Refer to the exhibit.

The virtual server is configured for destination 192.168.21.50 , service port 53 , but its Protocol is TCP . Standard DNS query traffic is normally sent using UDP port 53 . Because virtual-server traffic matching includes the configured IP protocol, UDP DNS packets will not match a virtual server configured exclusively for TCP, causing the queries to fail.

F5 ' s DNS configuration guidance explicitly instructs administrators creating a DNS query virtual server to configure service port 53 and select UDP as the protocol. F5 also recommends separate TCP and UDP virtual servers when both forms of DNS traffic must be supported; TCP port 53 is particularly relevant for operations such as DNS zone transfers.

Option C is not required. The existing source network 192.168.100.0/23 spans addresses from 192.168.100.0 through 192.168.101.255, so both 192.168.101.100 and 192.168.101.200 are already permitted by the source-address criterion. F5 documents that the Source Address setting can specify the network from which a virtual server accepts traffic.

Therefore, changing the virtual server ' s Protocol from TCP to UDP corrects the traffic-matching problem.

Reference Topics: Virtual Server traffic matching; Protocol selection; DNS over UDP/TCP; Source Address matching; Service Port 53.