HTTPfy

Port 531 (conference)

IANA assigns port 531 to conference: chat.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
conference TCP chat - -
conference UDP chat - -

Check what is listening on port 531

Linux: is anything listening on TCP 531?
sudo ss -tlnp 'sport = :531'
Linux: UDP 531
sudo ss -ulnp 'sport = :531'
Linux / macOS: which process owns the port
sudo lsof -nP -i :531
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':531 '
Windows (Command Prompt)
netstat -ano | findstr :531
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 531 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 531
From Windows
Test-NetConnection HOST -Port 531
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 531

Block port 531 in the firewall

ufw
sudo ufw deny 531/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 531 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 531 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 531' -Direction Inbound -Protocol TCP -LocalPort 531 -Action Block
ufw
sudo ufw deny 531/udp
iptables
sudo iptables -A INPUT -p udp --dport 531 -j DROP
nftables
sudo nft add rule inet filter input udp dport 531 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 531' -Direction Inbound -Protocol UDP -LocalPort 531 -Action Block

Related ports

529irc-serv530courier532netnews533netwall

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