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Port 89 (su-mit-tg)

IANA assigns port 89 to su-mit-tg: SU/MIT Telnet Gateway.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
su-mit-tg TCP SU/MIT Telnet Gateway Mark Crispin -
su-mit-tg UDP SU/MIT Telnet Gateway Mark Crispin -

Check what is listening on port 89

Linux: is anything listening on TCP 89?
sudo ss -tlnp 'sport = :89'
Linux: UDP 89
sudo ss -ulnp 'sport = :89'
Linux / macOS: which process owns the port
sudo lsof -nP -i :89
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':89 '
Windows (Command Prompt)
netstat -ano | findstr :89
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 89 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 89
From Windows
Test-NetConnection HOST -Port 89
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 89

Block port 89 in the firewall

ufw
sudo ufw deny 89/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 89 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 89 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 89' -Direction Inbound -Protocol TCP -LocalPort 89 -Action Block
ufw
sudo ufw deny 89/udp
iptables
sudo iptables -A INPUT -p udp --dport 89 -j DROP
nftables
sudo nft add rule inet filter input udp dport 89 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 89' -Direction Inbound -Protocol UDP -LocalPort 89 -Action Block

Related ports

87any private terminal link88Kerberos90dnsix91mit-dov

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