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: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 14 insertions, 7 deletions

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index afaebc766933..6640a224f9fb 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));