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MillersvilleEri's avatar
MillersvilleEri
Icon for Nimbostratus rankNimbostratus
Jan 16, 2026

can't access on prem dns when using F5LTM as a gateway

The title is the tl/dr.  I have a server on an internal network that is set up to use our F5 as a gateway.  I have all of the forwarding vip's set up and routed through snat pools.  and if i set the ...
dns
gateway
LTM F5
MillersvilleEri's avatar
MillersvilleEri
Icon for Nimbostratus rankNimbostratus
Jan 17, 2026

Hell Injeyan_Kostas​ 

Yes I am using the same internal dns on both the bigip and server.  And, as i am not 100% sure what you mean by creating a management route for internal dns i or subnet.  I'm going to guess that i have not. 

our dns servers are on a different vlan seperate from our external, management, and server vlan's.  however, the vlan with our dns servers is also a seperate internal network accessable by the bigip.  But access to this vlan through the bigip is not necessary.  

Injeyan_Kostas's avatar
Injeyan_Kostas
Icon for Nacreous rankNacreous
Jan 17, 2026

check the output of this command

tmsh list /sys management-route

is your dns server IP included?
If yes, your forwarding VIP will forward DNS traffic through MGMT interface


"our dns servers is also a seperate internal network accessable by the bigip"

Does this means that BIGIP has an interface in this vlan? 
Or there is a specific TMM route for thid vlan?

If any of the above is happening you will have to create a specific L4 VIP, matching only DNS destination and forward it to your TMM next hop or just use SNAT
Cause most probably you have asymetric routing issue