int hook_mask)
 217 {
 218         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
 219
 220         if (targinfosize != IPT_ALIGN(sizeof(struct ipt_tcpmss_info))) {
 221                 DEBUGP("ipt_tcpmss_checkentry: targinfosize %u != %u\n",
 222                        targinfosize, IPT_ALIGN(sizeof(struct ipt_tcpmss_info)));
 223                 return 0;
 224         }
 225
 226
 227         if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
 228                         ((hook_mask & ~((1 << NF_IP_FORWARD)
 229                                 | (1 << NF_IP_LOCAL_OUT)
 230                                 | (1 << NF_IP_POST_ROUTING))) != 0)) {
 231                 printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
 232                 return 0;
 233         }
 234
 235         if (e->ip.proto == IPPROTO_TCP
 236             && !(e->ip.invflags & IPT_INV_PROTO)
 237             && IPT_MATCH_ITERATE(e, find_syn_match))
 238                 return 1;
 239
 240         printk("TCPMSS: Only works on TCP SYN packets\n");
 241         return 0;
 242 }
 243
 244 static struct ipt_target ipt_tcpmss_reg = {
 245         .name           = "TCPMSS",
 246         .target         = ipt_tcpmss_target,
 247         .checkentry     = ipt_tcpmss_checkentry,
 248         .me             = THIS_MODULE,
 249 };
 250
 251 static int __init init(void)
 252 {
 253         return ipt_register_target(&ipt_tcpmss_reg);
 254 }
 255
 256 static void __exit fini(void)
 257 {
 258         ipt_unregister_target(&ipt_tcpmss_reg);
 259 }
 260
 261 module_init(init);
 262 module_exit(fini);