Windows Server 2008 R2 DFS Replication across two locations
If the domain controller is offline what happens?
Are users still able to access the DFS namespace? Does replication still occur?
Depends entirely on how this is setup and what's cached. Anything from "nothing will happen" to "it will break completely" are possible.
Basically, the Domain Controller here is needed for authentication to access the shares, and name resolution to translate your server names to IP addresses. If you have a client that has the DNS entry for its DFS server cached, and has its authentication cached by the DFS server(s), then it will be able to access DFS just fine, until the cached values expire (or are flushed). Likewise, if your DFS servers have each other's DNS entries cached, they should be able to replicate.
Anyone who doesn't have the DNS entries for these servers cached will be unable to access them by name, and anyone whose authentication isn't cached by the DFS servers will get a logon failure, because the DFS server will attempt to validate the logon attempt against the Domain Controller it can't reach (which generates an authentication failure).
Oh, and having only one Domain Controller is a problem, and a Bad Idea. Get another one.
If your domain controller goes offline (assuming you only have one) you have bigger problems than users not being able to access the DFS namespace. A myriad of complications arise, the most notable is the fact that authentication doesn't take place.
With that said, users will not be able to access the namespace and replication will not occur.
The default TTL for DFS Link referrals is 30 minutes and the default TTL for DFS Root referrals is 5 minutes.
I would assume that for a Domain based DFS namespace that the DFS links would still be accessible after the DC has gone down for clients that already have the Link referral cached, for the remaining life of the referral.