Enable DNS Hostname resolution with OpenVPN and DNSMasq

Question

I have configured OpenVPN as a server to host my own VPN and I want use DNSMasq to resolve hostnames on the VPN.

Say I have the OpenVPN server, two computers on the internal network, and one outside, all clients for the VPN (192.168.254.0/24):

• Internal Network: 192.168.1.0/24
  • server: IP: 192.168.1.1
  • A: IP: 192.168.1.2, VPN: 192.168.254.2
  • B: IP: 192.168.1.3, VPN: 192.168.254.3
• External Network: 192.168.2.0/24
  • C: IP: 192.168.2.1, VPN: 192.168.254.4

With my current setup, both A and B can resolve their hostnames via DNSMasq on the internal network. And, all of A, B, and C can access each other by direct IP. But, I want to allow C to access A and B by hostname (DNS resolution, not NetBIOS) without directing all network traffic through the VPN.

OpenVPN configuration:

proto tcp
dev tap
server 192.168.254.0 255.255.255.0
client-to-client
persist-key
persist-tun

Do I need to also configure the VPN server as a client? Do I need to push the domain from the Internal Network across the VPN? What do I need to do?

Solution

With great complexity, I have something approximating DNS over the VPN.

First, I had to run a script upon the addition of an address to OpenVPN. In the server configuration:

ifconfig-pool-persist ip-pool # Store mappings of CN,IP, 1 per line
script-security 2             # Allow OpenVPN to run user scripts
learn-address /path/to/learn-address.sh

I started with the learn-address.sh script from an old OpenVPN thread, but since I was running a TAP interface, I had to add script to parse the ip-pool file as well:

#!/bin/sh
# openvpn learn-address script to manage a hosts-like file
# - intended to allow dnsmasq to resolve openvpn clients
#   addn-hosts=/etc/hosts.openvpn-clients
#
# Changelog
# 2006-10-13 BDL original
# 2018-12-10 Palswim change to query OpenVPN Persistent pool for TAP interfaces

# replace with a sub-domain of your domain, use a sub-domain to
# prevent VPN clients from stealing existing names
DOMAIN=example

HOSTS=/etc/openvpn/hosts

h="hosts-openvpn-$DOMAIN"
LOCKFILE="/var/run/$h.lock"

IP="$2"
CN="$3"

if [ -z "$IP" ]; then
    echo "$0: IP not provided" >&2
    exit 1
else
    # In TAP mode, OpenVPN passes MAC instead of IP, since with TAP, clients can use protocols other than IP
    MAC="$IP"
    IP=$(grep "^$CN[[:space:]]*," ip-pool | head -n 1 | cut -d, -f 2)
    if [ -z "$IP" ]; then
        echo "$0: Failed to find IP in ipconfig-pool" >&2
        exit 0
    else
        echo "$0: Translated MAC ($MAC) to IP ($IP)"
    fi
fi

case "$1" in
    add|update)
        if [ -z "$CN" ]; then
            echo "$0: Common Name not provided" >&2
            exit 0
        fi
    ;;
    delete)
    ;;
    *)
        echo "$0: unknown operation [$1]" >&2
        exit 1
    ;;
esac

# serialise concurrent accesses
[ -x /bin/lock ] && /bin/lock "$LOCKFILE"

# clean up IP if we can
[ -x /bin/ipcalc ] && eval $(ipcalc "$IP")

FQDN="$CN"

# busybox mktemp must have exactly six X's
t=$(/bin/mktemp "/run/shm/$h.XXXXXX")
if [ $? -ne 0 ]; then
    echo "$0: mktemp failed" >&2
    exit 1
fi


case "$1" in
    add|update)
        /usr/bin/awk '
            # update/uncomment address|FQDN with new record, drop any duplicates:
            $1 == "'"$IP"'" || $1 == "#'"$IP"'" || $2 == "'"$FQDN"'" \
            { if (!m) print "'"$IP"'\t'"$FQDN"'"; m=1; next }
            { print }
            END { if (!m) print "'"$IP"'\t'"$FQDN"'" }           # add new address to end
        ' "$HOSTS" > "$t" && cat "$t" > "$HOSTS"
    ;;

    delete)
            /usr/bin/awk '
            # no FQDN, comment out all matching addresses (should only be one)
            $1 == "'"$IP"'" { print "#" $0; next }
            { print }
        ' "$HOSTS" > "$t" && cat "$t" > "$HOSTS"
    ;;

esac

# signal dnsmasq to reread hosts file
kill -HUP $(cat /var/run/dnsmasq/dnsmasq.pid)

rm "$t"

[ -x /bin/lock ] && /bin/lock -u "$LOCKFILE"
exit 0

I ended up running DNSMasq on one server for my own LAN, and a different server for the VPN. I had to update my configuration (/etc/dnsmasq.conf) on the VPN DNS server:

no-resolv                     # Didn't want to serve anything but VPN requests
interface=tap0
no-hosts                      # Don't use /etc/hosts
addn-hosts=/etc/openvpn/hosts # Target the output of the learn-address.sh script
expand-hosts
domain=example

Once I had this, I then had to push a few options via OpenVPN's DHCP server. Again, in the OpenVPN server configuration:

server 192.168.254.0 255.255.255.0 # Assuming this VPN network
push "dhcp-option DNS 192.168.254.1"
push "dhcp-option DOMAIN example"  # Push domain to clients

Unfortunately, only the Windows version of OpenVPN supports setting these options automatically. Linux clients will need to configure scripts to run on connection up/down. If you Linux system uses /etc/resolv.conf, ultimately, you need your VPN domain to appear in your search list, and your server IP to appear as a nameserver:

search example # you may have other strings here too, separated by a space
# ... other nameservers, then:
nameserver 192.168.254.1