Varnish and AWS ALBs

[Justin Lloyd]

Hi Guillaume!

It looks like you and Carlos are both correct. For some reason, before I was not seeing the Varnish XFF values from faked XFFs, not sure why, but now I’m seeing the fakes I’m using against one of my dev sites and I’m seeing the three values where it’s FAKED_IP, REAL_IP, ALB_IP. So with a little bit more VCL code (or probably easier once I move to Varnish Enterprise next year), I should be able to handle this. I’ll give it a whirl and see how it goes.

Thanks!

Justin

From: Guillaume Quintard <guillaume.quintard at gmail.com>
Sent: Thursday, August 19, 2021 2:00 PM
To: Carlos Abalde <carlos.abalde at gmail.com>
Cc: Justin Lloyd <justinl at arena.net>; varnish-misc at varnish-cache.org
Subject: Re: Varnish and AWS ALBs

Hi,

If I read this correctly: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/x-forwarded-headers.html , you can trust the before-last IP, because it was added by the ALB, always. (and using vmod_str makes it easy to retrieve https://github.com/varnish/varnish-modules/blob/master/src/vmod_str.vcc#L42)

Side question: would an NLB work? They support proxy-protocol, that would also solve your problem.

Cheers,

--
Guillaume Quintard


On Thu, Aug 19, 2021 at 1:52 PM Carlos Abalde <carlos.abalde at gmail.com<mailto:carlos.abalde at gmail.com>> wrote:
Hi,

No so sure about that. Let's assume the client address is 1.1.1.1. Two possible scenarios:

- The client request reaches the ALB without XFF. The ALB will inject XFF with value 1.1.1.1. Then Varnish will modify XFF adding the ALB's address (i.e., 1.1.1.1,<ALB IP>). Using the next-to-last IP you're using the right client address.

- The client request reaches the ALB with a forged XFF (e.g. 127.0.0.1). The ALB will will modify XFF (i.e. 127.0.0.1,1.1.1.1). The Varnish will do the same (i.e. 127.0.0.1,1.1.1.1,<ALB IP>). Using the next-to-last IP you're still using the right client address.

I've not checked using a ALB, but that should be the expected behaviour for me.

Best,

--
Carlos Abalde

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