Re: ICMP reply uses wrong source address as destination

[Jiri Kosina]

[ adding netdev@ to CC ]

On Mon, 28 Feb 2011, Anders Nilsson Plymoth wrote:

> Dear linux kernel enthusiasts,
> 
> I came upon an issue where ICMP reply packets were issued towards the
> IP address of the receiving interface, rather than the source IP
> address.
> Looking at the kernel code, I saw that this is caused by the following
> line in net/ipv4/icmp.c function icmp_reply:
> 
> daddr = ipc.addr = rt->rt_src;
> 
> For most cases the original line of code is ok, but in some situations
> doesn't arrive to the kernel from the network device, but through some
> other mechanism such as a userspace application. In these cases the
> receiving device in the skb appears to be the loopback interface, not
> a physical device. icmp_reply will thus issue the reply to the
> loopback IP address, rather than the source IP address as it should.
> 
> While googling to see if this issue have been submitted, I found this
> two threads that address the same problem:
> 
> ([PATCH] 2.6.22.6 NETWORKING [IPV4]: Always use source addr in skb to
> reply packet) on 17 sep 2007
> AND
> ([PATCH RESEND] 2.6.22.6 networking [ipv4]: fix wrong destination when
> reply packetes) on 20 sep 2007).
> 
> Nothing came out of these threads, and some of the questions there are
> easy to answer; such as this doesn't affect DNAT, and if source IP
> address is not set then you should not issue a reply for echo and
> timestamp anyway.
> 
> As to the statement:
> "... which IP address should be used as the source
> 1. the destination address of the packet that generated the message
> 
> or.
> 
> 2. the IP address that the machine would use by default if the machine
> were to generate a new connection to the destination."
> These may be relevant questions, but the ICMP RFC clearly states the
> answer is 1. 2. may seem relevant to multi-homing, but its not the
> role of the ICMP reply to resolve multi-homing issues.
> The following code will correct the issue.
> {
>    struct iphdr *ip = ip_hdr(skb);
>    daddr = ipc.addr = ip->saddr;
> }
> The only functions that use icmp_reply are icmp_echo and
> icmp_timestamp, and this change do not modify their behavior. After
> extensive testing, in regular setups and DNATed situations, I can
> verify this change works as intended.
> Thanks,

-- 
Jiri Kosina
SUSE Labs, Novell Inc.

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