The user was trying to connect to his VPN at our remote site. He uses Windows 7 and a split-tunneling VPN and has connected with no problems many times before. He connected successfully but no DNS. I try rebooting the computer, renewing the IP addresses (w ipconfig /renew, ipconfig /release), flushing the dns, nothing. I get out the Wireshark, don't see anything unusual, although I'm wondering if I failed to capture one of the DNS failures.
I am unable to 100% verify that the problem is on our end, but the user goes home, boots up, everything works. Comes back and everything works again. So, my question has 2 parts:
What is the likely cause? What do I do if it happens again?
For the likely cause, my suspects are Comcast DNS and my Sonicwall TZ210 firewall, in that order. My Sonicwall isn't going to mess with his VPN, but wondering if it could be misdirecting his DNS requests to the VPN. Comcast could be doing the same thing, and that seems more likely to me. There's a small chance the error was with the VPN client at the user's office, but it seems unlikely to me.
On what to do if it happens again, here's my thoughts so far: run him briefly outside the firewall to see if that clears things up. Use another DNS provider (I think Google is like 126.96.36.199). Rebuild his TCP/IP stack with netsh (is this really a good idea?).
Any suggestions, or just validations, would be welcome.