netdev - Re: [PATCH] ipv4/ipv6: check hop limit field on input

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Date:	Tue, 02 Jun 2009 11:30:43 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Florian Westphal <fw@...len.de>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv4/ipv6: check hop limit field on input

Eric Dumazet wrote:
> Nicolas Dichtel a écrit :
>> Le 01.06.2009 18:19, Florian Westphal a écrit :
>>> Nicolas Dichtel <nicolas.dichtel@....6wind.com> wrote:
>>>> when network stack receives a packet, it didn't check value of
>>>> ttl/hop limit
>>>> field. RFC indicates that a router must drop the packet if this field
>>>> is 0.
>>> Whats wrong with the checks in ip(6)_forward?
>> It's on forward, not on input. Router must not process it.
>> For example, if you try to ping (with ttl set to 0) the router, you will
>> receive a reply.
>>
> 
> You seem to mix requirements for routers and hosts. ttl processing
> is relevant for a gateway only, not for a host.
It's why I test the forwarding value in my patch. If forwarding is 
enable, it's a router.

> 
> (terminology : gateway / host in rfc 792)
> 
> I would say : who sent this ttl=0 packet at first ?
I prefere to ask: is this packet a "legal" packet?

> 
> ping -t 0 host
> ping: can't set unicast time-to-live: Invalid argument
> 
> So Linux is not able to do that, unless using tricks of course, or patching IP_TTL
> 
> BTW, sending ttl=0 packets to my cisco host (also a router but not relevant)
> is ok, it replies to this packets...
> 
> I wonder why Linux forbids sending ttl=0 packets, time to read again all these RFCs :)
I find a statement which tell to drop the packet but maybe I've missing 
something.


Nicolas
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