Anycast but for egress

Vimal j.vimal at
Tue Jul 27 22:20:39 UTC 2021

Hi all, great replies. :) Let me clarify my initial question, and then
respond one by one:

My intention is to run a web-crawling service on a public cloud. This
service is geographically distributed, and therefore will run in multiple
regions around the world inside AWS... this means there will be multiple
AWS VPCs, each with their own NAT gateway, and traffic destined to websites
that we crawl will appear to come from this NAT gateway's IP address.

The reason I want a predictable IP is to communicate this IP to
website owners so they can allow access from these IPs into their
networks.  I chose IP as an example; it can also be a subnet, but what I
don't want to provide is a list of 100 different IP addresses without any

I understand that this is not perfect, and would frankly not be my
preferred approach to solve the problem.... but we've had requests of this
nature from websites to create an allowlist of a limited number of
predictable IPs so it doesn't trip their IDSs/other systems they might
have... so we're trying to see how well it would work in practice.  For the
moment, let's set aside the issue as to whether AWS will even let me
advertise the same IP on all my VPC NAT gateways, and just look at whether
it's technically feasible.  My gut feeling is that this wouldn't work well
in practice, but I wanted to ask the experts here...

Also, pointers on what the best practices for solving this issue are most
welcome, so I can reference those who ask for IP addresses to this
discussion and follow recommendations here.

Onto the responses:

@owen at and @woody at athompson at
> Because there’s no good/reliable way to get the replies back to the
correct initiating host.

> 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.

Yes, this makes sense as the destination can be anywhere around the world,
and that routing is asymmetric as others mentioned.  However, if the
destination service is "close" (in the routing metric sense) to the
initiating host, anycast return IP ought to work well, right?  I understand
this is a very important caveat and impractical to implement correctly in
the real world.

> We use our IGP (IS-IS) for our Anycast services. We find it to be very
basic, and as such, very predictable.

This is interesting... I wonder whether Anycast will still have some
failure modes and break TCP connections if routing (configuration) were to
change?  I checked the PDF linked by Bill Woodcock... while the methodology
is the same from 20y ago, would the data still be the same (order of
magnitude)? :) (p38)
"Limited operational data shows underlying instability to be on
the order of one flow per ten thousand per hour of duration."

@daniel at, @matt at,
> Unless you’re twisting knobs, egress traffic should already exit your
network at the closest possible egress point to its origin.  Is your
intention to carry the traffic for longer than that?
No, but I hope my intention is more clear in this email.  It's to have a
predictable egress IP to simplify firewall rules.

thanks all!!

On Tue, Jul 27, 2021 at 12:25 PM Adam Thompson <athompson at>

> 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
> *Adam Thompson*
> Consultant, Infrastructure Services
> [image: 1593169877849]
> 100 - 135 Innovation Drive
> Winnipeg, MB, R3T 6A8
> (204) 977-6824 or 1-800-430-6404 (MB only)
> 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?
> --
> Vimal

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