NAS-Filter-Rule incorrectly encoded as string

Alan DeKok aland at
Tue Feb 16 14:38:10 CET 2021

On Feb 16, 2021, at 6:23 AM, Pablo Nogueira <pnogueira at> wrote:
> I'm trying to implement dynamic access lists for 802.1X authenticated
> users. To do so, I'm using attribute NAS-Filter-Rule, as defined in
> RFC 4849 and FreeRADIUS Version 3.0.21 as the RADIUS server.
> To do so, I've modified users file with the following entry
> user1      Cleartext-Password := "pass1"
>               Service-Type = Framed-User,
>               Nas-filter-Rule = "permit in tcp from any to",
>               Nas-filter-Rule += 0x00,
>               Nas-filter-Rule += "permit in ip from
> to",
>               Nas-filter-Rule += 0x00,
>               Nas-filter-Rule += "deny in ip from any to any",
>               Nas-filter-Rule += 0x00

  That really won't do what you want.

  The "+=" operator adds multiple attributes of the same name.  It doesn't concatenate strings for the same attribute.  See "man unlang".

  Further, adding "0x00" to a "string" attribute doesn't add a zero byte.  It appends the literal string "0x00".

> I've been able to make it work as I think it should adding the
> following line to my dictionary
> # override NAS-Filter-Rule to convey NUL character between rules
> ATTRIBUTE       NAS-Filter-Rule                         92      octets

  That still doesn't quite do what the RFC says.  That change sends *multiple* NAS-Filter-Rule attributes, some with string values, and some with a 0x00 byte.  It doesn't follow the RFC guidelines of packing all of the strings together.

  If it works... it's OK for now.  But it's not really the correct approach.

> My question is, is there any other way to force freeradius to send the
> entry rules as strings with NUL terminated character? Should I report
> this as an issue (bug) to freeradius developers so that they change
> the dictionary.rfc4849 entry?

  The developers read this list.  Don't worry about that.

  Changing the data type sort of works.  But it means that when the server is receiving proxied Access-Accept packets, it won't print out the NAS-Filter-Rule correctly.

  For now, your changes work for you.  We'll see if we can get some fixes into the next release.

  Alan DeKok.

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