Forum Discussion

Cirrus

Jul 15, 2024

Any way to do DNS loadbalancing without BIG-IP DNS module?

Hi, In our environment we have a number of domain controllers which act as DNS servers for everything internally. Now, we have one specific type of client that is only able to be configured with ...

dns

LTM VE

monitors

Moonlit
Jul 17, 2024
So yes, there was a L3 problem - which was that I chose the wrong interface on the BIG-IP VE to monitor traffic. Turned out the monitor packets were sent on the management interface because of a routing thing I just happened to remember.

Anyway: PROBLEM SOLVED. In order to monitor a DNS server by sending a query and checking for a correct response, I did it the hard way by hand-crafting the packets sent to replicate the bytestring as seen when doing a manual DNS lookup, through dumping the packet.

In the "Send string" field you can enter individual bytes by prefixing the hex value with "\x", so I copied the DNS header (including transaction id, query number etc) plus the actual query, converted it to \x format and put it in the Send string field.
In the Receive string field I entered just the ASCII IP address which I knew the correct query would result in if the server is healthy.

zamroni777

Nacreous

the dns monitor doesnt have such limitation.

you should try udp monitor with any send string.
if you dont see the trafic in the tcpdump, then very likely there is L3/L4 config problem.

Moonlit

Cirrus

Jul 17, 2024

So yes, there was a L3 problem - which was that I chose the wrong interface on the BIG-IP VE to monitor traffic. Turned out the monitor packets were sent on the management interface because of a routing thing I just happened to remember.

Anyway: PROBLEM SOLVED. In order to monitor a DNS server by sending a query and checking for a correct response, I did it the hard way by hand-crafting the packets sent to replicate the bytestring as seen when doing a manual DNS lookup, through dumping the packet.

In the "Send string" field you can enter individual bytes by prefixing the hex value with "\x", so I copied the DNS header (including transaction id, query number etc) plus the actual query, converted it to \x format and put it in the Send string field.
In the Receive string field I entered just the ASCII IP address which I knew the correct query would result in if the server is healthy.