Server Port Service Verification (S-Series)

Port service verification is used by LSNAT server load balancing and TWCB to assure that the remote server is up. LSNAT and TWCB configurations support the TCP and UDP probe protocols for port service verification.

TCP port service verification can be enabled on one or more real servers, in a server load balancing configuration, or cache servers, in a TWCB configuration. A connect request is sent out to the server port. If the connect request succeeds then the local application knows the remote server is up.

UDP port service verification can be enabled on one or more real servers, in a server load balancing configuration. LSNAT accomplishes this by sending a UDP packet with “\r\n” (Carriage Return / Line Feed) as data to the UDP port. If the server responds with an ICMP “Port Unreachable” message, it is concluded that the port is not active and the real server is reported as “DOWN”. Otherwise, if the LSNAT local application does not get any response at all, it is assumed that the port is active and the server is reported as “UP”. The lack of a response could also be the result of the server itself not being available and could produce an erroneous indication of the server being “UP”. To avoid this when the probe protocol is UDP, an ICMP ping is used in combination with UDP to ensure that the real server is available. By default LSNAT sets up a faildetect ICMP probe 1 ($slb_default). Set up a faildetect UPD probe 2 to use in conjunction with the ICMP probe.