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Date: Tue, 08 Jul 2014 19:55:58 +0000
From: "Poul-Henning Kamp" <>
Subject: Re: Varnish - no CVE == bug regression

In message <>, writes:

>Is it a supported configuration if auto_restart is off, and this
>hostname is specified using a DNS domain name?

Supported ?  Yes.

Recommended ?  No.

Common ?  No.

Actually ever seen ?  Only while developers debug.


The backend hostname is resolved only once, at the time the
administrator loads the configuration ("vcl.load" CLI command).

It is resolved through system services, however they are configured
to do it (/etc/hosts, NIS, DNS, DNSSEC, etc.)

The resolved IP#'s (one IPv4 and one IPv6 allowed, no more) are
then fixed in the compiled configuration and is stable for this
loaded configuration and automatic (or manual) restarts of
the child process will not change it.

This design were chosen to make sure that DNS system flakery only
impacts varnish management operations, but not Varnish operation.

If the hostname resolution fails, the load of the new configuration
obviously fails.

Since Varnish supports having multiple configurations loaded at the
same time, it keeps running its current active configuration while
you're trying to load the new configuration, and thus not affected
if you have to spend 20 minutes to deal with shit DNS service.

The resolved addresses can be verified from the CLI or via the
shared memory Log/Statistics facility, before the loaded 
configuration is activated ("vcl.use" CLI command)

>It seems that, if an attacker is occasionally successful at DNS
>spoofing (or spoofing at another level) and can thus trigger even one
>use of a rogue backend server, there's a long-lived denial of service.

First, if the attacker can spoof your DNS, what makes you think
your HTTP traffic will end up at your Varnish server to begin with ?
What about your emails ?

Second the attacker would need to time his attack to be coincident
with the administrator loading a configuration file.

Third ... which is using hostnames instead of IP#, which seem
to be preferred by many varnish admins because it takes DNS entirely
out of the picture.

Forth ... on a Varnish server which is not using some kind of split-
horizon name-resolution (/etc/hosts, NIS, S-H DNS)

Fifth ... against a Varnish admin who is not so worried about
DNS-spoofing that he uses DNSSEC.

I think there are a lot of things to worry about before this
attack makes top of the list.

>> By definition Varnish must explicitly and implicitly trust the
>> backend HTTP server
>With many realistic network designs, there isn't 100% assurance that
>this trusted server is always the actual origin of network traffic
>that appears to be from this server, and has a malicious HTTP header.

Oh, absolutely:  People can do BGP manipulation, they can cut and
splice fibers, they can do all sorts of nasty things, including
it seems, hijack alle your domains through the US justice system
because they don't like your abuse@ handling.

But it's not Varnish' task to worry about any of that.

Varnish sits on top of a TCP stack, and if people use hostnames,
we'll resolve them using the OS facility for hostname resolution,
just like we send and receive TCP streams using the OS facilities.

Varnish is no different from any other application in this respect.

>Thus, it seems that you are trusting data for which it's essentially
>impossible to know whether this data originated inside of your
>security boundary.

Me ?  I don't trust anybody.

The people who deploy Varnish have to make a lot of decisions, some
of them hard, some of them easy, a lot of them about security,
reliability and performance.  That's their job.

My job is to deliver at tool for them, a tool they can understand
and predict how will react, in particular I try to make it react
the way people would intutivily expect it to.  (POLA: Principle
Of Least Astonishment)

Policies, including risk-mitigation for their particular network
situation are entirely their own responsibility.

>If there shouldn't be CVE assignments, would it be possible to
>update your documentation so that "auto_restart off" is labeled
>as a known risk for long-lived DoS conditions?

The current description is: auto_restart
	200 132     
		Value is: on [bool] (default)
		Default is: on

		Restart child process automatically if it dies.

I have yet to meet an Varnish administrator who didn't understand
the meaning and value of this, in particular in relation to DoS
attaks, so I suspect that adding more verbiage might confuse
people more than it would help.

The only real problem we see with auto_restart is the opposite:
people misconfigure something else and Varnish restarts automatically
every X minutes, but they don't notice because autostart hides
their mistake.


Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@...eBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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