Commit | Line | Data |
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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 | */ | |
8bee4bad | 10 | #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt |
cdd289a2 PM |
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> | |
37c08387 JE |
16 | #include <net/dst.h> |
17 | #include <net/flow.h> | |
cdd289a2 | 18 | #include <net/ipv6.h> |
37c08387 | 19 | #include <net/route.h> |
cdd289a2 PM |
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 | ||
28 | MODULE_LICENSE("GPL"); | |
29 | MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); | |
2ae15b64 | 30 | MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment"); |
cdd289a2 PM |
31 | MODULE_ALIAS("ipt_TCPMSS"); |
32 | MODULE_ALIAS("ip6t_TCPMSS"); | |
33 | ||
34 | static inline unsigned int | |
35 | optlen(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 | ||
44 | static int | |
3db05fea | 45 | tcpmss_mangle_packet(struct sk_buff *skb, |
cdd289a2 | 46 | const struct xt_tcpmss_info *info, |
37c08387 | 47 | unsigned int in_mtu, |
cdd289a2 PM |
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 | ||
3db05fea | 57 | if (!skb_make_writable(skb, skb->len)) |
cdd289a2 PM |
58 | return -1; |
59 | ||
3db05fea HX |
60 | tcplen = skb->len - tcphoff; |
61 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); | |
cdd289a2 | 62 | |
10a19939 SA |
63 | /* Header cannot be larger than the packet */ |
64 | if (tcplen < tcph->doff*4) | |
cdd289a2 | 65 | return -1; |
cdd289a2 PM |
66 | |
67 | if (info->mss == XT_TCPMSS_CLAMP_PMTU) { | |
adf30907 | 68 | if (dst_mtu(skb_dst(skb)) <= minlen) { |
cdd289a2 | 69 | if (net_ratelimit()) |
8bee4bad | 70 | pr_err("xt_TCPMSS: " |
cdd289a2 | 71 | "unknown or invalid path-MTU (%u)\n", |
adf30907 | 72 | dst_mtu(skb_dst(skb))); |
cdd289a2 PM |
73 | return -1; |
74 | } | |
37c08387 JE |
75 | if (in_mtu <= minlen) { |
76 | if (net_ratelimit()) | |
8bee4bad | 77 | pr_err("xt_TCPMSS: unknown or " |
37c08387 JE |
78 | "invalid path-MTU (%u)\n", in_mtu); |
79 | return -1; | |
80 | } | |
adf30907 | 81 | newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen; |
cdd289a2 PM |
82 | } else |
83 | newmss = info->mss; | |
84 | ||
85 | opt = (u_int8_t *)tcph; | |
86 | for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) { | |
87 | if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS && | |
88 | opt[i+1] == TCPOLEN_MSS) { | |
89 | u_int16_t oldmss; | |
90 | ||
91 | oldmss = (opt[i+2] << 8) | opt[i+3]; | |
92 | ||
17008064 BL |
93 | /* Never increase MSS, even when setting it, as |
94 | * doing so results in problems for hosts that rely | |
95 | * on MSS being set correctly. | |
96 | */ | |
97 | if (oldmss <= newmss) | |
cdd289a2 PM |
98 | return 0; |
99 | ||
100 | opt[i+2] = (newmss & 0xff00) >> 8; | |
7c4e36bc | 101 | opt[i+3] = newmss & 0x00ff; |
cdd289a2 | 102 | |
be0ea7d5 PM |
103 | inet_proto_csum_replace2(&tcph->check, skb, |
104 | htons(oldmss), htons(newmss), | |
105 | 0); | |
cdd289a2 PM |
106 | return 0; |
107 | } | |
108 | } | |
109 | ||
10a19939 SA |
110 | /* There is data after the header so the option can't be added |
111 | without moving it, and doing so may make the SYN packet | |
112 | itself too large. Accept the packet unmodified instead. */ | |
113 | if (tcplen > tcph->doff*4) | |
114 | return 0; | |
115 | ||
cdd289a2 PM |
116 | /* |
117 | * MSS Option not found ?! add it.. | |
118 | */ | |
3db05fea HX |
119 | if (skb_tailroom(skb) < TCPOLEN_MSS) { |
120 | if (pskb_expand_head(skb, 0, | |
121 | TCPOLEN_MSS - skb_tailroom(skb), | |
2ca7b0ac | 122 | GFP_ATOMIC)) |
cdd289a2 | 123 | return -1; |
3db05fea | 124 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); |
cdd289a2 PM |
125 | } |
126 | ||
3db05fea | 127 | skb_put(skb, TCPOLEN_MSS); |
cdd289a2 PM |
128 | |
129 | opt = (u_int8_t *)tcph + sizeof(struct tcphdr); | |
130 | memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr)); | |
131 | ||
be0ea7d5 PM |
132 | inet_proto_csum_replace2(&tcph->check, skb, |
133 | htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1); | |
cdd289a2 PM |
134 | opt[0] = TCPOPT_MSS; |
135 | opt[1] = TCPOLEN_MSS; | |
136 | opt[2] = (newmss & 0xff00) >> 8; | |
7c4e36bc | 137 | opt[3] = newmss & 0x00ff; |
cdd289a2 | 138 | |
be0ea7d5 | 139 | inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0); |
cdd289a2 PM |
140 | |
141 | oldval = ((__be16 *)tcph)[6]; | |
142 | tcph->doff += TCPOLEN_MSS/4; | |
be0ea7d5 PM |
143 | inet_proto_csum_replace2(&tcph->check, skb, |
144 | oldval, ((__be16 *)tcph)[6], 0); | |
cdd289a2 PM |
145 | return TCPOLEN_MSS; |
146 | } | |
147 | ||
db1a75bd JE |
148 | static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb, |
149 | unsigned int family) | |
37c08387 | 150 | { |
db1a75bd | 151 | struct flowi fl = {}; |
37c08387 JE |
152 | const struct nf_afinfo *ai; |
153 | struct rtable *rt = NULL; | |
154 | u_int32_t mtu = ~0U; | |
155 | ||
db1a75bd JE |
156 | if (family == PF_INET) |
157 | fl.fl4_dst = ip_hdr(skb)->saddr; | |
158 | else | |
159 | fl.fl6_dst = ipv6_hdr(skb)->saddr; | |
160 | ||
37c08387 | 161 | rcu_read_lock(); |
db1a75bd | 162 | ai = nf_get_afinfo(family); |
37c08387 JE |
163 | if (ai != NULL) |
164 | ai->route((struct dst_entry **)&rt, &fl); | |
165 | rcu_read_unlock(); | |
166 | ||
167 | if (rt != NULL) { | |
168 | mtu = dst_mtu(&rt->u.dst); | |
169 | dst_release(&rt->u.dst); | |
170 | } | |
171 | return mtu; | |
172 | } | |
173 | ||
cdd289a2 | 174 | static unsigned int |
7eb35586 | 175 | tcpmss_tg4(struct sk_buff *skb, const struct xt_target_param *par) |
cdd289a2 | 176 | { |
3db05fea | 177 | struct iphdr *iph = ip_hdr(skb); |
cdd289a2 PM |
178 | __be16 newlen; |
179 | int ret; | |
180 | ||
7eb35586 | 181 | ret = tcpmss_mangle_packet(skb, par->targinfo, |
db1a75bd | 182 | tcpmss_reverse_mtu(skb, PF_INET), |
37c08387 | 183 | iph->ihl * 4, |
cdd289a2 PM |
184 | sizeof(*iph) + sizeof(struct tcphdr)); |
185 | if (ret < 0) | |
186 | return NF_DROP; | |
187 | if (ret > 0) { | |
3db05fea | 188 | iph = ip_hdr(skb); |
cdd289a2 | 189 | newlen = htons(ntohs(iph->tot_len) + ret); |
be0ea7d5 | 190 | csum_replace2(&iph->check, iph->tot_len, newlen); |
cdd289a2 PM |
191 | iph->tot_len = newlen; |
192 | } | |
193 | return XT_CONTINUE; | |
194 | } | |
195 | ||
196 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
197 | static unsigned int | |
7eb35586 | 198 | tcpmss_tg6(struct sk_buff *skb, const struct xt_target_param *par) |
cdd289a2 | 199 | { |
3db05fea | 200 | struct ipv6hdr *ipv6h = ipv6_hdr(skb); |
cdd289a2 PM |
201 | u8 nexthdr; |
202 | int tcphoff; | |
203 | int ret; | |
204 | ||
205 | nexthdr = ipv6h->nexthdr; | |
3db05fea | 206 | tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr); |
9dc0564e | 207 | if (tcphoff < 0) |
cdd289a2 | 208 | return NF_DROP; |
7eb35586 | 209 | ret = tcpmss_mangle_packet(skb, par->targinfo, |
db1a75bd | 210 | tcpmss_reverse_mtu(skb, PF_INET6), |
37c08387 | 211 | tcphoff, |
cdd289a2 PM |
212 | sizeof(*ipv6h) + sizeof(struct tcphdr)); |
213 | if (ret < 0) | |
214 | return NF_DROP; | |
215 | if (ret > 0) { | |
3db05fea | 216 | ipv6h = ipv6_hdr(skb); |
cdd289a2 PM |
217 | ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret); |
218 | } | |
219 | return XT_CONTINUE; | |
220 | } | |
221 | #endif | |
222 | ||
223 | #define TH_SYN 0x02 | |
224 | ||
225 | /* Must specify -p tcp --syn */ | |
e1931b78 | 226 | static inline bool find_syn_match(const struct xt_entry_match *m) |
cdd289a2 PM |
227 | { |
228 | const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data; | |
229 | ||
230 | if (strcmp(m->u.kernel.match->name, "tcp") == 0 && | |
231 | tcpinfo->flg_cmp & TH_SYN && | |
232 | !(tcpinfo->invflags & XT_TCP_INV_FLAGS)) | |
e1931b78 | 233 | return true; |
cdd289a2 | 234 | |
e1931b78 | 235 | return false; |
cdd289a2 PM |
236 | } |
237 | ||
af5d6dc2 | 238 | static bool tcpmss_tg4_check(const struct xt_tgchk_param *par) |
cdd289a2 | 239 | { |
af5d6dc2 JE |
240 | const struct xt_tcpmss_info *info = par->targinfo; |
241 | const struct ipt_entry *e = par->entryinfo; | |
dcea992a | 242 | const struct xt_entry_match *ematch; |
cdd289a2 PM |
243 | |
244 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
af5d6dc2 | 245 | (par->hook_mask & ~((1 << NF_INET_FORWARD) | |
6e23ae2a PM |
246 | (1 << NF_INET_LOCAL_OUT) | |
247 | (1 << NF_INET_POST_ROUTING))) != 0) { | |
8bee4bad JE |
248 | pr_info("path-MTU clamping only supported in " |
249 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
e1931b78 | 250 | return false; |
cdd289a2 | 251 | } |
dcea992a JE |
252 | xt_ematch_foreach(ematch, e) |
253 | if (find_syn_match(ematch)) | |
254 | return true; | |
8bee4bad | 255 | pr_info("Only works on TCP SYN packets\n"); |
e1931b78 | 256 | return false; |
cdd289a2 PM |
257 | } |
258 | ||
259 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
af5d6dc2 | 260 | static bool tcpmss_tg6_check(const struct xt_tgchk_param *par) |
cdd289a2 | 261 | { |
af5d6dc2 JE |
262 | const struct xt_tcpmss_info *info = par->targinfo; |
263 | const struct ip6t_entry *e = par->entryinfo; | |
dcea992a | 264 | const struct xt_entry_match *ematch; |
cdd289a2 PM |
265 | |
266 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
af5d6dc2 | 267 | (par->hook_mask & ~((1 << NF_INET_FORWARD) | |
6e23ae2a PM |
268 | (1 << NF_INET_LOCAL_OUT) | |
269 | (1 << NF_INET_POST_ROUTING))) != 0) { | |
8bee4bad JE |
270 | pr_info("path-MTU clamping only supported in " |
271 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
e1931b78 | 272 | return false; |
cdd289a2 | 273 | } |
dcea992a JE |
274 | xt_ematch_foreach(ematch, e) |
275 | if (find_syn_match(ematch)) | |
276 | return true; | |
8bee4bad | 277 | pr_info("Only works on TCP SYN packets\n"); |
e1931b78 | 278 | return false; |
cdd289a2 PM |
279 | } |
280 | #endif | |
281 | ||
d3c5ee6d | 282 | static struct xt_target tcpmss_tg_reg[] __read_mostly = { |
cdd289a2 | 283 | { |
ee999d8b | 284 | .family = NFPROTO_IPV4, |
cdd289a2 | 285 | .name = "TCPMSS", |
d3c5ee6d JE |
286 | .checkentry = tcpmss_tg4_check, |
287 | .target = tcpmss_tg4, | |
cdd289a2 PM |
288 | .targetsize = sizeof(struct xt_tcpmss_info), |
289 | .proto = IPPROTO_TCP, | |
290 | .me = THIS_MODULE, | |
291 | }, | |
292 | #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE) | |
293 | { | |
ee999d8b | 294 | .family = NFPROTO_IPV6, |
cdd289a2 | 295 | .name = "TCPMSS", |
d3c5ee6d JE |
296 | .checkentry = tcpmss_tg6_check, |
297 | .target = tcpmss_tg6, | |
cdd289a2 PM |
298 | .targetsize = sizeof(struct xt_tcpmss_info), |
299 | .proto = IPPROTO_TCP, | |
300 | .me = THIS_MODULE, | |
301 | }, | |
302 | #endif | |
303 | }; | |
304 | ||
d3c5ee6d | 305 | static int __init tcpmss_tg_init(void) |
cdd289a2 | 306 | { |
d3c5ee6d | 307 | return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
cdd289a2 PM |
308 | } |
309 | ||
d3c5ee6d | 310 | static void __exit tcpmss_tg_exit(void) |
cdd289a2 | 311 | { |
d3c5ee6d | 312 | xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
cdd289a2 PM |
313 | } |
314 | ||
d3c5ee6d JE |
315 | module_init(tcpmss_tg_init); |
316 | module_exit(tcpmss_tg_exit); |