HTTPfy

Port 6003 (x11)

IANA assigns port 6003 to x11: X Window System.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
x11 TCP X Window System (range 6000-6063) Stephen Gildea -
x11 UDP X Window System (range 6000-6063) Stephen Gildea -

Check what is listening on port 6003

Linux: is anything listening on TCP 6003?
sudo ss -tlnp 'sport = :6003'
Linux: UDP 6003
sudo ss -ulnp 'sport = :6003'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6003
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':6003 '
Windows (Command Prompt)
netstat -ano | findstr :6003
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 6003 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 6003
From Windows
Test-NetConnection HOST -Port 6003
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6003

Block port 6003 in the firewall

ufw
sudo ufw deny 6003/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 6003 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 6003 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 6003' -Direction Inbound -Protocol TCP -LocalPort 6003 -Action Block
ufw
sudo ufw deny 6003/udp
iptables
sudo iptables -A INPUT -p udp --dport 6003 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6003 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6003' -Direction Inbound -Protocol UDP -LocalPort 6003 -Action Block

Related ports

6000X116001x116002x116004x116005x116006x116007x116008x116009x116010x11

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