What did you mean, when you ask for 'backend'?
If you meant an storage, so it's not a .txt users file, I'm using db - radcheck table.
So guys, the I've solved the problem.
It wasn't consisted of kamailio functions or radius configuration.
So you're free to use: www_challenge("$fd", "1"), until up radius_www_authorize("$fd","$fU") comes up.
Qop parameter does what he does and changes nothing within radius authentication process.
My problem was about username column in radcheck table.
It's not enough to insert an username, you ought to use full URI, like: username@my.proxy.domain
Also don't forget about attributes of the row that belongs to a certain user agent.
So my part of table for one of users looks like that:
;-------------------------------------------------------------------------------------------------------------------;
;---id---;---username-------;------attribute---------;------op-------;----------value---------------------;
;-------------------------------------------------------------------------------------------------------------------;
;__1__;__ua@dom.com_;__User-Password_;___==_____;_____hereuapassowrd____;
;__2__;__ua@dom.com_;__Auth-Type_____;___:=______;_____Digest____________;
;__....
Actually, I don't know why, but there is just a few articles all over the net, that describes a bit the functionality and processing with auth_radius module.
I hope my case will be useful for others, who uses kamailio + radius/db
But I have a problem how to request AVPs for a certain user from RADIUS, I found some solutions with SIP-AVP attribute, but still haven't done it.
Now I have to databases, one for Kamailio (that contains users AVPs, that Kamailio gets by avp_db_query) and second for users credentials (that are used while authorization on INVITE, REGISTER requests).
And as for the future, I have a goal to store passwords in ha1, haven't started to discover this.