Firstly, thanks to all of you with being this patient with me. I think I've got a handle on the method. Please correct me if I'm wrong in either terminology or content. For the purpose of this mail, please use the following meanings: Server = RADIUS Server Client = "The thing that sends information to the server." OK, To construct a message that is received by the sever from the client, I can create a structure of the following type: typedef struct radius_packet{ int id, unsigned int code, uint8_t vector[16], VALUE_PAIR *vps; }RADIUS_PACKET The VALUE_PAIR struct is as defined currently. The radius server will receive the transmitted packet by passing a file descriptor and a data array to a system function call recvfromto. Data then holds the received data in the following uint8_t format. data[0] - code data[1] - identifier data[2] - length (HSB) data[3] - length (LSB) data[4] - vector[0] (16 octect authenticator) data[5] ...... data[19] - vector[15] data[20] - start of value_pair sequence. ....... If that is correct so far, radius than re-alignes to evaluate the value_pairs. This is where, again my understanding of the code is a bit hazy. Any help appreciated. :-) Therefore data[20] is mapped to a pointer attr. attr[0] - holds vp->attribute attr[1] - holds vp->length The server sanity checks the sum of the lengths of the attributes against the data length of the mesage. Once this happens, the servers mem copies the data array to RADIUS_PACKET -> data and treats it as it will. Many thanks and a promise to buy you a drink or three if I see you ;-) Ian Davies {02476 564662} Internal (x740 4662) IMS-SIPAC Software Development Engineer