HTTPfy

Port 6004 (x11)

IANA assigns port 6004 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 6004

Linux: is anything listening on TCP 6004?
sudo ss -tlnp 'sport = :6004'
Linux: UDP 6004
sudo ss -ulnp 'sport = :6004'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6004
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':6004 '
Windows (Command Prompt)
netstat -ano | findstr :6004
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 6004 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 6004
From Windows
Test-NetConnection HOST -Port 6004
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6004

Block port 6004 in the firewall

ufw
sudo ufw deny 6004/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 6004 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 6004 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 6004' -Direction Inbound -Protocol TCP -LocalPort 6004 -Action Block
ufw
sudo ufw deny 6004/udp
iptables
sudo iptables -A INPUT -p udp --dport 6004 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6004 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6004' -Direction Inbound -Protocol UDP -LocalPort 6004 -Action Block

Related ports

6000X116001x116002x116003x116005x116006x116007x116008x116009x116010x11

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