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[Openvpn-users] DNS conflicts using DHCP


  • Subject: [Openvpn-users] DNS conflicts using DHCP
  • From: Sean Atkinson <sean@xxxxxxxxxxxxxx>
  • Date: Wed, 04 Feb 2004 14:59:58 +0100

Hi,

I couldn't find a search feature for the list archives, so apologies if
this has been asked before.

I've been using OpenVPN to connect several Red Hat Linux 9 clients to an
internal office network for some months now, and it's been performing
very well.  However I've always had complications with the client's DNS
settings, and wondered if anybody might have some advice.

To route packets to the office LAN and other VPN clients I include the
following commands in the VPN startup script:

route add -net 10.51.0.0 netmask 255.255.0.0 gw $5
route add -net 10.1.1.0 netmask 255.255.255.0 gw $5

These routes are automatically deleted when the tun interface is
closed.  Then to override some defaults from any DHCP servers the
clients use to setup networking, I add the following to
/etc/dhclient-eth0.conf:

append domain-name "netproject.com";
prepend domain-name-servers 10.51.10.4;

I need the private DNS server to be prepended before any others since it
holds names for internal hosts in netproject.com, which fail if external
DNS is tried first.  DHCP negotiation then results in an
/etc/resolv.conf something like this:

search netproject.com
nameserver 10.51.10.4
nameserver 10.0.0.1
nameserver 10.0.0.2

So far so good, but there are two problems that I can't figure out nice
solutions for.  Firstly if openvpn is not running (e.g. it's not started
by default) then injecting the unreachable DNS server messes up
resolving.  Secondly if a laptop roams then automatically starting the
VPN confuses networking if it's already on the office LAN, but until
it's manually started DNS is a problem on other networks if the
unreachable private server is included.

Somehow I'd like an intelligent test so the VPN isn't started if the
office LAN is already locally available.  In addition I'd like the DNS
server to be added and removed on the fly with the VPN connection to it,
rather than just being forced on or off.  However local DHCP details
must also be honoured, which makes things more complicated.

Any ideas?

Thanks in advance,

Sean.

-- 
Sean Atkinson <sean@xxxxxxxxxxxxxx>
Netproject



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