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11#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
12#include <linux/module.h>
13#include <linux/skbuff.h>
14#include <linux/ip.h>
15#include <linux/gfp.h>
16#include <linux/ipv6.h>
17#include <linux/tcp.h>
18#include <net/dst.h>
19#include <net/flow.h>
20#include <net/ipv6.h>
21#include <net/route.h>
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
30MODULE_LICENSE("GPL");
31MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
32MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
33MODULE_ALIAS("ipt_TCPMSS");
34MODULE_ALIAS("ip6t_TCPMSS");
35
36static inline unsigned int
37optlen(const u_int8_t *opt, unsigned int offset)
38{
39
40 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
41 return 1;
42 else
43 return opt[offset+1];
44}
45
46static int
47tcpmss_mangle_packet(struct sk_buff *skb,
48 const struct xt_action_param *par,
49 unsigned int in_mtu,
50 unsigned int tcphoff,
51 unsigned int minlen)
52{
53 const struct xt_tcpmss_info *info = par->targinfo;
54 struct tcphdr *tcph;
55 int len, tcp_hdrlen;
56 unsigned int i;
57 __be16 oldval;
58 u16 newmss;
59 u8 *opt;
60
61
62 if (par->fragoff != 0)
63 return 0;
64
65 if (!skb_make_writable(skb, skb->len))
66 return -1;
67
68 len = skb->len - tcphoff;
69 if (len < (int)sizeof(struct tcphdr))
70 return -1;
71
72 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
73 tcp_hdrlen = tcph->doff * 4;
74
75 if (len < tcp_hdrlen)
76 return -1;
77
78 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
79 if (dst_mtu(skb_dst(skb)) <= minlen) {
80 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
81 dst_mtu(skb_dst(skb)));
82 return -1;
83 }
84 if (in_mtu <= minlen) {
85 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
86 in_mtu);
87 return -1;
88 }
89 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
90 } else
91 newmss = info->mss;
92
93 opt = (u_int8_t *)tcph;
94 for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
95 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
96 u_int16_t oldmss;
97
98 oldmss = (opt[i+2] << 8) | opt[i+3];
99
100
101
102
103
104 if (oldmss <= newmss)
105 return 0;
106
107 opt[i+2] = (newmss & 0xff00) >> 8;
108 opt[i+3] = newmss & 0x00ff;
109
110 inet_proto_csum_replace2(&tcph->check, skb,
111 htons(oldmss), htons(newmss),
112 0);
113 return 0;
114 }
115 }
116
117
118
119
120
121 if (len > tcp_hdrlen)
122 return 0;
123
124
125
126
127 if (skb_tailroom(skb) < TCPOLEN_MSS) {
128 if (pskb_expand_head(skb, 0,
129 TCPOLEN_MSS - skb_tailroom(skb),
130 GFP_ATOMIC))
131 return -1;
132 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
133 }
134
135 skb_put(skb, TCPOLEN_MSS);
136
137
138
139
140
141
142
143
144 if (par->family == NFPROTO_IPV4)
145 newmss = min(newmss, (u16)536);
146 else
147 newmss = min(newmss, (u16)1220);
148
149 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
150 memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
151
152 inet_proto_csum_replace2(&tcph->check, skb,
153 htons(len), htons(len + TCPOLEN_MSS), 1);
154 opt[0] = TCPOPT_MSS;
155 opt[1] = TCPOLEN_MSS;
156 opt[2] = (newmss & 0xff00) >> 8;
157 opt[3] = newmss & 0x00ff;
158
159 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
160
161 oldval = ((__be16 *)tcph)[6];
162 tcph->doff += TCPOLEN_MSS/4;
163 inet_proto_csum_replace2(&tcph->check, skb,
164 oldval, ((__be16 *)tcph)[6], 0);
165 return TCPOLEN_MSS;
166}
167
168static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
169 unsigned int family)
170{
171 struct flowi fl;
172 const struct nf_afinfo *ai;
173 struct rtable *rt = NULL;
174 u_int32_t mtu = ~0U;
175
176 if (family == PF_INET) {
177 struct flowi4 *fl4 = &fl.u.ip4;
178 memset(fl4, 0, sizeof(*fl4));
179 fl4->daddr = ip_hdr(skb)->saddr;
180 } else {
181 struct flowi6 *fl6 = &fl.u.ip6;
182
183 memset(fl6, 0, sizeof(*fl6));
184 fl6->daddr = ipv6_hdr(skb)->saddr;
185 }
186 rcu_read_lock();
187 ai = nf_get_afinfo(family);
188 if (ai != NULL)
189 ai->route(&init_net, (struct dst_entry **)&rt, &fl, false);
190 rcu_read_unlock();
191
192 if (rt != NULL) {
193 mtu = dst_mtu(&rt->dst);
194 dst_release(&rt->dst);
195 }
196 return mtu;
197}
198
199static unsigned int
200tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
201{
202 struct iphdr *iph = ip_hdr(skb);
203 __be16 newlen;
204 int ret;
205
206 ret = tcpmss_mangle_packet(skb, par,
207 tcpmss_reverse_mtu(skb, PF_INET),
208 iph->ihl * 4,
209 sizeof(*iph) + sizeof(struct tcphdr));
210 if (ret < 0)
211 return NF_DROP;
212 if (ret > 0) {
213 iph = ip_hdr(skb);
214 newlen = htons(ntohs(iph->tot_len) + ret);
215 csum_replace2(&iph->check, iph->tot_len, newlen);
216 iph->tot_len = newlen;
217 }
218 return XT_CONTINUE;
219}
220
221#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
222static unsigned int
223tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
224{
225 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
226 u8 nexthdr;
227 __be16 frag_off;
228 int tcphoff;
229 int ret;
230
231 nexthdr = ipv6h->nexthdr;
232 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
233 if (tcphoff < 0)
234 return NF_DROP;
235 ret = tcpmss_mangle_packet(skb, par,
236 tcpmss_reverse_mtu(skb, PF_INET6),
237 tcphoff,
238 sizeof(*ipv6h) + sizeof(struct tcphdr));
239 if (ret < 0)
240 return NF_DROP;
241 if (ret > 0) {
242 ipv6h = ipv6_hdr(skb);
243 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
244 }
245 return XT_CONTINUE;
246}
247#endif
248
249
250static inline bool find_syn_match(const struct xt_entry_match *m)
251{
252 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
253
254 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
255 tcpinfo->flg_cmp & TCPHDR_SYN &&
256 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
257 return true;
258
259 return false;
260}
261
262static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
263{
264 const struct xt_tcpmss_info *info = par->targinfo;
265 const struct ipt_entry *e = par->entryinfo;
266 const struct xt_entry_match *ematch;
267
268 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
269 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
270 (1 << NF_INET_LOCAL_OUT) |
271 (1 << NF_INET_POST_ROUTING))) != 0) {
272 pr_info("path-MTU clamping only supported in "
273 "FORWARD, OUTPUT and POSTROUTING hooks\n");
274 return -EINVAL;
275 }
276 xt_ematch_foreach(ematch, e)
277 if (find_syn_match(ematch))
278 return 0;
279 pr_info("Only works on TCP SYN packets\n");
280 return -EINVAL;
281}
282
283#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
284static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
285{
286 const struct xt_tcpmss_info *info = par->targinfo;
287 const struct ip6t_entry *e = par->entryinfo;
288 const struct xt_entry_match *ematch;
289
290 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
291 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
292 (1 << NF_INET_LOCAL_OUT) |
293 (1 << NF_INET_POST_ROUTING))) != 0) {
294 pr_info("path-MTU clamping only supported in "
295 "FORWARD, OUTPUT and POSTROUTING hooks\n");
296 return -EINVAL;
297 }
298 xt_ematch_foreach(ematch, e)
299 if (find_syn_match(ematch))
300 return 0;
301 pr_info("Only works on TCP SYN packets\n");
302 return -EINVAL;
303}
304#endif
305
306static struct xt_target tcpmss_tg_reg[] __read_mostly = {
307 {
308 .family = NFPROTO_IPV4,
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 IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
317 {
318 .family = NFPROTO_IPV6,
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);
341
