The classic "pick a memorable port" number — QEMU's debug stub and VLC's stream examples both land here.
The IANA registration is search-agent, "Infoseek Search Agent", which has no practical presence today. What actually uses 1234 is convention: QEMU's -s flag makes the guest's gdbstub listen on TCP 1234 so GDB can attach, and VLC's UDP/RTP streaming examples use 1234 as the destination port. Because it is short and easy to remember, ad-hoc test servers and tutorials pick it constantly, so an open 1234 usually means someone's debugger, stream, or throwaway listener rather than a defined service.
Do not expose
A QEMU gdbstub is unauthenticated full control of the guest, including memory writes — bind it to localhost. Ad-hoc listeners on 1234 are, by nature, not hardened.
$ sudo lsof -nP -iTCP:1234 -sTCP:LISTEN-s listens for gdb on TCP port 1234)qemu.org
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