Not responding when a user is unknown

[perl-list]

We have a couple customers whose FreeRADIUS servers do not respond if a user does not exist on their FreeRADIUS system. If a user authenticates using username: idontexist and password: notarealpass the FreeRADIUS server does NOT send an access denied response, and indeed sends no response at all. I vaguely remember that this may have been an available setting on FreeRADIUS, but can find no information about it on the internet. I have been unable to recreate this problem in our lab and the customers who administer these servers seem to be unable to find any explanation. 


Please note I have no access to the servers that this happens on. The servers are owned by customers of ours. 


So basically, is there a setting that causes the FreeRADIUS server to not respond to Access-Request packets if the username contained there-in is not found in whatever database it is using? 
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