Does anyone know what the DNS type value 0 ("T_INVALID") is used for?
Most DNS resource records of interest are type value 1 (an IP
address), or type value 5 (a canonical name.)

But I've noticed some sites return type value 0 and an IP address.
For example, the domain "albaath.com" returns the following answer:

C0 0C 00 00 00 01 00 00 0A F5 00 04 CC 0D A0 81

Here the type value is 0, but the data returned is a valid IP address.

I checked the lib source code for gethostbyname(), where I found the
following enumeration:

typedef enum __ns_type {
ns_t_invalid = 0, /* Cookie. */
ns_t_a = 1, /* Host address. */
ns_t_ns = 2, /* Authoritative server. */
ns_t_md = 3, /* Mail destination. */
ns_t_mf = 4, /* Mail forwarder. */
ns_t_cname = 5, /* Canonical name. */

etc...

Here type value 0 is a described as a cookie. What is a cookie in the
context of a DNS response? And why do some websites return type value
0 along with an IP address?