Problems with gif tunnels

[Greg 'groggy' Lehey]

On Wednesday,  8 June 2005 at 12:40:53 +0200, Jeremie Le Hen wrote:
>>> It's currently pushing 7:30 pm, and I was going to send out a reply
>>> tomorrow.  But indeed, it seems that Linux people prefer GRE tunnels,
>>> we prefer (with good reason) IP tunnels, and the whole issue was one
>>> of documentation.  After changing my tunnel from GRE to IP, it worked
>>> (and works) like a charm.
>
> IIRC,
>     - Linux uses the ipip module to do IP-over-IP tunnel
>     - FreeBSD uses the gre(4) interface to do GRE tunnels
>     - GRE is a Cisco product and means ``Generic Routing
>       Encapsulation''.  I don't know what they mean with the term
>       "Generic" because I have only seen IP encapsulated tunnel so far.
>       According to the GRE header, I guess GRE is far more powerful
>       than a simple IP-over-IP encapsulation, and I would be glad if
>       someone could explain us what are the benefits of this protocol.
>       I would conclude by saying that indeed Linux users tend to use
>       GRE tunnels whereas a IP-over-IP tunnel would be enough, because
>       they used to be trendy.
>
>> What is the difference between gre and gif tunnels anyway... the man mages
>> were not that informative...
>
> Read above.  Usually gre(4) tunnels are used as simple IP-over-IP tunnel,
> so a gif(4) would do the same with less overload (due to GRE header size).
> GRE seems far more powerful, but I don't know its benefits.

My understanding is that GRE is to IP as PPP is to SLIP: it allows
multiple protocols to be encapsulated.  I've done some tracing with
Ethereal, and the only difference is a four-byte header in front of
the payload for GRE; in an IP tunnel, it's simply missing.  I've
written this up in my diary
(http://www.lemis.com/grog/diary-jun2005.html#8), along with the
traces.

Greg
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