HTTPfy

Port 31 (msg-auth)

IANA assigns port 31 to msg-auth: MSG Authentication.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
msg-auth TCP MSG Authentication Robert Thomas -
msg-auth UDP MSG Authentication Robert Thomas -

Check what is listening on port 31

Linux: is anything listening on TCP 31?
sudo ss -tlnp 'sport = :31'
Linux: UDP 31
sudo ss -ulnp 'sport = :31'
Linux / macOS: which process owns the port
sudo lsof -nP -i :31
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':31 '
Windows (Command Prompt)
netstat -ano | findstr :31
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 31 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 31
From Windows
Test-NetConnection HOST -Port 31
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 31

Block port 31 in the firewall

ufw
sudo ufw deny 31/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 31 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 31 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 31' -Direction Inbound -Protocol TCP -LocalPort 31 -Action Block
ufw
sudo ufw deny 31/udp
iptables
sudo iptables -A INPUT -p udp --dport 31 -j DROP
nftables
sudo nft add rule inet filter input udp dport 31 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 31' -Direction Inbound -Protocol UDP -LocalPort 31 -Action Block

Related ports

27nsw-fe29msg-icp33dsp35any private printer server

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