7.1.2 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 LOOP_CHECK(rval, cmd)                                             \
	do {                                                              \
		rval = cmd;                                               \
	} while (rval == GNUTLS_E_AGAIN || rval == GNUTLS_E_INTERRUPTED); \
	assert(rval >= 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,
				     "www.example.com",
				     strlen("www.example.com")));

	gnutls_session_set_verify_cert(session, "www.example.com", 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);
	}

	LOOP_CHECK(ret, gnutls_record_send(session, MSG, strlen(MSG)));

	LOOP_CHECK(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;
}