Background:
I have a windows server 2012 install with AD, DNS and DHCP.
I also have a netopia router/modem providing my WAN access.
My router/modem is configured with DHCP off and connected to a 24 port switch.
My WS2012 has static IP and uses the router/gateway as its default gateway for internet traffic and uses loopback for DNS. This WS2012 is also connected to same 24port switch.
The Problem:
My DNS server has forwarding enabled to my ISP's dns servers.
Here is the problem:
All my workstations on the network are getting extremely slow internet access (.1-.2 Mbps). Connecting directly to the modem and setting static IP (and completely bypassing the WS2012) gets about 4-5 Mbps. The reason I think this is a DNS issue is because whenever I go into DNS manager and clear the Cache the internet on all machines runs at 4-5 Mbps for about 10-20 minutes and then slows to crawl again.
I have already checked my reverse lookup records and forward records and they seem to be in-tact.
This issue just arose about 1 week ago and this WS2012 has been in production for 6-7 months without a single hickup. No configuration changes have been made to this machine. Nobody has even logged into this machine (headless) since put into production besides a monthly reboot (from login screen via RDP).
PS. All internal traffic runs at full speed.
Best Answer
So it worked fine with 8.8.8.8 as the original forwarder but not with your ISP's DNS servers?
That would seem to be the issue then, that your forwarders you have in place are causing slow DNS resolutions. You can easily confirm if DNS is causing this. Just use FileZilla and FTP into an FTP site via IP address (not FQDN) and then transfer a large file from that FTP site to you. If it runs at the right speed, then your issue is DNS resolution (and probably just your forwarders).