Our IIS server seems to have encountered some problems recently and I'm not sure where to start looking for causes. The problems lie within it's ability to resolve the sites externally, where the response and loading times seem to be extremely slow and often fails. Internally however everything works fine and is blazingly fast. What's strange about this is that up to around a few days ago, everything was working fine with no problems.
In regards to the server setup, it's running IIS on a virtual machine with Windows Server 2007 Standard. This box is connected to a domain controller hosted on another Windows Server 2008 R2 VM, which also contains a DNS Server. The DNS server is being forwarded to our Fortigate 60C Router which handles external relays to our ISP's DNS Servers. All computers in our organisation uses the DNS server on the windows server as part of the domain network.
We also have an external OWA access enabled on our network which is mapped under domain.com/owa. This is hosted on another IIS server on our exchange server. The connection to the OWA dropped around the same time that we started to have the performance dips hence, I'm not sure whether this is related the main issue or a completely different one.
I don't think it'd be an IIS issue since nothing has been changed by anyone, and none of the apps on it are suffering from performance issues internally. I'm more inclined to think that it may be a network/DNS issue, but I'm not sure where to start looking. If it was a DNS issue however, our users would have problems with their internet speed and access which they don't so I find it quite confusing.
Could anyone give me some insight perhaps on where I may start looking and if possible, what's causing this to suddenly happen? Thank-you for reading.