Register CN of OpenVPN clients in DNS Resolver
Use case: Serial port servers deployed in the field connect to the office via OpenVPN. In order to collect the data, a machine connects to each serial port server via proprietary protocols over the VPN.
Configuring fixed IP addresses for each OpenVPN client is cumbersome and error prone. Instead it would be great if the VPN server would register the common name (CN) of connected clients in the DNS Resolver.
This is currently possible using a
learn-address script in OpenVPN as described in this ml-post
I'm using the following learn-address script (
#!/bin/sh # openvpn learn-address script maintaining DNS entries of connected clients in # unbound config. DOMAIN="$1" OP="$2" IP="$3" CN="$4" case "$1" in add|update) # Remove all configs which mention the CN grep -l -null -F "$CN.$DOMAIN" /var/unbound/openvpn.client.*.conf | xargs -0 rm rm -f /var/unbound/openvpn.client.$IP.conf # Add new local-data entry. ( echo "local-data-ptr: \"$IP $CN.$DOMAIN\"" && echo "local-data: \"$CN.$DOMAIN A $IP\"" && echo "local-data: \"$CN A $IP\"" ) > /var/unbound/openvpn.client.$IP.conf /bin/pkill -HUP -F /var/run/unbound.pid ;; delete) # CN is not set on delete. rm -f /var/unbound/openvpn.client.$IP.conf && /bin/pkill -HUP -F /var/run/unbound.pid ;; esac exit 0
This is along with the following row in OpenVPN server custom options:
learn-address "/usr/local/sbin/openvpn.learn-address.sh vpn.example.com"
Finally the following custom options for DNS Resolver:
I'm willing to write a patch if people are interested in having this functionality built-in. I might just duplicate the "Register DHCP leases in the DNS Resolver" checkbox in the "DNS Resolver" configuration. If it is checked, the OpenVPN server automatically will use the script and unbound will include the necessary files.
I'm not quite sure how robust unbound is regarding invalid host names. A bullet-proof learn-address script probably should reject writing invalid unbound config.