RADIUS attribute from rfc5090
This attribute holds the opaque parameter that is passed to the HTTP-style client. The HTTP-style client will pass this value back to the server (i.e., the RADIUS client) without modification. This attribute MUST only be used in Access- Request and Access-Challenge packets. Type 116 for Digest-Opaque Length >= 3 Text In Access-Requests, the RADIUS client takes the value of the opaque directive (opaque-value according to ) without surrounding quotes from the HTTP-style request it wants to authenticate and puts it into this attribute. In Access- Challenge packets, the RADIUS server MAY include this attribute.