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Port 4881 (socp-t)

IANA assigns port 4881 to socp-t: SOCP Time Synchronization Protocol.

Registered (user) port, 1024-49151 UDP

Port 4881 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

IANA assignment

Service nameProtoDescriptionAssigneeReference
socp-t UDP SOCP Time Synchronization Protocol Joe Haver -

Registry dates for this port: registered 2010-05-28.

Check what is listening on port 4881

Linux: UDP 4881
sudo ss -ulnp 'sport = :4881'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4881
Windows (Command Prompt)
netstat -ano | findstr :4881
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 4881

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4881

Block port 4881 in the firewall

ufw
sudo ufw deny 4881/udp
iptables
sudo iptables -A INPUT -p udp --dport 4881 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4881 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4881' -Direction Inbound -Protocol UDP -LocalPort 4881 -Action Block

Related ports

4879wsdl-event4880hislip4882socp-c4883wmlserver

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