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Port 673 (cimplex)

IANA assigns port 673 to cimplex: CIMPLEX.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
cimplex TCP CIMPLEX Ulysses G Smith Jr -
cimplex UDP CIMPLEX Ulysses G Smith Jr -

Check what is listening on port 673

Linux: is anything listening on TCP 673?
sudo ss -tlnp 'sport = :673'
Linux: UDP 673
sudo ss -ulnp 'sport = :673'
Linux / macOS: which process owns the port
sudo lsof -nP -i :673
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':673 '
Windows (Command Prompt)
netstat -ano | findstr :673
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 673 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 673
From Windows
Test-NetConnection HOST -Port 673
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 673

Block port 673 in the firewall

ufw
sudo ufw deny 673/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 673 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 673 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 673' -Direction Inbound -Protocol TCP -LocalPort 673 -Action Block
ufw
sudo ufw deny 673/udp
iptables
sudo iptables -A INPUT -p udp --dport 673 -j DROP
nftables
sudo nft add rule inet filter input udp dport 673 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 673' -Direction Inbound -Protocol UDP -LocalPort 673 -Action Block

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