- 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 191: Prospero Directory Service
Port 191 is assigned by IANA to prospero (Prospero Directory Service).
191
How prospero works
Directory & authenticationThe client proves who it is on port 191 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 191 dangerous?
We have no specific security note for port 191. Its risk depends on the program actually listening on it. General advice for directory & authentication services:
IANA registry entries for port 191
| Service | Port | Transport | Description | Reference | Updated |
|---|---|---|---|---|---|
| prospero | 191 | TCP, UDP | Prospero Directory Service | - | - |
// source: IANA service name and port number registry
Which program uses port 191, and is it reachable?
Linux
sudo ss -ltnup 'sport = :191' # process listening on 191
sudo lsof -nP -i :191macOS
sudo lsof -nP -i :191
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 191 -State Listen | Select LocalAddress, OwningProcess Get-NetUDPEndpoint -LocalPort 191 netstat -ano | findstr :191
From another machine
nmap -p 191 -sV <host> sudo nmap -sU -p 191 <host> nc -zv <host> 191
Block it in the firewall
sudo ufw deny 191/tcp # Linux (ufw) New-NetFirewallRule -DisplayName "Block 191" -Direction Inbound -LocalPort 191 -Protocol TCP -Action Block # Windows
What is port 191 used for?
Port 191 is assigned by IANA to prospero (Prospero Directory Service).
Is port 191 TCP or UDP?
Port 191 is registered for TCP and UDP.
Is it safe to open port 191?
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 191 is open?
On Linux run "ss -ltnup 'sport = :191'", on macOS "lsof -nP -i :191", and on Windows "netstat -ano | findstr :191" to see the local process. From another machine, "nmap -p 191 <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