The link state routing channel between Exchange 2000/2003 routing group masters.
Exchange 2000 and 2003 used a link state algorithm to share the health of connectors between routing groups, and the routing group master exchanged that state over TCP 691. Microsoft dropped routing groups in Exchange 2007 in favour of Active Directory sites, so this port disappeared from supported deployments nearly two decades ago. Nmap still labels it "resvc" after the Exchange Routing Engine service. Seeing it open today means an unpatched, long-unsupported Exchange server is still running.
Do not expose
It is an internal mail-infrastructure control channel, and its presence implies an end-of-life Exchange server that should not be internet-facing at all.
$ nmap -sV -p 691 TARGETSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
Get Beige Box →also available as JSON · Markdown
Corrections or a missing port? Reply @rimrocksystems.