The Port That Does Not Exist

Port 0, the reserved TCP/UDP port number that most tools skip by convention, is quietly hosting live services across the IPv4 internet, according to a full-internet scan by the blog ACIDVEGAS published November 6, 2026. The author, whose IRC network already runs on port 0, scanned all of IPv4 for listeners on the port and found unexpected neighbors, arguing that because RFC 9293 sets no rule against zero and IANA merely reserves 0/tcp and 0/udp under RFC 6335, nothing on the wire prevents packets from being addressed there. The confusion stems from the sockets API rather than the protocol: passing port 0 to bind() asks the kernel to pick an ephemeral port, typically drawn from Linux's default 32768–60999 range set by net.ipv4.ip_local_port_range, and no CAP_NET_BIND_SERVICE privilege is required. The kernel's selection logic in inet_connection_sock.c starts at a random offset and forces odd parity to avoid colliding with connect() users. The finding matters because port 0 sits outside default scanner lists and browser behavior, making it an overlooked place to hide services. What to watch next is whether the scan results prompt operators to audit their own port-0 listeners or push for clearer standards language on the reserved value.