Port 2855: Unassigned — FreeSWITCH's Default MSRP Port

What Range This Port Belongs To

Port 2855 falls in the registered ports range (1024–49151), sometimes called user ports. These are managed by IANA, and vendors or standards bodies can formally register a port for their service. Port 2855 has not been registered — IANA lists it as unassigned.¹

That doesn't mean it's idle.

The Unofficial Occupant: FreeSWITCH and MSRP

FreeSWITCH, the open-source telecommunications platform, uses port 2855 as the default listening port for its MSRP module.² Port 2856 is the companion port for MSRPS (MSRP over TLS).

MSRP — Message Session Relay Protocol — is defined in RFC 4975. It exists to solve a specific problem: once a SIP call is established, how do you send instant messages inside that session? Not SMS. Not email. Real-time text that's part of the call context.

MSRP doesn't use IANA-assigned ports the way HTTP uses 80 or HTTPS uses 443. MSRP URIs negotiate their own connection endpoints during session setup via SDP. FreeSWITCH has to listen somewhere by default, so it chose 2855.³

If you run FreeSWITCH and haven't touched the msrp.conf.xml configuration file, something is listening on 2855.

Security Considerations

FreeSWITCH users have reported that ports 2855 and 2856 open automatically when FreeSWITCH starts, even when MSRP functionality isn't intentionally configured — and early versions provided no easy way to disable them.⁴ If you're running FreeSWITCH and don't use MSRP, verify whether this port is unnecessarily exposed.

MSRP traffic on port 2855 is unencrypted. MSRPS on port 2856 uses TLS. Prefer 2856 for any production deployment.

How to Check What's Listening on This Port

Linux/macOS:

# See what process is using port 2855
ss -tlnp | grep 2855
# or
lsof -i :2855

Windows:

netstat -ano | findstr :2855

From outside the machine (careful — only scan systems you own):

nmap -p 2855 <host>

Why Unassigned Ports Matter

IANA's registry isn't just bureaucracy. It's a coordination mechanism — a way to prevent two different applications from fighting over the same number. When software uses unassigned ports without registering them, you get ambiguity: is port 2855 on this server FreeSWITCH, something else, or malware?

The registered port range has 48,128 slots. Fewer than 10,000 are formally assigned. The rest are a first-come, first-served informal landscape where convention and configuration files do the work that the registry doesn't.

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