[Devel] Parse Errors, error_route and fixing common NAT ALG bugs

[Klaus Darilion]

Aron Rosenberg wrote:
> This is a discussion question mostly, but part bug report. Our
> application targets general consumers and as such we see a huge amount
> of "broken" SIP message traffic come to our servers. These are packets
> where "friendly" NATs/routers try to fixup the SIP traffic to undo the
> NAT behavior. While most of these routers do the right thing, we have
> noticed several that have bugs in their implementations. These bugs
> manifest themselves as a mangling of the SIP headers in one or another.
> 

Yes - many ALGs sucks. A workaround is to put your SIP proxy to another 
port - not 5060. Then, many ALGs do not detect the SIP traffic anymore.
This is why my opensers are listening on port 5050 to.

> The major issue that we see in consumer routers is that the Via: header
> has been reconstructed incorrectly. The OpenSER bug that we see here is
> that an incorrect Via header does NOT result in error_route being
> invoked, nor does it result in a 400 error packet being sent to the
> originator either.
> 
>  
> 
> The discussion point is should OpenSER try to "fix" these broken routers
> so that the traffic can be used correctly. Two of the most common errors
> that our servers see are below
> 
>  
> 
> Issue 1. The via header has a port that is totally out of range. This
> error comes from the default DSL modem that Embarq / Sprint.net ships to
> their customers.
> 
>  
> 
> Via: SIP/2.0/UDP
> 1.1.1.1:13088001;branch=z9hG4bK-d87543-0865e7125c508513-1--d87543-;rport

IMO, this should be forwarded to the routing-script as the rport allows 
correct routing.

> Issue 2. The via header originally had a rport param from our client,
> but the router stripped it off incorrectly, notice how it ends in just a
> ; We believe this error is caused by Linksys WRT54G routers that don't
> have recent firmware. 
> 
> Via: SIP/2.0/UDP
> 1.1.1.1:56261;branch=z9hG4bK-d87543-9b572d31e0515e44-1--d87543-;

I have seen this many times too. The Via parser detects an error and the 
message will be discarded before entering the routing script. IMO it 
also makes sense to allow a ";" at the end - maybe turned on with a 
parameter: "relaxed_via_checking=yes"

regards
klaus

> 
> In both of these cases the NAT / router is changing the existing VIA
> header rather than adding a new VIA line
> 
>  
> 
>  
> 
> The question is should OpenSER allows these errors to proceed to the
> script layer? Right now it just swallows the packet.
> 
>  
> 
> Issue 1, if allowed to reach the script layer would be fixed by our call
> to nat_uac_test("23") and fix_nated_contact(). 
> 
>  
> 
> Issue 2, I am not so sure what the behavior would be if it reached the
> script layer.
> 
>  
> 
> Thoughts?
> 
>  
> 
> -Aron Rosenberg
> 
> SightSpeed Inc.
> 
> http://www.sightspeed.com <http://www.sightspeed.com/> 
> 
>  
> 
> 
> 
> 
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-- 
Klaus Darilion
nic.at