netdev - Re: R: Re: mtu issue with ipsec tunnel and netfilter snat

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Date:	Thu, 10 Jan 2013 18:57:04 +0100 (CET)
From:	Jan Engelhardt <jengelh@...i.de>
To:	"pupilla@...ero.it" <pupilla@...ero.it>
cc:	netdev@...r.kernel.org
Subject: Re: R: Re: mtu issue with ipsec tunnel and netfilter snat

On Thursday 2013-01-10 17:46, pupilla@...ero.it wrote:

>jengelh@...i.de wrote:
>>>
>>>But why linux_gw_snat is not sending icmp need to frag packets to
>>>10.148.12.23?
>>
>>Perhaps because ICMP was blocked erroneously?
>
>well, I don't see the icmp packets because tcpdump 'see' only the 
>incoming ipsec clear packets. Is there a way to see the outgoing clear 
>ipsec packets with tcpdump?

Not with AF_PACKET sockets (tcpdump uses such), but you could with 
iptables -j LOG, NFLOG and TRACE they can be made visible. If you need 
the full packet, you can either patch up LOG to call the kernel hexdump 
functions, or use NFLOG - I think - to deliver it to any userspace 
program to do further processing. If all else fails, there is also 
NFQUEUE with which the packet can also be copied to userspace.
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