DNS Lookup

I had the syntax way wrong for RESOLV::conf. Please check out the wiki:

 

 

http://devcentral.f5.com/wiki/default.aspx/iRules/resolv__lookup.html

 

 

Thanks for the updates. Would RESOLVE::lookup been too intuitive for a name? :D

 

 

Any idea why this command wouldn't be valid in RULE_INIT? Logically, it seems like it wouldn't need to be tied to a connection.

 

 

Thanks,

 

Aaron

 

 

Hi Tom,

 

 

That was the easiest event to test it with (and not have to add the rule to a VIP and make a request). I could imagine it might be useful to only perform the lookup once if it's not a value that is expected to change frequently. I guess it's more of a question of why not.

 

 

Aaron

Hi,

 

 

I am trying to make RESOLV::lookup more robust by resolving against more than one DNS server. I have seen elsewhere in the forums that if you configure a DNS VIP that load balances to a pool of DNS servers (eg 1.1.1.1:53) and then specify this VIP in the RESOLV command i.e. (RESOLV::lookup @1.1.1.1 www.test.com) this should work.

 

 

Well, in 10.2.1 i cannot get this to work. is this possible? I have tried using the "virtual" command within the RESOLV::lookup and that doesn't work either.

 

 

Anybody managed to get this to work??

Hi Evan,

 

 

If you query the DNS virtual server directly does it work? dig @1.1.1.1 www.example.com

 

 

Aaron

Hey aaron,

 

 

Yep, if i do a dig @VIP from the Bigip CLI (or from an external client) this works. but within the iRule, the lookup times out...

 

 

I have a workaround where i set the resolve retries and timouts to be small and then sequentially try the next server if there is a failure i.e

 

 

set ips [RESOLV::lookup @$static::DNS_RESOLVER $host]

 

set _ipaddress [lindex $ips 0]

 

if {$_ipaddress eq "" }{

 

set ips [RESOLV::lookup @$static::DNS_RESOLVER_BACKUP $host]

 

set _ipaddress [lindex $ips 0]

 

}

 

 

It would be MUCH better if we could avoid the timeout and go directly to the working DNS server once Bigip has marked the other one down...

Could you specifically check each of the nodes with LB::status?

Something like:

set status1 [LB::status pool  member  ]
set status2 [LB::status pool  member  ]

if {$status1 eq "up"} {
  set ips [RESOLV::lookup @$static::DNS_RESOLVER $host]
  set _ipaddress [lindex $ips 0]
} elseif {$status2 eq "up"}{
  set ips [RESOLV::lookup @$static::DNS_RESOLVER_BACKUP $host]
  set _ipaddress [lindex $ips 0]
} 

This way you wouldn't have to endure the timeout.

Colin

I expected the VS IP to work with RESOLV::lookup. It would be great if you could open a case with F5 Support if this isn't working to document the issue and ideally get it fixed (if it's a bug or limitation).

 

 

Aaron

Hey did you ever open up a case on this one? I'm looking to do something similar in the future.. Would you mind posting what you found/find?

 

 

Thanks!

Colin - thanks for the suggestion, i may give it a try in the short term.

 

 

In the long term, i'm opening a case... will update you all with what i find...

 

 

Cheers,

 

Evan.