[Buildbot-devel] proxy support?

F. Oliver Gathmann gathmann at cenix-bioscience.com
Thu Feb 8 13:57:05 UTC 2007

Wow - thank you both, Benoit and Brian, for your elaborate replies!
Using a ssh tunnel is really a creative solution, although the idea of
having to constantly re-open the tunnel under cygwin scared me a bit. We
ended up using a port forwarder running on our proxy server (rinet under
Linux) just as Brian suggested and now the slave buildbots run fine (a
great piece of software, Brian, thank you!).

I also enjoyed reading Brian's views on the almost philosophical issue
of opening unsual ports in the company firewall. I suppose the general
sentiment of sysadmins is to prefer to err on the safe side - when you
are, like our sysadmin, charged with ensuring the reliable operation of
60 or so machines running three different operating systems and operated
mostly by people with little concept of the dangers lurking out there,
you better make sure you control what's going out and what's coming in,
if only to cover your own back in case something goes wrong. But you are
right - sysadmins should not go unchecked, either, and luckily ours is a
quite reasonable chap :-) .

Thank you again for your time,



Brian Warner wrote:
>> We have a situation where our buildbot master for an Open Source project
>> we use lives outside the company, but we would like to run buildbot
>> slaves for two target platforms inside the company firewall.
> If you don't have direct network connectivity from the buildslave to the
> buildmaster, then yes, you have to use some sort of port-forwarding or
> firewall. The buildslave-to-buildmaster connection is designed to easily work
> around NAT boxes, but not against restrictive filtering policies.
> Benoit's suggestion of using an ssh-based port forwarding is one possibility,
> if you have a suitable target host that both (A) you are able to log into,
> and (B) has a better chance of connecting to the buildmaster than the
> original machine.
> Many firewalls offer a SOCKS server, which is basically a service that runs
> on the firewall/proxy box that offers to make connections to external hosts
> for you. You could run the buildslave under the 'tsocks' utility to make it
> use the SOCKS server instead of direct connections: that tool uses
> shared-library tricks to cause the unsuspecting application's
> networking-related system calls to use the SOCKS server. (actually you can
> configure a specified subset of their connections to use the proxy, so
> in-perimeter targets get direct connections, etc).
> If your network policies allow direct connections to certain ports (say, port
> 80), then you could rig a port forwarder on some external host that listens
> on port 80 and relays the data to the buildmaster. Or, you could tell the
> buildmaster to listen on port 80 (and patch the kernel to remove that
> pointless low-numbered-ports-must-be-opened-as-root misfeature).
> In general, if you can get *some* connectivity to the outside work, you can
> jury-rig some sort of proxy that will let the program of your choice get to
> the target of your choice.
> Now that I've proposed a couple of solutions, allow me to complain about the
> problem for a moment :-)
>> Our sysadmin (rightly) refuses to just open the port the buildbot master is
>> listening on (9989),
> <rant directed_against="that sysadmin's policies, not you!">
> I have to disagree with that "rightly" there. I know it seems to be
> fashionable in contemporary IT circles to lock down everything inbound and
> outbound, and to refuse to honor requests to open ports, but what purpose
> does this serve other than to inhibit their customers (i.e. the employees)
> from getting their job done?
> I appreciate that network-using applications are too hard to implement safely
> and represent too large of an attack surface, such that prohibiting arbitrary
> inbound connections represents a reasonable compromise between safety and
> utility: the practice is meant to protect internal machines that are too
> buggy to protect themselves, at the expense of being unable to use the
> network in new and interesting ways.
> And to protect certain services on external machines, some environments
> require that all connections to, say, port 25 must go through an
> accounting/proxy box, prohibiting any direct connection to external hosts at
> this port. This allows the proxy box to function as a monitoring tool to
> detect when an internal machine is spewing spam, probably because it has been
> converted into a spam-zombie by a virus that arrived through some other
> vector. This is a compromise between protecting external hosts and reducing
> the robustness and efficiency of the network (now all your outbound mail is
> dependent upon that one accounting/proxy box, even though your local machine
> is perfectly capable of speaking SMTP itself).
> But what benefit can there be to prohibiting outbound connections to a
> non-well-known port like 9989? There aren't any public services being offered
> there (except for the buildmaster you're trying to connect to), so there's
> nothing external to protect. Malware that takes over a local machine that
> wants to phone home will go through some other port, usually 80, to make
> itself look less suspicious, and will probably just take advantage of the
> same proxy settings as everything else on your system.
> If the goal of the proxy/firewall is monitoring, then the sysadmin would want
> the different kinds of traffic to look as different as possible. Forcing both
> virus authors and application developers to make their traffic pretend to be
> HTTP on port 80 just to get past these filters is only reducing the amount of
> information available, and making it more expensive to inspect the packets to
> figure out what they're for.
> (in a related vein, requiring that applications run as root to listen on port
> 80 increases the capabilites of the server, which makes any bug or server
> compromise much more dangerous, and/or requires code to shed root after
> obtaining the port which is easy to get wrong)
> It's hard for me to imagine a reason for these sorts of policies that doesn't
> stem from either a desire for power/control or from a lack of analysis. If
> this were my office, I'd ask the sysadmin to reconsider that policy, and if
> they wouldn't, I'd ask the management to reconsider the sysadmin.
> In an engineering environment, IT is there to provide safe services to the
> engineers. If there is an antagonistic relationship between the two,
> something is wrong.
> </rant>
> Whew! :)
> So I'd ask your sysadmin if they would allow just your buildslave to make a
> direct connection to the buildmaster, at that one port: a hole in the
> firewall that is specific to the 3-tuple of (buildslave_ipaddr,
> buildmaster_ipaddr, buildmaster_port). If they refuse, ask them what security
> benefit is gained by that prohibition (at least make them think about the
> question.. they may benefit in other ways from doing that sort of analysis).
> Then rig up some sort of proxy to work around the bug.
> cheers,
>  -Brian

F. Oliver Gathmann, Ph.D.
Director IT Unit
Cenix BioScience GmbH
Tatzberg 47                       phone: +49 (351) 4173-136
D-01307 Dresden, Germany          fax:   +49 (351) 4173-109

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