[VOIPSEC] TLS and Firewalls

Thu Feb 10 17:51:23 CST 2005

I should have been clearer in my last email. The term 'proxy' which I used
in my last email is not a SIP specific term. In firewall world, transparent
proxies are used to terminate the connections and create new connections
without changing any IP, TCP address fields and normally they don't change
any application data. One can call this as virtual termination. It only
gives implementation flexibility to inspect the payloads for access control
and for detecting any application specific attacks.  This can be extended to
virtual termination of clear connections and making TLS connections.  I
guess, since actual clients and servers don't know about the existence of
this proxy, it is called transparent proxy.  With respect to this, will it
be considered as violation if SIP message is not updated with 'VIA' header?
What functionality problems would it have, if the SIP messages are not
updated?

Srini

  _____  

From: Brian Raymond [mailto:Brian.Raymond at dataline.com] 
Sent: Thursday, February 10, 2005 11:40 AM
To: Srinivasa Rao Addepalli; Thomas Howe; Michael Sandee
Cc: VoIPsec
Subject: Re: [VOIPSEC] TLS and Firewalls

I have a long history with more legacy protocols, H.320, H.323 etc. so I'm
reliving a lot of the same things with SIP that people attempted to address
with H.323. In this case the mention of ALGs on firewalls brings back a lot
of memories.

One thing to consider when using the firewall as an ALG for traffic (be it a
B2BUA, proxy) is the amount of overhead it needs to deal with for both SDP
parsing and TLS termination if that is being utilized. In the end though
just as in H.323 the data that is most important to keep confidential is the
RTP data itself, the signaling was a secondary concern but never as
important. H.323 has H.235 to solve that problem and it is effective at what
it does, albeit not widely implemented. There isn't really a way around this
problem but I think more research needs to be done to determine the best way
to maintain a good security profile without demanding too much in the way of
hardware resources.

Transparent proxies might be something to consider but if you are indeed
terminating connections (which is required for looking inside TLS
connections) then you would need to rewrite the SDP messages with the
address of the termination point. That in itself will cause possible
problems with SIP and violate the protocol. SIP can do proxy routing, which
is within the bounds of the standard and I would think a much more viable
approach to solve this problem.

- Brian

On 2/10/05 1:28 PM, "Srinivasa Rao Addepalli" <srao at intoto.com> wrote:

>
> I agree with you. TLS is one reason, which I feel future firewall
> implementations would need to implement proxies. Also, if you observe the
> type of bugs that are being found in SIP implementations, it is not
entirely
> possible to detect and stop them, if firewalls do packet based inspection.
> Application firewalls, i.e. proxies is one way to detect and protect
> internal SIP devices. Proxies need not be B2BUA and IMO, firewall
> implementers go with transparent proxies. Transparent proxies have similar
> capabilities as normal proxies in terms of terminating client connections
> and making new connections, without SIP peers knowing about it.
>
> Srini
>
> -----Original Message-----
> From: Voipsec-bounces at voipsa.org [mailto:Voipsec-bounces at voipsa.org] On
> Behalf Of Thomas Howe
> Sent: Thursday, February 10, 2005 7:00 AM
> To: Michael Sandee
> Cc: Voipsec at voipsa.org
> Subject: Re: [VOIPSEC] TLS and Firewalls
>
> Hi Michael -
>
> Yes, you are certainly right on that point.  It is very useful to look
> at real world practice when working on the standards.  I have to side
> with Brian though, and feel that as cumbersome as the standards bodies
> are, they are the only real way to solve the long term problem. And more
> than that, they have clearly identified SIP as the protocol for handling
> session management, and the industry is cleary behind it now.
>
> Here's my question about SIP, firewalls and architectures - please tell
> me where I'm right/wrong/inbetween....
>
> Given that SIP signalling uses TLS for security, doesn't that sort of
> imply that most firewalls will be using some sort of back to back user
> agent, instead of packet inspection? I mean, how will the firewall be
> able to decrypt the streams appropriately without being a terminating
> point for the call?
>
>
> Tom
>
>
>
> Michael Sandee wrote on 2/9/2005, 4:28 PM:
>
>> Hi Thomas,
>>
>> I accept your comments in general, however they are not true for IAX.
>> But I know this is nearly impossible to know without looking at the
>> protocol in detail. Dispite earlier comment a RFC is still in the works,
>> and given the dozen of standards and extensions of earlier VoIP
>> protocols it might be better to first analyse a protocol in _practice_
>> before filing it as RFC.
>>
>> Ein Reich, Ein Protocol... or "One Protocol to rule them all"... I think
>> we are not looking at IAX as a replacement of any existing protocol, but
>> it has, as earlier pointed out, clear advantages over other protocols to
>> deal with _current_ problems.
>>
>> The IAX protocol has a mechanism to hand off calls between 2 endpoints,
>> but this can be disabled on a call by call basis. Two endpoints behind
>> NAT naturally cannot communicate with each other in a direct path, so
>> this will proceed through a central/public gateway (This is usually only
>> an issue with Internet Telephony, or very large Internetworks).
>>
>> For scalability this is ideal, because the entire call can be offloaded
>> to a different server, perhaps after authentication to one of the
>> frontends. In relation to SIP this can be done for the control channel
>> by using 302 Moved signalling, possibly a similar mechanism exists for
>> H323.
>>
>> Billing between VoIP/IP endpoints is something which is not important
>> (for us). These are free calls and only interesting for statistics.
>> However there is an extension to the protocol to actually handle this.
>>
>> Billing is usually done on the PSTN gateways.
>>
>> Regarding the security relation this might be interesting:
>> Given that you cannot do billing on your speech detection system, IVR,
>> or whatever.
>> Given that it is a blackbox speech processor.
>> Would you expose it to a direct media path to the user?
>>
>> [IAX ENDPOINT] - IP Network - [IAX GATEWAY] - SIP/H323/MGCP/SCCP/IAX -
>> [BACKEND SPEECH PROCESSING]
>> You will have a frontend either way, but it might be your security rules
>> are different than mine.
>>
>> Regards,
>>
>> Michael
>>
>> Thomas Howe wrote:
>>
>>> Michael -
>>>
>>> Let me weigh in on that one.  A single port for transport has some
>>> serious scaling and implementation issues.  A single port makes an
>>> assumption that the media and control signalling are both going to the
>>> same place.  Maybe that might be an OK assumption for an IP phone, but
>>> what about any large scale device? Would you really have all the media
>>> flow through the control processor, then out again into the media
>>> processor?  That becomes very cumbersome, very quickly.
>>>
>>> It also has a problem with disaggregation.  If you wanted to add an
>>> speech detection system into your network, how would that work? Would
>>> you put one into every IP phone? Or maybe you could come up with a call
>>> control scheme that would hand off the call to another system.  Well, if
>>> you completely handed off the call, how would you do billing and
>>> authorization? Or, if you didn't, can you imagine the convoluted call
>>> control ladders to get that done? And if you could, then try to scale
>>> that puppy, and you could get a discount price on Tylonol for your
>>> headache.
>>>
>>> It would be so much simpler to send the RTP somewhere else for a while,
>>> thus requiring a separate port for it. (Of course, two if you count
>> RTCP).
>>>
>>> This help any?
>>>
>>> Tom
>>>
>>>
>>>
>>> Michael Sandee wrote on 2/9/2005, 2:15 PM:
>>>
>>>> Brian,
>>>>
>>>> RTP and the problems surrounding firewalls, NAT/PAT have been
>> around for
>>>> quite a few years, being it H323, SIP or...
>>>> Trying to globally solve this is a nice goal to set, but
>> (apparently)
>>>> impossible to accomplish. There are workarounds like STUN which work
>>>> with _some_ devices.
>>>>
>>>> If one protocol comes forward which has some distinct advantages
>> over
>>>> the alternatives, it cannot be considered a "Not Invented Here"
>>>> protocol. The advantages are not only a single port, but also
>> trunking
>>>> and some other features which are very useful in a practical pbx
>>>> environment.
>>>>
>>>> Can you please elaborate on why exactly IAX is bad for choosing a
>> single
>>>> port as transport?
>>>>
>>>> Michael
>>>>
>>>> Brian Rosen wrote:
>>>>
>>>>> Ultimately, this is the problem with IAX.  It's a special protocol,
>>>>> promulgated by a small group, without a rigorous process.
>>>>>
>>>>> It's not in the general interest of the Internet Community
>> (whatever
>>>> that
>>>>> is) to have multiple ways to do the same thing.  SIP is the way the
>>> IETF
>>>>> decided to do session management, including voice, video and text
>>>> (although
>>>>> there are other IM protocols).  IETF is not the only game in
>> town, of
>>>>> course.
>>>>>
>>>>> I think that, actually, the IAX one port idea is a bad way to
>> handle
>>>>> signaling and multiple media streams related to the same session.
>>>> The fact
>>>>> that it makes it easier on the firewalls is not enough to
>> overcome the
>>>>> limitations it has.  We're better off working to make SIP and
>>>> firewalls work
>>>>> better together.
>>>>>
>>>>> Brian
>>>>>
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Voipsec-bounces at voipsa.org
>>> [mailto:Voipsec-bounces at voipsa.org] On
>>>>>> Behalf Of Diana Cionoiu
>>>>>> Sent: Wednesday, February 09, 2005 12:09 PM
>>>>>> To: Alexander
>>>>>> Cc: Voipsec at voipsa.org
>>>>>> Subject: Re: [VOIPSEC] TLS and Firewalls
>>>>>>
>>>>>> If you find any RFC avaibile for IAX let me know. Until now we
>> have
>>>>>> implement IAX based on what we have been able to learn from other
>>>> people
>>>>>> code. The problem with IAX secure is that of course there is no
>>>> standard
>>>>>> and we have to get all developers from different projects
>> together and
>>>>>> "maybe" we are lucky enough to convince them to make it work
>> right.
>>>>>>> From my experience each project has his own IAX version.
>>>>>>
>>>>>> Diana
>>>>>>
>>>>>>
>>>>>>
>>>>>>>> one port. The problem with IAX is that are no devices around. We
>>> hope
>>>>>>>>
>>>>>>>>
>>>>>> that
>>>>>>
>>>>>>
>>>>>>>  There are some devices with IAX support, and the trend is,
>> there
>>>>>>>  will be more soon. Just few of them:
>>>>>>>
>>>>>>>  http://www.iaxtalk.com/
>>>>>>>  http://www.digium.com/index.php?menu=iaxy
>>>>>>>  http://www.farfon.com/
>>>>>>>
>>>>>>> Regards,
>>>>>>> /Al
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Voipsec mailing list
>>>>>>> Voipsec at voipsa.org
>>>>>>> http://voipsa.org/mailman/listinfo/voipsec_voipsa.org
>>>>>>>
>>>>>>>
>>>>>> _______________________________________________
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>>>>>> Voipsec at voipsa.org
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>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Voipsec mailing list
>>>>> Voipsec at voipsa.org
>>>>> http://voipsa.org/mailman/listinfo/voipsec_voipsa.org
>>>>>
>>>>>
>>>>>
>>>>
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>>>
>>>
>>>
>>>
>>>
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>
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