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// Other services

Port 1903: Local Link Name Resolution

Port 1903 is assigned by IANA to linkname (Local Link Name Resolution).

1903

How linkname works

Other services
client:1903linknameresponse client:1903linknameresponse

The client sends a request to port 1903 and the linkname 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
Registered (1024-49151)
Category
Other services
01 Security

Is port 1903 dangerous?

We have no specific security note for port 1903. 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 1903

ServicePortTransportDescriptionReferenceUpdated
linkname 1903 TCP, UDP Local Link Name Resolution - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 1903, and is it reachable?

Linux

sudo ss -ltnup 'sport = :1903'      # process listening on 1903
sudo lsof -nP -i :1903

macOS

sudo lsof -nP -i :1903

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1903 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 1903
netstat -ano | findstr :1903

From another machine

nmap -p 1903 -sV <host>
sudo nmap -sU -p 1903 <host>
nc -zv <host> 1903

Block it in the firewall

sudo ufw deny 1903/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 1903" -Direction Inbound -LocalPort 1903 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 1903 used for?

Port 1903 is assigned by IANA to linkname (Local Link Name Resolution).

Is port 1903 TCP or UDP?

Port 1903 is registered for TCP and UDP.

Is it safe to open port 1903?

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 1903 is open?

On Linux run "ss -ltnup 'sport = :1903'", on macOS "lsof -nP -i :1903", and on Windows "netstat -ano | findstr :1903" to see the local process. From another machine, "nmap -p 1903 <host>" shows whether it is reachable.

05 Related
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