DNS pulling BGP routes?

Blake Dunlap ikiris at gmail.com
Wed Oct 6 18:47:38 UTC 2021

Yes, it really is common to announce sink routes via bgp from destination
services / proxies and to have those announcements be dynamically based on
service viability.

On Wed, Oct 6, 2021, 12:56 Jared Mauch <jared at puck.nether.net> wrote:

> This is quite common to tie an underlying service announcement to BGP
> announcements in an Anycast or similar environment.  It doesn’t have to be
> externally visible like this event for that to be the case.
> I would say more like Application availability caused the BGP routes to be
> withdrawn.
> I know several network operators that run DNS internally (even on
> raspberry pi devices)  and may have OSPF or BGP announcements internally to
> ensure things work well.  If the process dies (crash, etc) they want to
> route to the next nearest cluster.
> Of course if they all are down there’s negative outcomes.
> - Jared
> > On Oct 6, 2021, at 1:42 PM, Michael Thomas <mike at mtcc.com> wrote:
> >
> > So if I understand their post correctly, their DNS servers have the
> ability to withdraw routes if they determine are sub-optimal (fsvo). I can
> certainly understand for the DNS servers to not give answers they think are
> unreachable but there is always the problem that they may be partitioned
> and not the routes themselves. At a minimum, I would think they'd need some
> consensus protocol that says that it's broken across multiple servers.
> >
> > But I just don't understand why this is a good idea at all. Network
> topology is not DNS's bailiwick so using it as a trigger to withdraw routes
> seems really strange and fraught with unintended consequences. Why is it a
> good idea to withdraw the route if it doesn't seem reachable from the DNS
> server? Give answers that are reachable, sure, but to actually make a
> topology decision? Yikes. And what happens to the cached answers that still
> point to the supposedly dead route? They're going to fail until the TTL
> expires anyway so why is it preferable withdraw the route too?
> >
> > My guess is that their post while more clear that most doesn't go into
> enough detail, but is it me or does it seem like this is a really weird
> thing to do?
> >
> > Mike
> >
> >
> > On 10/5/21 11:56 PM, Bjørn Mork wrote:
> >> Masataka Ohta <mohta at necom830.hpcl.titech.ac.jp> writes:
> >>
> >>> As long as name servers with expired zone data won't serve
> >>> request from outside of facebook, whether BGP routes to the
> >>> name servers are announced or not is unimportant.
> >> I am not convinced this is true.  You'd normally serve some semi-static
> >> content, especially wrt stuff you need yourself to manage your network.
> >> Removing all DNS servers at the same time is never a good idea, even in
> >> the situation where you believe they are all failing.
> >>
> >> The problem is of course that you can't let the servers take the
> >> decision to withdraw from anycast if you want to prevent this
> >> catastrophe.  The servers have no knowledge of the rest of the network.
> >> They only know that they've lost contact with it.  So they all make the
> >> same stupid decision.
> >>
> >> But if the servers can't withdraw, then they will serve stale content if
> >> the data center loses backbone access. And with a large enough network
> >> then that is probably something which happens on a regular basis.
> >>
> >> This is a very hard problem to solve.
> >>
> >> Thanks a lot to facebook for making the detailed explanation available
> >> to the public.  I'm crossing my fingers hoping they follow up with
> >> details about the solutions they come up with.  The problem affects any
> >> critical anycast DNS service. And it doesn't have to be as big as
> >> facebook to be locally critical to an enterprise, ISP or whatever.
> >>
> >>
> >>
> >> Bjørn
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