Port 9123 (grcp)

IANA assigns port 9123 to grcp: Global Relay compliant instant messaging protocol.

Registered (user) port, 1024-49151 TCP

IANA assignment

Service nameProtoDescriptionAssigneeReference

grcp TCP Global Relay compliant instant messaging protocol Global Relay Communications Inc -

Registry dates for this port: registered 2012-11-28, last modified 2025-06-12.

Linux: is anything listening on TCP 9123?

sudo ss -tlnp 'sport = :9123'

Linux / macOS: which process owns the port

sudo lsof -nP -i :9123

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':9123 '

Windows (Command Prompt)

netstat -ano | findstr :9123

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 9123 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 9123

From Windows

Test-NetConnection HOST -Port 9123

ufw

sudo ufw deny 9123/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 9123 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 9123 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 9123' -Direction Inbound -Protocol TCP -LocalPort 9123 -Action Block

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