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 <net/route.h>
22 #include <linux/netfilter_ipv4/ip_tables.h>
23 #include <linux/netfilter_ipv6/ip6_tables.h>
24 #include <linux/netfilter/x_tables.h>
25 #include <linux/netfilter/xt_tcpudp.h>
26 #include <linux/netfilter/xt_TCPMSS.h>
28 MODULE_LICENSE("GPL");
29 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
30 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
31 MODULE_ALIAS("ipt_TCPMSS");
32 MODULE_ALIAS("ip6t_TCPMSS");
34 static inline unsigned int
35 optlen(const u_int8_t *opt, unsigned int offset)
37 /* Beware zero-length options: make finite progress */
38 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
45 tcpmss_mangle_packet(struct sk_buff *skb,
46 const struct xt_tcpmss_info *info,
52 unsigned int tcplen, i;
57 if (!skb_make_writable(skb, skb->len))
60 tcplen = skb->len - tcphoff;
61 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
63 /* Since it passed flags test in tcp match, we know it is is
64 not a fragment, and has data >= tcp header length. SYN
65 packets should not contain data: if they did, then we risk
66 running over MTU, sending Frag Needed and breaking things
68 if (tcplen != tcph->doff*4) {
70 printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
75 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
76 if (dst_mtu(skb_dst(skb)) <= minlen) {
78 printk(KERN_ERR "xt_TCPMSS: "
79 "unknown or invalid path-MTU (%u)\n",
80 dst_mtu(skb_dst(skb)));
83 if (in_mtu <= minlen) {
85 printk(KERN_ERR "xt_TCPMSS: unknown or "
86 "invalid path-MTU (%u)\n", in_mtu);
89 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
93 opt = (u_int8_t *)tcph;
94 for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
95 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
96 opt[i+1] == TCPOLEN_MSS) {
99 oldmss = (opt[i+2] << 8) | opt[i+3];
101 /* Never increase MSS, even when setting it, as
102 * doing so results in problems for hosts that rely
103 * on MSS being set correctly.
105 if (oldmss <= newmss)
108 opt[i+2] = (newmss & 0xff00) >> 8;
109 opt[i+3] = newmss & 0x00ff;
111 inet_proto_csum_replace2(&tcph->check, skb,
112 htons(oldmss), htons(newmss),
119 * MSS Option not found ?! add it..
121 if (skb_tailroom(skb) < TCPOLEN_MSS) {
122 if (pskb_expand_head(skb, 0,
123 TCPOLEN_MSS - skb_tailroom(skb),
126 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
129 skb_put(skb, TCPOLEN_MSS);
131 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
132 memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
134 inet_proto_csum_replace2(&tcph->check, skb,
135 htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
137 opt[1] = TCPOLEN_MSS;
138 opt[2] = (newmss & 0xff00) >> 8;
139 opt[3] = newmss & 0x00ff;
141 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
143 oldval = ((__be16 *)tcph)[6];
144 tcph->doff += TCPOLEN_MSS/4;
145 inet_proto_csum_replace2(&tcph->check, skb,
146 oldval, ((__be16 *)tcph)[6], 0);
150 static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
153 struct flowi fl = {};
154 const struct nf_afinfo *ai;
155 struct rtable *rt = NULL;
158 if (family == PF_INET)
159 fl.fl4_dst = ip_hdr(skb)->saddr;
161 fl.fl6_dst = ipv6_hdr(skb)->saddr;
164 ai = nf_get_afinfo(family);
166 ai->route((struct dst_entry **)&rt, &fl);
170 mtu = dst_mtu(&rt->u.dst);
171 dst_release(&rt->u.dst);
177 tcpmss_tg4(struct sk_buff *skb, const struct xt_target_param *par)
179 struct iphdr *iph = ip_hdr(skb);
183 ret = tcpmss_mangle_packet(skb, par->targinfo,
184 tcpmss_reverse_mtu(skb, PF_INET),
186 sizeof(*iph) + sizeof(struct tcphdr));
191 newlen = htons(ntohs(iph->tot_len) + ret);
192 csum_replace2(&iph->check, iph->tot_len, newlen);
193 iph->tot_len = newlen;
198 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
200 tcpmss_tg6(struct sk_buff *skb, const struct xt_target_param *par)
202 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
207 nexthdr = ipv6h->nexthdr;
208 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
211 ret = tcpmss_mangle_packet(skb, par->targinfo,
212 tcpmss_reverse_mtu(skb, PF_INET6),
214 sizeof(*ipv6h) + sizeof(struct tcphdr));
218 ipv6h = ipv6_hdr(skb);
219 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
227 /* Must specify -p tcp --syn */
228 static inline bool find_syn_match(const struct xt_entry_match *m)
230 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
232 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
233 tcpinfo->flg_cmp & TH_SYN &&
234 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
240 static bool tcpmss_tg4_check(const struct xt_tgchk_param *par)
242 const struct xt_tcpmss_info *info = par->targinfo;
243 const struct ipt_entry *e = par->entryinfo;
245 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
246 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
247 (1 << NF_INET_LOCAL_OUT) |
248 (1 << NF_INET_POST_ROUTING))) != 0) {
249 printk("xt_TCPMSS: path-MTU clamping only supported in "
250 "FORWARD, OUTPUT and POSTROUTING hooks\n");
253 if (IPT_MATCH_ITERATE(e, find_syn_match))
255 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
259 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
260 static bool tcpmss_tg6_check(const struct xt_tgchk_param *par)
262 const struct xt_tcpmss_info *info = par->targinfo;
263 const struct ip6t_entry *e = par->entryinfo;
265 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
266 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
267 (1 << NF_INET_LOCAL_OUT) |
268 (1 << NF_INET_POST_ROUTING))) != 0) {
269 printk("xt_TCPMSS: path-MTU clamping only supported in "
270 "FORWARD, OUTPUT and POSTROUTING hooks\n");
273 if (IP6T_MATCH_ITERATE(e, find_syn_match))
275 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
280 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
282 .family = NFPROTO_IPV4,
284 .checkentry = tcpmss_tg4_check,
285 .target = tcpmss_tg4,
286 .targetsize = sizeof(struct xt_tcpmss_info),
287 .proto = IPPROTO_TCP,
290 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
292 .family = NFPROTO_IPV6,
294 .checkentry = tcpmss_tg6_check,
295 .target = tcpmss_tg6,
296 .targetsize = sizeof(struct xt_tcpmss_info),
297 .proto = IPPROTO_TCP,
303 static int __init tcpmss_tg_init(void)
305 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
308 static void __exit tcpmss_tg_exit(void)
310 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
313 module_init(tcpmss_tg_init);
314 module_exit(tcpmss_tg_exit);