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Port 20003 (commtact-https)

IANA assigns port 20003 to commtact-https: Commtact HTTPS.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
commtact-https TCP Commtact HTTPS Tomas Svoboda -
commtact-https UDP Commtact HTTPS Tomas Svoboda -

Registry dates for this port: registered 2004-12.

Check what is listening on port 20003

Linux: is anything listening on TCP 20003?
sudo ss -tlnp 'sport = :20003'
Linux: UDP 20003
sudo ss -ulnp 'sport = :20003'
Linux / macOS: which process owns the port
sudo lsof -nP -i :20003
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':20003 '
Windows (Command Prompt)
netstat -ano | findstr :20003
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 20003 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 20003
From Windows
Test-NetConnection HOST -Port 20003
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 20003

Block port 20003 in the firewall

ufw
sudo ufw deny 20003/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 20003 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 20003 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 20003' -Direction Inbound -Protocol TCP -LocalPort 20003 -Action Block
ufw
sudo ufw deny 20003/udp
iptables
sudo iptables -A INPUT -p udp --dport 20003 -j DROP
nftables
sudo nft add rule inet filter input udp dport 20003 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 20003' -Direction Inbound -Protocol UDP -LocalPort 20003 -Action Block

Related ports

20001microsan20002commtact-http20005openwebnet20012ss-idi-disc

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