2 * This is a module which is used for setting the MSS option in TCP packets.
4 * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
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.
11 #include <linux/module.h>
12 #include <linux/skbuff.h>
17 #include <linux/netfilter_ipv4/ip_tables.h>
18 #include <linux/netfilter_ipv4/ipt_TCPMSS.h>
20 MODULE_LICENSE("GPL");
21 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
22 MODULE_DESCRIPTION("iptables TCP MSS modification module");
27 #define DEBUGP(format, args...)
31 cheat_check(u_int32_t oldvalinv
, u_int32_t newval
, u_int16_t oldcheck
)
33 u_int32_t diffs
[] = { oldvalinv
, newval
};
34 return csum_fold(csum_partial((char *)diffs
, sizeof(diffs
),
38 static inline unsigned int
39 optlen(const u_int8_t
*opt
, unsigned int offset
)
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];
47 ipt_tcpmss_target(struct sk_buff
**pskb
,
48 const struct net_device
*in
,
49 const struct net_device
*out
,
51 const struct xt_target
*target
,
55 const struct ipt_tcpmss_info
*tcpmssinfo
= targinfo
;
58 u_int16_t tcplen
, newtotlen
, oldval
, newmss
;
62 if (!skb_make_writable(pskb
, (*pskb
)->len
))
65 if (((*pskb
)->ip_summed
== CHECKSUM_PARTIAL
||
66 (*pskb
)->ip_summed
== CHECKSUM_COMPLETE
) &&
67 skb_checksum_help(*pskb
))
70 iph
= (*pskb
)->nh
.iph
;
71 tcplen
= (*pskb
)->len
- iph
->ihl
*4;
73 tcph
= (void *)iph
+ iph
->ihl
*4;
75 /* Since it passed flags test in tcp match, we know it is is
76 not a fragment, and has data >= tcp header length. SYN
77 packets should not contain data: if they did, then we risk
78 running over MTU, sending Frag Needed and breaking things
80 if (tcplen
!= tcph
->doff
*4) {
83 "ipt_tcpmss_target: bad length (%d bytes)\n",
88 if(tcpmssinfo
->mss
== IPT_TCPMSS_CLAMP_PMTU
) {
92 "ipt_tcpmss_target: no dst?! can't determine path-MTU\n");
93 return NF_DROP
; /* or IPT_CONTINUE ?? */
96 if(dst_mtu((*pskb
)->dst
) <= (sizeof(struct iphdr
) + sizeof(struct tcphdr
))) {
99 "ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", dst_mtu((*pskb
)->dst
));
100 return NF_DROP
; /* or IPT_CONTINUE ?? */
103 newmss
= dst_mtu((*pskb
)->dst
) - sizeof(struct iphdr
) - sizeof(struct tcphdr
);
105 newmss
= tcpmssinfo
->mss
;
107 opt
= (u_int8_t
*)tcph
;
108 for (i
= sizeof(struct tcphdr
); i
< tcph
->doff
*4; i
+= optlen(opt
, i
)){
109 if ((opt
[i
] == TCPOPT_MSS
) &&
110 ((tcph
->doff
*4 - i
) >= TCPOLEN_MSS
) &&
111 (opt
[i
+1] == TCPOLEN_MSS
)) {
114 oldmss
= (opt
[i
+2] << 8) | opt
[i
+3];
116 if((tcpmssinfo
->mss
== IPT_TCPMSS_CLAMP_PMTU
) &&
120 opt
[i
+2] = (newmss
& 0xff00) >> 8;
121 opt
[i
+3] = (newmss
& 0x00ff);
123 tcph
->check
= cheat_check(htons(oldmss
)^0xFFFF,
127 DEBUGP(KERN_INFO
"ipt_tcpmss_target: %u.%u.%u.%u:%hu"
128 "->%u.%u.%u.%u:%hu changed TCP MSS option"
129 " (from %u to %u)\n",
130 NIPQUAD((*pskb
)->nh
.iph
->saddr
),
132 NIPQUAD((*pskb
)->nh
.iph
->daddr
),
140 * MSS Option not found ?! add it..
142 if (skb_tailroom((*pskb
)) < TCPOLEN_MSS
) {
143 struct sk_buff
*newskb
;
145 newskb
= skb_copy_expand(*pskb
, skb_headroom(*pskb
),
146 TCPOLEN_MSS
, GFP_ATOMIC
);
149 printk(KERN_ERR
"ipt_tcpmss_target:"
150 " unable to allocate larger skb\n");
156 iph
= (*pskb
)->nh
.iph
;
157 tcph
= (void *)iph
+ iph
->ihl
*4;
160 skb_put((*pskb
), TCPOLEN_MSS
);
162 opt
= (u_int8_t
*)tcph
+ sizeof(struct tcphdr
);
163 memmove(opt
+ TCPOLEN_MSS
, opt
, tcplen
- sizeof(struct tcphdr
));
165 tcph
->check
= cheat_check(htons(tcplen
) ^ 0xFFFF,
166 htons(tcplen
+ TCPOLEN_MSS
), tcph
->check
);
167 tcplen
+= TCPOLEN_MSS
;
170 opt
[1] = TCPOLEN_MSS
;
171 opt
[2] = (newmss
& 0xff00) >> 8;
172 opt
[3] = (newmss
& 0x00ff);
174 tcph
->check
= cheat_check(~0, *((u_int32_t
*)opt
), tcph
->check
);
176 oldval
= ((u_int16_t
*)tcph
)[6];
177 tcph
->doff
+= TCPOLEN_MSS
/4;
178 tcph
->check
= cheat_check(oldval
^ 0xFFFF,
179 ((u_int16_t
*)tcph
)[6], tcph
->check
);
181 newtotlen
= htons(ntohs(iph
->tot_len
) + TCPOLEN_MSS
);
182 iph
->check
= cheat_check(iph
->tot_len
^ 0xFFFF,
183 newtotlen
, iph
->check
);
184 iph
->tot_len
= newtotlen
;
186 DEBUGP(KERN_INFO
"ipt_tcpmss_target: %u.%u.%u.%u:%hu"
187 "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
188 NIPQUAD((*pskb
)->nh
.iph
->saddr
),
190 NIPQUAD((*pskb
)->nh
.iph
->daddr
),
200 static inline int find_syn_match(const struct ipt_entry_match
*m
)
202 const struct ipt_tcp
*tcpinfo
= (const struct ipt_tcp
*)m
->data
;
204 if (strcmp(m
->u
.kernel
.match
->name
, "tcp") == 0
205 && (tcpinfo
->flg_cmp
& TH_SYN
)
206 && !(tcpinfo
->invflags
& IPT_TCP_INV_FLAGS
))
212 /* Must specify -p tcp --syn/--tcp-flags SYN */
214 ipt_tcpmss_checkentry(const char *tablename
,
216 const struct xt_target
*target
,
218 unsigned int targinfosize
,
219 unsigned int hook_mask
)
221 const struct ipt_tcpmss_info
*tcpmssinfo
= targinfo
;
222 const struct ipt_entry
*e
= e_void
;
224 if((tcpmssinfo
->mss
== IPT_TCPMSS_CLAMP_PMTU
) &&
225 ((hook_mask
& ~((1 << NF_IP_FORWARD
)
226 | (1 << NF_IP_LOCAL_OUT
)
227 | (1 << NF_IP_POST_ROUTING
))) != 0)) {
228 printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
232 if (IPT_MATCH_ITERATE(e
, find_syn_match
))
234 printk("TCPMSS: Only works on TCP SYN packets\n");
238 static struct ipt_target ipt_tcpmss_reg
= {
240 .target
= ipt_tcpmss_target
,
241 .targetsize
= sizeof(struct ipt_tcpmss_info
),
242 .proto
= IPPROTO_TCP
,
243 .checkentry
= ipt_tcpmss_checkentry
,
247 static int __init
ipt_tcpmss_init(void)
249 return ipt_register_target(&ipt_tcpmss_reg
);
252 static void __exit
ipt_tcpmss_fini(void)
254 ipt_unregister_target(&ipt_tcpmss_reg
);
257 module_init(ipt_tcpmss_init
);
258 module_exit(ipt_tcpmss_fini
);