Local vs VPN DNS Name Resolution

NOYB

Using Wireshark to capture both the WiFi and VPN networks. Filter on port 53.

Query for nslookup yahoo.con goes to the VPN DNS only. As desired.

However,
Browse (IE) to http://yahoo.com and the name queries go to both the local (192.168.1.1) and VPN (192.168.2.1) DNS servers. And resolved addresses are received from both.

Seems like this could be a big security hole. Possible for a public hot spot network to send clients to malicious sites even though they are using a VPN.

johnpoz

That is your browser doing that - not the OS.. Don't use IE for starters if your worried about security ;)

Well yeah if you query them, they will answer if they can ;)

NOYB

It's not just IE that this OpenVPN DNS behavior occurs with.

johnpoz

Its not openvpn behavior.. Openvpn has nothing to do with dns, what your os or application does with what its handed has nothing to do with openvpn.

I don't see this behavior, are you talking about queries for such like wpad ?

You see in first picture that hey I am doing dns to the dns server on my openvpn interface..

Look what happens if I ask for something like pfsense.local.lan - it asks my openvpn dns, he says NX.. Sorry.. there is no domain like that.. So client gives up.. Now if dns server had not answered, or returned serv fail, then sure would of keep trying to resolve via methods it knows how to use to resolve.

You sure your not seeing broadcasts for names – we dns your set to says fails to respond windows will get desperate and even broadcast for your fqdn even though it doesn't exist, etc.

Lets see a specific example of what your concerned with.

querydns.png_thumb

nxreturned.png_thumb

NOYB

Attachment Details:

Wireshark capture on both the WiFi and OpenVPN interfaces with capture filter 'port 53'.

192.168.1.x is local WiFi hotspot network.
192.168.22.x is OpenVPN to pfSense (configured to force all traffic through VPN and provide DNS servers list.

Browse to: https://wwws.ameritrade.com/apps/LogIn/

DNS queries for the non-local name are sent to both the local WiFi hotspot network DNS server and the OpenVPN DNS server.
DNS response received from both. Making it possible for the local WiFi hotspot network DNS to provide an address that sends the client to a malicious site.

OpenVPN Forum Thread:
https://forums.openvpn.net/viewtopic.php?t=15939

![OpenVPN DNS.jpg](/public/imported_attachments/1/OpenVPN DNS.jpg)
![OpenVPN DNS.jpg_thumb](/public/imported_attachments/1/OpenVPN DNS.jpg_thumb)

kpa

Are you handing out one or two DNS forwarders during the OpenVPN tunnel set up? If you're handing out just one forwarder Windows may keep the second forwarder from the initial DHCP setup and combine it with the one it got from the OpenVPN tunnel set up. Try giving the client at least two DNS forwarders and see how it behaves then.

But yes, this has almost nothing to do with OpenVPN itself. Windows is known for its "creative adaptations" of internet standards and this is just one of them.

johnpoz

Well my home box that I showed was static IP on its local interface..

Its quite possible windows does something odd with dhcp.. It sure and the F shouldn't but as stated MS likes to go their own route sometimes when it comes to standards.

I vpn to my home network pretty much every single day from work, so happy to validate what your seeing. That second query should not be happening - unless the fist 1 failed to answer or returned serv fail. But again this has NOTHING to do with openvpn.. Openvpn just hands the client dns to use, it can not control the how the OS uses or doesn't use that info.

You did update your binding btw? Your screenshot was of providers.. keep in mind if you enable or disable interfaces this order can and does change.. So double check it, from your sniff where the local dns was .001 second later I would assume vpn connection is first.. Its best to rename your interfaces so you know exactly what they are vs local connection 1, local connection 2 ;)

remoteaccess.png_thumb

johnpoz

Ok - from a quick test when I got into the office, I set client to redirect-gateway def1

now get this in my route print for default, before I was not redirecting

Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 10.56.41.1 10.56.41.36 10
0.0.0.0 128.0.0.0 10.0.200.5 10.0.200.6 20

Now what I noticed right off the bat was that it was sending dns to my local configured dns at 10.56.144.11 and 10.56.144.21 down my vpn connection. This is all it was doing it was not sending anything to my 192.168.1.253 dns (pfsense) down the vpn..

So as suggested I added couple more entries fro dns to openvpn config.. so it handed out 192.168.1.253 3 times

Ethernet adapter vpn:

Connection-specific DNS Suffix . : local.lan
Description . . . . . . . . . . . : TAP-Windows Adapter V9
Physical Address. . . . . . . . . : 00-FF-5A-2F-7E-EA
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IPv4 Address. . . . . . . . . . . : 10.0.200.6(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.252
Lease Obtained. . . . . . . . . . : Tuesday, May 27, 2014 10:19:14 AM
Lease Expires . . . . . . . . . . : Wednesday, May 27, 2015 10:19:14 AM
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . : 10.0.200.5
DNS Servers . . . . . . . . . . . : 192.168.1.253
192.168.1.253
192.168.1.253
NetBIOS over Tcpip. . . . . . . . : Enabled

Now watching dns queries with wireshark - I only see dns queries to my pfsense box down the vpn.. Nothing going local at all.

Ok something is weird, I disconnected and connected again and now its only sending queries to my local dns - but down the gateway.. I don't have dns on my local segment to test that scenario. Windows can do some odd shit for sure.. In this scenario you would be good because the queries would just go nowhere down your tunnel to those addresses.. If your worried about dns leakage, I would prob use a proxy on your vpn side and set your browser to use that proxy and setup in firefox for example the socks dns setting.

querydns.png_thumb

NOYB

Pippin, Maybe this would be a better place to continue the "block-outside-dns" discussion from https://forum.pfsense.org/index.php?topic=110910.0
https://forum.pfsense.org/index.php?topic=110910.msg617893#msg617893

So verb 4 shows the logs as you have too. So my only issue with this is that it eliminates resolution of local names. Need those some places.

johnpoz

continue?? this thread was from 2014 ;)

Pippin

Hmm, ok I see, the difference is that I'm not in a road worrier situation.
Connecting from home to my external server (NAS) which runs (adblocking) DNS and I can still talk to local.

Maybe ask over at OpenVPN Community Forum or even better on the users mailing list?

johnpoz

"Connecting from home to my external server"

How is that not a road warrior connection?? You don't have a site to site setup do you? If its not site to site then your road warrior.

Pippin

I mean as in local is my home and my home is not switching networks :)

NOYB

@johnpoz:

continue?? this thread was from 2014 ;)

Continuing from the other thread that we had hijacked.
Yes this thread is from 2014. Is my thread that I started on the specific subject of local vs. VPN DNS name resolution, or more concisely put "DNS leakage", and previously there had not been a solid resolution to the issue. So it is a very direct fit for the newly implemented "block-outside-dns" option discussion.

johnpoz

So what exactly do you want again.

So you want to use your vpn dns, but not your local dns to resolve stuff other than local? Is that what your after? Yeah not going to happen.. How do you control your client to only ask the local dns where its at for stuff that is local, and ask your vpn dns for everything else..

Or am I missing something?

kpa

I believe the following is possible when the client is MS Windows:

The TUN/TAP adapter on the client gets an adapter specific DNS domain name.
The DNS resolvers pushed from the OpenVPN server are associated with the TUN/TAP adapter on the client.
DNS names ending with this adapter specific DNS domain name get resolved by the adapter specific DNS forwarders which can be directed over the VPN tunnel using routes.

Of course none of this will work on any other operating system.

johnpoz

"DNS names ending with this adapter specific DNS domain name get resolved by the adapter specific DNS forwarders which can be directed over the VPN tunnel using routes."

Where did you get that idea from?? Where is the MS article that says dns is used based upon connection specific domains or search order?

That is not how it works…
https://technet.microsoft.com/en-us/library/dn305896.aspx

Now per the above there was an API that allows a program do do name resolution out a specific interface... Are you writing the program to tell your browser what to resolve and from where to use this API? Or any of your other applications you might use? They made some changes to the binding order of how name resolution is done. But I have not read anywhere that if I put a specific domain name on a specific interface that that is the dns servers it uses for name resolution of that domain name.. But will for sure test it on monday when get to work.. Because what I can tell you from every single day using vpn into home network and resolving different things both locally and remotely.. That is not how it works on windows 7 for sure.. But sure I will double check it..

NOYB

Pippin,
I'm thinking/wondering if maybe the name resolution you are getting for local services is WINS. With the block outside dns enabled I can for example ping via local host name even though there is no DNS resolution of the name. Though there is a slight second or two delay in resolving the address since WINS is later in the resolution order.

John,
Pretty much. Yes, I want to have my cake and eat it too. I want local names resolution for local resources only, and VPN provided DNS only for everything else. I know it doesn't work like that. But that's how I want it to work.

Pippin

Will try to find why its working for me. That machine as I just discovered, is getting detected through NetBIOS so yes maybe its WINS.

Under Network select View-Details, then right click on the columns (Name-Category-Workgroup-etc.) and select "Discovery Method".
Then you can see how they`re discovered, obviously :o

I hope thats clear enough because I dont have English version of Windows :)