Looking through the code of ec_http.c I thought it was determining if it was a username or password. Now I see the dissector picks out possible user id and pass fields and looks through etter_fields.
For those curious / confused like myself this function at the bottom of ec_http.c shows whats happening
Code:
int http_fields_init(void)
{
FILE *f;
struct http_field_entry *d;
char line[128];
char *ptr;
int pass_flag = USER;
/* open the file */
f = open_data("share", ETTER_FIELDS, FOPEN_READ_TEXT);
if (f == NULL) {
USER_MSG("Cannot open %s", ETTER_FIELDS);
return -EINVALID;
}