[Kamailio-Users] DNS unreachable

[Henning Westerholt]

On Monday 26 January 2009, Zappasodi Daniele wrote:
> I'm testing the proxy behaviour (Openser 1.3.2) when DNS servers are
> unreachable and I observe that with few requests that require the DNS, I
> have the proxy completely blocked, while it could serve all the other
> methods that don't need the DNS (method with IP adddress or incoming
> REGISTERs). I observe that each child process remains suspended until all
> DNS requests are finished (NAPTR, SRV and A). If I configure two DNS server
> and X retransmissions after Y seconds, the child remain suspended for: 2 *
> X * Y * 3 seconds (3=NAPTR+SRV+A queries)
> Even if I set low values for X and Y (X=2 retr and Y=2 sec.) the child is
> suspended for a long interval: 2*2*2*3=24 seconds.
>
> Is it possible to avoid that chlidren remain suspended?

Hi Zappasodi,

the blocking is caused because the server tries to resolve the address 
synchronously. So its not possible to avoid this blocking completely.

> Is there any configuration param that can reduce this interval?

Yes, take a look at the dns_retr_time and dns_retr_no parameter. But the 
lowest timeout value possible (with glibc) is 1s, if one want to have a lower 
timeout, the used resolver needs to be changed.

Another thing that is useful to avoid/ minimize blocking is the use of a local 
DNS cache, which also caches failed resolve attempts.

> Is it possibile to introduce a little change in sip_resolvehost (check
> errno) in order to avoid the SRV and A queries if the NAPTR fails because
> the DNS servers are unreachable?

If you've a patch that improves the behaviour, just add it to our tracker [1]. 
In newer versions of the server the DNS implementation was also changed, 
perhaps you can review if the problem is already fixed there.

Cheers,

Henning


[1] https://sourceforge.net/tracker/?group_id=139143&atid=743022