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Asterisk slow to load and run if DNS server is not available

Posted on 2011-09-04
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Last Modified: 2012-05-12
I am running a couple of asterisk servers.  They are set to use a local DNS server - which forwards unknown domains to an external DNS.  I am seeing the following issues with Asterisk and DNS and any help is appreciated.

1. If the external resolution DNS is not available, when I try to load the dialplan, it takes a very long time.  you can see it loading 1 line at a time every few seconds.

2. If the local DNS server is down for any reason even after the asterisk load, it starts acting slow and erratic.

Is this an Asterisk issue or a CentOS issue.  And what can I do to avoid these slowdowns if the DNS's are down.
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Question by:thetechgroup
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Expert Comment

by:grblades
ID: 36483095
I would suggest you run a caching only nameserver on the asterisk box. You can set a forwarder to your regular DNS server aswell if you wish.
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Author Comment

by:thetechgroup
ID: 36483562
Would you then point the resolv to the local DNS instead of the general DNS?  Seems like that could work.  Do you have any idea what it is looking for that causes it to slow down considerably?
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Expert Comment

by:grblades
ID: 36483573
Yes you would point resolv.conf to the loopback address 127.0.0.1.
I am not sure whats causing it to slow down. You could run tcpdump on port 53 so it shows you all the dns requests.
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Author Comment

by:thetechgroup
ID: 36488067
Sounds like a good idea.  Let me try and report back.
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by:Ron M
ID: 36497447
You could try changing trunk host registrations, to IP address rather than Hostname...then it won't need to resolve them.

See this article and tell me if you think it might apply to your situation.
http://www.voip-info.org/wiki/view/Asterisk+SIP+srvlookup
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Expert Comment

by:Ron M
ID: 36497481
...also, how about these options in sip.conf...

http://www.asteriskdocs.org/html/apas02.html

externhost
externhost takes a fully qualified domain name as its argument. If Asterisk is behind NAT, the SIP header will normally use the private IP address assigned to the server. If you set this option, Asterisk will perform periodic DNS lookups on the hostname and replace the private IP address with the IP address returned from the DNS lookup.

    externhost=my.hostname.tld

 Warning
The use of externhost is not recommended in production systems, because if the IP address of the server changes, the wrong IP address will be set in the SIP headers until the next lookup is performed. The use of externip is recommended instead.
 

externip
externip takes an IP address as its argument. If Asterisk is behind NAT, the SIP header will normally use the private IP address assigned to the server. The remote server will not know how to route back to this address; thus, it must be replaced with a valid, routable address.

    externip=216.239.39.104
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Author Comment

by:thetechgroup
ID: 36498164
Actually, all our trunk registrations are IP based and we do not use externip or externhost.  The production PBX's do not reside behind a NAT.  Is it possibly not an Asterisk issue, but a LINUX requirement to have a valid and live DNS active at all times?
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Expert Comment

by:Ron M
ID: 36498814
Have you checked /var/log/asterisk/messages   for any errors?

Or /var/log/messages for system errors?
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Author Comment

by:thetechgroup
ID: 36584681
I still have not had a chance to install bind on the asterisk boxes yet.  Other issues came ahead, but this is still a modification we would like to make. What is the best way to configure bind as a caching server?
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grblades earned 250 total points
ID: 36585469
Depends on the distribution. If you are running Centos/Redhat then there is a package called caching-nameserver or something similarly named which you just install and set to start automatically.
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Author Closing Comment

by:thetechgroup
ID: 36587874
Thanks.  That makes perfect sense.
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