7.1.4 Simple datagram TLS client example

This is a client that uses UDP to connect to a server. This is the DTLS equivalent to the TLS example with X.509 certificates.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <assert.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/dtls.h>

/* A very basic Datagram TLS client, over UDP with X.509 authentication.
 */

#define CHECK(x) assert((x)>=0)

#define MAX_BUF 1024
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern int udp_connect(void);
extern void udp_close(int sd);
extern int verify_certificate_callback(gnutls_session_t session);

int main(void)
{
        int ret, sd, ii;
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        gnutls_certificate_credentials_t xcred;

        if (gnutls_check_version("3.1.4") == NULL) {
                fprintf(stderr, "GnuTLS 3.1.4 or later is required for this example\n");
                exit(1);
        }

        /* for backwards compatibility with gnutls < 3.3.0 */
        CHECK(gnutls_global_init());

        /* X509 stuff */
        CHECK(gnutls_certificate_allocate_credentials(&xcred));

        /* sets the system trusted CAs for Internet PKI */
        CHECK(gnutls_certificate_set_x509_system_trust(xcred));

        /* Initialize TLS session */
        CHECK(gnutls_init(&session, GNUTLS_CLIENT | GNUTLS_DATAGRAM));

        /* Use default priorities */
        CHECK(gnutls_set_default_priority(session));

        /* put the x509 credentials to the current session */
        CHECK(gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred));
        CHECK(gnutls_server_name_set(session, GNUTLS_NAME_DNS, "my_host_name",
                                     strlen("my_host_name")));

        gnutls_session_set_verify_cert(session, "my_host_name", 0);

        /* connect to the peer */
        sd = udp_connect();

        gnutls_transport_set_int(session, sd);

        /* set the connection MTU */
        gnutls_dtls_set_mtu(session, 1000);
        /* gnutls_dtls_set_timeouts(session, 1000, 60000); */

        /* Perform the TLS handshake */
        do {
                ret = gnutls_handshake(session);
        }
        while (ret == GNUTLS_E_INTERRUPTED || ret == GNUTLS_E_AGAIN);
        /* Note that DTLS may also receive GNUTLS_E_LARGE_PACKET */

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        CHECK(gnutls_record_send(session, MSG, strlen(MSG)));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (ret == 0) {
                printf("- Peer has closed the TLS connection\n");
                goto end;
        } else if (ret < 0 && gnutls_error_is_fatal(ret) == 0) {
                fprintf(stderr, "*** Warning: %s\n", gnutls_strerror(ret));
        } else if (ret < 0) {
                fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
                goto end;
        }

        if (ret > 0) {
                printf("- Received %d bytes: ", ret);
                for (ii = 0; ii < ret; ii++) {
                        fputc(buffer[ii], stdout);
                }
                fputs("\n", stdout);
        }

        /* It is suggested not to use GNUTLS_SHUT_RDWR in DTLS
         * connections because the peer's closure message might
         * be lost */
        CHECK(gnutls_bye(session, GNUTLS_SHUT_WR));

      end:

        udp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);

        gnutls_global_deinit();

        return 0;
}