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Port 84 (ctf)

IANA assigns port 84 to ctf: Common Trace Facility.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
ctf TCP Common Trace Facility Hugh Thomas -
ctf UDP Common Trace Facility Hugh Thomas -

Check what is listening on port 84

Linux: is anything listening on TCP 84?
sudo ss -tlnp 'sport = :84'
Linux: UDP 84
sudo ss -ulnp 'sport = :84'
Linux / macOS: which process owns the port
sudo lsof -nP -i :84
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':84 '
Windows (Command Prompt)
netstat -ano | findstr :84
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 84 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 84
From Windows
Test-NetConnection HOST -Port 84
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 84

Block port 84 in the firewall

ufw
sudo ufw deny 84/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 84 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 84 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 84' -Direction Inbound -Protocol TCP -LocalPort 84 -Action Block
ufw
sudo ufw deny 84/udp
iptables
sudo iptables -A INPUT -p udp --dport 84 -j DROP
nftables
sudo nft add rule inet filter input udp dport 84 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 84' -Direction Inbound -Protocol UDP -LocalPort 84 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.