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Port 18 (msp)

IANA assigns port 18 to msp: Message Send Protocol (historic).

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
msp TCP Message Send Protocol (historic) Rina Nethaniel -
msp UDP Message Send Protocol (historic) Rina Nethaniel -

Check what is listening on port 18

Linux: is anything listening on TCP 18?
sudo ss -tlnp 'sport = :18'
Linux: UDP 18
sudo ss -ulnp 'sport = :18'
Linux / macOS: which process owns the port
sudo lsof -nP -i :18
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':18 '
Windows (Command Prompt)
netstat -ano | findstr :18
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 18 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 18
From Windows
Test-NetConnection HOST -Port 18
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 18

Block port 18 in the firewall

ufw
sudo ufw deny 18/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 18 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 18 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 18' -Direction Inbound -Protocol TCP -LocalPort 18 -Action Block
ufw
sudo ufw deny 18/udp
iptables
sudo iptables -A INPUT -p udp --dport 18 -j DROP
nftables
sudo nft add rule inet filter input udp dport 18 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 18' -Direction Inbound -Protocol UDP -LocalPort 18 -Action Block

Related ports

2438msp15Unassigned [was netstat]17qotd19chargen20FTP (data, active mode)

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