Anycast but for egress

Adam Thompson athompson at
Tue Jul 27 19:25:35 UTC 2021

Without any sarcasm: to make it harder to block.
If, say, Google, always crawled your site from (random made-up example) then you would see a not-insignificant number of hosts and networks null-routing that IP.  I have no idea why someone would do so, but I've seen it done many times.  Mostly by people who don't understand how un-special they are on the internet.  Also it would trigger IDS/IPS systems all over the place, having gobs and gobs of connections coming from a single IP.

That's setting aside the technical issues involved; routing is often asymmetric, i.e. the return packet takes a different path than the inbound packet.  So it would, as Owen implied, be nearly impossible to ensure the reply packets got back to the correct TCP stack.  As an example, I'm multi-homed and use path-prepending, so if a packet claiming to be from arrived on one of my commercial links, I would send the reply out the cheapest link, which in my case is a flat-rate R&E network (that has a path to Google), thus ensuring the reply does not get to the originating anycast node.

When my clients make connections outbound to anycast addresses, the destination is more-or-less stable, and the replies come back to the client's unique IP, so anycast works in that direction.  The guarantees are not present in the reverse direction.

The logical extremity of this is that it would be nearly impossible for two anycast addresses to establish a TCP connection to each other.  (In general.  There will be lots of local cases where it does happen to work, by coincidence.)

You'll find that even anycast nodes do not make connections outbound using their anycast address, pretty much for these reasons.


Adam Thompson
Consultant, Infrastructure Services
100 - 135 Innovation Drive
Winnipeg, MB, R3T 6A8
(204) 977-6824 or 1-800-430-6404 (MB only)
athompson at<mailto:athompson at><>
From: NANOG < at> on behalf of Vimal <j.vimal at>
Sent: July 27, 2021 12:54
To: nanog at <nanog at>
Subject: Anycast but for egress

(Unsure if this is the right forum to ask this question, but here goes:)

>From what I understand, IP Anycast can be used to steer traffic into a server that's close to the client.

I am curious if anyone here has/encountered a setup where they use anycast IP on their gateways... to have a predictable egress IP for their traffic, regardless of where they are located?

For example, a search engine crawler could in principle have the same IP advertised all over the world, but it looks like they don't...  I wonder why?


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