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/*
* This is a module which is used for setting the MSS option in TCP packets.
*
* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/tcp.h>
#include <net/ipv6.h>
#include <net/tcp.h>
#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv6/ip6_tables.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_tcpudp.h>
#include <linux/netfilter/xt_TCPMSS.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
MODULE_ALIAS("ipt_TCPMSS");
MODULE_ALIAS("ip6t_TCPMSS");
static inline unsigned int
optlen(const u_int8_t *opt, unsigned int offset)
{
/* Beware zero-length options: make finite progress */
if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
return 1;
else
return opt[offset+1];
}
static int
tcpmss_mangle_packet(struct sk_buff *skb,
const struct xt_tcpmss_info *info,
unsigned int tcphoff,
unsigned int minlen)
{
struct tcphdr *tcph;
unsigned int tcplen, i;
__be16 oldval;
u16 newmss;
u8 *opt;
if (!skb_make_writable(skb, skb->len))
return -1;
tcplen = skb->len - tcphoff;
tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
/* Since it passed flags test in tcp match, we know it is is
not a fragment, and has data >= tcp header length. SYN
packets should not contain data: if they did, then we risk
running over MTU, sending Frag Needed and breaking things
badly. --RR */
if (tcplen != tcph->doff*4) {
if (net_ratelimit())
printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
skb->len);
return -1;
}
if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
if (dst_mtu(skb->dst) <= minlen) {
if (net_ratelimit())
printk(KERN_ERR "xt_TCPMSS: "
"unknown or invalid path-MTU (%u)\n",
dst_mtu(skb->dst));
return -1;
}
newmss = dst_mtu(skb->dst) - minlen;
} else
newmss = info->mss;
opt = (u_int8_t *)tcph;
for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
opt[i+1] == TCPOLEN_MSS) {
u_int16_t oldmss;
oldmss = (opt[i+2] << 8) | opt[i+3];
/* Never increase MSS, even when setting it, as
* doing so results in problems for hosts that rely
* on MSS being set correctly.
*/
if (oldmss <= newmss)
return 0;
opt[i+2] = (newmss & 0xff00) >> 8;
opt[i+3] = newmss & 0x00ff;
inet_proto_csum_replace2(&tcph->check, skb,
htons(oldmss), htons(newmss),
0);
return 0;
}
}
/*
* MSS Option not found ?! add it..
*/
if (skb_tailroom(skb) < TCPOLEN_MSS) {
if (pskb_expand_head(skb, 0,
TCPOLEN_MSS - skb_tailroom(skb),
GFP_ATOMIC))
return -1;
tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
}
skb_put(skb, TCPOLEN_MSS);
opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
inet_proto_csum_replace2(&tcph->check, skb,
htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
opt[0] = TCPOPT_MSS;
opt[1] = TCPOLEN_MSS;
opt[2] = (newmss & 0xff00) >> 8;
opt[3] = newmss & 0x00ff;
inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
oldval = ((__be16 *)tcph)[6];
tcph->doff += TCPOLEN_MSS/4;
inet_proto_csum_replace2(&tcph->check, skb,
oldval, ((__be16 *)tcph)[6], 0);
return TCPOLEN_MSS;
}
static unsigned int
tcpmss_tg4(struct sk_buff *skb, const struct net_device *in,
const struct net_device *out, unsigned int hooknum,
const struct xt_target *target, const void *targinfo)
{
struct iphdr *iph = ip_hdr(skb);
__be16 newlen;
int ret;
ret = tcpmss_mangle_packet(skb, targinfo, iph->ihl * 4,
sizeof(*iph) + sizeof(struct tcphdr));
if (ret < 0)
return NF_DROP;
if (ret > 0) {
iph = ip_hdr(skb);
newlen = htons(ntohs(iph->tot_len) + ret);
csum_replace2(&iph->check, iph->tot_len, newlen);
iph->tot_len = newlen;
}
return XT_CONTINUE;
}
#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
static unsigned int
tcpmss_tg6(struct sk_buff *skb, const struct net_device *in,
const struct net_device *out, unsigned int hooknum,
const struct xt_target *target, const void *targinfo)
{
struct ipv6hdr *ipv6h = ipv6_hdr(skb);
u8 nexthdr;
int tcphoff;
int ret;
nexthdr = ipv6h->nexthdr;
tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
if (tcphoff < 0)
return NF_DROP;
ret = tcpmss_mangle_packet(skb, targinfo, tcphoff,
sizeof(*ipv6h) + sizeof(struct tcphdr));
if (ret < 0)
return NF_DROP;
if (ret > 0) {
ipv6h = ipv6_hdr(skb);
ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
}
return XT_CONTINUE;
}
#endif
#define TH_SYN 0x02
/* Must specify -p tcp --syn */
static inline bool find_syn_match(const struct xt_entry_match *m)
{
const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
tcpinfo->flg_cmp & TH_SYN &&
!(tcpinfo->invflags & XT_TCP_INV_FLAGS))
return true;
return false;
}
static bool
tcpmss_tg4_check(const char *tablename, const void *entry,
const struct xt_target *target, void *targinfo,
unsigned int hook_mask)
{
const struct xt_tcpmss_info *info = targinfo;
const struct ipt_entry *e = entry;
if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
(hook_mask & ~((1 << NF_INET_FORWARD) |
(1 << NF_INET_LOCAL_OUT) |
(1 << NF_INET_POST_ROUTING))) != 0) {
printk("xt_TCPMSS: path-MTU clamping only supported in "
"FORWARD, OUTPUT and POSTROUTING hooks\n");
return false;
}
if (IPT_MATCH_ITERATE(e, find_syn_match))
return true;
printk("xt_TCPMSS: Only works on TCP SYN packets\n");
return false;
}
#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
static bool
tcpmss_tg6_check(const char *tablename, const void *entry,
const struct xt_target *target, void *targinfo,
unsigned int hook_mask)
{
const struct xt_tcpmss_info *info = targinfo;
const struct ip6t_entry *e = entry;
if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
(hook_mask & ~((1 << NF_INET_FORWARD) |
(1 << NF_INET_LOCAL_OUT) |
(1 << NF_INET_POST_ROUTING))) != 0) {
printk("xt_TCPMSS: path-MTU clamping only supported in "
"FORWARD, OUTPUT and POSTROUTING hooks\n");
return false;
}
if (IP6T_MATCH_ITERATE(e, find_syn_match))
return true;
printk("xt_TCPMSS: Only works on TCP SYN packets\n");
return false;
}
#endif
static struct xt_target tcpmss_tg_reg[] __read_mostly = {
{
.family = AF_INET,
.name = "TCPMSS",
.checkentry = tcpmss_tg4_check,
.target = tcpmss_tg4,
.targetsize = sizeof(struct xt_tcpmss_info),
.proto = IPPROTO_TCP,
.me = THIS_MODULE,
},
#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
{
.family = AF_INET6,
.name = "TCPMSS",
.checkentry = tcpmss_tg6_check,
.target = tcpmss_tg6,
.targetsize = sizeof(struct xt_tcpmss_info),
.proto = IPPROTO_TCP,
.me = THIS_MODULE,
},
#endif
};
static int __init tcpmss_tg_init(void)
{
return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
}
static void __exit tcpmss_tg_exit(void)
{
xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
}
module_init(tcpmss_tg_init);
module_exit(tcpmss_tg_exit);
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