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>
14 #include <linux/ipv6.h>
15 #include <linux/tcp.h>
19 #include <linux/netfilter_ipv4/ip_tables.h>
20 #include <linux/netfilter_ipv6/ip6_tables.h>
21 #include <linux/netfilter/x_tables.h>
22 #include <linux/netfilter/xt_tcpudp.h>
23 #include <linux/netfilter/xt_TCPMSS.h>
25 MODULE_LICENSE("GPL");
26 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
27 MODULE_DESCRIPTION("x_tables TCP MSS modification module");
28 MODULE_ALIAS("ipt_TCPMSS");
29 MODULE_ALIAS("ip6t_TCPMSS");
31 static inline unsigned int
32 optlen(const u_int8_t
*opt
, unsigned int offset
)
34 /* Beware zero-length options: make finite progress */
35 if (opt
[offset
] <= TCPOPT_NOP
|| opt
[offset
+1] == 0)
42 tcpmss_mangle_packet(struct sk_buff
*skb
,
43 const struct xt_tcpmss_info
*info
,
48 unsigned int tcplen
, i
;
53 if (!skb_make_writable(skb
, skb
->len
))
56 tcplen
= skb
->len
- tcphoff
;
57 tcph
= (struct tcphdr
*)(skb_network_header(skb
) + tcphoff
);
59 /* Since it passed flags test in tcp match, we know it is is
60 not a fragment, and has data >= tcp header length. SYN
61 packets should not contain data: if they did, then we risk
62 running over MTU, sending Frag Needed and breaking things
64 if (tcplen
!= tcph
->doff
*4) {
66 printk(KERN_ERR
"xt_TCPMSS: bad length (%u bytes)\n",
71 if (info
->mss
== XT_TCPMSS_CLAMP_PMTU
) {
72 if (dst_mtu(skb
->dst
) <= minlen
) {
74 printk(KERN_ERR
"xt_TCPMSS: "
75 "unknown or invalid path-MTU (%u)\n",
79 newmss
= dst_mtu(skb
->dst
) - minlen
;
83 opt
= (u_int8_t
*)tcph
;
84 for (i
= sizeof(struct tcphdr
); i
< tcph
->doff
*4; i
+= optlen(opt
, i
)) {
85 if (opt
[i
] == TCPOPT_MSS
&& tcph
->doff
*4 - i
>= TCPOLEN_MSS
&&
86 opt
[i
+1] == TCPOLEN_MSS
) {
89 oldmss
= (opt
[i
+2] << 8) | opt
[i
+3];
91 if (info
->mss
== XT_TCPMSS_CLAMP_PMTU
&&
95 opt
[i
+2] = (newmss
& 0xff00) >> 8;
96 opt
[i
+3] = newmss
& 0x00ff;
98 inet_proto_csum_replace2(&tcph
->check
, skb
,
99 htons(oldmss
), htons(newmss
),
106 * MSS Option not found ?! add it..
108 if (skb_tailroom(skb
) < TCPOLEN_MSS
) {
109 if (pskb_expand_head(skb
, 0,
110 TCPOLEN_MSS
- skb_tailroom(skb
),
113 tcph
= (struct tcphdr
*)(skb_network_header(skb
) + tcphoff
);
116 skb_put(skb
, TCPOLEN_MSS
);
118 opt
= (u_int8_t
*)tcph
+ sizeof(struct tcphdr
);
119 memmove(opt
+ TCPOLEN_MSS
, opt
, tcplen
- sizeof(struct tcphdr
));
121 inet_proto_csum_replace2(&tcph
->check
, skb
,
122 htons(tcplen
), htons(tcplen
+ TCPOLEN_MSS
), 1);
124 opt
[1] = TCPOLEN_MSS
;
125 opt
[2] = (newmss
& 0xff00) >> 8;
126 opt
[3] = newmss
& 0x00ff;
128 inet_proto_csum_replace4(&tcph
->check
, skb
, 0, *((__be32
*)opt
), 0);
130 oldval
= ((__be16
*)tcph
)[6];
131 tcph
->doff
+= TCPOLEN_MSS
/4;
132 inet_proto_csum_replace2(&tcph
->check
, skb
,
133 oldval
, ((__be16
*)tcph
)[6], 0);
138 tcpmss_tg4(struct sk_buff
*skb
, const struct net_device
*in
,
139 const struct net_device
*out
, unsigned int hooknum
,
140 const struct xt_target
*target
, const void *targinfo
)
142 struct iphdr
*iph
= ip_hdr(skb
);
146 ret
= tcpmss_mangle_packet(skb
, targinfo
, iph
->ihl
* 4,
147 sizeof(*iph
) + sizeof(struct tcphdr
));
152 newlen
= htons(ntohs(iph
->tot_len
) + ret
);
153 csum_replace2(&iph
->check
, iph
->tot_len
, newlen
);
154 iph
->tot_len
= newlen
;
159 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
161 tcpmss_tg6(struct sk_buff
*skb
, const struct net_device
*in
,
162 const struct net_device
*out
, unsigned int hooknum
,
163 const struct xt_target
*target
, const void *targinfo
)
165 struct ipv6hdr
*ipv6h
= ipv6_hdr(skb
);
170 nexthdr
= ipv6h
->nexthdr
;
171 tcphoff
= ipv6_skip_exthdr(skb
, sizeof(*ipv6h
), &nexthdr
);
174 ret
= tcpmss_mangle_packet(skb
, targinfo
, tcphoff
,
175 sizeof(*ipv6h
) + sizeof(struct tcphdr
));
179 ipv6h
= ipv6_hdr(skb
);
180 ipv6h
->payload_len
= htons(ntohs(ipv6h
->payload_len
) + ret
);
188 /* Must specify -p tcp --syn */
189 static inline bool find_syn_match(const struct xt_entry_match
*m
)
191 const struct xt_tcp
*tcpinfo
= (const struct xt_tcp
*)m
->data
;
193 if (strcmp(m
->u
.kernel
.match
->name
, "tcp") == 0 &&
194 tcpinfo
->flg_cmp
& TH_SYN
&&
195 !(tcpinfo
->invflags
& XT_TCP_INV_FLAGS
))
202 tcpmss_tg4_check(const char *tablename
, const void *entry
,
203 const struct xt_target
*target
, void *targinfo
,
204 unsigned int hook_mask
)
206 const struct xt_tcpmss_info
*info
= targinfo
;
207 const struct ipt_entry
*e
= entry
;
209 if (info
->mss
== XT_TCPMSS_CLAMP_PMTU
&&
210 (hook_mask
& ~((1 << NF_INET_FORWARD
) |
211 (1 << NF_INET_LOCAL_OUT
) |
212 (1 << NF_INET_POST_ROUTING
))) != 0) {
213 printk("xt_TCPMSS: path-MTU clamping only supported in "
214 "FORWARD, OUTPUT and POSTROUTING hooks\n");
217 if (IPT_MATCH_ITERATE(e
, find_syn_match
))
219 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
223 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
225 tcpmss_tg6_check(const char *tablename
, const void *entry
,
226 const struct xt_target
*target
, void *targinfo
,
227 unsigned int hook_mask
)
229 const struct xt_tcpmss_info
*info
= targinfo
;
230 const struct ip6t_entry
*e
= entry
;
232 if (info
->mss
== XT_TCPMSS_CLAMP_PMTU
&&
233 (hook_mask
& ~((1 << NF_INET_FORWARD
) |
234 (1 << NF_INET_LOCAL_OUT
) |
235 (1 << NF_INET_POST_ROUTING
))) != 0) {
236 printk("xt_TCPMSS: path-MTU clamping only supported in "
237 "FORWARD, OUTPUT and POSTROUTING hooks\n");
240 if (IP6T_MATCH_ITERATE(e
, find_syn_match
))
242 printk("xt_TCPMSS: Only works on TCP SYN packets\n");
247 static struct xt_target tcpmss_tg_reg
[] __read_mostly
= {
251 .checkentry
= tcpmss_tg4_check
,
252 .target
= tcpmss_tg4
,
253 .targetsize
= sizeof(struct xt_tcpmss_info
),
254 .proto
= IPPROTO_TCP
,
257 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
261 .checkentry
= tcpmss_tg6_check
,
262 .target
= tcpmss_tg6
,
263 .targetsize
= sizeof(struct xt_tcpmss_info
),
264 .proto
= IPPROTO_TCP
,
270 static int __init
tcpmss_tg_init(void)
272 return xt_register_targets(tcpmss_tg_reg
, ARRAY_SIZE(tcpmss_tg_reg
));
275 static void __exit
tcpmss_tg_exit(void)
277 xt_unregister_targets(tcpmss_tg_reg
, ARRAY_SIZE(tcpmss_tg_reg
));
280 module_init(tcpmss_tg_init
);
281 module_exit(tcpmss_tg_exit
);