[IPSEC]: Add XFRM_STATE_NOPMTUDISC flag

This patch adds the flag XFRM_STATE_NOPMTUDISC for xfrm states.  It is
similar to the nopmtudisc on IPIP/GRE tunnels.  It only has an effect
on IPv4 tunnel mode states.  For these states, it will ensure that the
DF flag is always cleared.

This is primarily useful to work around ICMP blackholes.

In future this flag could also allow a larger MTU to be set within the
tunnel just like IPIP/GRE tunnels.  This could be useful for short haul
tunnels where temporary fragmentation outside the tunnel is desired over
smaller fragments inside the tunnel.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: James Morris <jmorris@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/ipv4/xfrm4_output.c b/net/ipv4/xfrm4_output.c
index af2392ae5769..66620a95942a 100644
--- a/net/ipv4/xfrm4_output.c
+++ b/net/ipv4/xfrm4_output.c
@@ -33,6 +33,7 @@ static void xfrm4_encap(struct sk_buff *skb)
         struct dst_entry *dst = skb->dst;
         struct xfrm_state *x = dst->xfrm;
         struct iphdr *iph, *top_iph;
+        int flags;
 
         iph = skb->nh.iph;
         skb->h.ipiph = iph;
@@ -51,10 +52,13 @@ static void xfrm4_encap(struct sk_buff *skb)
 
         /* DS disclosed */
         top_iph->tos = INET_ECN_encapsulate(iph->tos, iph->tos);
-        if (x->props.flags & XFRM_STATE_NOECN)
+
+        flags = x->props.flags;
+        if (flags & XFRM_STATE_NOECN)
                 IP_ECN_clear(top_iph);
 
-        top_iph->frag_off = iph->frag_off & htons(IP_DF);
+        top_iph->frag_off = (flags & XFRM_STATE_NOPMTUDISC) ?
+                0 : (iph->frag_off & htons(IP_DF));
         if (!top_iph->frag_off)
                 __ip_select_ident(top_iph, dst, 0);