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 <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/dtls.h>

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

#define MAX_BUF 1024
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#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];
        const char *err;
        gnutls_certificate_credentials_t xcred;

        gnutls_global_init();

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

        /* sets the trusted cas file */
        gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);
        gnutls_certificate_set_verify_function(xcred,
                                               verify_certificate_callback);

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

        /* Use default priorities */
        ret = gnutls_priority_set_direct(session, "NORMAL", &err);
        if (ret < 0) {
                if (ret == GNUTLS_E_INVALID_REQUEST) {
                        fprintf(stderr, "Syntax error at: %s\n", err);
                }
                exit(1);
        }

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

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

        gnutls_transport_set_int(session, sd);

        /* set the connection MTU */
        gnutls_dtls_set_mtu(session, 1000);
        gnutls_handshake_set_timeout(session,
                                     GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

        /* 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);
        }

        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 */
        gnutls_bye(session, GNUTLS_SHUT_WR);

      end:

        udp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);

        gnutls_global_deinit();

        return 0;
}

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