SharePoint 2013 Search: "The server is unavailable and could not be accessed. The server is probably disconnected from the network."

I recently experienced an issue of of a single content source not crawling in the SharePoint 2013 Search Service Application. The crawl log gave the following error:

"The server is unavailable and could not be accessed. The server is probably disconnected from the network."

I had already added the infamous DisableLoopBackCheck fix when I initially deployed the farm, so I knew that wasn't going to be the solution.

The following steps fixed the issue for me:

1) Open the registry on the Central Administration server.
2) Navigate to HKLM\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0.
3) Right-click MSV1_0 and add a new Multi-String Value. Give it a name of BackConnectionHostNames.
4) Double-click to modify the newly created BackConnectionHostNames item. Add the FQDN of each web application you want to crawl, one per line. Select OK.
5) Perform an IIS reset.

Should be able to successfully crawl the content source now.