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cdd289a2 PM |
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/ipv6.h> | |
17 | #include <net/tcp.h> | |
18 | ||
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> | |
24 | ||
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"); | |
30 | ||
31 | static inline unsigned int | |
32 | optlen(const u_int8_t *opt, unsigned int offset) | |
33 | { | |
34 | /* Beware zero-length options: make finite progress */ | |
35 | if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) | |
36 | return 1; | |
37 | else | |
38 | return opt[offset+1]; | |
39 | } | |
40 | ||
41 | static int | |
42 | tcpmss_mangle_packet(struct sk_buff **pskb, | |
43 | const struct xt_tcpmss_info *info, | |
44 | unsigned int tcphoff, | |
45 | unsigned int minlen) | |
46 | { | |
47 | struct tcphdr *tcph; | |
48 | unsigned int tcplen, i; | |
49 | __be16 oldval; | |
50 | u16 newmss; | |
51 | u8 *opt; | |
52 | ||
53 | if (!skb_make_writable(pskb, (*pskb)->len)) | |
54 | return -1; | |
55 | ||
56 | tcplen = (*pskb)->len - tcphoff; | |
d56f90a7 | 57 | tcph = (struct tcphdr *)(skb_network_header(*pskb) + tcphoff); |
cdd289a2 PM |
58 | |
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 | |
63 | badly. --RR */ | |
64 | if (tcplen != tcph->doff*4) { | |
65 | if (net_ratelimit()) | |
66 | printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n", | |
67 | (*pskb)->len); | |
68 | return -1; | |
69 | } | |
70 | ||
71 | if (info->mss == XT_TCPMSS_CLAMP_PMTU) { | |
72 | if (dst_mtu((*pskb)->dst) <= minlen) { | |
73 | if (net_ratelimit()) | |
74 | printk(KERN_ERR "xt_TCPMSS: " | |
75 | "unknown or invalid path-MTU (%u)\n", | |
76 | dst_mtu((*pskb)->dst)); | |
77 | return -1; | |
78 | } | |
79 | newmss = dst_mtu((*pskb)->dst) - minlen; | |
80 | } else | |
81 | newmss = info->mss; | |
82 | ||
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) { | |
87 | u_int16_t oldmss; | |
88 | ||
89 | oldmss = (opt[i+2] << 8) | opt[i+3]; | |
90 | ||
91 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
92 | oldmss <= newmss) | |
93 | return 0; | |
94 | ||
95 | opt[i+2] = (newmss & 0xff00) >> 8; | |
96 | opt[i+3] = (newmss & 0x00ff); | |
97 | ||
98 | nf_proto_csum_replace2(&tcph->check, *pskb, | |
99 | htons(oldmss), htons(newmss), 0); | |
100 | return 0; | |
101 | } | |
102 | } | |
103 | ||
104 | /* | |
105 | * MSS Option not found ?! add it.. | |
106 | */ | |
107 | if (skb_tailroom((*pskb)) < TCPOLEN_MSS) { | |
108 | struct sk_buff *newskb; | |
109 | ||
110 | newskb = skb_copy_expand(*pskb, skb_headroom(*pskb), | |
111 | TCPOLEN_MSS, GFP_ATOMIC); | |
112 | if (!newskb) | |
113 | return -1; | |
114 | kfree_skb(*pskb); | |
115 | *pskb = newskb; | |
d56f90a7 | 116 | tcph = (struct tcphdr *)(skb_network_header(*pskb) + tcphoff); |
cdd289a2 PM |
117 | } |
118 | ||
119 | skb_put((*pskb), TCPOLEN_MSS); | |
120 | ||
121 | opt = (u_int8_t *)tcph + sizeof(struct tcphdr); | |
122 | memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr)); | |
123 | ||
124 | nf_proto_csum_replace2(&tcph->check, *pskb, | |
125 | htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1); | |
126 | opt[0] = TCPOPT_MSS; | |
127 | opt[1] = TCPOLEN_MSS; | |
128 | opt[2] = (newmss & 0xff00) >> 8; | |
129 | opt[3] = (newmss & 0x00ff); | |
130 | ||
131 | nf_proto_csum_replace4(&tcph->check, *pskb, 0, *((__be32 *)opt), 0); | |
132 | ||
133 | oldval = ((__be16 *)tcph)[6]; | |
134 | tcph->doff += TCPOLEN_MSS/4; | |
135 | nf_proto_csum_replace2(&tcph->check, *pskb, | |
136 | oldval, ((__be16 *)tcph)[6], 0); | |
137 | return TCPOLEN_MSS; | |
138 | } | |
139 | ||
140 | static unsigned int | |
141 | xt_tcpmss_target4(struct sk_buff **pskb, | |
142 | const struct net_device *in, | |
143 | const struct net_device *out, | |
144 | unsigned int hooknum, | |
145 | const struct xt_target *target, | |
146 | const void *targinfo) | |
147 | { | |
eddc9ec5 | 148 | struct iphdr *iph = ip_hdr(*pskb); |
cdd289a2 PM |
149 | __be16 newlen; |
150 | int ret; | |
151 | ||
152 | ret = tcpmss_mangle_packet(pskb, targinfo, iph->ihl * 4, | |
153 | sizeof(*iph) + sizeof(struct tcphdr)); | |
154 | if (ret < 0) | |
155 | return NF_DROP; | |
156 | if (ret > 0) { | |
eddc9ec5 | 157 | iph = ip_hdr(*pskb); |
cdd289a2 PM |
158 | newlen = htons(ntohs(iph->tot_len) + ret); |
159 | nf_csum_replace2(&iph->check, iph->tot_len, newlen); | |
160 | iph->tot_len = newlen; | |
161 | } | |
162 | return XT_CONTINUE; | |
163 | } | |
164 | ||
165 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
166 | static unsigned int | |
167 | xt_tcpmss_target6(struct sk_buff **pskb, | |
168 | const struct net_device *in, | |
169 | const struct net_device *out, | |
170 | unsigned int hooknum, | |
171 | const struct xt_target *target, | |
172 | const void *targinfo) | |
173 | { | |
0660e03f | 174 | struct ipv6hdr *ipv6h = ipv6_hdr(*pskb); |
cdd289a2 PM |
175 | u8 nexthdr; |
176 | int tcphoff; | |
177 | int ret; | |
178 | ||
179 | nexthdr = ipv6h->nexthdr; | |
180 | tcphoff = ipv6_skip_exthdr(*pskb, sizeof(*ipv6h), &nexthdr); | |
181 | if (tcphoff < 0) { | |
182 | WARN_ON(1); | |
183 | return NF_DROP; | |
184 | } | |
185 | ret = tcpmss_mangle_packet(pskb, targinfo, tcphoff, | |
186 | sizeof(*ipv6h) + sizeof(struct tcphdr)); | |
187 | if (ret < 0) | |
188 | return NF_DROP; | |
189 | if (ret > 0) { | |
0660e03f | 190 | ipv6h = ipv6_hdr(*pskb); |
cdd289a2 PM |
191 | ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret); |
192 | } | |
193 | return XT_CONTINUE; | |
194 | } | |
195 | #endif | |
196 | ||
197 | #define TH_SYN 0x02 | |
198 | ||
199 | /* Must specify -p tcp --syn */ | |
200 | static inline int find_syn_match(const struct xt_entry_match *m) | |
201 | { | |
202 | const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data; | |
203 | ||
204 | if (strcmp(m->u.kernel.match->name, "tcp") == 0 && | |
205 | tcpinfo->flg_cmp & TH_SYN && | |
206 | !(tcpinfo->invflags & XT_TCP_INV_FLAGS)) | |
207 | return 1; | |
208 | ||
209 | return 0; | |
210 | } | |
211 | ||
212 | static int | |
213 | xt_tcpmss_checkentry4(const char *tablename, | |
214 | const void *entry, | |
215 | const struct xt_target *target, | |
216 | void *targinfo, | |
217 | unsigned int hook_mask) | |
218 | { | |
219 | const struct xt_tcpmss_info *info = targinfo; | |
220 | const struct ipt_entry *e = entry; | |
221 | ||
222 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
223 | (hook_mask & ~((1 << NF_IP_FORWARD) | | |
224 | (1 << NF_IP_LOCAL_OUT) | | |
225 | (1 << NF_IP_POST_ROUTING))) != 0) { | |
226 | printk("xt_TCPMSS: path-MTU clamping only supported in " | |
227 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
228 | return 0; | |
229 | } | |
230 | if (IPT_MATCH_ITERATE(e, find_syn_match)) | |
231 | return 1; | |
232 | printk("xt_TCPMSS: Only works on TCP SYN packets\n"); | |
233 | return 0; | |
234 | } | |
235 | ||
236 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
237 | static int | |
238 | xt_tcpmss_checkentry6(const char *tablename, | |
239 | const void *entry, | |
240 | const struct xt_target *target, | |
241 | void *targinfo, | |
242 | unsigned int hook_mask) | |
243 | { | |
244 | const struct xt_tcpmss_info *info = targinfo; | |
245 | const struct ip6t_entry *e = entry; | |
246 | ||
247 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
248 | (hook_mask & ~((1 << NF_IP6_FORWARD) | | |
249 | (1 << NF_IP6_LOCAL_OUT) | | |
250 | (1 << NF_IP6_POST_ROUTING))) != 0) { | |
251 | printk("xt_TCPMSS: path-MTU clamping only supported in " | |
252 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
253 | return 0; | |
254 | } | |
255 | if (IP6T_MATCH_ITERATE(e, find_syn_match)) | |
256 | return 1; | |
257 | printk("xt_TCPMSS: Only works on TCP SYN packets\n"); | |
258 | return 0; | |
259 | } | |
260 | #endif | |
261 | ||
262 | static struct xt_target xt_tcpmss_reg[] = { | |
263 | { | |
264 | .family = AF_INET, | |
265 | .name = "TCPMSS", | |
266 | .checkentry = xt_tcpmss_checkentry4, | |
267 | .target = xt_tcpmss_target4, | |
268 | .targetsize = sizeof(struct xt_tcpmss_info), | |
269 | .proto = IPPROTO_TCP, | |
270 | .me = THIS_MODULE, | |
271 | }, | |
272 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
273 | { | |
274 | .family = AF_INET6, | |
275 | .name = "TCPMSS", | |
276 | .checkentry = xt_tcpmss_checkentry6, | |
277 | .target = xt_tcpmss_target6, | |
278 | .targetsize = sizeof(struct xt_tcpmss_info), | |
279 | .proto = IPPROTO_TCP, | |
280 | .me = THIS_MODULE, | |
281 | }, | |
282 | #endif | |
283 | }; | |
284 | ||
285 | static int __init xt_tcpmss_init(void) | |
286 | { | |
287 | return xt_register_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg)); | |
288 | } | |
289 | ||
290 | static void __exit xt_tcpmss_fini(void) | |
291 | { | |
292 | xt_unregister_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg)); | |
293 | } | |
294 | ||
295 | module_init(xt_tcpmss_init); | |
296 | module_exit(xt_tcpmss_fini); |