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

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Date:	Mon, 01 Jun 2009 20:43:45 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	nicolas.dichtel@...nd.com
CC:	Florian Westphal <fw@...len.de>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv4/ipv6: check hop limit field on input

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.

(terminology : gateway / host in rfc 792)

I would say : who sent this ttl=0 packet at first ?

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 :)


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