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>
5 * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License version 2 as
9 * published by the Free Software Foundation.
11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
12 #include <linux/module.h>
13 #include <linux/skbuff.h>
15 #include <linux/gfp.h>
16 #include <linux/ipv6.h>
17 #include <linux/tcp.h>
21 #include <net/route.h>
24 #include <linux/netfilter_ipv4/ip_tables.h>
25 #include <linux/netfilter_ipv6/ip6_tables.h>
26 #include <linux/netfilter/x_tables.h>
27 #include <linux/netfilter/xt_tcpudp.h>
28 #include <linux/netfilter/xt_TCPMSS.h>
30 MODULE_LICENSE("GPL");
31 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
32 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
33 MODULE_ALIAS("ipt_TCPMSS");
34 MODULE_ALIAS("ip6t_TCPMSS");
36 static inline unsigned int
37 optlen(const u_int8_t *opt, unsigned int offset)
39 /* Beware zero-length options: make finite progress */
40 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
47 tcpmss_mangle_packet(struct sk_buff *skb,
48 const struct xt_action_param *par,
53 const struct xt_tcpmss_info *info = par->targinfo;
55 unsigned int tcplen, i;
60 /* This is a fragment, no TCP header is available */
61 if (par->fragoff != 0)
64 if (!skb_make_writable(skb, skb->len))
67 tcplen = skb->len - tcphoff;
68 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
70 /* Header cannot be larger than the packet */
71 if (tcplen < tcph->doff*4)
74 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
75 if (dst_mtu(skb_dst(skb)) <= minlen) {
76 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
77 dst_mtu(skb_dst(skb)));
80 if (in_mtu <= minlen) {
81 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
85 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
89 opt = (u_int8_t *)tcph;
90 for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
91 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
92 opt[i+1] == TCPOLEN_MSS) {
95 oldmss = (opt[i+2] << 8) | opt[i+3];
97 /* Never increase MSS, even when setting it, as
98 * doing so results in problems for hosts that rely
99 * on MSS being set correctly.
101 if (oldmss <= newmss)
104 opt[i+2] = (newmss & 0xff00) >> 8;
105 opt[i+3] = newmss & 0x00ff;
107 inet_proto_csum_replace2(&tcph->check, skb,
108 htons(oldmss), htons(newmss),
114 /* There is data after the header so the option can't be added
115 without moving it, and doing so may make the SYN packet
116 itself too large. Accept the packet unmodified instead. */
117 if (tcplen > tcph->doff*4)
121 * MSS Option not found ?! add it..
123 if (skb_tailroom(skb) < TCPOLEN_MSS) {
124 if (pskb_expand_head(skb, 0,
125 TCPOLEN_MSS - skb_tailroom(skb),
128 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
131 skb_put(skb, TCPOLEN_MSS);
134 * IPv4: RFC 1122 states "If an MSS option is not received at
135 * connection setup, TCP MUST assume a default send MSS of 536".
136 * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
137 * length IPv6 header of 60, ergo the default MSS value is 1220
138 * Since no MSS was provided, we must use the default values
140 if (par->family == NFPROTO_IPV4)
141 newmss = min(newmss, (u16)536);
143 newmss = min(newmss, (u16)1220);
145 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
146 memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
148 inet_proto_csum_replace2(&tcph->check, skb,
149 htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
151 opt[1] = TCPOLEN_MSS;
152 opt[2] = (newmss & 0xff00) >> 8;
153 opt[3] = newmss & 0x00ff;
155 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
157 oldval = ((__be16 *)tcph)[6];
158 tcph->doff += TCPOLEN_MSS/4;
159 inet_proto_csum_replace2(&tcph->check, skb,
160 oldval, ((__be16 *)tcph)[6], 0);
164 static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
168 const struct nf_afinfo *ai;
169 struct rtable *rt = NULL;
172 if (family == PF_INET) {
173 struct flowi4 *fl4 = &fl.u.ip4;
174 memset(fl4, 0, sizeof(*fl4));
175 fl4->daddr = ip_hdr(skb)->saddr;
177 struct flowi6 *fl6 = &fl.u.ip6;
179 memset(fl6, 0, sizeof(*fl6));
180 fl6->daddr = ipv6_hdr(skb)->saddr;
183 ai = nf_get_afinfo(family);
185 ai->route(&init_net, (struct dst_entry **)&rt, &fl, false);
189 mtu = dst_mtu(&rt->dst);
190 dst_release(&rt->dst);
196 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
198 struct iphdr *iph = ip_hdr(skb);
202 ret = tcpmss_mangle_packet(skb, par,
203 tcpmss_reverse_mtu(skb, PF_INET),
205 sizeof(*iph) + sizeof(struct tcphdr));
210 newlen = htons(ntohs(iph->tot_len) + ret);
211 csum_replace2(&iph->check, iph->tot_len, newlen);
212 iph->tot_len = newlen;
217 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
219 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
221 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
227 nexthdr = ipv6h->nexthdr;
228 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
231 ret = tcpmss_mangle_packet(skb, par,
232 tcpmss_reverse_mtu(skb, PF_INET6),
234 sizeof(*ipv6h) + sizeof(struct tcphdr));
238 ipv6h = ipv6_hdr(skb);
239 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
245 /* Must specify -p tcp --syn */
246 static inline bool find_syn_match(const struct xt_entry_match *m)
248 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
250 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
251 tcpinfo->flg_cmp & TCPHDR_SYN &&
252 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
258 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
260 const struct xt_tcpmss_info *info = par->targinfo;
261 const struct ipt_entry *e = par->entryinfo;
262 const struct xt_entry_match *ematch;
264 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
265 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
266 (1 << NF_INET_LOCAL_OUT) |
267 (1 << NF_INET_POST_ROUTING))) != 0) {
268 pr_info("path-MTU clamping only supported in "
269 "FORWARD, OUTPUT and POSTROUTING hooks\n");
272 xt_ematch_foreach(ematch, e)
273 if (find_syn_match(ematch))
275 pr_info("Only works on TCP SYN packets\n");
279 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
280 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
282 const struct xt_tcpmss_info *info = par->targinfo;
283 const struct ip6t_entry *e = par->entryinfo;
284 const struct xt_entry_match *ematch;
286 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
287 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
288 (1 << NF_INET_LOCAL_OUT) |
289 (1 << NF_INET_POST_ROUTING))) != 0) {
290 pr_info("path-MTU clamping only supported in "
291 "FORWARD, OUTPUT and POSTROUTING hooks\n");
294 xt_ematch_foreach(ematch, e)
295 if (find_syn_match(ematch))
297 pr_info("Only works on TCP SYN packets\n");
302 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
304 .family = NFPROTO_IPV4,
306 .checkentry = tcpmss_tg4_check,
307 .target = tcpmss_tg4,
308 .targetsize = sizeof(struct xt_tcpmss_info),
309 .proto = IPPROTO_TCP,
312 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
314 .family = NFPROTO_IPV6,
316 .checkentry = tcpmss_tg6_check,
317 .target = tcpmss_tg6,
318 .targetsize = sizeof(struct xt_tcpmss_info),
319 .proto = IPPROTO_TCP,
325 static int __init tcpmss_tg_init(void)
327 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
330 static void __exit tcpmss_tg_exit(void)
332 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
335 module_init(tcpmss_tg_init);
336 module_exit(tcpmss_tg_exit);