OpenVPN: DNS and dnsmasq configuration

In addition to the OpenVPN: OpenVPN Access Server set up and AWS VPC peering configuration post – DNS settings example.

We have the ci.example.com domain, which has to be resolved to its Public IP (of the AWS EC2 instance) if it’s requested from thу Internet – or to its Private IP – if requested via a VPN connection.

To achieve this – you can use the dnsmasq service installed on the OpenVPN AS host.

Install it:

$ sudo apt -y install dnsmasq

Create a /etc/dnsmasq.hosts file with hardcoded Private IPs for necessary services:

10.0.5.10 ci.example.com 
172.31.36.107 nexus-repo.example.com 
10.0.3.105 rabbitadmin-production.example.com 
10.0.1.6 monitor.example.com

Next – update /etc/dnsmasq.conf file and add the addn-hosts option so it will look like next:

listen-address=127.0.0.1
listen-address=10.0.10.4
bind-interfaces
log-queries
addn-hosts=/etc/dnsmasq.hosts

Restart the dnsmasq service:

root@openvpnas2:~# service dnsmasq restart

Check locally:

root@openvpnas2:~# dig @localhost ci.example.com +short
10.0.5.10

Good.

Now go to your OpenVPN’s admin page => VPN Settings and set the Have clients use specific DNS servers to Yes:

In the Primary DNS Server field set your EC2’s Private IP where is your OpenVPN server running.

Restart your VPN connection on a workstation:

$ sudo openvpn --config vpnroot-client.ovpn
...
Fri Feb 22 16:53:58 2019 /usr/bin/ip link set dev tun0 up mtu 1500
Fri Feb 22 16:53:58 2019 /usr/bin/ip addr add dev tun0 172.27.240.25/20 broadcast 172.27.255.255
Fri Feb 22 16:53:58 2019 /etc/openvpn/update-resolv-conf tun0 1500 1553 172.27.240.25 255.255.240.0 init
dhcp-option DNS 10.0.10.4
...

Check local resolv.conf:

$ cat /etc/resolv.conf
# Generated by resolvconf
nameserver 10.0.10.4

Check DNS resolution:

$ dig ci.example.com +short
10.0.5.10

And any other:

$ dig google.com +short
74.125.193.100
74.125.193.101
74.125.193.102

Done.

UPD: But there is much more correct solution using DNS resolution via AWS Peerings.

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