Is it safe to open
SIP and VoIP ports are scanned constantly by toll-fraud bots looking for weak extensions. Allow only your provider's IP addresses on the firewall where possible, use long SIP passwords, fail2ban, and disable SIP ALG on the router.
How to open port 8021
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 8021 -j ACCEPT # Ubuntu ufw ufw allow 8021/tcp # firewalld firewall-cmd --permanent --add-port=8021/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=8021 action=accept place-before=0 comment="port 8021" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 8021 TCP" -Direction Inbound -Protocol TCP -LocalPort 8021 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 8021 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 8021 -j DROP ufw allow from 203.0.113.0/24 to any port 8021 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=8021 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=8021 action=drop
If the server is behind a router, you also need a port forward (dst-nat). Behind ISP CGNAT (100.64.0.0/10 on the WAN) forwarding will not help: you need a public IP or a VPN.
What is listening
Check on the server
# what listens on the port ss -lntup | grep ':8021 ' netstat -ano | findstr :8021 # Windows lsof -i :8021 # macOS # test from another machine nc -vz example.com 8021