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_HW
&&
66 skb_checksum_help(*pskb
, out
== NULL
))
69 iph
= (*pskb
)->nh
.iph
;
70 tcplen
= (*pskb
)->len
- iph
->ihl
*4;
72 tcph
= (void *)iph
+ iph
->ihl
*4;
74 /* Since it passed flags test in tcp match, we know it is is
75 not a fragment, and has data >= tcp header length. SYN
76 packets should not contain data: if they did, then we risk
77 running over MTU, sending Frag Needed and breaking things
79 if (tcplen
!= tcph
->doff
*4) {
82 "ipt_tcpmss_target: bad length (%d bytes)\n",
87 if(tcpmssinfo
->mss
== IPT_TCPMSS_CLAMP_PMTU
) {
91 "ipt_tcpmss_target: no dst?! can't determine path-MTU\n");
92 return NF_DROP
; /* or IPT_CONTINUE ?? */
95 if(dst_mtu((*pskb
)->dst
) <= (sizeof(struct iphdr
) + sizeof(struct tcphdr
))) {
98 "ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", dst_mtu((*pskb
)->dst
));
99 return NF_DROP
; /* or IPT_CONTINUE ?? */
102 newmss
= dst_mtu((*pskb
)->dst
) - sizeof(struct iphdr
) - sizeof(struct tcphdr
);
104 newmss
= tcpmssinfo
->mss
;
106 opt
= (u_int8_t
*)tcph
;
107 for (i
= sizeof(struct tcphdr
); i
< tcph
->doff
*4; i
+= optlen(opt
, i
)){
108 if ((opt
[i
] == TCPOPT_MSS
) &&
109 ((tcph
->doff
*4 - i
) >= TCPOLEN_MSS
) &&
110 (opt
[i
+1] == TCPOLEN_MSS
)) {
113 oldmss
= (opt
[i
+2] << 8) | opt
[i
+3];
115 if((tcpmssinfo
->mss
== IPT_TCPMSS_CLAMP_PMTU
) &&
119 opt
[i
+2] = (newmss
& 0xff00) >> 8;
120 opt
[i
+3] = (newmss
& 0x00ff);
122 tcph
->check
= cheat_check(htons(oldmss
)^0xFFFF,
126 DEBUGP(KERN_INFO
"ipt_tcpmss_target: %u.%u.%u.%u:%hu"
127 "->%u.%u.%u.%u:%hu changed TCP MSS option"
128 " (from %u to %u)\n",
129 NIPQUAD((*pskb
)->nh
.iph
->saddr
),
131 NIPQUAD((*pskb
)->nh
.iph
->daddr
),
139 * MSS Option not found ?! add it..
141 if (skb_tailroom((*pskb
)) < TCPOLEN_MSS
) {
142 struct sk_buff
*newskb
;
144 newskb
= skb_copy_expand(*pskb
, skb_headroom(*pskb
),
145 TCPOLEN_MSS
, GFP_ATOMIC
);
148 printk(KERN_ERR
"ipt_tcpmss_target:"
149 " unable to allocate larger skb\n");
155 iph
= (*pskb
)->nh
.iph
;
156 tcph
= (void *)iph
+ iph
->ihl
*4;
159 skb_put((*pskb
), TCPOLEN_MSS
);
161 opt
= (u_int8_t
*)tcph
+ sizeof(struct tcphdr
);
162 memmove(opt
+ TCPOLEN_MSS
, opt
, tcplen
- sizeof(struct tcphdr
));
164 tcph
->check
= cheat_check(htons(tcplen
) ^ 0xFFFF,
165 htons(tcplen
+ TCPOLEN_MSS
), tcph
->check
);
166 tcplen
+= TCPOLEN_MSS
;
169 opt
[1] = TCPOLEN_MSS
;
170 opt
[2] = (newmss
& 0xff00) >> 8;
171 opt
[3] = (newmss
& 0x00ff);
173 tcph
->check
= cheat_check(~0, *((u_int32_t
*)opt
), tcph
->check
);
175 oldval
= ((u_int16_t
*)tcph
)[6];
176 tcph
->doff
+= TCPOLEN_MSS
/4;
177 tcph
->check
= cheat_check(oldval
^ 0xFFFF,
178 ((u_int16_t
*)tcph
)[6], tcph
->check
);
180 newtotlen
= htons(ntohs(iph
->tot_len
) + TCPOLEN_MSS
);
181 iph
->check
= cheat_check(iph
->tot_len
^ 0xFFFF,
182 newtotlen
, iph
->check
);
183 iph
->tot_len
= newtotlen
;
185 DEBUGP(KERN_INFO
"ipt_tcpmss_target: %u.%u.%u.%u:%hu"
186 "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
187 NIPQUAD((*pskb
)->nh
.iph
->saddr
),
189 NIPQUAD((*pskb
)->nh
.iph
->daddr
),
199 static inline int find_syn_match(const struct ipt_entry_match
*m
)
201 const struct ipt_tcp
*tcpinfo
= (const struct ipt_tcp
*)m
->data
;
203 if (strcmp(m
->u
.kernel
.match
->name
, "tcp") == 0
204 && (tcpinfo
->flg_cmp
& TH_SYN
)
205 && !(tcpinfo
->invflags
& IPT_TCP_INV_FLAGS
))
211 /* Must specify -p tcp --syn/--tcp-flags SYN */
213 ipt_tcpmss_checkentry(const char *tablename
,
215 const struct xt_target
*target
,
217 unsigned int targinfosize
,
218 unsigned int hook_mask
)
220 const struct ipt_tcpmss_info
*tcpmssinfo
= targinfo
;
221 const struct ipt_entry
*e
= e_void
;
223 if((tcpmssinfo
->mss
== IPT_TCPMSS_CLAMP_PMTU
) &&
224 ((hook_mask
& ~((1 << NF_IP_FORWARD
)
225 | (1 << NF_IP_LOCAL_OUT
)
226 | (1 << NF_IP_POST_ROUTING
))) != 0)) {
227 printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
231 if (IPT_MATCH_ITERATE(e
, find_syn_match
))
233 printk("TCPMSS: Only works on TCP SYN packets\n");
237 static struct ipt_target ipt_tcpmss_reg
= {
239 .target
= ipt_tcpmss_target
,
240 .targetsize
= sizeof(struct ipt_tcpmss_info
),
241 .proto
= IPPROTO_TCP
,
242 .checkentry
= ipt_tcpmss_checkentry
,
246 static int __init
init(void)
248 return ipt_register_target(&ipt_tcpmss_reg
);
251 static void __exit
fini(void)
253 ipt_unregister_target(&ipt_tcpmss_reg
);