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Port 6623 (ktelnet)

IANA assigns port 6623 to ktelnet: Kerberos V5 Telnet.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
ktelnet TCP Kerberos V5 Telnet Robert J Scott -
ktelnet UDP Kerberos V5 Telnet Robert J Scott -

Registry dates for this port: registered 2005-08.

Check what is listening on port 6623

Linux: is anything listening on TCP 6623?
sudo ss -tlnp 'sport = :6623'
Linux: UDP 6623
sudo ss -ulnp 'sport = :6623'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6623
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':6623 '
Windows (Command Prompt)
netstat -ano | findstr :6623
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 6623 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 6623
From Windows
Test-NetConnection HOST -Port 6623
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6623

Block port 6623 in the firewall

ufw
sudo ufw deny 6623/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 6623 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 6623 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 6623' -Direction Inbound -Protocol TCP -LocalPort 6623 -Action Block
ufw
sudo ufw deny 6623/udp
iptables
sudo iptables -A INPUT -p udp --dport 6623 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6623 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6623' -Direction Inbound -Protocol UDP -LocalPort 6623 -Action Block

Related ports

6621kftp6622mcftp6624datascaler-db6625datascaler-ctl

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.