Incase you didn't know- when you set your index server to use a dedicated server for crawling, SharePoint updates the host file on the index server with the IP address of the WFE. However, there are times where SharePoint adds an invalid host entry when there are 2 or more WFE's.
Example - Say there have 2 WFE's - MOSSWFE01 & MOSSWFE02. Setting MOSSWFE02 as the dedicated crawler will cause SharePoint to update the host file in the index server. It adds an entry for MOSSWFE01 with the IP address of MOSSWFE02. Hence, the index server cannot access the index file location on MOSSWFE01. No use trying to update the host file manually since SharePoint reverts your change every few minutes.
With the invalid host file entry you'll start seeing errors on both the index server and the WFE (see below). The errors are due to the index servers inability to propagate the index file to the WFE. If you modify the index server to use all WFE's for crawling then no entries are added to the index file.
I couldn't find a supported resolution for this, but there is a 'hack' to overcome this! Instead of setting a dedicated crawler, select the option to use all WFE's for crawling. Next open the host file on the index server and add the host names of the web apps along with the IP address of the WFE you want to act as the crawler.
Here is the TechNet article on this:http://technet.microsoft.com/en-us/library/cc261810.aspx
Error on WFE:
Description:
Application Server Administration job failed for service instance Microsoft.Office.Server.Search.Administration.SearchServiceInstance (4d1c24af-411b-42b0-9f05-f657644e7d4b).
Reason: Object 0ad5f391-9f4b-4f49-8f85-3550343690e1 not found.
Techinal Support Details:
System.Collections.Generic.KeyNotFoundException: Object 0ad5f391-9f4b-4f49-8f85-3550343690e1 not found.
at Microsoft.Office.Server.Search.Administration.SearchApi.get_App()
at Microsoft.Office.Server.Search.Administration.SearchServiceInstance.Synchronize()
at Microsoft.Office.Server.Administration.ApplicationServerJob.ProvisionLocalSharedServiceInstances(Boolean isAdministrationServiceJob)
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Error in Index Server:
Description:
The Execute method of job definition Microsoft.Office.Server.Search.Administration.AddQueryServerJob (ID bcd3ce3e-b058-46ef-81a9-1bda53fa6c45) threw an exception. More information is included below.
Index location '\\MOSSWFE01\searchindexpropagation' not found on indexer 'M0SSWFE1'. Aborting query server deployment for application SharedServices1.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.