[VoiceOps] What does an ALG actually do?
abalashov at evaristesys.com
Sat Mar 2 17:17:35 EST 2013
Correct me if I'm wrong, but last time I looked, Linux's netfilter kernel module for SIP, ip_conntrack_sip, still is ignorant of SDP entirely.
Scott Berkman <scott at sberkman.net> wrote:
>How reliable and predictable an ALG is really varies vendor by vendor.
>standard firewalls' and routers' ALG do cause more problems (for
>most Cisco stuff), but the SIP specific vendors usually do a much
>job. My personal favorite is Edgewater Edgemarcs.
>Most generally what they do is provide layer 5+ (OSI) NAT,
>replacing addresses in the SIP and SDP headers. In most cases they
>also handle RTP, doing things like making sure outside ports are unique
>open based on following the SDP on the signaling side.
>From: voiceops-bounces at voiceops.org
>[mailto:voiceops-bounces at voiceops.org]
>On Behalf Of Tim Bray
>Sent: Thursday, February 28, 2013 6:45 AM
>To: voiceops at voiceops.org
>Subject: Re: [VoiceOps] What does an ALG actually do?
>On 27/02/13 21:33, John Levine wrote:
>> I realize that an ALG is a hack in a router that is supposed to allow
>> SIP packets to go through a NAT router. I also realize that for
>> modern SIP equipment, ALG usually causes more problems than it
>> and that it's described in RFCs 2663, 3424, and others.
>> What I can't find anywhere is what a SIP ALG actually does to the
>> packets. Is that written down anywhere, or is it just network
>The simple answer is `break stuff`.
>The marketing answer is `Sip is the next big thing, and we want to say
>are "SIP READY" so we put an ALG in`.
>The OKish ALGs are passive and sniff the ports for Qos etc.
>Most NAT passing ones just search and replace the IP addresses in the
>SIP and SDP. Mainly though, I've seen them swap one IP, but not the
>other. Or misread the port number. Very basic search and replace
>than properly parsing the messages. Bad idea.
Sent from my mobile, and thus lacking in the refinement one might expect from a fully-fledged keyboard.
Alex Balashov - Principal
Evariste Systems LLC
235 E Ponce de Leon Ave
Decatur, GA 30030
Web: http://www.evaristesys.com/, http://www.alexbalashov.com/
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