[C] 가변 리스트 (pack된 속성)에 대한 고찰, Variable list of packed attribute

Strongswan 소스코드에서, 추측해볼만한 사실들,

1. 가변 길이 배열은 unit8_t attributes[], 즉, []형 자료형으로 선언한다.

(https://github.com/strongswan/strongswan/blob/master/src/libradius/radius_message.c)

 

 

 

 

 

- 코드

/**
 * RADIUS message header
 */
struct rmsg_t {
	/** message code, radius_message_code_t */
	uint8_t code;
	/** message identifier */
	uint8_t identifier;
	/** length of Code, Identifier, Length, Authenticator and Attributes */
	uint16_t length;
	/** message authenticator, MD5 hash */
	uint8_t authenticator[HASH_SIZE_MD5];
	/** variable list of packed attributes */
	uint8_t attributes[];
} __attribute__((packed));

 

 

   Attributes

      The Attribute field is variable in length, and contains the list
      of Attributes that are required for the type of service, as well
      as any desired optional Attributes.

 

 

관련 스펙

4.  Packet Types

   The RADIUS Packet type is determined by the Code field in the first
   octet of the Packet.

4.1.  Access-Request

   Description

      Access-Request packets are sent to a RADIUS server, and convey
      information used to determine whether a user is allowed access to
      a specific NAS, and any special services requested for that user.
      An implementation wishing to authenticate a user MUST transmit a
      RADIUS packet with the Code field set to 1 (Access-Request).

      Upon receipt of an Access-Request from a valid client, an
      appropriate reply MUST be transmitted.

      An Access-Request SHOULD contain a User-Name attribute.  It MUST
      contain either a NAS-IP-Address attribute or a NAS-Identifier
      attribute (or both).

      An Access-Request MUST contain either a User-Password or a CHAP-
      Password or a State.  An Access-Request MUST NOT contain both a
      User-Password and a CHAP-Password.  If future extensions allow
      other kinds of authentication information to be conveyed, the
      attribute for that can be used in an Access-Request instead of
      User-Password or CHAP-Password.

      An Access-Request SHOULD contain a NAS-Port or NAS-Port-Type
      attribute or both unless the type of access being requested does
      not involve a port or the NAS does not distinguish among its
      ports.

      An Access-Request MAY contain additional attributes as a hint to
      the server, but the server is not required to honor the hint.

      When a User-Password is present, it is hidden using a method based
      on the RSA Message Digest Algorithm MD5 [3].












Rigney, et al.              Standards Track                    [Page 17]

---

RFC 2865                         RADIUS                        June 2000


   A summary of the Access-Request packet format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Request Authenticator                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      1 for Access-Request.

   Identifier

      The Identifier field MUST be changed whenever the content of the
      Attributes field changes, and whenever a valid reply has been
      received for a previous request.  For retransmissions, the
      Identifier MUST remain unchanged.

   Request Authenticator

      The Request Authenticator value MUST be changed each time a new
      Identifier is used.

   Attributes

      The Attribute field is variable in length, and contains the list
      of Attributes that are required for the type of service, as well
      as any desired optional Attributes.