How to Check Subject Alternative Names for a SSL/TLS Certificate?

Question

Is there a way to programmatically check the Subject Alternative Names of a SAN SSL cert?

Using, for instance, the following command I can get many info but not all

Answer 1

Is there a way to programmatically check the Alternative Names of a SAN SSL cert?

There could be multiple SANs in a X509 certificate. The following is from the OpenSSL wiki at SSL/TLS Client. It loops over the names and prints them.

You get the X509* from a function like SSL_get_peer_certificate from a TLS connection, d2i_X509 from memory or PEM_read_bio_X509 from the filesystem.

void print_san_name(const char* label, X509* const cert)
{
    int success = 0;
    GENERAL_NAMES* names = NULL;
    unsigned char* utf8 = NULL;

    do
    {
        if(!cert) break; /* failed */

        names = X509_get_ext_d2i(cert, NID_subject_alt_name, 0, 0 );
        if(!names) break;

        int i = 0, count = sk_GENERAL_NAME_num(names);
        if(!count) break; /* failed */

        for( i = 0; i < count; ++i )
        {
            GENERAL_NAME* entry = sk_GENERAL_NAME_value(names, i);
            if(!entry) continue;

            if(GEN_DNS == entry->type)
            {
                int len1 = 0, len2 = -1;

                len1 = ASN1_STRING_to_UTF8(&utf8, entry->d.dNSName);
                if(utf8) {
                    len2 = (int)strlen((const char*)utf8);
                }

                if(len1 != len2) {
                    fprintf(stderr, "  Strlen and ASN1_STRING size do not match (embedded null?): %d vs %d\n", len2, len1);
                }

                /* If there's a problem with string lengths, then     */
                /* we skip the candidate and move on to the next.     */
                /* Another policy would be to fails since it probably */
                /* indicates the client is under attack.              */
                if(utf8 && len1 && len2 && (len1 == len2)) {
                    fprintf(stdout, "  %s: %s\n", label, utf8);
                    success = 1;
                }

                if(utf8) {
                    OPENSSL_free(utf8), utf8 = NULL;
                }
            }
            else
            {
                fprintf(stderr, "  Unknown GENERAL_NAME type: %d\n", entry->type);
            }
        }

    } while (0);

    if(names)
        GENERAL_NAMES_free(names);

    if(utf8)
        OPENSSL_free(utf8);

    if(!success)
        fprintf(stdout, "  %s: \n", label);

}