From: Phil Oester <kernel@xxxxxxxxxxxx>
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: Phil Oester <kernel@xxxxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
net/netfilter/xt_TCPMSS.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index afaebc7..6640a22 100644
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -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
*/
- newmss = min(newmss, (u16)536);
+ 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));
--
1.7.10.4
--
To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
