vpnbook offers free VPN services that include OpenVPN configuration files packaged as ZIP archives. The “OpenVPN UDP 53 ZIP” typically refers to a ZIP file containing configuration files for connecting to vpnbook’s OpenVPN servers over UDP using port 53 (the DNS port). Using UDP on port 53 can help the VPN traffic bypass some restrictive firewalls that block common VPN ports, since port 53 is normally used for DNS.
This technique is often called or Protocol Obfuscation. VPNBook provides this specific zip file explicitly to help users bypass censorship or captive portals that block standard VPN ports. vpnbook com openvpn udp 53 zip