Port 172 (cl-1, cl/1)

IANA assigns port 172 to cl-1, cl/1: Network Innovations CL/1 IANA assigned this well-formed service name as a replacement for "cl/1"..

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

IANA assignment

Service nameProtoDescriptionAssigneeReference

cl-1 TCP Network Innovations CL/1 IANA assigned this well-formed service name as a replacement for "cl/1". Kevin DeVault -

cl/1 TCP Network Innovations CL/1
This entry is an alias to "cl-1". This entry is now historic, not usable for use with many common service discovery mechanisms. Kevin DeVault -

cl-1 UDP Network Innovations CL/1 IANA assigned this well-formed service name as a replacement for "cl/1". Kevin DeVault -

cl/1 UDP Network Innovations CL/1
This entry is an alias to "cl-1". This entry is now historic, not usable for use with many common service discovery mechanisms. Kevin DeVault -

Check what is listening on port 172

Linux: is anything listening on TCP 172?

sudo ss -tlnp 'sport = :172'

Linux: UDP 172

sudo ss -ulnp 'sport = :172'

Linux / macOS: which process owns the port

sudo lsof -nP -i :172

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':172 '

Windows (Command Prompt)

netstat -ano | findstr :172

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 172 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 172

From Windows

Test-NetConnection HOST -Port 172

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 172

Block port 172 in the firewall

ufw

sudo ufw deny 172/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 172 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 172 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 172' -Direction Inbound -Protocol TCP -LocalPort 172 -Action Block

ufw

sudo ufw deny 172/udp