Commit 70d19f80 authored by Phil Oester's avatar Phil Oester Committed by Pablo Neira Ayuso
Browse files

netfilter: xt_TCPMSS: Fix IPv6 default MSS too 



As a followup to commit 409b545a ("netfilter: xt_TCPMSS: Fix violation
of RFC879 in absence of MSS option"), John Heffner points out that IPv6
has a higher MTU than IPv4, and thus a higher minimum MSS. Update TCPMSS
target to account for this, and update RFC comment.

While at it, point to more recent reference RFC1122 instead of RFC879.

Signed-off-by: default avatarPhil Oester <kernel@linuxace.com>
Signed-off-by: default avatarPablo Neira Ayuso <pablo@netfilter.org>
parent ed82c437

net/netfilter/xt_TCPMSS.c

+14 −7
Original line number Diff line number Diff line
@@ -45,11 +45,12 @@ optlen(const u_int8_t *opt, unsigned int offset) 


static int

tcpmss_mangle_packet(struct sk_buff *skb,

		     const struct xt_tcpmss_info *info,

		     const struct xt_action_param *par,

		     unsigned int in_mtu,

		     unsigned int tcphoff,

		     unsigned int minlen)

{

	const struct xt_tcpmss_info *info = par->targinfo;

	struct tcphdr *tcph;

	unsigned int tcplen, i;

	__be16 oldval;
@@ -125,11 +126,17 @@ tcpmss_mangle_packet(struct sk_buff *skb, 


	skb_put(skb, TCPOLEN_MSS);



	/* RFC 879 states that the default MSS is 536 without specific

	 * knowledge that the destination host is prepared to accept larger.

	 * Since no MSS was provided, we MUST NOT set a value > 536.

	/*

	 * IPv4: RFC 1122 states "If an MSS option is not received at

	 * connection setup, TCP MUST assume a default send MSS of 536".

	 * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum

	 * length IPv6 header of 60, ergo the default MSS value is 1220

	 * Since no MSS was provided, we must use the default values

	 */

	if (par->family == NFPROTO_IPV4)

		newmss = min(newmss, (u16)536);

	else

		newmss = min(newmss, (u16)1220);



	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);

	memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
@@ -188,7 +195,7 @@ tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par) 
	__be16 newlen;

	int ret;



	ret = tcpmss_mangle_packet(skb, par->targinfo,

	ret = tcpmss_mangle_packet(skb, par,

				   tcpmss_reverse_mtu(skb, PF_INET),

				   iph->ihl * 4,

				   sizeof(*iph) + sizeof(struct tcphdr));
@@ -217,7 +224,7 @@ tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par) 
	tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);

	if (tcphoff < 0)

		return NF_DROP;

	ret = tcpmss_mangle_packet(skb, par->targinfo,

	ret = tcpmss_mangle_packet(skb, par,

				   tcpmss_reverse_mtu(skb, PF_INET6),

				   tcphoff,

				   sizeof(*ipv6h) + sizeof(struct tcphdr));

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