[Openvpn-users] HOWTO: Integrating client names into server-side DNS

[Ben Low <ben@xxxxxxxxx>]

This describes a mechanism for integrating OpenVPN client assignments  
into a local (server) DNS, such that you can reference VPN clients by  
name. e.g. 'common-name.vpn.example.net', where common-name is the  
VPN client's authenticated username (typically the certificate's  
Common Name).

The learn-address.sh script below maintains a separate /etc/hosts- 
style file, adding and removing (commenting out) IP - name records as  
openvpn clients connect and disconnect. Using this with the dnsmasq  
DNS server's support for an 'additional hosts' file gives the desired  
result.


Server setup:

- OpenVPN 2.0.x
- dnsmasq 2.27 (or equivalent)
- the learn-address.sh script below

Client setup:
- client certificate's Common Name will be used as the client's hostname


Edit your server config files, and create the learn-address script as  
follows:

=== openvpn.conf (relevant extracts) ===
learn-address /var/lib/openvpn/learn-address.sh

=== dnsmasq.conf (relevant extracts) ===
addn-hosts=/etc/hosts.openvpn-clients

=== /var/lib/openvpn/learn-address.sh ===
#!/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
# - written for openwrt (busybox), but should work most anywhere
#
# Changelog
# 2006-10-13 BDL original

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

HOSTS=/etc/hosts.openvpn-clients

h=$(/usr/bin/basename "$HOSTS")
LOCKFILE="/var/run/$h.lock"

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

case "$1" in
   add|update)
     if [ -z "$IP" -o -z "$CN" ]; then
         echo "$0: IP and/or Common Name not provided" >&2
         exit 0
     fi
   ;;
   delete)
     if [ -z "$IP" ]; then
         echo "$0: IP not provided" >&2
         exit 0
     fi
   ;;
   *)
     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="$CN.$DOMAIN"

# busybox mktemp must have exactly six X's
t=$(/bin/mktemp "/tmp/$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
/bin/kill -HUP $(cat /var/run/dnsmasq.pid)

/bin/rm "$t"

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