[Users] Best practice for DNS failover using OpenSER?

Bogdan-Andrei Iancu bogdan at voice-system.ro
Tue Jan 16 16:46:33 CET 2007

Hi Klaus,

Klaus Darilion wrote:

>> Retransmissions are good (most of the time).  Somehow the ser
>> script would need to know that there is another thread tending to
>> a DNS lookup.  How does my script know if a message is original,
>> or a retransmission anyway?
>> It seems to me that the tm module should be reaping the retransmits.
> At first the proxy should reply 100 trying to stop retransmissions.

this is very and elegant to do with the new flags in t_reply(). You can 
statelessly send a 100 and configure t_relay() not to do it again. See:

letting t_relay() to auto generate the 100 is in most of the case 
futile, as it is too late.

> Further, the retransmissions should be absorbed by tm - if not then we 
> should analyze it.

the TM functions do that, but again, in most of the case it, as the 
relaying is done at the end of processing, it can be a wast of 
computation and time - just to apply all changes and perform ops to 
figure out at the end that the request was a retransmission.
So, you can use the t_check_trans() function from the beginning:

the great part is that the function does not affect the further 
processing and has no computation penalties.


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