[PATCH 04/19] ipv4: fix path MTU discovery with connection tracking

kaber
2012-08-09T20:18:37Z

From: Patrick McHardy <kaber@trash.net>

IPv4 conntrack defragments incoming packet at the PRE_ROUTING hook and
(in case of forwarded packets) refragments them at POST_ROUTING
independant of the IP_DF flag. Refragmentation uses the dst_mtu() of
the local route without caring about the original fragment sizes,
thereby breaking PMTUD.

This patch fixes this by keeping track of the largest received fragment
with IP_DF set and generates an ICMP fragmentation required error during
refragmentation if that size exceeds the MTU.

Signed-off-by: Patrick McHardy <kaber@trash.net>
---
include/net/inet_frag.h | 2 ++
include/net/ip.h | 2 ++
net/ipv4/ip_fragment.c | 8 +++++++-
net/ipv4/ip_output.c | 4 +++-
4 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
index 2431cf8..5098ee7 100644
--- a/include/net/inet_frag.h
+++ b/include/net/inet_frag.h
@@ -29,6 +29,8 @@ struct inet_frag_queue {
#define INET_FRAG_COMPLETE 4
#define INET_FRAG_FIRST_IN 2
#define INET_FRAG_LAST_IN 1
+
+ u16 max_size;
};

#define INETFRAGS_HASHSZ 64
diff --git a/include/net/ip.h b/include/net/ip.h
index bd5e444..613053e 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -42,6 +42,8 @@ struct inet_skb_parm {
#define IPSKB_XFRM_TRANSFORMED 4
#define IPSKB_FRAG_COMPLETE 8
#define IPSKB_REROUTED 16
+
+ u16 frag_max_size;
};

static inline unsigned int ip_hdrlen(const struct sk_buff *skb)
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 8d07c97..fa6a12c 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -523,6 +523,10 @@ found:
if (offset == 0)
qp->q.last_in |= INET_FRAG_FIRST_IN;

+ if (ip_hdr(skb)->frag_off & htons(IP_DF) &&
+ skb->len + ihl > qp->q.max_size)
+ qp->q.max_size = skb->len + ihl;
+
if (qp->q.last_in == (INET_FRAG_FIRST_IN | INET_FRAG_LAST_IN) &&
qp->q.meat == qp->q.len)
return ip_frag_reasm(qp, prev, dev);
@@ -646,9 +650,11 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
head->next = NULL;
head->dev = dev;
head->tstamp = qp->q.stamp;
+ IPCB(head)->frag_max_size = qp->q.max_size;

iph = ip_hdr(head);
- iph->frag_off = 0;
+ /* max_size != 0 implies at least one fragment had IP_DF set */
+ iph->frag_off = qp->q.max_size ? htons(IP_DF) : 0;
iph->tot_len = htons(len);
iph->tos |= ecn;
IP_INC_STATS_BH(net, IPSTATS_MIB_REASMOKS);
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 76dde25..ae33b99 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -467,7 +467,9 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))

iph = ip_hdr(skb);

- if (unlikely((iph->frag_off & htons(IP_DF)) && !skb->local_df)) {
+ if (unlikely(((iph->frag_off & htons(IP_DF)) && !skb->local_df) ||
+ (IPCB(skb)->frag_max_size &&
+ IPCB(skb)->frag_max_size > dst_mtu(&rt->dst)))) {
IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
htonl(ip_skb_dst_mtu(skb)));
--
1.7.1