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Port 24 (any private mail system)

IANA assigns port 24 to any private mail system.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
- TCP any private mail system Rick Adams -
- UDP any private mail system Rick Adams -

Check what is listening on port 24

Linux: is anything listening on TCP 24?
sudo ss -tlnp 'sport = :24'
Linux: UDP 24
sudo ss -ulnp 'sport = :24'
Linux / macOS: which process owns the port
sudo lsof -nP -i :24
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':24 '
Windows (Command Prompt)
netstat -ano | findstr :24
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 24 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 24
From Windows
Test-NetConnection HOST -Port 24
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 24

Block port 24 in the firewall

ufw
sudo ufw deny 24/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 24 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 24 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 24' -Direction Inbound -Protocol TCP -LocalPort 24 -Action Block
ufw
sudo ufw deny 24/udp
iptables
sudo iptables -A INPUT -p udp --dport 24 -j DROP
nftables
sudo nft add rule inet filter input udp dport 24 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 24' -Direction Inbound -Protocol UDP -LocalPort 24 -Action Block

Related ports

22SSH23Telnet25SMTP27nsw-fe

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