Attribute Number



3 or more octets



Allowed in

Access-Request, Access-Accept

Prohibited in

Access-Reject, Access-Challenge

Presence in Packet

Not required

Maximum Iterations


The value of this attribute is the number to which the RADIUS client gear should return a call to the authenticating user. Depending on what packet this attribute is found in, different actions may be configured. For instance, if Callback-Number is found in an Access-Request packet, the implementation may assume that the end user is requesting callback service. If the attribute is found in the Access-Accept packet, it can mean anything that the administrator configuring the gear wants it to mean. In fact, in some cases, Callback-ID and Callback-Number will not be found together in one packet.

Coupled with the Callback-ID attribute, this attribute is one of several RADIUS security measures. In addition to being more convenient and cost-effective for companies with employees in hotels needing access to corporate IT resources, the callback mechanism is also a security device. The implementation could be configured to call a certain number when a certain username requests access. This way, if a hacker is located somewhere other than where the genuine user normally connects from, the hacker would not be able to authenticate.

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