Port 5666 (Nagios NRPE)

Nagios Remote Plugin Executor; Nagios and Icinga run checks on remote hosts through it.

Registered (user) port, 1024-49151 TCP Monitoring and logging Do not expose to the internet

What port 5666 is used for

Nagios Remote Plugin Executor; Nagios and Icinga run checks on remote hosts through it.

Security: should port 5666 be open?

Do not expose to the internet. Allow only the monitoring server and never enable dont_blame_nrpe (command arguments), which can lead to command injection.

IANA assignment

Service nameProtoDescriptionAssigneeReference

nrpe TCP Nagios Remote Plugin Executor Nagios Enterprises LLC -

Registry dates for this port: registered 2017-09-06.

Check what is listening on port 5666

Linux: is anything listening on TCP 5666?

sudo ss -tlnp 'sport = :5666'

Linux / macOS: which process owns the port

sudo lsof -nP -i :5666

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':5666 '

Windows (Command Prompt)

netstat -ano | findstr :5666

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 5666 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 5666

From Windows

Test-NetConnection HOST -Port 5666

Block port 5666 in the firewall

ufw

sudo ufw deny 5666/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 5666 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 5666 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 5666' -Direction Inbound -Protocol TCP -LocalPort 5666 -Action Block

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