Add a very simple blocking TLS client demo

This blocking client is intended to be used to explain how to implement
a simple client in the documentation.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21133)

2 files changed, 289 insertions, 0 deletions

diff --git a/demos/guide/Makefile b/demos/guide/Makefile
new file mode 100644
index 0000000000..54c6979f43
--- /dev/null
+++ b/demos/guide/Makefile
@@ -0,0 +1,16 @@
+#
+# To run the demos when linked with a shared library (default):
+#
+#    LD_LIBRARY_PATH=../.. ./tls-client-block
+
+CFLAGS = -I../../include -g
+LDFLAGS = -L../..
+LDLIBS = -lcrypto -lssl
+
+all: tls-client-block
+
+tls-client-block: tls-client-block.c
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $< $(LDLIBS)
+
+clean:
+	$(RM) *.o tls-client-block
diff --git a/demos/guide/tls-client-block.c b/demos/guide/tls-client-block.c
new file mode 100644
index 0000000000..56133aab64
--- /dev/null
+++ b/demos/guide/tls-client-block.c
@@ -0,0 +1,273 @@
+/*
+ *  Copyright 2023 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ *  Licensed under the Apache License 2.0 (the "License").  You may not use
+ *  this file except in compliance with the License.  You can obtain a copy
+ *  in the file LICENSE in the source distribution or at
+ *  https://www.openssl.org/source/license.html
+ */
+
+/*
+ * NB: Changes to this file should also be reflected in
+ * doc/man7/ossl-guide-tls-client-block.pod
+ */
+
+#include <string.h>
+
+/* Include the appropriate header file for SOCK_STREAM */
+#ifdef _WIN32 /* Windows */
+# include <winsock2.h>
+#else /* Linux/Unix */
+# include <sys/socket.h>
+#endif
+
+#include <openssl/bio.h>
+#include <openssl/ssl.h>
+#include <openssl/err.h>
+
+/* Helper function to create a BIO connected to the server */
+static BIO *create_socket_bio(const char *hostname, const char *port)
+{
+    int sock = -1;
+    BIO_ADDRINFO *res;
+    const BIO_ADDRINFO *ai = NULL;
+    BIO *bio;
+
+    /*
+     * Lookup IP address info for the server.
+     */
+    if (!BIO_lookup_ex(hostname, port, BIO_LOOKUP_CLIENT, 0, SOCK_STREAM, 0,
+                       &res)) {
+        BIO_closesocket(sock);
+        return NULL;
+    }
+
+    /*
+     * Loop through all the possible addresses for the server and find one
+     * we can connect to.
+     */
+    for (ai = res; ai != NULL; ai = BIO_ADDRINFO_next(ai)) {
+        /*
+         * Create a TCP socket. We could equally use non-OpenSSL calls such
+         * as "socket" here for this and the subsequent connect and close
+         * functions. But for portability reasons and also so that we get
+         * errors on the OpenSSL stack in the event of a failure we use
+         * OpenSSL's versions of these functions.
+         */
+        sock = BIO_socket(BIO_ADDRINFO_family(ai), SOCK_STREAM, 0, 0);
+        if (sock == -1)
+            continue;
+
+        /* Connect the socket to the server's address */
+        if (!BIO_connect(sock, BIO_ADDRINFO_address(ai), BIO_SOCK_NODELAY)) {
+            BIO_closesocket(sock);
+            sock = -1;
+            continue;
+        }
+
+        /* We have a connected socket so break out of the loop */
+        break;
+    }
+
+    /* Free the address information resources we allocated earlier */
+    BIO_ADDRINFO_free(res);
+
+    /* If sock is -1 then we've been unable to connect to the server */
+    if (sock == -1)
+        return NULL;
+
+    /* Create a BIO to wrap the socket*/
+    bio = BIO_new(BIO_s_socket());
+    if (bio == NULL)
+        BIO_closesocket(sock);
+
+    /*
+     * Associate the newly created BIO with the underlying socket. By
+     * passing BIO_CLOSE here the socket will be automatically closed when
+     * the BIO is freed. Alternatively you can use BIO_NOCLOSE, in which
+     * case you must close the socket explicitly when it is no longer
+     * needed.
+     */
+    BIO_set_fd(bio, sock, BIO_CLOSE);
+
+    return bio;
+}
+
+/* Server hostname and port details */
+#define HOSTNAME "www.example.com"
+#define PORT     "443"
+
+/*
+ * Simple application to send a basic HTTP/1.0 request to a server and
+ * print the response on the screen.
+ */
+int main(void)
+{
+    SSL_CTX *ctx = NULL;
+    SSL *ssl;
+    BIO *bio = NULL;
+    int res = EXIT_FAILURE;
+    int ret;
+    const char *request =
+        "GET / HTTP/1.0\r\nConnection: close\r\nHost: "HOSTNAME"\r\n\r\n";
+    size_t written, readbytes;
+    char buf[160];
+
+    /*
+     * Create an SSL_CTX which we can use to create SSL objects from. We
+     * want an SSL_CTX for creating clients so we use TLS_client_method()
+     * here.
+     */
+    ctx = SSL_CTX_new(TLS_client_method());
+    if (ctx == NULL) {
+        printf("Failed to create the SSL_CTX\n");
+        goto end;
+    }
+
+    /*
+     * Configure the client to abort the handshake if certificate
+     * verification fails. Virtually all clients should do this unless you
+     * really know what you are doing.
+     */
+    SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, NULL);
+
+    /* Use the default trusted certificate store */
+    if (!SSL_CTX_set_default_verify_paths(ctx)) {
+        printf("Failed to set the default trusted certificate store\n");
+        goto end;
+    }
+
+    /*
+     * TLSv1.1 or earlier are deprecated by IETF and are generally to be
+     * avoided if possible. We require a mimimum TLS version of TLSv1.2.
+     */
+    if (!SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION)) {
+        printf("Failed to set the minimum TLS protocol version\n");
+        goto end;
+    }
+
+    /* Create an SSL object to represent the TLS connection */
+    ssl = SSL_new(ctx);
+    if (ssl == NULL) {
+        printf("Failed to create the SSL object\n");
+        goto end;
+    }
+
+    /*
+     * Create the underlying transport socket/BIO and associate it with the
+     * connection
+     */
+    bio = create_socket_bio(HOSTNAME, PORT);
+    if (bio == NULL) {
+        printf("Failed to crete the BIO\n");
+        goto end;
+    }
+    SSL_set_bio(ssl, bio, bio);
+
+    /*
+     * Tell the server during the handshake which hostname we are attempting
+     * to connect to in case the server supports multiple hosts.
+     */
+    if (!SSL_set_tlsext_host_name(ssl, HOSTNAME)) {
+        printf("Failed to set the SNI hostname\n");
+        goto end;
+    }
+
+    /*
+     * Ensure we check during certificate verification that the server has
+     * supplied a certificate for the hostname that we were expecting.
+     * Virtually all clients should do this unless you really know what you
+     * are doing.
+     */
+    if (!SSL_set1_host(ssl, HOSTNAME)) {
+        printf("Failed to set the certificate verification hostname");
+        goto end;
+    }
+
+    /* Do the handshake with the server */
+    if (SSL_connect(ssl) < 1) {
+        printf("Failed to connect to the server\n");
+        /*
+         * If the failure is due to a verification error we can get more
+         * information about it from SSL_get_verify_result().
+         */
+        if (SSL_get_verify_result(ssl) != X509_V_OK)
+            printf("Verify error: %s\n",
+                X509_verify_cert_error_string(SSL_get_verify_result(ssl)));
+        goto end;
+    }
+
+    /* Write an HTTP GET request to the peer */
+    if (!SSL_write_ex(ssl, request, strlen(request), &written)) {
+        printf("Failed to write HTTP request\n");
+        goto end;
+    }
+
+    /*
+     * Get up to sizeof(buf) bytes of the response. We keep reading until the
+     * server closes the connection.
+     */
+    while (SSL_read_ex(ssl, buf, sizeof(buf), &readbytes)) {
+        /*
+        * OpenSSL does not guarantee that the returned data is a string or
+        * that it is NUL terminated so we use fwrite() to write the exact
+        * number of bytes that we read. The data could be non-printable or
+        * have NUL characters in the middle of it. For this simple example
+        * we're going to print it to stdout anyway.
+        */
+        fwrite(buf, 1, readbytes, stdout);
+    }
+    /* In case the response didn't finish with a newline we add one now */
+    printf("\n");
+
+    /*
+     * Check whether we finished the while loop above normally or as the
+     * result of an error. The 0 argument to SSL_get_error() is the return
+     * code we received from the SSL_read_ex() call. It must be 0 in order
+     * to get here. Normal completion is indicated by SSL_ERROR_ZERO_RETURN.
+     */
+    if (SSL_get_error(ssl, 0) != SSL_ERROR_ZERO_RETURN) {
+        /*
+         * Some error occurred other than a graceful close down by the
+         * peer
+         */
+        printf ("Failed reading remaining data\n");
+        goto end;
+    }
+
+    /*
+     * The peer already shutdown gracefully (we know this because of the
+     * SSL_ERROR_ZERO_RETURN above). We should do the same back.
+     */
+    ret = SSL_shutdown(ssl);
+    if (ret < 1) {
+        /*
+         * ret < 0 indicates an error. ret == 0 would be unexpected here
+         * because that means "we've sent a close_notify and we're waiting
+         * for one back". But we already know we got one from the peer
+         * because of the SSL_ERROR_ZERO_RETURN above.
+         */
+        printf("Error shuting down\n");
+        goto end;
+    }
+
+    /* Success! */
+    res = EXIT_SUCCESS;
+ end:
+    /*
+     * If something bad happened then we will dump the contents of the
+     * OpenSSL error stack to stderr. There might be some useful diagnostic
+     * information there.
+     */
+    if (res == EXIT_FAILURE)
+        ERR_print_errors_fp(stderr);
+
+    /*
+     * Free the resources we allocated. We do not free the BIO object here
+     * because ownership of it was immediately transferred to the SSL object
+     * via SSL_set_bio(). The BIO will be freed when we free the SSL object.
+     */
+    SSL_free(ssl);
+    SSL_CTX_free(ctx);
+    return res;
+}