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.
10 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
11 #include <linux/module.h>
12 #include <linux/skbuff.h>
14 #include <linux/gfp.h>
15 #include <linux/ipv6.h>
16 #include <linux/tcp.h>
20 #include <net/route.h>
23 #include <linux/netfilter_ipv4/ip_tables.h>
24 #include <linux/netfilter_ipv6/ip6_tables.h>
25 #include <linux/netfilter/x_tables.h>
26 #include <linux/netfilter/xt_tcpudp.h>
27 #include <linux/netfilter/xt_TCPMSS.h>
29 MODULE_LICENSE("GPL");
30 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
31 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
32 MODULE_ALIAS("ipt_TCPMSS");
33 MODULE_ALIAS("ip6t_TCPMSS");
35 static inline unsigned int
36 optlen(const u_int8_t *opt, unsigned int offset)
38 /* Beware zero-length options: make finite progress */
39 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
46 tcpmss_mangle_packet(struct sk_buff *skb,
47 const struct xt_action_param *par,
52 const struct xt_tcpmss_info *info = par->targinfo;
60 /* This is a fragment, no TCP header is available */
61 if (par->fragoff != 0)
64 if (!skb_make_writable(skb, skb->len))
67 len = skb->len - tcphoff;
68 if (len < (int)sizeof(struct tcphdr))
71 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
72 tcp_hdrlen = tcph->doff * 4;
77 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
78 if (dst_mtu(skb_dst(skb)) <= minlen) {
80 pr_err("unknown or invalid path-MTU (%u)\n",
81 dst_mtu(skb_dst(skb)));
84 if (in_mtu <= minlen) {
86 pr_err("unknown or invalid path-MTU (%u)\n",
90 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
94 opt = (u_int8_t *)tcph;
95 for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
96 if (opt[i] == TCPOPT_MSS && 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),
118 /* There is data after the header so the option can't be added
119 * without moving it, and doing so may make the SYN packet
120 * itself too large. Accept the packet unmodified instead.
122 if (len > tcp_hdrlen)
126 * MSS Option not found ?! add it..
128 if (skb_tailroom(skb) < TCPOLEN_MSS) {
129 if (pskb_expand_head(skb, 0,
130 TCPOLEN_MSS - skb_tailroom(skb),
133 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
136 skb_put(skb, TCPOLEN_MSS);
138 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
139 memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
141 inet_proto_csum_replace2(&tcph->check, skb,
142 htons(len), htons(len + TCPOLEN_MSS), 1);
144 opt[1] = TCPOLEN_MSS;
145 opt[2] = (newmss & 0xff00) >> 8;
146 opt[3] = newmss & 0x00ff;
148 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
150 oldval = ((__be16 *)tcph)[6];
151 tcph->doff += TCPOLEN_MSS/4;
152 inet_proto_csum_replace2(&tcph->check, skb,
153 oldval, ((__be16 *)tcph)[6], 0);
157 static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
161 const struct nf_afinfo *ai;
162 struct rtable *rt = NULL;
165 if (family == PF_INET) {
166 struct flowi4 *fl4 = &fl.u.ip4;
167 memset(fl4, 0, sizeof(*fl4));
168 fl4->daddr = ip_hdr(skb)->saddr;
170 struct flowi6 *fl6 = &fl.u.ip6;
172 memset(fl6, 0, sizeof(*fl6));
173 ipv6_addr_copy(&fl6->daddr, &ipv6_hdr(skb)->saddr);
176 ai = nf_get_afinfo(family);
178 ai->route(&init_net, (struct dst_entry **)&rt, &fl, false);
182 mtu = dst_mtu(&rt->dst);
183 dst_release(&rt->dst);
189 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
191 struct iphdr *iph = ip_hdr(skb);
195 ret = tcpmss_mangle_packet(skb, par,
196 tcpmss_reverse_mtu(skb, PF_INET),
198 sizeof(*iph) + sizeof(struct tcphdr));
203 newlen = htons(ntohs(iph->tot_len) + ret);
204 csum_replace2(&iph->check, iph->tot_len, newlen);
205 iph->tot_len = newlen;
210 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
212 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
214 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
219 nexthdr = ipv6h->nexthdr;
220 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
223 ret = tcpmss_mangle_packet(skb, par,
224 tcpmss_reverse_mtu(skb, PF_INET6),
226 sizeof(*ipv6h) + sizeof(struct tcphdr));
230 ipv6h = ipv6_hdr(skb);
231 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
237 /* Must specify -p tcp --syn */
238 static inline bool find_syn_match(const struct xt_entry_match *m)
240 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
242 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
243 tcpinfo->flg_cmp & TCPHDR_SYN &&
244 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
250 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
252 const struct xt_tcpmss_info *info = par->targinfo;
253 const struct ipt_entry *e = par->entryinfo;
254 const struct xt_entry_match *ematch;
256 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
257 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
258 (1 << NF_INET_LOCAL_OUT) |
259 (1 << NF_INET_POST_ROUTING))) != 0) {
260 pr_info("path-MTU clamping only supported in "
261 "FORWARD, OUTPUT and POSTROUTING hooks\n");
264 xt_ematch_foreach(ematch, e)
265 if (find_syn_match(ematch))
267 pr_info("Only works on TCP SYN packets\n");
271 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
272 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
274 const struct xt_tcpmss_info *info = par->targinfo;
275 const struct ip6t_entry *e = par->entryinfo;
276 const struct xt_entry_match *ematch;
278 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
279 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
280 (1 << NF_INET_LOCAL_OUT) |
281 (1 << NF_INET_POST_ROUTING))) != 0) {
282 pr_info("path-MTU clamping only supported in "
283 "FORWARD, OUTPUT and POSTROUTING hooks\n");
286 xt_ematch_foreach(ematch, e)
287 if (find_syn_match(ematch))
289 pr_info("Only works on TCP SYN packets\n");
294 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
296 .family = NFPROTO_IPV4,
298 .checkentry = tcpmss_tg4_check,
299 .target = tcpmss_tg4,
300 .targetsize = sizeof(struct xt_tcpmss_info),
301 .proto = IPPROTO_TCP,
304 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
306 .family = NFPROTO_IPV6,
308 .checkentry = tcpmss_tg6_check,
309 .target = tcpmss_tg6,
310 .targetsize = sizeof(struct xt_tcpmss_info),
311 .proto = IPPROTO_TCP,
317 static int __init tcpmss_tg_init(void)
319 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
322 static void __exit tcpmss_tg_exit(void)
324 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
327 module_init(tcpmss_tg_init);
328 module_exit(tcpmss_tg_exit);