DNS setup

[Ronny_7]

Hi,

I have a system configured as DNS client pointing to three DNS servers. I have the entries for all the three DNS servers ip adresses in the /etc/resolv.conf.

But when I disconnect the first DNS server from the network, my DNS client seems not able to auto communicate to the second DNS server.

When I keyed "nslooklup", it hang there. But if I connect the first DNS server back into the network, the nslookup command will return the first DNS server name and ip address.

Any advise?

Regards,
Ronny

[Ralf Puchner]

Read the man page of resolv.conf:

"the algorithm used is to try a name server, and if the query times out, try the next, until out of name servers"

So the query must timeout. DNS sends a paket and waits for a response if there is no response within a while, the next dns server will be used.

[Ralf Puchner]

Last time I looked nslookup specifies the timeout=5 and the retries=4, which makes up for 5+10+20+40=75 secondes before nslookup skips to the next entry in resolv.conf.

[Ronny_7]

Hi Ralf,

Thank you for the replies.

1. Are you saying I did not wait long enough for the first query to timeout?

2. My resolv.conf looks like this,
domain m1.com.sg
nameserver 202.79.64.21
nameserver 202.56.128.30
nameserver 202.56.136.46

Is there any problem with it?

3. The man page says max three name servers, does it includes the domain entry for my case, since I have four entries.

4. Is there any way I can check the current timeout and retry settings in the "nslookup"?

5. Is there any way if I can change the timeout and retry settings?

Regards,
Ronny

[Ralf Puchner]

yes you must wait a little bit longer.
you can set the timeout and retry values within nslookup (set retry=, set timeout=). but this information is well documented within the man pages of nslookup.

[Al Licause · Accepted solution]

From your description, it is also possible that your second and possibly third nameserver entries are pointing to servers you either cannot reach or that do not offer a dns service.

If you comment or remove the first entry, then try another nslookup and get no response, the remaining resolv.conf entry may be invalid.

To confirm that those other services are being tried, use tcpdump to monitor port 53 for queries. If you see that a query has been sent to that service, but you get no response, then either the server doesn't have an entry for the host you're requesting in which case it may go off to another server or the root servers in an attempt to find it, or the host simply doesn't offer the service.

If the later you need to recheck with your dns admins to see if you have the correct entries in your resolv.conf file.

As Ralf says, try shortening the timeout parameters as documented in the man page for resolv.conf

Good luck.
Al

[Ronny_7]

As per replies.