How to set a static IP for client on OpenVPN server with duplicate-cn?

Youran

6/7/24, 8:37 PM

To set static IP for clients with distinct certs, we can set static IP for clients following jdmorei's answer. However, If duplicate-cn is set on the server side, so that many clients share the same cert, how can I set static IP for a specific client?

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Youran

6/7/24, 8:37 PM

Thanks to IP Management With duplicate-cn, we can use --client-connect cmd to write different configurations to a dynamically generated temporary file for different clients. In Reference manual for OpenVPN 2.4, it says

OpenVPN's internal client IP address selection algorithm works as follows:

1 Use --client-connect script generated file for static IP (first choice).
2 Use --client-config-dir file for static IP (next choice).
3 Use --ifconfig-pool allocation for dynamic IP (last choice).

So we can overwrite the server's ifconfig-pool with this script. In my case, I distinguish different clients by their IP and set static IP for a special client. When the special IP x.x.x.x appears, a static IP 172.0.0.3 will be distributed to this client. The script for client-connect is

$ cat /etc/openvpn/client-connect.sh 
#!/bin/bash

if [ $trusted_ip = "x.x.x.x" ]; then
    echo "static ip triggered for" $trusted_ip  
    echo "ifconfig-push 172.0.0.3 255.255.255.0" >> ${@:-1}
else
    echo "still random ip for" $trusted_ip
fi

exit 0

I also set the ifconfig-pool on the server side to avoid conflict with 172.0.0.3

mode server
server 172.0.0.0 255.255.255.0 'nopool'
ifconfig-pool 172.0.0.16 172.0.0.128 255.255.255.0

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