Nmap's Nping echo server — it shows you exactly how NAT and firewalls rewrote your packets in transit.
Nping's echo mode splits into a server and a client: the client sends crafted probe packets to the server, the server captures them off the wire and sends verbatim copies back over a separate encrypted TCP side channel using the Nping Echo Protocol. Comparing sent and echoed packets makes NAT rewrites, traffic shapers, and firewalls that strip TCP options immediately visible, and distinguishes "the request was dropped" from "the reply was dropped". The documented default for --echo-server is port 9929. It is unassigned at IANA; Nmap's own service database lists it as nping-echo, and Nmap's public scanme.nmap.org host has long run one, which is where most people first encounter the port.
Expose with care
The channel is encrypted and requires a shared passphrase, and running a public echo server is a deliberate choice Nmap supports. It still lets clients drive packet capture on your host, so run it only when you mean to and shut it down afterwards.
$ nping --echo-client "public" TARGET --tcp -p 80Seen 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.