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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> | |
f229f6ce | 5 | * Copyright (C) 2007 Patrick McHardy <kaber@trash.net> |
cdd289a2 PM |
6 | * |
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. | |
10 | */ | |
8bee4bad | 11 | #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt |
cdd289a2 PM |
12 | #include <linux/module.h> |
13 | #include <linux/skbuff.h> | |
14 | #include <linux/ip.h> | |
5a0e3ad6 | 15 | #include <linux/gfp.h> |
cdd289a2 PM |
16 | #include <linux/ipv6.h> |
17 | #include <linux/tcp.h> | |
37c08387 JE |
18 | #include <net/dst.h> |
19 | #include <net/flow.h> | |
cdd289a2 | 20 | #include <net/ipv6.h> |
37c08387 | 21 | #include <net/route.h> |
cdd289a2 PM |
22 | #include <net/tcp.h> |
23 | ||
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> | |
29 | ||
30 | MODULE_LICENSE("GPL"); | |
31 | MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); | |
2ae15b64 | 32 | MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment"); |
cdd289a2 PM |
33 | MODULE_ALIAS("ipt_TCPMSS"); |
34 | MODULE_ALIAS("ip6t_TCPMSS"); | |
35 | ||
36 | static inline unsigned int | |
37 | optlen(const u_int8_t *opt, unsigned int offset) | |
38 | { | |
39 | /* Beware zero-length options: make finite progress */ | |
40 | if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) | |
41 | return 1; | |
42 | else | |
43 | return opt[offset+1]; | |
44 | } | |
45 | ||
7722e0d1 G |
46 | static u_int32_t tcpmss_reverse_mtu(struct net *net, |
47 | const struct sk_buff *skb, | |
de1389b1 G |
48 | unsigned int family) |
49 | { | |
50 | struct flowi fl; | |
51 | const struct nf_afinfo *ai; | |
52 | struct rtable *rt = NULL; | |
53 | u_int32_t mtu = ~0U; | |
54 | ||
55 | if (family == PF_INET) { | |
56 | struct flowi4 *fl4 = &fl.u.ip4; | |
57 | memset(fl4, 0, sizeof(*fl4)); | |
58 | fl4->daddr = ip_hdr(skb)->saddr; | |
59 | } else { | |
60 | struct flowi6 *fl6 = &fl.u.ip6; | |
61 | ||
62 | memset(fl6, 0, sizeof(*fl6)); | |
63 | fl6->daddr = ipv6_hdr(skb)->saddr; | |
64 | } | |
65 | rcu_read_lock(); | |
66 | ai = nf_get_afinfo(family); | |
67 | if (ai != NULL) | |
7722e0d1 | 68 | ai->route(net, (struct dst_entry **)&rt, &fl, false); |
de1389b1 G |
69 | rcu_read_unlock(); |
70 | ||
71 | if (rt != NULL) { | |
72 | mtu = dst_mtu(&rt->dst); | |
73 | dst_release(&rt->dst); | |
74 | } | |
75 | return mtu; | |
76 | } | |
77 | ||
cdd289a2 | 78 | static int |
3db05fea | 79 | tcpmss_mangle_packet(struct sk_buff *skb, |
70d19f80 | 80 | const struct xt_action_param *par, |
de1389b1 | 81 | unsigned int family, |
cdd289a2 PM |
82 | unsigned int tcphoff, |
83 | unsigned int minlen) | |
84 | { | |
70d19f80 | 85 | const struct xt_tcpmss_info *info = par->targinfo; |
cdd289a2 | 86 | struct tcphdr *tcph; |
71ffe9c7 PNA |
87 | int len, tcp_hdrlen; |
88 | unsigned int i; | |
cdd289a2 PM |
89 | __be16 oldval; |
90 | u16 newmss; | |
91 | u8 *opt; | |
92 | ||
b396966c PO |
93 | /* This is a fragment, no TCP header is available */ |
94 | if (par->fragoff != 0) | |
1205e1fa | 95 | return 0; |
b396966c | 96 | |
3db05fea | 97 | if (!skb_make_writable(skb, skb->len)) |
cdd289a2 PM |
98 | return -1; |
99 | ||
71ffe9c7 PNA |
100 | len = skb->len - tcphoff; |
101 | if (len < (int)sizeof(struct tcphdr)) | |
102 | return -1; | |
103 | ||
3db05fea | 104 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); |
71ffe9c7 | 105 | tcp_hdrlen = tcph->doff * 4; |
cdd289a2 | 106 | |
71ffe9c7 | 107 | if (len < tcp_hdrlen) |
cdd289a2 | 108 | return -1; |
cdd289a2 PM |
109 | |
110 | if (info->mss == XT_TCPMSS_CLAMP_PMTU) { | |
613dbd95 | 111 | struct net *net = xt_net(par); |
7722e0d1 | 112 | unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family); |
50f4c7b7 | 113 | unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu); |
de1389b1 | 114 | |
50f4c7b7 | 115 | if (min_mtu <= minlen) { |
e87cc472 | 116 | net_err_ratelimited("unknown or invalid path-MTU (%u)\n", |
50f4c7b7 | 117 | min_mtu); |
cdd289a2 PM |
118 | return -1; |
119 | } | |
50f4c7b7 | 120 | newmss = min_mtu - minlen; |
cdd289a2 PM |
121 | } else |
122 | newmss = info->mss; | |
123 | ||
124 | opt = (u_int8_t *)tcph; | |
71ffe9c7 PNA |
125 | for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) { |
126 | if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) { | |
cdd289a2 PM |
127 | u_int16_t oldmss; |
128 | ||
129 | oldmss = (opt[i+2] << 8) | opt[i+3]; | |
130 | ||
17008064 BL |
131 | /* Never increase MSS, even when setting it, as |
132 | * doing so results in problems for hosts that rely | |
133 | * on MSS being set correctly. | |
134 | */ | |
135 | if (oldmss <= newmss) | |
cdd289a2 PM |
136 | return 0; |
137 | ||
138 | opt[i+2] = (newmss & 0xff00) >> 8; | |
7c4e36bc | 139 | opt[i+3] = newmss & 0x00ff; |
cdd289a2 | 140 | |
be0ea7d5 PM |
141 | inet_proto_csum_replace2(&tcph->check, skb, |
142 | htons(oldmss), htons(newmss), | |
4b048d6d | 143 | false); |
cdd289a2 PM |
144 | return 0; |
145 | } | |
146 | } | |
147 | ||
10a19939 | 148 | /* There is data after the header so the option can't be added |
71ffe9c7 PNA |
149 | * without moving it, and doing so may make the SYN packet |
150 | * itself too large. Accept the packet unmodified instead. | |
151 | */ | |
152 | if (len > tcp_hdrlen) | |
10a19939 SA |
153 | return 0; |
154 | ||
cdd289a2 PM |
155 | /* |
156 | * MSS Option not found ?! add it.. | |
157 | */ | |
3db05fea HX |
158 | if (skb_tailroom(skb) < TCPOLEN_MSS) { |
159 | if (pskb_expand_head(skb, 0, | |
160 | TCPOLEN_MSS - skb_tailroom(skb), | |
2ca7b0ac | 161 | GFP_ATOMIC)) |
cdd289a2 | 162 | return -1; |
3db05fea | 163 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); |
cdd289a2 PM |
164 | } |
165 | ||
3db05fea | 166 | skb_put(skb, TCPOLEN_MSS); |
cdd289a2 | 167 | |
70d19f80 PO |
168 | /* |
169 | * IPv4: RFC 1122 states "If an MSS option is not received at | |
170 | * connection setup, TCP MUST assume a default send MSS of 536". | |
171 | * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum | |
172 | * length IPv6 header of 60, ergo the default MSS value is 1220 | |
173 | * Since no MSS was provided, we must use the default values | |
409b545a | 174 | */ |
613dbd95 | 175 | if (xt_family(par) == NFPROTO_IPV4) |
70d19f80 PO |
176 | newmss = min(newmss, (u16)536); |
177 | else | |
178 | newmss = min(newmss, (u16)1220); | |
409b545a | 179 | |
cdd289a2 | 180 | opt = (u_int8_t *)tcph + sizeof(struct tcphdr); |
71ffe9c7 | 181 | memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr)); |
cdd289a2 | 182 | |
be0ea7d5 | 183 | inet_proto_csum_replace2(&tcph->check, skb, |
4b048d6d | 184 | htons(len), htons(len + TCPOLEN_MSS), true); |
cdd289a2 PM |
185 | opt[0] = TCPOPT_MSS; |
186 | opt[1] = TCPOLEN_MSS; | |
187 | opt[2] = (newmss & 0xff00) >> 8; | |
7c4e36bc | 188 | opt[3] = newmss & 0x00ff; |
cdd289a2 | 189 | |
4b048d6d | 190 | inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false); |
cdd289a2 PM |
191 | |
192 | oldval = ((__be16 *)tcph)[6]; | |
193 | tcph->doff += TCPOLEN_MSS/4; | |
be0ea7d5 | 194 | inet_proto_csum_replace2(&tcph->check, skb, |
4b048d6d | 195 | oldval, ((__be16 *)tcph)[6], false); |
cdd289a2 PM |
196 | return TCPOLEN_MSS; |
197 | } | |
198 | ||
199 | static unsigned int | |
4b560b44 | 200 | tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par) |
cdd289a2 | 201 | { |
3db05fea | 202 | struct iphdr *iph = ip_hdr(skb); |
cdd289a2 PM |
203 | __be16 newlen; |
204 | int ret; | |
205 | ||
70d19f80 | 206 | ret = tcpmss_mangle_packet(skb, par, |
de1389b1 | 207 | PF_INET, |
37c08387 | 208 | iph->ihl * 4, |
cdd289a2 PM |
209 | sizeof(*iph) + sizeof(struct tcphdr)); |
210 | if (ret < 0) | |
211 | return NF_DROP; | |
212 | if (ret > 0) { | |
3db05fea | 213 | iph = ip_hdr(skb); |
cdd289a2 | 214 | newlen = htons(ntohs(iph->tot_len) + ret); |
be0ea7d5 | 215 | csum_replace2(&iph->check, iph->tot_len, newlen); |
cdd289a2 PM |
216 | iph->tot_len = newlen; |
217 | } | |
218 | return XT_CONTINUE; | |
219 | } | |
220 | ||
c0cd1156 | 221 | #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) |
cdd289a2 | 222 | static unsigned int |
4b560b44 | 223 | tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par) |
cdd289a2 | 224 | { |
3db05fea | 225 | struct ipv6hdr *ipv6h = ipv6_hdr(skb); |
cdd289a2 | 226 | u8 nexthdr; |
d6b3347b | 227 | __be16 frag_off, oldlen, newlen; |
cdd289a2 PM |
228 | int tcphoff; |
229 | int ret; | |
230 | ||
231 | nexthdr = ipv6h->nexthdr; | |
75f2811c | 232 | tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off); |
9dc0564e | 233 | if (tcphoff < 0) |
cdd289a2 | 234 | return NF_DROP; |
70d19f80 | 235 | ret = tcpmss_mangle_packet(skb, par, |
de1389b1 | 236 | PF_INET6, |
37c08387 | 237 | tcphoff, |
cdd289a2 PM |
238 | sizeof(*ipv6h) + sizeof(struct tcphdr)); |
239 | if (ret < 0) | |
240 | return NF_DROP; | |
241 | if (ret > 0) { | |
3db05fea | 242 | ipv6h = ipv6_hdr(skb); |
d6b3347b ED |
243 | oldlen = ipv6h->payload_len; |
244 | newlen = htons(ntohs(oldlen) + ret); | |
245 | if (skb->ip_summed == CHECKSUM_COMPLETE) | |
246 | skb->csum = csum_add(csum_sub(skb->csum, oldlen), | |
247 | newlen); | |
248 | ipv6h->payload_len = newlen; | |
cdd289a2 PM |
249 | } |
250 | return XT_CONTINUE; | |
251 | } | |
252 | #endif | |
253 | ||
cdd289a2 | 254 | /* Must specify -p tcp --syn */ |
e1931b78 | 255 | static inline bool find_syn_match(const struct xt_entry_match *m) |
cdd289a2 PM |
256 | { |
257 | const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data; | |
258 | ||
259 | if (strcmp(m->u.kernel.match->name, "tcp") == 0 && | |
a3433f35 | 260 | tcpinfo->flg_cmp & TCPHDR_SYN && |
cdd289a2 | 261 | !(tcpinfo->invflags & XT_TCP_INV_FLAGS)) |
e1931b78 | 262 | return true; |
cdd289a2 | 263 | |
e1931b78 | 264 | return false; |
cdd289a2 PM |
265 | } |
266 | ||
135367b8 | 267 | static int tcpmss_tg4_check(const struct xt_tgchk_param *par) |
cdd289a2 | 268 | { |
af5d6dc2 JE |
269 | const struct xt_tcpmss_info *info = par->targinfo; |
270 | const struct ipt_entry *e = par->entryinfo; | |
dcea992a | 271 | const struct xt_entry_match *ematch; |
cdd289a2 PM |
272 | |
273 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
af5d6dc2 | 274 | (par->hook_mask & ~((1 << NF_INET_FORWARD) | |
6e23ae2a PM |
275 | (1 << NF_INET_LOCAL_OUT) | |
276 | (1 << NF_INET_POST_ROUTING))) != 0) { | |
8bee4bad JE |
277 | pr_info("path-MTU clamping only supported in " |
278 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
d6b00a53 | 279 | return -EINVAL; |
cdd289a2 | 280 | } |
55917a21 PNA |
281 | if (par->nft_compat) |
282 | return 0; | |
283 | ||
dcea992a JE |
284 | xt_ematch_foreach(ematch, e) |
285 | if (find_syn_match(ematch)) | |
d6b00a53 | 286 | return 0; |
8bee4bad | 287 | pr_info("Only works on TCP SYN packets\n"); |
d6b00a53 | 288 | return -EINVAL; |
cdd289a2 PM |
289 | } |
290 | ||
c0cd1156 | 291 | #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) |
135367b8 | 292 | static int tcpmss_tg6_check(const struct xt_tgchk_param *par) |
cdd289a2 | 293 | { |
af5d6dc2 JE |
294 | const struct xt_tcpmss_info *info = par->targinfo; |
295 | const struct ip6t_entry *e = par->entryinfo; | |
dcea992a | 296 | const struct xt_entry_match *ematch; |
cdd289a2 PM |
297 | |
298 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && | |
af5d6dc2 | 299 | (par->hook_mask & ~((1 << NF_INET_FORWARD) | |
6e23ae2a PM |
300 | (1 << NF_INET_LOCAL_OUT) | |
301 | (1 << NF_INET_POST_ROUTING))) != 0) { | |
8bee4bad JE |
302 | pr_info("path-MTU clamping only supported in " |
303 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); | |
d6b00a53 | 304 | return -EINVAL; |
cdd289a2 | 305 | } |
55917a21 PNA |
306 | if (par->nft_compat) |
307 | return 0; | |
308 | ||
dcea992a JE |
309 | xt_ematch_foreach(ematch, e) |
310 | if (find_syn_match(ematch)) | |
d6b00a53 | 311 | return 0; |
8bee4bad | 312 | pr_info("Only works on TCP SYN packets\n"); |
d6b00a53 | 313 | return -EINVAL; |
cdd289a2 PM |
314 | } |
315 | #endif | |
316 | ||
d3c5ee6d | 317 | static struct xt_target tcpmss_tg_reg[] __read_mostly = { |
cdd289a2 | 318 | { |
ee999d8b | 319 | .family = NFPROTO_IPV4, |
cdd289a2 | 320 | .name = "TCPMSS", |
d3c5ee6d JE |
321 | .checkentry = tcpmss_tg4_check, |
322 | .target = tcpmss_tg4, | |
cdd289a2 PM |
323 | .targetsize = sizeof(struct xt_tcpmss_info), |
324 | .proto = IPPROTO_TCP, | |
325 | .me = THIS_MODULE, | |
326 | }, | |
c0cd1156 | 327 | #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) |
cdd289a2 | 328 | { |
ee999d8b | 329 | .family = NFPROTO_IPV6, |
cdd289a2 | 330 | .name = "TCPMSS", |
d3c5ee6d JE |
331 | .checkentry = tcpmss_tg6_check, |
332 | .target = tcpmss_tg6, | |
cdd289a2 PM |
333 | .targetsize = sizeof(struct xt_tcpmss_info), |
334 | .proto = IPPROTO_TCP, | |
335 | .me = THIS_MODULE, | |
336 | }, | |
337 | #endif | |
338 | }; | |
339 | ||
d3c5ee6d | 340 | static int __init tcpmss_tg_init(void) |
cdd289a2 | 341 | { |
d3c5ee6d | 342 | return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
cdd289a2 PM |
343 | } |
344 | ||
d3c5ee6d | 345 | static void __exit tcpmss_tg_exit(void) |
cdd289a2 | 346 | { |
d3c5ee6d | 347 | xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
cdd289a2 PM |
348 | } |
349 | ||
d3c5ee6d JE |
350 | module_init(tcpmss_tg_init); |
351 | module_exit(tcpmss_tg_exit); |