BIRDv2 OSPF: Stub for loopback potentially broken: Invalid Prefix in LSA

[Ondrej Zajicek]

On Wed, Apr 24, 2019 at 01:10:31PM +0000, Joakim Tjernlund wrote:
> On Tue, 2019-04-23 at 21:03 +0000, Joakim Tjernlund wrote:
> > On Tue, 2019-04-23 at 19:33 +0200, Ondrej Zajicek wrote:
> > > On Tue, Apr 23, 2019 at 12:40:04PM +0000, Joakim Tjernlund wrote:
> > > > > I think the standard just does not consider the case of 'unnumbered'
> > > > > link with both local and remote address but without subnet and assumes
> > > > > 'real' unnumbered PtP link with no local IP address associated with
> > > > > the iface.
> > > > 
> > > > There is always some local address, in linux you have to assign it to the I/F,
> > > > but in others, like Cisco, you can assign an IP address to a dummy I/F and then
> > > > tell unnumbered I/Fs to use the dummy I/Fs' IP address.
> > > > There has to be some SRC IP address on pkgs sent by OSPF
> > > 
> > > Yes, but the way how the RFC is written it (IMHO) assumes the second case -
> > > unnumbered ifaces using SRC IP from some dummy iface. Therefore stub
> > 
> > yes, the second case, agreed.
> > 
> > > node is associated with the dummy iface and not with the unnumbered PtP iface.
> > 
> > No, this is a true unnumbered I/F(it has no local IP address) and will not generate
> > any stub route.
> > 
> > > 
> > > > Since Linux always has an local IP adress it is not possible to deduce if the
> > > > user intended the link to be unnumbered or not, unless one wants to add explicit
> > > > config "unnumbered", it would be best to assume unnumbered I think.
> > > 
> > > I do not understand what you mean in this article. Linux could have active iface
> > > without any IP address and it can be used for PtP link (with SRC IP from some
> > > other iface), but it is not implemented in BIRD.
> > 
> > I expressed myself badly, I meant if one want to have control over what SRC ip is used,
> > you need to have it on the local I/F. Maybe there is a way to do that these days differently
> > but I don't think any PtoP impl. on Linux uses that.
> > 
> > > It is true that even a link with local /32 IP without specified remote peer could
> > > be used for OSPF PtP link. BIRD in this case assumes it is a stub /32 and i am not
> > > sure whether it could be configured as regular PtP. I did not realized before that
> > > this is a relevant case for PtP link.
> > 
> > This case is as close one gets to a true unnumbered I/F in Linux I think and pppd supports
> > such links. I think should treat /32 links as unnumbered even if there is an remote
> > IP address or make it configurable.
> > 
> > > > Consider the use case with many /32 ptp links, all with the same local IP, there would
> > > > be a lot of redundant host routes in the Router LSA. Better to let the user
> > > > add an explicit stub network for all PtoP's if one needs it.
> > > 
> > > Well, that is solvable by not putting the same stub multiple times in the
> > > Router LSA.
> > 
> > Maybe but since it is the remote ip one puts here you will only save "space" when
> > you have multiple ptop links between the same two routers.
> > 
> > > More debatable is a case like this:
> > > 
> > > eth0 10.0.0.1/24
> > > eth1 10.0.0.1/32 peer 10.0.1.1/32
> > > 
> > > 10.0.0.1 is already reachable by route for eth0 (either stub or network),
> > > so it is technically not necessary to have it in the Router LSA as
> > > a separate /32 stub. So it makes sense to me to have it configurable
> > > with three values: yes, no, if-not-covered.
> > 
> > Yes, this is what I was getting at too. Maybe have a conf option "unnumbered" to treat
> > eth1 as a "true" unnumbred I/F(no stub network).
> > Quietion is, what should be default?
> > What is default in current 2.0 Bird?
> > 
> >  Jocke
> > 
> 
> I have a hard time understanding the code for section 12.4.1.1 Option 1 and Option 2:
> 
>     /* Now we will originate stub area if there is no primary */
>     if (net_lsa ||
> 	(ifa->type == OSPF_IT_VLINK) ||
> 	((ifa->addr->flags & IA_PEER) && ! ifa->cf->stub) ||
> 	configured_stubnet(oa, ifa->addr))
>       continue;
> Is the above always true for a ptp I/F? I am not clear over how IA_HOST, IA_PEER and ifa->cf->stub works.

IA_HOST means /32, no peer address
IA_PEER means /32, and peer address

ifa->cf->stub means explicitly configured stub/pasive mode.


> Also, important to agree if a /32 mask constitutes a subnet or not?

/32 does not consistute subnet because it does not cover both local and remote addresses.


> That would explain the below, there is no Option 1 

Yes, we do not announce neighboring router's IP (Option 1) (unless it is passive interface).

>       /* Host or network stub entry */
>     if ((ifa->addr->flags & IA_HOST) ||
> 	(ifa->state == OSPF_IS_LOOP) ||
> 	(ifa->type == OSPF_IT_PTMP))
>       add_rt2_lsa_link(p, LSART_STUB, ipa_to_u32(ifa->addr->ip), 0xffffffff, 0);
>     else
>       add_rt2_lsa_link(p, LSART_STUB, ip4_to_u32(net4_prefix(&ifa->addr->prefix)),
> 		       u32_mkmask(net4_pxlen(&ifa->addr->prefix)), ifa->cost);


-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."