Merge master.kernel.org:/pub/scm/linux/kernel/git/acme/net-2.6
[linux-2.6] / net / ipv4 / netfilter / ipt_TCPMSS.c
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
14 #include <linux/ip.h>
15 #include <net/tcp.h>
16
17 #include <linux/netfilter_ipv4/ip_tables.h>
18 #include <linux/netfilter_ipv4/ipt_TCPMSS.h>
19
20 MODULE_LICENSE("GPL");
21 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
22 MODULE_DESCRIPTION("iptables TCP MSS modification module");
23
24 #if 0
25 #define DEBUGP printk
26 #else
27 #define DEBUGP(format, args...)
28 #endif
29
30 static u_int16_t
31 cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck)
32 {
33         u_int32_t diffs[] = { oldvalinv, newval };
34         return csum_fold(csum_partial((char *)diffs, sizeof(diffs),
35                                       oldcheck^0xFFFF));
36 }
37
38 static inline unsigned int
39 optlen(const u_int8_t *opt, unsigned int offset)
40 {
41         /* Beware zero-length options: make finite progress */
42         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) return 1;
43         else return opt[offset+1];
44 }
45
46 static unsigned int
47 ipt_tcpmss_target(struct sk_buff **pskb,
48                   const struct net_device *in,
49                   const struct net_device *out,
50                   unsigned int hooknum,
51                   const void *targinfo,
52                   void *userinfo)
53 {
54         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
55         struct tcphdr *tcph;
56         struct iphdr *iph;
57         u_int16_t tcplen, newtotlen, oldval, newmss;
58         unsigned int i;
59         u_int8_t *opt;
60
61         if (!skb_make_writable(pskb, (*pskb)->len))
62                 return NF_DROP;
63
64         if ((*pskb)->ip_summed == CHECKSUM_HW &&
65             skb_checksum_help(*pskb, out == NULL))
66                 return NF_DROP;
67
68         iph = (*pskb)->nh.iph;
69         tcplen = (*pskb)->len - iph->ihl*4;
70
71         tcph = (void *)iph + iph->ihl*4;
72
73         /* Since it passed flags test in tcp match, we know it is is
74            not a fragment, and has data >= tcp header length.  SYN
75            packets should not contain data: if they did, then we risk
76            running over MTU, sending Frag Needed and breaking things
77            badly. --RR */
78         if (tcplen != tcph->doff*4) {
79                 if (net_ratelimit())
80                         printk(KERN_ERR
81                                "ipt_tcpmss_target: bad length (%d bytes)\n",
82                                (*pskb)->len);
83                 return NF_DROP;
84         }
85
86         if(tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
87                 if(!(*pskb)->dst) {
88                         if (net_ratelimit())
89                                 printk(KERN_ERR
90                                         "ipt_tcpmss_target: no dst?! can't determine path-MTU\n");
91                         return NF_DROP; /* or IPT_CONTINUE ?? */
92                 }
93
94                 if(dst_mtu((*pskb)->dst) <= (sizeof(struct iphdr) + sizeof(struct tcphdr))) {
95                         if (net_ratelimit())
96                                 printk(KERN_ERR
97                                         "ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", dst_mtu((*pskb)->dst));
98                         return NF_DROP; /* or IPT_CONTINUE ?? */
99                 }
100
101                 newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) - sizeof(struct tcphdr);
102         } else
103                 newmss = tcpmssinfo->mss;
104
105         opt = (u_int8_t *)tcph;
106         for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)){
107                 if ((opt[i] == TCPOPT_MSS) &&
108                     ((tcph->doff*4 - i) >= TCPOLEN_MSS) &&
109                     (opt[i+1] == TCPOLEN_MSS)) {
110                         u_int16_t oldmss;
111
112                         oldmss = (opt[i+2] << 8) | opt[i+3];
113
114                         if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
115                                 (oldmss <= newmss))
116                                         return IPT_CONTINUE;
117
118                         opt[i+2] = (newmss & 0xff00) >> 8;
119                         opt[i+3] = (newmss & 0x00ff);
120
121                         tcph->check = cheat_check(htons(oldmss)^0xFFFF,
122                                                   htons(newmss),
123                                                   tcph->check);
124
125                         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
126                                "->%u.%u.%u.%u:%hu changed TCP MSS option"
127                                " (from %u to %u)\n", 
128                                NIPQUAD((*pskb)->nh.iph->saddr),
129                                ntohs(tcph->source),
130                                NIPQUAD((*pskb)->nh.iph->daddr),
131                                ntohs(tcph->dest),
132                                oldmss, newmss);
133                         goto retmodified;
134                 }
135         }
136
137         /*
138          * MSS Option not found ?! add it..
139          */
140         if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
141                 struct sk_buff *newskb;
142
143                 newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
144                                          TCPOLEN_MSS, GFP_ATOMIC);
145                 if (!newskb) {
146                         if (net_ratelimit())
147                                 printk(KERN_ERR "ipt_tcpmss_target:"
148                                        " unable to allocate larger skb\n");
149                         return NF_DROP;
150                 }
151
152                 kfree_skb(*pskb);
153                 *pskb = newskb;
154                 iph = (*pskb)->nh.iph;
155                 tcph = (void *)iph + iph->ihl*4;
156         }
157
158         skb_put((*pskb), TCPOLEN_MSS);
159
160         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
161         memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
162
163         tcph->check = cheat_check(htons(tcplen) ^ 0xFFFF,
164                                   htons(tcplen + TCPOLEN_MSS), tcph->check);
165         tcplen += TCPOLEN_MSS;
166
167         opt[0] = TCPOPT_MSS;
168         opt[1] = TCPOLEN_MSS;
169         opt[2] = (newmss & 0xff00) >> 8;
170         opt[3] = (newmss & 0x00ff);
171
172         tcph->check = cheat_check(~0, *((u_int32_t *)opt), tcph->check);
173
174         oldval = ((u_int16_t *)tcph)[6];
175         tcph->doff += TCPOLEN_MSS/4;
176         tcph->check = cheat_check(oldval ^ 0xFFFF,
177                                   ((u_int16_t *)tcph)[6], tcph->check);
178
179         newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
180         iph->check = cheat_check(iph->tot_len ^ 0xFFFF,
181                                  newtotlen, iph->check);
182         iph->tot_len = newtotlen;
183
184         DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
185                "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
186                NIPQUAD((*pskb)->nh.iph->saddr),
187                ntohs(tcph->source),
188                NIPQUAD((*pskb)->nh.iph->daddr),
189                ntohs(tcph->dest),
190                newmss);
191
192  retmodified:
193         return IPT_CONTINUE;
194 }
195
196 #define TH_SYN 0x02
197
198 static inline int find_syn_match(const struct ipt_entry_match *m)
199 {
200         const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
201
202         if (strcmp(m->u.kernel.match->name, "tcp") == 0
203             && (tcpinfo->flg_cmp & TH_SYN)
204             && !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
205                 return 1;
206
207         return 0;
208 }
209
210 /* Must specify -p tcp --syn/--tcp-flags SYN */
211 static int
212 ipt_tcpmss_checkentry(const char *tablename,
213                       const struct ipt_entry *e,
214                       void *targinfo,
215                       unsigned int targinfosize,
216                       unsigned int hook_mask)
217 {
218         const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
219
220         if (targinfosize != IPT_ALIGN(sizeof(struct ipt_tcpmss_info))) {
221                 DEBUGP("ipt_tcpmss_checkentry: targinfosize %u != %u\n",
222                        targinfosize, IPT_ALIGN(sizeof(struct ipt_tcpmss_info)));
223                 return 0;
224         }
225
226
227         if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) && 
228                         ((hook_mask & ~((1 << NF_IP_FORWARD)
229                                 | (1 << NF_IP_LOCAL_OUT)
230                                 | (1 << NF_IP_POST_ROUTING))) != 0)) {
231                 printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
232                 return 0;
233         }
234
235         if (e->ip.proto == IPPROTO_TCP
236             && !(e->ip.invflags & IPT_INV_PROTO)
237             && IPT_MATCH_ITERATE(e, find_syn_match))
238                 return 1;
239
240         printk("TCPMSS: Only works on TCP SYN packets\n");
241         return 0;
242 }
243
244 static struct ipt_target ipt_tcpmss_reg = {
245         .name           = "TCPMSS",
246         .target         = ipt_tcpmss_target,
247         .checkentry     = ipt_tcpmss_checkentry,
248         .me             = THIS_MODULE,
249 };
250
251 static int __init init(void)
252 {
253         return ipt_register_target(&ipt_tcpmss_reg);
254 }
255
256 static void __exit fini(void)
257 {
258         ipt_unregister_target(&ipt_tcpmss_reg);
259 }
260
261 module_init(init);
262 module_exit(fini);