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Port 5766 (openmailns)

IANA assigns port 5766 to openmailns: OpenMail NewMail Server.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
openmailns TCP OpenMail NewMail Server OpenMail Encyclopedi, Don Loughry -
openmailns UDP OpenMail NewMail Server OpenMail Encyclopedi, Don Loughry -

Check what is listening on port 5766

Linux: is anything listening on TCP 5766?
sudo ss -tlnp 'sport = :5766'
Linux: UDP 5766
sudo ss -ulnp 'sport = :5766'
Linux / macOS: which process owns the port
sudo lsof -nP -i :5766
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':5766 '
Windows (Command Prompt)
netstat -ano | findstr :5766
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 5766 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 5766
From Windows
Test-NetConnection HOST -Port 5766
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 5766

Block port 5766 in the firewall

ufw
sudo ufw deny 5766/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 5766 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 5766 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 5766' -Direction Inbound -Protocol TCP -LocalPort 5766 -Action Block
ufw
sudo ufw deny 5766/udp
iptables
sudo iptables -A INPUT -p udp --dport 5766 -j DROP
nftables
sudo nft add rule inet filter input udp dport 5766 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 5766' -Direction Inbound -Protocol UDP -LocalPort 5766 -Action Block

Related ports

5755openmailg5757x500ms5767s-openmail5768openmailpxy

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