- Keep directory and authentication services on internal networks.
- Require signing or TLS and disable anonymous queries.
- Monitor authentication failures for spraying and brute force.
Port 491: go-login
Port 491 is assigned by IANA to go-login (go-login).
491
How go-login works
Directory & authenticationThe client proves who it is on port 491 and gets an answer (or a ticket) it can use with other services.
- 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
- Directory & authentication
Is port 491 dangerous?
We have no specific security note for port 491. Its risk depends on the program actually listening on it. General advice for directory & authentication services:
IANA registry entries for port 491
| Service | Port | Transport | Description | Reference | Updated |
|---|---|---|---|---|---|
| go-login | 491 | TCP, UDP | go-login | - | - |
// source: IANA service name and port number registry
Which program uses port 491, and is it reachable?
Linux
sudo ss -ltnup 'sport = :491' # process listening on 491
sudo lsof -nP -i :491macOS
sudo lsof -nP -i :491
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 491 -State Listen | Select LocalAddress, OwningProcess Get-NetUDPEndpoint -LocalPort 491 netstat -ano | findstr :491
From another machine
nmap -p 491 -sV <host> sudo nmap -sU -p 491 <host> nc -zv <host> 491
Block it in the firewall
sudo ufw deny 491/tcp # Linux (ufw) New-NetFirewallRule -DisplayName "Block 491" -Direction Inbound -LocalPort 491 -Protocol TCP -Action Block # Windows
What is port 491 used for?
Port 491 is assigned by IANA to go-login (go-login).
Is port 491 TCP or UDP?
Port 491 is registered for TCP and UDP.
Is it safe to open port 491?
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. Keep directory and authentication services on internal networks.
How do I check if port 491 is open?
On Linux run "ss -ltnup 'sport = :491'", on macOS "lsof -nP -i :491", and on Windows "netstat -ano | findstr :491" to see the local process. From another machine, "nmap -p 491 <host>" shows whether it is reachable.
Other directory & authentication ports
:389 LDAP tcp · udp · high risk :636 LDAPS tcp · udp · moderate risk :88 Kerberos tcp · udp · high risk :3268 Global Catalog (LDAP) tcp · udp · high risk :1812 RADIUS udp · tcp · moderate risk :31 MSG Authentication tcp · udp :49 Login Host Protocol (TACACS) tcp · udp :56 XNS Authentication tcp · udp