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In our Windows AD domain, we have 2 DCs that also act as our DNS servers which allow the client computers to update their A records. We have a lot of outside salespeople, so some of our laptops have to go off-site for long periods of time and connect in through our (full tunnel) SSL VPN, using a Fortinet VPN client. DNS over the VPN tunnel works fine, VPN clients are able to resolve local hostnames perfectly.

The problem is that the clients connecting in over the VPN do not update the DNS records with their SSLVPN Adapter IP address. In fact, they don't update the DNS server at all. From my research, I've determined that clients are supposed to send an update to the DNS server "when a change occurs," but that doesn't seem to happen when the SSL VPN adapter connects and gets an IP address.

I have thought about deploying PowerShell scripts to all of the computers that employ the DNSCMD command when it detects that the SSL VPN adapter has an IP address, but that solution is far from ideal, feels overly complicated and very messy. I am hoping there is a simpler solution to this that I have not been able to dig up.

  • 1) Client-to-site SSLVPN (full-client, full-tunnel) FortiClient is the VPN software specifically. 2) I need to be able to run several powershell scripts against their computers regardless if they are in the office or in the field, but I don't know where they are going to be at the time the script is run, thus I need DNS to update with the proper IP if they are on the VPN. – MartinC Apr 23 '14 at 14:04
  • I need to be able to run several powershell scripts against their computers Why not use Group Policy? – HopelessN00b Apr 23 '14 at 14:07
  • Use GP to do what exactly? I'm not a GP guru so the only thing I can think of is putting the script on each computer then scheduling a task through GP. This might be OK for some scripts, but I don't want to actually allow scripting on the client computers and sometimes I want to run these scripts on-demand, I'm gathering what I need through Get-WmiObject for the most part. – MartinC Apr 23 '14 at 14:16
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    Are the VPN clients getting their IP from the Fortinet or from a Windows DHCP server? When a client is connected over VPN can they manually register via ipconfig /registerdns or does that not work either? Also, in the SSLVPN adapter, in TCP/IP properties, DNS, is Register this connection's addresses in DNS actually checked? – TheCleaner Apr 23 '14 at 14:28
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    @MartinC - I'll set it as an answer for now, but if you find it still isn't working I'll retract it later. – TheCleaner Apr 23 '14 at 15:10
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In the SSLVPN adapter, in TCP/IP properties, DNS, make sure Register this connection's addresses in DNS actually checked.

Often on a VPN connection it isn't...

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    Worked like a charm! – MartinC Apr 23 '14 at 16:06
  • @TheCleaner are you aware of any way to do this in Linux? – Steve Lorimer Oct 19 '18 at 17:26
  • @SteveLorimer - this is about the best you'll get if the DNS server is Windows based: serverfault.com/a/20082/7861 – TheCleaner Oct 22 '18 at 13:45
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Your clients are not registering their IP addresses with DNS for the simple reason that they're not configure to, when connecting to the VPN. This behavior is called Dynamic DNS Registration, and in Windows, is a per-network adapter setting. Through the GUI, you'd enable or disable this on the network adapter Properties, the Networking tab, TCP/IP Properties, Advanced, DNS tab, as shown below.

enter image description here

Fortunately, it's possible to correct this for all your users. Unfortunately, it's not as simple as it should be - there's no built in GPO that controls this specific setting, which leaves this task up to scripts, or manual intervention. Hopefully the manual intervention road needs no real explanation (tell your users to do it, and/or do it for the ones who can't/won't), but there are three approaches to this problem that may merit some explanation.

  1. Offload the DDNS registration work to your DHCP server.
    • This may be a configurable option on your Fortinet (to register DHCP clients with a DNS server you provide).
  2. Script the DDNS registration on the clients.
  3. Check the VPN client installer documentation to see if this is a configurable option.
    • When I had a similar issue at an old job, using the Cisco VPN client, DDNS registration was configurable via a simple config file in the installer package, which I altered to suit our needs and then baked into all our laptop images.
  • I'll probably go with #2, thanks for the additional info :) – MartinC Apr 23 '14 at 16:05
  • @HopelessN00b are you aware of any way to do this in Linux? – Steve Lorimer Oct 19 '18 at 17:25
  • @SteveLorimer No, but that sounds like a good question to ask. – HopelessN00b Oct 19 '18 at 17:27
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Make an exclusion of a scope of ips in your dhcp server in the DC's for the range 192.168.x.x, and assign this scope to your dhcp server in the forti client, so when your clients connect through the vpn the dhcp assigns an ip from the exclude range, and the Operative System updates the A record in your dns with the ip which is assigned.

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