Re: Skype through a firewall?

[Kevin <kkadow () gmail com>]

On 8/25/06, Paul D. Robertson <paul () compuwar net> wrote:
> On Thu, 24 Aug 2006, Kevin wrote:
> > Is anybody permitting Skype through a HTTP or SOCKS proxy?
> > I've been instructed to "make Skype work", and short of opening up the
>
> Whenever you have a "this application must work," you should look at what
> the actual requirement is...

I wish I could.

Unfortunately, when a request comes down from a personality spoken of
primarily by their three-letter first name, bearing the title "SVP/
CTO $REMOTESITE", the actual requirement is that the buzzword-friendly
Skype desktop application must work.  No excuses.

If I could show Skype itself (or the firewall policy changes to enable
it) pose "an immediate threat to the security, performance or
stability of the corporate intranet", then I can use policy to say no,
even to a SVP or CTO.

What little I know from my own testing and from published research is
that the binary is encrypted and debugger-resistant, as is the
protocol, and that the P2P nature of Skype makes me very
uncomfortable.  But that's not enough to deny this V(I)P's request.


> > outbound policy to permit TCP and UDP to every possible destination IP
> > on every possible port, the next best thing seems to be to use the
> > HTTPS and SOCKS5 proxy settings included in most platforms/versions of
> > Skype.

Opening a HTTPS proxy for Skype requires at a minimum permitting
outbound "CONNECT" to every possible destination IP on port 443, and
disabling any IPS or other device which might detect that the protocol
running across port 443 isn't really SSL.  Many proxy gateways
currently don't inspect the protocol, this is how Skype works through
Squid and other web proxies.


> > I'm running into some odd issues while trying to write a reasonable
> > proxy policy for Skype and still have reliable calling and reasonable
> > audio quality.
> >
> > Any hints?
>
> 1.  Terminal Service to a TS in the DMZ with the client loaded.

Thanks, that's an interesting idea.
I know RDP can route audio outbound to the client, but how do I get
the microphone audio back out?


> 2.  Asterisk PBX in the DMZ as a gateway (much more fun) with IAX2 or SIP
> client access from the LAN.  Do all the conference bridge stuff on
> Asterisk and gateway a single Skype call at a time if you need to using
> psgw_linux ($20.)
> 3.  Deny the request as unreasonablely out of kilter with the security
> policy in place and make them do the requirement over.

See above :(

Kevin
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