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Port 300 (tacacss)

IANA assigns port 300 to tacacss: TLS Secure Login Host Protocol (TACACSS).

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
tacacss TCP TLS Secure Login Host Protocol (TACACSS) IESG RFC 9887

Registry dates for this port: registered 2025-07-15.

Check what is listening on port 300

Linux: is anything listening on TCP 300?
sudo ss -tlnp 'sport = :300'
Linux / macOS: which process owns the port
sudo lsof -nP -i :300
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':300 '
Windows (Command Prompt)
netstat -ano | findstr :300
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 300 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 300
From Windows
Test-NetConnection HOST -Port 300

Block port 300 in the firewall

ufw
sudo ufw deny 300/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 300 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 300 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 300' -Direction Inbound -Protocol TCP -LocalPort 300 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.