[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>

2 files changed, 15 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);
 
diff --git a/net/ipv4/xfrm4_state.c b/net/ipv4/xfrm4_state.c
index 223a2e83853f..050611d7a967 100644
--- a/net/ipv4/xfrm4_state.c
+++ b/net/ipv4/xfrm4_state.c
@@ -7,12 +7,20 @@
  *
  */
 
+#include <net/ip.h>
 #include <net/xfrm.h>
 #include <linux/pfkeyv2.h>
 #include <linux/ipsec.h>
 
 static struct xfrm_state_afinfo xfrm4_state_afinfo;
 
+static int xfrm4_init_flags(struct xfrm_state *x)
+{
+        if (ipv4_config.no_pmtu_disc)
+                x->props.flags |= XFRM_STATE_NOPMTUDISC;
+        return 0;
+}
+
 static void
 __xfrm4_init_tempsel(struct xfrm_state *x, struct flowi *fl,
                      struct xfrm_tmpl *tmpl,
@@ -109,6 +117,7 @@ __xfrm4_find_acq(u8 mode, u32 reqid, u8 proto,
 static struct xfrm_state_afinfo xfrm4_state_afinfo = {
         .family                 = AF_INET,
         .lock                   = RW_LOCK_UNLOCKED,
+        .init_flags             = xfrm4_init_flags,
         .init_tempsel           = __xfrm4_init_tempsel,
         .state_lookup           = __xfrm4_state_lookup,
         .find_acq               = __xfrm4_find_acq,