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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>"); | |
2ae15b64 | 27 | MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment"); |
cdd289a2 PM |
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 | |
3db05fea | 42 | tcpmss_mangle_packet(struct sk_buff *skb, |
cdd289a2 PM |
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 | ||
3db05fea | 53 | if (!skb_make_writable(skb, skb->len)) |
cdd289a2 PM |
54 | return -1; |
55 | ||
3db05fea HX |
56 | tcplen = skb->len - tcphoff; |
57 | tcph = (struct tcphdr *)(skb_network_header(skb) + 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", | |
3db05fea | 67 | skb->len); |
cdd289a2 PM |
68 | return -1; |
69 | } | |
70 | ||
71 | if (info->mss == XT_TCPMSS_CLAMP_PMTU) { | |
3db05fea | 72 | if (dst_mtu(skb->dst) <= minlen) { |
cdd289a2 PM |
73 | if (net_ratelimit()) |
74 | printk(KERN_ERR "xt_TCPMSS: " | |
75 | "unknown or invalid path-MTU (%u)\n", | |
3db05fea | 76 | dst_mtu(skb->dst)); |
cdd289a2 PM |
77 | return -1; |
78 | } | |
3db05fea | 79 | newmss = dst_mtu(skb->dst) - minlen; |
cdd289a2 PM |
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 | ||
17008064 BL |
91 | /* Never increase MSS, even when setting it, as |
92 | * doing so results in problems for hosts that rely | |
93 | * on MSS being set correctly. | |
94 | */ | |
95 | if (oldmss <= newmss) | |
cdd289a2 PM |
96 | return 0; |
97 | ||
98 | opt[i+2] = (newmss & 0xff00) >> 8; | |
7c4e36bc | 99 | opt[i+3] = newmss & 0x00ff; |
cdd289a2 | 100 | |
be0ea7d5 PM |
101 | inet_proto_csum_replace2(&tcph->check, skb, |
102 | htons(oldmss), htons(newmss), | |
103 | 0); | |
cdd289a2 PM |
104 | return 0; |
105 | } | |
106 | } | |
107 | ||
108 | /* | |
109 | * MSS Option not found ?! add it.. | |
110 | */ | |
3db05fea HX |
111 | if (skb_tailroom(skb) < TCPOLEN_MSS) { |
112 | if (pskb_expand_head(skb, 0, | |
113 | TCPOLEN_MSS - skb_tailroom(skb), | |
2ca7b0ac | 114 | GFP_ATOMIC)) |
cdd289a2 | 115 | return -1; |
3db05fea | 116 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); |
cdd289a2 PM |
117 | } |
118 | ||
3db05fea | 119 | skb_put(skb, TCPOLEN_MSS); |
cdd289a2 PM |
120 | |
121 | opt = (u_int8_t *)tcph + sizeof(struct tcphdr); | |
122 | memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr)); | |
123 | ||
be0ea7d5 PM |
124 | inet_proto_csum_replace2(&tcph->check, skb, |
125 | htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1); | |
cdd289a2 PM |
126 | opt[0] = TCPOPT_MSS; |
127 | opt[1] = TCPOLEN_MSS; | |
128 | opt[2] = (newmss & 0xff00) >> 8; | |
7c4e36bc | 129 | opt[3] = newmss & 0x00ff; |
cdd289a2 | 130 | |
be0ea7d5 | 131 | inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0); |
cdd289a2 PM |
132 | |
133 | oldval = ((__be16 *)tcph)[6]; | |
134 | tcph->doff += TCPOLEN_MSS/4; | |
be0ea7d5 PM |
135 | inet_proto_csum_replace2(&tcph->check, skb, |
136 | oldval, ((__be16 *)tcph)[6], 0); | |
cdd289a2 PM |
137 | return TCPOLEN_MSS; |
138 | } | |
139 | ||
140 | static unsigned int | |
d3c5ee6d JE |
141 | tcpmss_tg4(struct sk_buff *skb, const struct net_device *in, |
142 | const struct net_device *out, unsigned int hooknum, | |
143 | const struct xt_target *target, const void *targinfo) | |
cdd289a2 | 144 | { |
3db05fea | 145 | struct iphdr *iph = ip_hdr(skb); |
cdd289a2 PM |
146 | __be16 newlen; |
147 | int ret; | |
148 | ||
3db05fea | 149 | ret = tcpmss_mangle_packet(skb, targinfo, iph->ihl * 4, |
cdd289a2 PM |
150 | sizeof(*iph) + sizeof(struct tcphdr)); |
151 | if (ret < 0) | |
152 | return NF_DROP; | |
153 | if (ret > 0) { | |
3db05fea | 154 | iph = ip_hdr(skb); |
cdd289a2 | 155 | newlen = htons(ntohs(iph->tot_len) + ret); |
be0ea7d5 | 156 | csum_replace2(&iph->check, iph->tot_len, newlen); |
cdd289a2 PM |
157 | iph->tot_len = newlen; |
158 | } | |
159 | return XT_CONTINUE; | |
160 | } | |
161 | ||
162 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
163 | static unsigned int | |
d3c5ee6d JE |
164 | tcpmss_tg6(struct sk_buff *skb, const struct net_device *in, |
165 | const struct net_device *out, unsigned int hooknum, | |
166 | const struct xt_target *target, const void *targinfo) | |
cdd289a2 | 167 | { |
3db05fea | 168 | struct ipv6hdr *ipv6h = ipv6_hdr(skb); |
cdd289a2 PM |
169 | u8 nexthdr; |
170 | int tcphoff; | |
171 | int ret; | |
172 | ||
173 | nexthdr = ipv6h->nexthdr; | |
3db05fea | 174 | tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr); |
9dc0564e | 175 | if (tcphoff < 0) |
cdd289a2 | 176 | return NF_DROP; |
3db05fea | 177 | ret = tcpmss_mangle_packet(skb, targinfo, tcphoff, |
cdd289a2 PM |
178 | sizeof(*ipv6h) + sizeof(struct tcphdr)); |
179 | if (ret < 0) | |
180 | return NF_DROP; | |
181 | if (ret > 0) { | |
3db05fea | 182 | ipv6h = ipv6_hdr(skb); |
cdd289a2 PM |
183 | ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret); |
184 | } | |
185 | return XT_CONTINUE; | |
186 | } | |
187 | #endif | |
188 | ||
189 | #define TH_SYN 0x02 | |
190 | ||
191 | /* Must specify -p tcp --syn */ | |
e1931b78 | 192 | static inline bool find_syn_match(const struct xt_entry_match *m) |
cdd289a2 PM |
193 | { |
194 | const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data; | |
195 | ||
196 | if (strcmp(m->u.kernel.match->name, "tcp") == 0 && | |
197 | tcpinfo->flg_cmp & TH_SYN && | |
198 | !(tcpinfo->invflags & XT_TCP_INV_FLAGS)) | |
e1931b78 | 199 | return true; |
cdd289a2 | 200 | |
e1931b78 | 201 | return false; |
cdd289a2 PM |
202 | } |
203 | ||
e1931b78 | 204 | static bool |
d3c5ee6d JE |
205 | tcpmss_tg4_check(const char *tablename, const void *entry, |
206 | const struct xt_target *target, void *targinfo, | |
207 | unsigned int hook_mask) | |
cdd289a2 PM |
208 | { |
209 | const struct xt_tcpmss_info *info = targinfo; | |
210 | const struct ipt_entry *e = entry; | |
211 | ||
212 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
6e23ae2a PM |
213 | (hook_mask & ~((1 << NF_INET_FORWARD) | |
214 | (1 << NF_INET_LOCAL_OUT) | | |
215 | (1 << NF_INET_POST_ROUTING))) != 0) { | |
cdd289a2 PM |
216 | printk("xt_TCPMSS: path-MTU clamping only supported in " |
217 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
e1931b78 | 218 | return false; |
cdd289a2 PM |
219 | } |
220 | if (IPT_MATCH_ITERATE(e, find_syn_match)) | |
e1931b78 | 221 | return true; |
cdd289a2 | 222 | printk("xt_TCPMSS: Only works on TCP SYN packets\n"); |
e1931b78 | 223 | return false; |
cdd289a2 PM |
224 | } |
225 | ||
226 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
e1931b78 | 227 | static bool |
d3c5ee6d JE |
228 | tcpmss_tg6_check(const char *tablename, const void *entry, |
229 | const struct xt_target *target, void *targinfo, | |
230 | unsigned int hook_mask) | |
cdd289a2 PM |
231 | { |
232 | const struct xt_tcpmss_info *info = targinfo; | |
233 | const struct ip6t_entry *e = entry; | |
234 | ||
235 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
6e23ae2a PM |
236 | (hook_mask & ~((1 << NF_INET_FORWARD) | |
237 | (1 << NF_INET_LOCAL_OUT) | | |
238 | (1 << NF_INET_POST_ROUTING))) != 0) { | |
cdd289a2 PM |
239 | printk("xt_TCPMSS: path-MTU clamping only supported in " |
240 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
e1931b78 | 241 | return false; |
cdd289a2 PM |
242 | } |
243 | if (IP6T_MATCH_ITERATE(e, find_syn_match)) | |
e1931b78 | 244 | return true; |
cdd289a2 | 245 | printk("xt_TCPMSS: Only works on TCP SYN packets\n"); |
e1931b78 | 246 | return false; |
cdd289a2 PM |
247 | } |
248 | #endif | |
249 | ||
d3c5ee6d | 250 | static struct xt_target tcpmss_tg_reg[] __read_mostly = { |
cdd289a2 PM |
251 | { |
252 | .family = AF_INET, | |
253 | .name = "TCPMSS", | |
d3c5ee6d JE |
254 | .checkentry = tcpmss_tg4_check, |
255 | .target = tcpmss_tg4, | |
cdd289a2 PM |
256 | .targetsize = sizeof(struct xt_tcpmss_info), |
257 | .proto = IPPROTO_TCP, | |
258 | .me = THIS_MODULE, | |
259 | }, | |
260 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
261 | { | |
262 | .family = AF_INET6, | |
263 | .name = "TCPMSS", | |
d3c5ee6d JE |
264 | .checkentry = tcpmss_tg6_check, |
265 | .target = tcpmss_tg6, | |
cdd289a2 PM |
266 | .targetsize = sizeof(struct xt_tcpmss_info), |
267 | .proto = IPPROTO_TCP, | |
268 | .me = THIS_MODULE, | |
269 | }, | |
270 | #endif | |
271 | }; | |
272 | ||
d3c5ee6d | 273 | static int __init tcpmss_tg_init(void) |
cdd289a2 | 274 | { |
d3c5ee6d | 275 | return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
cdd289a2 PM |
276 | } |
277 | ||
d3c5ee6d | 278 | static void __exit tcpmss_tg_exit(void) |
cdd289a2 | 279 | { |
d3c5ee6d | 280 | xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
cdd289a2 PM |
281 | } |
282 | ||
d3c5ee6d JE |
283 | module_init(tcpmss_tg_init); |
284 | module_exit(tcpmss_tg_exit); |