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Commit 5f2d04f1 authored by Patrick McHardy's avatar Patrick McHardy
Browse files

ipv4: fix path MTU discovery with connection tracking 



IPv4 conntrack defragments incoming packet at the PRE_ROUTING hook and
(in case of forwarded packets) refragments them at POST_ROUTING
independent 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: default avatarPatrick McHardy <kaber@trash.net>
Acked-by: default avatarEric Dumazet <edumazet@google.com>
Acked-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 0fa7fa98

include/net/inet_frag.h

+2 −0
Original line number Diff line number Diff line
@@ -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

include/net/ip.h

+2 −0
Original line number Diff line number Diff line
@@ -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)

net/ipv4/ip_fragment.c

+7 −1
Original line number Diff line number Diff line
@@ -523,6 +523,10 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb) 
	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);

net/ipv4/ip_output.c

+3 −1
Original line number Diff line number Diff line
@@ -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)));