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// Directory & authentication

Port 221: Berkeley rlogind with SPX auth

Port 221 is assigned by IANA to fln-spx (Berkeley rlogind with SPX auth).

221

How fln-spx works

Directory & authentication
userdirectoryfln-spxresponse userdirectoryfln-spxresponse

The client proves who it is on port 221 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
01 Security

Is port 221 dangerous?

We have no specific security note for port 221. Its risk depends on the program actually listening on it. General advice for directory & authentication services:

  • Keep directory and authentication services on internal networks.
  • Require signing or TLS and disable anonymous queries.
  • Monitor authentication failures for spraying and brute force.
02 Registry

IANA registry entries for port 221

ServicePortTransportDescriptionReferenceUpdated
fln-spx 221 TCP, UDP Berkeley rlogind with SPX auth - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 221, and is it reachable?

Linux

sudo ss -ltnup 'sport = :221'      # process listening on 221
sudo lsof -nP -i :221

macOS

sudo lsof -nP -i :221

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 221 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 221
netstat -ano | findstr :221

From another machine

nmap -p 221 -sV <host>
sudo nmap -sU -p 221 <host>
nc -zv <host> 221

Block it in the firewall

sudo ufw deny 221/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 221" -Direction Inbound -LocalPort 221 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 221 used for?

Port 221 is assigned by IANA to fln-spx (Berkeley rlogind with SPX auth).

Is port 221 TCP or UDP?

Port 221 is registered for TCP and UDP.

Is it safe to open port 221?

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 221 is open?

On Linux run "ss -ltnup 'sport = :221'", on macOS "lsof -nP -i :221", and on Windows "netstat -ano | findstr :221" to see the local process. From another machine, "nmap -p 221 <host>" shows whether it is reachable.

05 Related
: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

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