GTM Load Balancing of pool of external DNS servers won't work

Haarith,

Thank you for your replying,

But I resolved this problem finally.

I found I had to add "snat automap" settings in ltm virtual "DNS_listener".

With this settings, I can resolve desired results.

Packet flow was like this.

(without snat automap)

[client]->[GTM]->[external DNS server]->[client]

- response packet was dropped in FW&LB between [external DNS server] and [client]

OR

-[client] dropped response packet.

(with snat automap)

[client]->[GTM]->[external DNS server]->[GTM]->[client]

(tmos) list /ltm virtual DNS_listener
ltm virtual DNS_listener {
    destination 172.16.0.11:domain
    ip-protocol udp
    mask 255.255.255.255
    pool DNS_pool
    profiles {
        dns { }
        udp_gtm_dns { }
    }
    snat automap
    translate-port disabled
}