2 * This is a module which is used for setting the MSS option in TCP packets.
4 * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
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.
11 #include <linux/module.h>
12 #include <linux/skbuff.h>
14 #include <linux/ipv6.h>
15 #include <linux/tcp.h>
19 #include <linux/netfilter_ipv4/ip_tables.h>
20 #include <linux/netfilter_ipv6/ip6_tables.h>
21 #include <linux/netfilter/x_tables.h>
22 #include <linux/netfilter/xt_tcpudp.h>
23 #include <linux/netfilter/xt_TCPMSS.h>
25 MODULE_LICENSE("GPL");
26 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
27 MODULE_DESCRIPTION("x_tables TCP MSS modification module");
28 MODULE_ALIAS("ipt_TCPMSS");
29 MODULE_ALIAS("ip6t_TCPMSS");
31 static inline unsigned int
32 optlen(const u_int8_t *opt, unsigned int offset)
34 /* Beware zero-length options: make finite progress */
35 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
42 tcpmss_mangle_packet(struct sk_buff *skb,
43 const struct xt_tcpmss_info *info,
48 unsigned int tcplen, i;
53 if (!skb_make_writable(skb, skb->len))
56 tcplen = skb->len - tcphoff;
57 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
59 /* Since it passed flags test in tcp match, we know it is is
60 not a fragment, and has data >= tcp header length. SYN
61 packets should not contain data: if they did, then we risk
62 running over MTU, sending Frag Needed and breaking things
64 if (tcplen != tcph->doff*4) {
66 printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
71 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
72 if (dst_mtu(skb->dst) <= minlen) {
74 printk(KERN_ERR "xt_TCPMSS: "
75 "unknown or invalid path-MTU (%u)\n",
79 newmss = dst_mtu(skb->dst) - minlen;
83 opt = (u_int8_t *)tcph;
84 for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
85 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
86 opt[i+1] == TCPOLEN_MSS) {
89 oldmss = (opt[i+2] << 8) | opt[i+3];
91 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
95 opt[i+2] = (newmss & 0xff00) >> 8;
96 opt[i+3] = newmss & 0x00ff;
98 inet_proto_csum_replace2(&tcph->check, skb,
99 htons(oldmss), htons(newmss),
106 * MSS Option not found ?! add it..
108 if (skb_tailroom(skb) < TCPOLEN_MSS) {
109 if (pskb_expand_head(skb, 0,
110 TCPOLEN_MSS - skb_tailroom(skb),
113 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
116 skb_put(skb, TCPOLEN_MSS);
118 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
119 memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
121 inet_proto_csum_replace2(&tcph->check, skb,
122 htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
124 opt[1] = TCPOLEN_MSS;
125 opt[2] = (newmss & 0xff00) >> 8;
126 opt[3] = newmss & 0x00ff;
128 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
130 oldval = ((__be16 *)tcph)[6];
131 tcph->doff += TCPOLEN_MSS/4;
132 inet_proto_csum_replace2(&tcph->check, skb,
133 oldval, ((__be16 *)tcph)[6], 0);
138 tcpmss_tg4(struct sk_buff *skb, const struct net_device *in,
139 const struct net_device *out, unsigned int hooknum,
140 const struct xt_target *target, const void *targinfo)
142 struct iphdr *iph = ip_hdr(skb);
146 ret = tcpmss_mangle_packet(skb, targinfo, iph->ihl * 4,
147 sizeof(*iph) + sizeof(struct tcphdr));
152 newlen = htons(ntohs(iph->tot_len) + ret);
153 csum_replace2(&iph->check, iph->tot_len, newlen);
154 iph->tot_len = newlen;
159 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
161 tcpmss_tg6(struct sk_buff *skb, const struct net_device *in,
162 const struct net_device *out, unsigned int hooknum,
163 const struct xt_target *target, const void *targinfo)
165 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
170 nexthdr = ipv6h->nexthdr;
171 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
174 ret = tcpmss_mangle_packet(skb, targinfo, tcphoff,
175 sizeof(*ipv6h) + sizeof(struct tcphdr));
179 ipv6h = ipv6_hdr(skb);
180 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
188 /* Must specify -p tcp --syn */
189 static inline bool find_syn_match(const struct xt_entry_match *m)
191 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
193 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
194 tcpinfo->flg_cmp & TH_SYN &&
195 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
202 tcpmss_tg4_check(const char *tablename, const void *entry,
203 const struct xt_target *target, void *targinfo,
204 unsigned int hook_mask)
206 const struct xt_tcpmss_info *info = targinfo;
207 const struct ipt_entry *e = entry;
209 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
210 (hook_mask & ~((1 << NF_INET_FORWARD) |
211 (1 << NF_INET_LOCAL_OUT) |
212 (1 << NF_INET_POST_ROUTING))) != 0) {
213 printk("xt_TCPMSS: path-MTU clamping only supported in "
214 "FORWARD, OUTPUT and POSTROUTING hooks\n");
217 if (IPT_MATCH_ITERATE(e, find_syn_match))
219 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
223 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
225 tcpmss_tg6_check(const char *tablename, const void *entry,
226 const struct xt_target *target, void *targinfo,
227 unsigned int hook_mask)
229 const struct xt_tcpmss_info *info = targinfo;
230 const struct ip6t_entry *e = entry;
232 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
233 (hook_mask & ~((1 << NF_INET_FORWARD) |
234 (1 << NF_INET_LOCAL_OUT) |
235 (1 << NF_INET_POST_ROUTING))) != 0) {
236 printk("xt_TCPMSS: path-MTU clamping only supported in "
237 "FORWARD, OUTPUT and POSTROUTING hooks\n");
240 if (IP6T_MATCH_ITERATE(e, find_syn_match))
242 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
247 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
251 .checkentry = tcpmss_tg4_check,
252 .target = tcpmss_tg4,
253 .targetsize = sizeof(struct xt_tcpmss_info),
254 .proto = IPPROTO_TCP,
257 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
261 .checkentry = tcpmss_tg6_check,
262 .target = tcpmss_tg6,
263 .targetsize = sizeof(struct xt_tcpmss_info),
264 .proto = IPPROTO_TCP,
270 static int __init tcpmss_tg_init(void)
272 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
275 static void __exit tcpmss_tg_exit(void)
277 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
280 module_init(tcpmss_tg_init);
281 module_exit(tcpmss_tg_exit);