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Port 6040 (x11)

IANA assigns port 6040 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 6040

Linux: is anything listening on TCP 6040?
sudo ss -tlnp 'sport = :6040'
Linux: UDP 6040
sudo ss -ulnp 'sport = :6040'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6040
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':6040 '
Windows (Command Prompt)
netstat -ano | findstr :6040
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 6040 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 6040
From Windows
Test-NetConnection HOST -Port 6040
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6040

Block port 6040 in the firewall

ufw
sudo ufw deny 6040/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 6040 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 6040 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 6040' -Direction Inbound -Protocol TCP -LocalPort 6040 -Action Block
ufw
sudo ufw deny 6040/udp
iptables
sudo iptables -A INPUT -p udp --dport 6040 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6040 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6040' -Direction Inbound -Protocol UDP -LocalPort 6040 -Action Block

Related ports

6000X116001x116002x116003x116004x116005x116006x116007x116008x116009x11

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