A penetration tester performs the following scan:nmap -sU -p 53,161,162 192.

In PenTest+ network enumeration, UDP scanning is emphasized as inherently less reliable than TCP because many UDP services do not respond unless they receive an application-valid request, and many firewalls silently drop unsolicited UDP probes. With Nmap -sU, if the scanner does not receive either (1) an application-layer UDP response indicating the service is listening or (2) an ICMP “port unreachable” message indicating the port is closed, Nmap cannot definitively classify the port. In that case, it reports open|filtered, meaning the port may be open but nonresponsive to the probe, or traffic may be filtered.

SNMP commonly requires a correct community string and properly formed requests before returning data. snmpwalk generates valid SNMP queries and can succeed even when Nmap’s generic probe yields no reply, resulting in an open|filtered label. Root privilege issues do not explain this state, and rate limiting is possible but not the best fit given the standard UDP classification behavior.