RIPng advertisement hop count 1 (should be 255 per RFC)

[Simon Dickhoven]

Hi, again

If you were paying attention (unlike myself) you may have noticed that 
the below fix doesn't actually make BIRD RFC-compliant.

Rather, it makes BIRD interoperate with other RFC-compliant RIPng routers.

After all, the RFC doesn't state that route advertisements must be sent 
with an HLIM of 255 (though that's implied, of course), but rather that 
routers must _check_ that the HLIM is 255 when they _receive_ routing 
updates.

I tried getting that to work by checking s->ttl in the rip_rx function. 
However, that always returns 255 (or, I suspect, whatever rif->sock->ttl 
was set to in the new_iface function) regardless of the incoming 
packet's HLIM.

I then tried using the sk_set_min_ttl function on the socket in the 
new_iface function but got this error:

     Kernel does not support IPv6 TTL security

(i.e. the socket protocol doesn't support that option). Since I'm on 
Linux (Debian) this error comes from sysdep/linux/sysio.h.

Anyway, I am not familiar enough with the BIRD code to understand where 
I can obtain the actual HLIM (TTL) of the incoming packet in order to 
ensure that the HLIM (TTL) is 255.

I'll keep digging but if anybody has any suggestions or pointers to get 
me moving in the right direction I'd appreciate it.

Thanks.

- Simon

On 06/14/2013 05:41 PM, Simon Dickhoven wrote:
> OK. I looked at proto/rip/rip.c a bit more and figured that I might as
> well give it a shot and hack around a little bit. I ended up making this
> tiny mod:
>
> --- a/proto/rip/rip.c
> +++ b/proto/rip/rip.c
> @@ -706,7 +706,11 @@
>      rif->sock->dport = P_CF->port;
>      if (new)
>        {
> +#ifndef IPV6
>          rif->sock->ttl = 1;
> +#else
> +      rif->sock->ttl = 255;
> +#endif
>          rif->sock->tos = IP_PREC_INTERNET_CONTROL;
>          rif->sock->flags = SKF_LADDR_RX;
>        }
>
> Subsequently, I did a full Debian package build based on
>
> http://backports.debian.org/debian-backports/pool/main/b/bird/bird_1.3.7-1~bpo60+1.diff.gz
>
> I added the above patch to the debian/patches dir and appended the patch
> file name (I named it "011-ripng_hopcount.patch") to debian/patches/series.
>
> The package built fine. I installed it on my test box and lo and behold:
> Vyatta/Quagga is now happy and I'm seeing my IPv6 routes propagate via
> RIPng.
>
> Tcpdump reveals that RIP(v2) is still using a TTL of 1 and RIPng is
> using an HLIM (IPv6 equivalent of TTL) of 255.
>
> Thanks.
>
> - Simon
>
> On 06/14/2013 03:04 PM, Simon Dickhoven wrote:
>> Hi,
>>
>> I just started experimenting with BIRD for an IPv6 deployment. I am
>> using Vyatta VC6.6R1 router VMs on either side of my BIRD VM (which runs
>> on a customized Debian Squeeze release with kernel 3.3.1). I installed
>> bird/bird6 1.3.7 from the squeeze-backports repository.
>>
>> Here my setup.
>>
>> Lab Net --- Vyatta --- BIRD on Debian --- Vyatta --- Stub Net
>>
>> Anyway, I don't have any problems with my configs or anything like that.
>> My problem is that Vyatta's ripngd (part of Quagga) complains about an
>> RFC violation when it receives RIPng advertisements from BIRD:
>>
>> Jun 14 21:43:40 vyatta ripngd[1682]: RIPng packet comes with non 255 hop
>> count 1 from fe80::20c:29ff:fef8:cbc5
>>
>> I looked at the source code in rip.c and see this line:
>>
>>          rif->sock->ttl = 1;
>>
>> which is the only reference I can find to TTL/Hop Count. So I'm guessing
>> this is the culprit. The latest source code (1.3.10) is identical in
>> this respect.
>>
>> RFC 2080 states
>>
>> [...]
>> As an additional check, periodic advertisements must have their hop
>> counts set to 255, and inbound, multicast packets sent from the RIPng
>> port (i.e. periodic advertisement or triggered update packets) must be
>> examined to ensure that the hop count is 255.
>> [...]
>>
>> The use of the term "must" leads me to believe that this is not optional
>> and is therefore required for RFC-compliance.
>>
>> There seems to be no such requirement for RIP (v1/v2) so simply changing
>> the source code to indiscriminately set the TTL to 255 is probably not
>> the right thing to do.
>>
>> Have others encountered this problem and is there possibly a patch or
>> something for getting RFC-compliance and hence interoperability with
>> Vyatta/Quagga(ripngd)?
>>
>> Thanks.
>>
>> - Simon
>>