tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
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#include <linux/ip.h>
14#include <linux/ipv6.h>
15#include <linux/tcp.h>
16#include <net/dst.h>
17#include <net/flow.h>
18#include <net/ipv6.h>
19#include <net/route.h>
20#include <net/tcp.h>
21
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>
27
28MODULE_LICENSE("GPL");
29MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
30MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
31MODULE_ALIAS("ipt_TCPMSS");
32MODULE_ALIAS("ip6t_TCPMSS");
33
34static inline unsigned int
35optlen(const u_int8_t *opt, unsigned int offset)
36{
37 /* Beware zero-length options: make finite progress */
38 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
39 return 1;
40 else
41 return opt[offset+1];
42}
43
44static int
45tcpmss_mangle_packet(struct sk_buff *skb,
46 const struct xt_tcpmss_info *info,
47 unsigned int in_mtu,
48 unsigned int tcphoff,
49 unsigned int minlen)
50{
51 struct tcphdr *tcph;
52 unsigned int tcplen, i;
53 __be16 oldval;
54 u16 newmss;
55 u8 *opt;
56
57 if (!skb_make_writable(skb, skb->len))
58 return -1;
59
60 tcplen = skb->len - tcphoff;
61 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
62
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
67 badly. --RR */
68 if (tcplen != tcph->doff*4) {
69 if (net_ratelimit())
70 printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
71 skb->len);
72 return -1;
73 }
74
75 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
76 if (dst_mtu(skb->dst) <= minlen) {
77 if (net_ratelimit())
78 printk(KERN_ERR "xt_TCPMSS: "
79 "unknown or invalid path-MTU (%u)\n",
80 dst_mtu(skb->dst));
81 return -1;
82 }
83 if (in_mtu <= minlen) {
84 if (net_ratelimit())
85 printk(KERN_ERR "xt_TCPMSS: unknown or "
86 "invalid path-MTU (%u)\n", in_mtu);
87 return -1;
88 }
89 newmss = min(dst_mtu(skb->dst), in_mtu) - minlen;
90 } else
91 newmss = info->mss;
92
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) {
97 u_int16_t oldmss;
98
99 oldmss = (opt[i+2] << 8) | opt[i+3];
100
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.
104 */
105 if (oldmss <= newmss)
106 return 0;
107
108 opt[i+2] = (newmss & 0xff00) >> 8;
109 opt[i+3] = newmss & 0x00ff;
110
111 inet_proto_csum_replace2(&tcph->check, skb,
112 htons(oldmss), htons(newmss),
113 0);
114 return 0;
115 }
116 }
117
118 /*
119 * MSS Option not found ?! add it..
120 */
121 if (skb_tailroom(skb) < TCPOLEN_MSS) {
122 if (pskb_expand_head(skb, 0,
123 TCPOLEN_MSS - skb_tailroom(skb),
124 GFP_ATOMIC))
125 return -1;
126 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
127 }
128
129 skb_put(skb, TCPOLEN_MSS);
130
131 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
132 memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
133
134 inet_proto_csum_replace2(&tcph->check, skb,
135 htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
136 opt[0] = TCPOPT_MSS;
137 opt[1] = TCPOLEN_MSS;
138 opt[2] = (newmss & 0xff00) >> 8;
139 opt[3] = newmss & 0x00ff;
140
141 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
142
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);
147 return TCPOLEN_MSS;
148}
149
150static u_int32_t tcpmss_reverse_mtu4(const struct iphdr *iph)
151{
152 struct flowi fl = {
153 .fl4_dst = iph->saddr,
154 };
155 const struct nf_afinfo *ai;
156 struct rtable *rt = NULL;
157 u_int32_t mtu = ~0U;
158
159 rcu_read_lock();
160 ai = nf_get_afinfo(AF_INET);
161 if (ai != NULL)
162 ai->route((struct dst_entry **)&rt, &fl);
163 rcu_read_unlock();
164
165 if (rt != NULL) {
166 mtu = dst_mtu(&rt->u.dst);
167 dst_release(&rt->u.dst);
168 }
169 return mtu;
170}
171
172static unsigned int
173tcpmss_tg4(struct sk_buff *skb, const struct net_device *in,
174 const struct net_device *out, unsigned int hooknum,
175 const struct xt_target *target, const void *targinfo)
176{
177 struct iphdr *iph = ip_hdr(skb);
178 __be16 newlen;
179 int ret;
180
181 ret = tcpmss_mangle_packet(skb, targinfo, tcpmss_reverse_mtu4(iph),
182 iph->ihl * 4,
183 sizeof(*iph) + sizeof(struct tcphdr));
184 if (ret < 0)
185 return NF_DROP;
186 if (ret > 0) {
187 iph = ip_hdr(skb);
188 newlen = htons(ntohs(iph->tot_len) + ret);
189 csum_replace2(&iph->check, iph->tot_len, newlen);
190 iph->tot_len = newlen;
191 }
192 return XT_CONTINUE;
193}
194
195#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
196static u_int32_t tcpmss_reverse_mtu6(const struct ipv6hdr *iph)
197{
198 struct flowi fl = {
199 .fl6_dst = iph->saddr,
200 };
201 const struct nf_afinfo *ai;
202 struct rtable *rt = NULL;
203 u_int32_t mtu = ~0U;
204
205 rcu_read_lock();
206 ai = nf_get_afinfo(AF_INET6);
207 if (ai != NULL)
208 ai->route((struct dst_entry **)&rt, &fl);
209 rcu_read_unlock();
210
211 if (rt != NULL) {
212 mtu = dst_mtu(&rt->u.dst);
213 dst_release(&rt->u.dst);
214 }
215 return mtu;
216}
217
218static unsigned int
219tcpmss_tg6(struct sk_buff *skb, const struct net_device *in,
220 const struct net_device *out, unsigned int hooknum,
221 const struct xt_target *target, const void *targinfo)
222{
223 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
224 u8 nexthdr;
225 int tcphoff;
226 int ret;
227
228 nexthdr = ipv6h->nexthdr;
229 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
230 if (tcphoff < 0)
231 return NF_DROP;
232 ret = tcpmss_mangle_packet(skb, targinfo, tcpmss_reverse_mtu6(ipv6h),
233 tcphoff,
234 sizeof(*ipv6h) + sizeof(struct tcphdr));
235 if (ret < 0)
236 return NF_DROP;
237 if (ret > 0) {
238 ipv6h = ipv6_hdr(skb);
239 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
240 }
241 return XT_CONTINUE;
242}
243#endif
244
245#define TH_SYN 0x02
246
247/* Must specify -p tcp --syn */
248static inline bool find_syn_match(const struct xt_entry_match *m)
249{
250 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
251
252 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
253 tcpinfo->flg_cmp & TH_SYN &&
254 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
255 return true;
256
257 return false;
258}
259
260static bool
261tcpmss_tg4_check(const char *tablename, const void *entry,
262 const struct xt_target *target, void *targinfo,
263 unsigned int hook_mask)
264{
265 const struct xt_tcpmss_info *info = targinfo;
266 const struct ipt_entry *e = entry;
267
268 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
269 (hook_mask & ~((1 << NF_INET_FORWARD) |
270 (1 << NF_INET_LOCAL_OUT) |
271 (1 << NF_INET_POST_ROUTING))) != 0) {
272 printk("xt_TCPMSS: path-MTU clamping only supported in "
273 "FORWARD, OUTPUT and POSTROUTING hooks\n");
274 return false;
275 }
276 if (IPT_MATCH_ITERATE(e, find_syn_match))
277 return true;
278 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
279 return false;
280}
281
282#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
283static bool
284tcpmss_tg6_check(const char *tablename, const void *entry,
285 const struct xt_target *target, void *targinfo,
286 unsigned int hook_mask)
287{
288 const struct xt_tcpmss_info *info = targinfo;
289 const struct ip6t_entry *e = entry;
290
291 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
292 (hook_mask & ~((1 << NF_INET_FORWARD) |
293 (1 << NF_INET_LOCAL_OUT) |
294 (1 << NF_INET_POST_ROUTING))) != 0) {
295 printk("xt_TCPMSS: path-MTU clamping only supported in "
296 "FORWARD, OUTPUT and POSTROUTING hooks\n");
297 return false;
298 }
299 if (IP6T_MATCH_ITERATE(e, find_syn_match))
300 return true;
301 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
302 return false;
303}
304#endif
305
306static struct xt_target tcpmss_tg_reg[] __read_mostly = {
307 {
308 .family = AF_INET,
309 .name = "TCPMSS",
310 .checkentry = tcpmss_tg4_check,
311 .target = tcpmss_tg4,
312 .targetsize = sizeof(struct xt_tcpmss_info),
313 .proto = IPPROTO_TCP,
314 .me = THIS_MODULE,
315 },
316#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
317 {
318 .family = AF_INET6,
319 .name = "TCPMSS",
320 .checkentry = tcpmss_tg6_check,
321 .target = tcpmss_tg6,
322 .targetsize = sizeof(struct xt_tcpmss_info),
323 .proto = IPPROTO_TCP,
324 .me = THIS_MODULE,
325 },
326#endif
327};
328
329static int __init tcpmss_tg_init(void)
330{
331 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
332}
333
334static void __exit tcpmss_tg_exit(void)
335{
336 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
337}
338
339module_init(tcpmss_tg_init);
340module_exit(tcpmss_tg_exit);