CyberHunter
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// Other services

Port 657: RMC

Port 657 is assigned by IANA to rmc (RMC).

657

How rmc works

Other services
client:657rmcresponse client:657rmcresponse

The client sends a request to port 657 and the rmc service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Other services
01 Security

Is port 657 dangerous?

We have no specific security note for port 657. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 657

ServicePortTransportDescriptionReferenceUpdated
rmc 657 TCP, UDP RMC - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 657, and is it reachable?

Linux

sudo ss -ltnup 'sport = :657'      # process listening on 657
sudo lsof -nP -i :657

macOS

sudo lsof -nP -i :657

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 657 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 657
netstat -ano | findstr :657

From another machine

nmap -p 657 -sV <host>
sudo nmap -sU -p 657 <host>
nc -zv <host> 657

Block it in the firewall

sudo ufw deny 657/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 657" -Direction Inbound -LocalPort 657 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 657 used for?

Port 657 is assigned by IANA to rmc (RMC).

Is port 657 TCP or UDP?

Port 657 is registered for TCP and UDP.

Is it safe to open port 657?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Close the port if nothing on the host needs it.

How do I check if port 657 is open?

On Linux run "ss -ltnup 'sport = :657'", on macOS "lsof -nP -i :657", and on Windows "netstat -ano | findstr :657" to see the local process. From another machine, "nmap -p 657 <host>" shows whether it is reachable.

05 Related
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