netdev - Re: ICMP reply uses wrong source address as destinatio

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Date:	Tue, 01 Mar 2011 11:27:43 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Anders Nilsson Plymoth <lml@...hoc.se>
Cc:	netdev@...r.kernel.org
Subject: Re: ICMP reply uses wrong source address as destinatio

Le mardi 01 mars 2011 à 09:49 +0100, Anders Nilsson Plymoth a écrit :
> 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 posts that address the same problem:
> h**p://www.mail-archive.com/linux-kernel@...r.kernel.org/msg209746.html
> h**p://www.mail-archive.com/linux-kernel@...r.kernel.org/msg208272.html
> Some of the questions there are easy to answer; such as this doesn't
> affect DNAT, and if source address is not set then you can' reply
> 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.

I suspect you could respin Lepton Wu patch (from msg209746.html) on top
of net-next-2.6 and add your "Acked-by or Tested-by"

Keep in mind changelog should be very very detailed because patch was
delayed because of this.



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