FreeRadius sends Access-Reject for MAC-AUTH, if shared secret on NAS and server differ

Phani Siriki yvsg.phanis at
Mon Apr 15 01:43:03 CEST 2019

Hi Alan

Need some inputs on Message-Authenticator attribute. For PAP, Is it
recommended to send this attribute from NAS? RFC 3579 says it is not
required but can prevent other attacks.

>From RFC 2869:

Access-Request packets with a User-Password establish the identity of
   both the user and the NAS sending the Access-Request, because of the
   way the shared secret between NAS and RADIUS server is used.
   Access-Request packets with CHAP-Password or EAP-Message do not have
   a User-Password attribute, so the Message-Authenticator attribute
   should be used in access-request packets that do not have a User-
   Password, in order to establish the identity of the NAS sending the

>From RFC 3579:

This attribute is not required in Access-Requests which include
      the User-Password attribute, but is useful for preventing attacks
      on other types of authentication.  This attribute is intended to
      thwart attempts by an attacker to setup a "rogue" NAS, and perform
      online dictionary attacks against the RADIUS server.

Best Regards

On Sun, Apr 14, 2019 at 3:26 PM Phani Siriki <yvsg.phanis at> wrote:
> Hi Alan
> Thanks for the reply. I checked this already and NAS is not sending
> Message-Authenticator attribute in this case. I will further check
> this. Thanks.
> Best Regards
> Phani
> On Sun, Apr 14, 2019 at 3:20 PM Alan DeKok <aland at> wrote:
> >
> > On Apr 14, 2019, at 6:04 PM, Phani Siriki <yvsg.phanis at> wrote:
> > > Yes, you are correct. But in case of MAC-AUTH which is doing PAP
> > > authentication, Access-Reject is sent. FreeRadius should have dropped
> > > the request without sending Access-Reject right?
> >
> >   No.
> >
> > > Can we make
> > > FreeRadius not reply in case MAC-auth if shared secret is wrong.
> >
> >   No.
> >
> >   If there is a Message-Authenticator attribute, then the server knows that the shared secret is wrong, and drops the packet.
> >
> >   If there is no Message-Authenticator attribute, then the server guesses that the shared secret *might* be wrong, but it's not sure.  Because there's no way of knowing for sure.
> >
> >   If you want to know why, read the RFCs.  If you're not going to read the RFCs, then trust that the server does the Right Thing.  It's been doing RADIUS for 20 years, which is likely longer than you've been doing it.
> >
> >   Alan DeKok.
> >
> >
> > -
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