How to use FreeRADIUS proxy to set an attribute value only if not provided by end RADIUS server ?

[John Butala]

We would like to use FreeRADIUS (acting as a proxy server) to set the 
Primary-DNS-Server and Secondary-DNS-server attributes in the auth 
response to the RADIUS client only if these attributes are not provied 
by the end RADIUS server (which we don't control).   Is there anyway to 
do this without making a FreeRADIUS source code change ?

Thanks much,

John Butala
Senior Staff Engineer
Qwest