I have had for about a year very poor call quality from my voip phone and have called my voip service provider and my internet isp and they both could not find a reason for this poor call quality problem. I have always never been a complete fan of voip but it is the wave of the future so I continue to use it. They have run S&R test, changed configuration on my ATA adapter and even have come out to my home to change out some connection outside of of my home thinking it was something hardware. This has been going on for about a year and a half. Just last night I decided to telnet into my voip phone and look at the settings and was surprised to find that the dns servers chosen were ones that possibly is unique to my voip provider but for what ever reason I decided to run ping test on those dns servers that they had provided. The reply times were horrible 240ms on the average. They had specified 6 different IP addresses for dns server. I changed them to dns servers that I know perform better then what they had specified(19-25ms). Now my voip phone are 1000 times better. please explain to me why a dns server would have made a difference in voip quality if it is only dealing with ip addresses and not name translation?

If the VOIP software has the VOIP server hard coded by name it has to do a lookup for that. Even if it has it hard coded by IP address it may be the software is running some reverse lookup (puts in IP to find the name rather than name to find the IP).

I see this for example on NetBackup. It connects to a server fine then attempts to do a reverse lookup AFTER the connection to be sure it is where it thinks it is. If it went in via a different NIC than the "name" I had provided it will find the name I've assigned for that and bail out on me.

e.g. hostname = NIC1's IP
hostnameB = NIC2's IP

If the software went in successfully via NIC2 then did a "hostname" it would see the one for NIC1 and complain. The solution would be to add both to DNS (or even local /etc/hosts file) even though I knew it was using NIC2 for the backup.

The real question here is why is your VOIP or DNS provider's DNS resolution so slow. Do you have them first in your /etc/resolv.conf or something else first? Simply changing the order of their servers might help you. Also there is a flag you can set that reduces the delay between trying one DNS server and the next in resolv.conf.

On my LAN I have my BIND server setup to forward any request that it cant resolve to my external ISP DNS servers which has excellent response time (19-25ms). My VOIP ATA is in front of my network and my VOIP provider has program the ATA with it very slow DNS servers (150ms-244ms). I just simply telneted to the ATA and changed the DNS server to point to my kick but DNS servers and that completely improved my VOIP call quality. The calls dropped, had a lot of static and just plain and simply terrible call quality. Just that one change alone made a world of difference. I guess what you said about maybe the ATA device does some reverse DNS lookups and when there is a delay then it can affect any voice or data. Wow that was big lesson learned.

Of course sometimes you can bypass all that by just adding the entries you know you need to your local hosts file. hosts and nslookup won't see them but the C routines that do lookups will so long as you've made sure to list both "files" and "dns" in your nsswitch.conf.

Many thanks for the clarification!