484 lines
18 KiB
Plaintext
484 lines
18 KiB
Plaintext
.\" Automatically generated by Pod::Man 4.11 (Pod::Simple 3.35)
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.\" ========================================================================
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.\" ========================================================================
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.\"
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.IX Title "OSSL-GUIDE-TLS-SERVER-BLOCK 7ossl"
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.TH OSSL-GUIDE-TLS-SERVER-BLOCK 7ossl "2024-10-22" "3.4.0" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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ossl\-guide\-tls\-server\-block
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\&\- OpenSSL Guide: Writing a simple blocking TLS server
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.SH "SIMPLE BLOCKING TLS SERVER EXAMPLE"
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.IX Header "SIMPLE BLOCKING TLS SERVER EXAMPLE"
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This page will present various source code samples demonstrating how to write a
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simple, non-concurrent, \s-1TLS\s0 \*(L"echo\*(R" server application which accepts one client
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connection at a time, echoing input from the client back to the same client.
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Once the current client disconnects, the next client connection is accepted.
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.PP
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Both the acceptor socket and client connections are \*(L"blocking\*(R". A more typical
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server might use nonblocking sockets with an event loop and callbacks for I/O
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events.
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.PP
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The complete source code for this example blocking \s-1TLS\s0 server is available in
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the \fBdemos/guide\fR directory of the OpenSSL source distribution in the file
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\&\fBtls\-server\-block.c\fR. It is also available online at
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<https://github.com/openssl/openssl/blob/master/demos/guide/tls\-server\-block.c>.
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.PP
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We assume that you already have OpenSSL installed on your system; that you
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already have some fundamental understanding of OpenSSL concepts and \s-1TLS\s0 (see
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\&\fBossl\-guide\-libraries\-introduction\fR\|(7) and \fBossl\-guide\-tls\-introduction\fR\|(7));
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and that you know how to write and build C code and link it against the
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libcrypto and libssl libraries that are provided by OpenSSL. It also assumes
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that you have a basic understanding of \s-1TCP/IP\s0 and sockets.
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.SS "Creating the \s-1SSL_CTX\s0 and \s-1SSL\s0 objects"
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.IX Subsection "Creating the SSL_CTX and SSL objects"
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The first step is to create an \fB\s-1SSL_CTX\s0\fR object for our server. We use the
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\&\fBSSL_CTX_new\fR\|(3) function for this purpose. We could alternatively use
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\&\fBSSL_CTX_new_ex\fR\|(3) if we want to associate the \fB\s-1SSL_CTX\s0\fR with a particular
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\&\fB\s-1OSSL_LIB_CTX\s0\fR (see \fBossl\-guide\-libraries\-introduction\fR\|(7) to learn about
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\&\fB\s-1OSSL_LIB_CTX\s0\fR). We pass as an argument the return value of the function
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\&\fBTLS_server_method\fR\|(3). You should use this method whenever you are writing a
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\&\s-1TLS\s0 server. This method will automatically use \s-1TLS\s0 version negotiation to select
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the highest version of the protocol that is mutually supported by both the
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server and the client.
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.PP
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.Vb 9
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\& /*
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\& * An SSL_CTX holds shared configuration information for multiple
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\& * subsequent per\-client SSL connections.
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\& */
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\& ctx = SSL_CTX_new(TLS_server_method());
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\& if (ctx == NULL) {
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\& ERR_print_errors_fp(stderr);
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\& errx(res, "Failed to create server SSL_CTX");
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\& }
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.Ve
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.PP
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We would also like to restrict the \s-1TLS\s0 versions that we are willing to accept to
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TLSv1.2 or above. \s-1TLS\s0 protocol versions earlier than that are generally to be
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avoided where possible. We can do that using
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\&\fBSSL_CTX_set_min_proto_version\fR\|(3):
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.PP
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.Vb 9
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\& /*
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\& * TLS versions older than TLS 1.2 are deprecated by IETF and SHOULD
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\& * be avoided if possible.
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\& */
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\& if (!SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION)) {
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\& SSL_CTX_free(ctx);
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\& ERR_print_errors_fp(stderr);
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\& errx(res, "Failed to set the minimum TLS protocol version");
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\& }
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.Ve
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.PP
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Next we configure some option flags, see \fBSSL_CTX_set_options\fR\|(3) for details:
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.PP
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.Vb 6
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\& /*
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\& * Tolerate clients hanging up without a TLS "shutdown". Appropriate in all
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\& * application protocols which perform their own message "framing", and
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\& * don\*(Aqt rely on TLS to defend against "truncation" attacks.
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\& */
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\& opts = SSL_OP_IGNORE_UNEXPECTED_EOF;
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\&
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\& /*
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\& * Block potential CPU\-exhaustion attacks by clients that request frequent
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\& * renegotiation. This is of course only effective if there are existing
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\& * limits on initial full TLS handshake or connection rates.
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\& */
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\& opts |= SSL_OP_NO_RENEGOTIATION;
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\&
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\& /*
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\& * Most servers elect to use their own cipher preference rather than that of
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\& * the client.
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\& */
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\& opts |= SSL_OP_CIPHER_SERVER_PREFERENCE;
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\&
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\& /* Apply the selection options */
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\& SSL_CTX_set_options(ctx, opts);
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.Ve
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.PP
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Servers need a private key and certificate. Though anonymous ciphers (no
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server certificate) are possible in \s-1TLS 1.2,\s0 they are rarely applicable, and
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are not currently defined for \s-1TLS 1.3.\s0 Additional intermediate issuer \s-1CA\s0
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certificates are often also required, and both the server (end-entity or \s-1EE\s0)
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certificate and the issuer (\*(L"chain\*(R") certificates are most easily configured in
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a single \*(L"chain file\*(R". Below we load such a chain file (the \s-1EE\s0 certificate
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must appear first), and then load the corresponding private key, checking that
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it matches the server certificate. No checks are performed to check the
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integrity of the chain (\s-1CA\s0 signatures or certificate expiration dates, for
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example).
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.PP
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.Vb 10
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|
\& /*
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|
\& * Load the server\*(Aqs certificate *chain* file (PEM format), which includes
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\& * not only the leaf (end\-entity) server certificate, but also any
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\& * intermediate issuer\-CA certificates. The leaf certificate must be the
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\& * first certificate in the file.
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\& *
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\& * In advanced use\-cases this can be called multiple times, once per public
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\& * key algorithm for which the server has a corresponding certificate.
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\& * However, the corresponding private key (see below) must be loaded first,
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\& * *before* moving on to the next chain file.
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\& */
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\& if (SSL_CTX_use_certificate_chain_file(ctx, "chain.pem") <= 0) {
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\& SSL_CTX_free(ctx);
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\& ERR_print_errors_fp(stderr);
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\& errx(res, "Failed to load the server certificate chain file");
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\& }
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\&
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\& /*
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\& * Load the corresponding private key, this also checks that the private
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\& * key matches the just loaded end\-entity certificate. It does not check
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\& * whether the certificate chain is valid, the certificates could be
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\& * expired, or may otherwise fail to form a chain that a client can validate.
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\& */
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\& if (SSL_CTX_use_PrivateKey_file(ctx, "pkey.pem", SSL_FILETYPE_PEM) <= 0) {
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\& SSL_CTX_free(ctx);
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\& ERR_print_errors_fp(stderr);
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\& errx(res, "Error loading the server private key file, "
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\& "possible key/cert mismatch???");
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\& }
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.Ve
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.PP
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Next we enable session caching, which makes it possible for clients to more
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efficiently make additional \s-1TLS\s0 connections after completing an initial full
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\&\s-1TLS\s0 handshake. With \s-1TLS 1.3,\s0 session resumption typically still performs a fresh
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key agreement, but the certificate exchange is avoided.
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.PP
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.Vb 7
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\& /*
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\& * Servers that want to enable session resumption must specify a cache id
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\& * byte array, that identifies the server application, and reduces the
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\& * chance of inappropriate cache sharing.
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\& */
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\& SSL_CTX_set_session_id_context(ctx, (void *)cache_id, sizeof(cache_id));
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\& SSL_CTX_set_session_cache_mode(ctx, SSL_SESS_CACHE_SERVER);
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\&
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\& /*
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\& * How many client TLS sessions to cache. The default is
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\& * SSL_SESSION_CACHE_MAX_SIZE_DEFAULT (20k in recent OpenSSL versions),
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\& * which may be too small or too large.
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\& */
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\& SSL_CTX_sess_set_cache_size(ctx, 1024);
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\&
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\& /*
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\& * Sessions older than this are considered a cache miss even if still in
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\& * the cache. The default is two hours. Busy servers whose clients make
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\& * many connections in a short burst may want a shorter timeout, on lightly
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\& * loaded servers with sporadic connections from any given client, a longer
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\& * time may be appropriate.
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\& */
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\& SSL_CTX_set_timeout(ctx, 3600);
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.Ve
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.PP
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|
Most servers, including this one, do not solicit client certificates. We
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therefore do not need a \*(L"trust store\*(R" and allow the handshake to complete even
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when the client does not present a certificate. Note: Even if a client did
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present a trusted ceritificate, for it to be useful, the server application
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would still need custom code to use the verified identity to grant nondefault
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access to that particular client. Some servers grant access to all clients
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with certificates from a private \s-1CA,\s0 this then requires processing of
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certificate revocation lists to deauthorise a client. It is often simpler and
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|
more secure to instead keep a list of authorised public keys.
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.PP
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Though this is the default setting, we explicitly call the
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\&\fBSSL_CTX_set_verify\fR\|(3) function and pass the \fB\s-1SSL_VERIFY_NONE\s0\fR value to it.
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The final argument to this function is a callback that you can optionally
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supply to override the default handling for certificate verification. Most
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applications do not need to do this so this can safely be set to \s-1NULL\s0 to get
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the default handling.
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.PP
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.Vb 12
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\& /*
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\& * Clients rarely employ certificate\-based authentication, and so we don\*(Aqt
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\& * require "mutual" TLS authentication (indeed there\*(Aqs no way to know
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\& * whether or how the client authenticated the server, so the term "mutual"
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\& * is potentially misleading).
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\& *
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\& * Since we\*(Aqre not soliciting or processing client certificates, we don\*(Aqt
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\& * need to configure a trusted\-certificate store, so no call to
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\& * SSL_CTX_set_default_verify_paths() is needed. The server\*(Aqs own
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\& * certificate chain is assumed valid.
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\& */
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\& SSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, NULL);
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.Ve
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.PP
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|
That is all the setup that we need to do for the \fB\s-1SSL_CTX\s0\fR. Next we create an
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acceptor \s-1BIO\s0 on which to accept client connections. This just records the
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intended port (and optional \*(L"host:\*(R" prefix), without actually creating the
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socket. This delayed processing allows the programmer to specify additional
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|
behaviours before the listening socket is actually created.
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.PP
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.Vb 10
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\& /*
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\& * Create a listener socket wrapped in a BIO.
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\& * The first call to BIO_do_accept() initialises the socket
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\& */
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\& acceptor_bio = BIO_new_accept(hostport);
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\& if (acceptor_bio == NULL) {
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\& SSL_CTX_free(ctx);
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|
\& ERR_print_errors_fp(stderr);
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\& errx(res, "Error creating acceptor bio");
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\& }
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.Ve
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.PP
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Servers almost always want to use the \*(L"\s-1SO_REUSEADDR\*(R"\s0 option to avoid startup
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|
failures if there are still lingering client connections, so we do that before
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|
making the \fBfirst\fR call to \fBBIO_do_accept\fR\|(3) which creates the listening
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|
socket, without accepting a client connection. Subsequent calls to the same
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|
function will accept new connections.
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.PP
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.Vb 6
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\& BIO_set_bind_mode(acceptor_bio, BIO_BIND_REUSEADDR);
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\& if (BIO_do_accept(acceptor_bio) <= 0) {
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\& SSL_CTX_free(ctx);
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\& ERR_print_errors_fp(stderr);
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\& errx(res, "Error setting up acceptor socket");
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|
\& }
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.Ve
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|
.SS "Server loop"
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|
.IX Subsection "Server loop"
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|
The server now enters a \*(L"forever\*(R" loop handling one client connection at a
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time. Before each connection we clear the OpenSSL error stack, so that any
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error reports are related to just the new connection.
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.PP
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.Vb 2
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|
\& /* Pristine error stack for each new connection */
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|
\& ERR_clear_error();
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.Ve
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|
.PP
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|
At this point the server blocks to accept the next client:
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.PP
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|
.Vb 5
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|
\& /* Wait for the next client to connect */
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\& if (BIO_do_accept(acceptor_bio) <= 0) {
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\& /* Client went away before we accepted the connection */
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|
\& continue;
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|
\& }
|
|
.Ve
|
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.PP
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|
On success the accepted client connection has been wrapped in a fresh \s-1BIO\s0 and
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pushed onto the end of the acceptor \s-1BIO\s0 chain. We pop it off returning the
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acceptor \s-1BIO\s0 to its initial state.
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.PP
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|
.Vb 3
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\& /* Pop the client connection from the BIO chain */
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\& client_bio = BIO_pop(acceptor_bio);
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\& fprintf(stderr, "New client connection accepted\en");
|
|
.Ve
|
|
.PP
|
|
Next, we create an \fB\s-1SSL\s0\fR object by calling the \fB\fBSSL_new\fB\|(3)\fR function and
|
|
passing the \fB\s-1SSL_CTX\s0\fR we created as an argument. The client connection \s-1BIO\s0 is
|
|
configured as the I/O conduit for this \s-1SSL\s0 handle. SSL_set_bio transfers
|
|
ownership of the \s-1BIO\s0 or BIOs involved (our \fBclient_bio\fR) to the \s-1SSL\s0 handle.
|
|
.PP
|
|
.Vb 8
|
|
\& /* Associate a new SSL handle with the new connection */
|
|
\& if ((ssl = SSL_new(ctx)) == NULL) {
|
|
\& ERR_print_errors_fp(stderr);
|
|
\& warnx("Error creating SSL handle for new connection");
|
|
\& BIO_free(client_bio);
|
|
\& continue;
|
|
\& }
|
|
\& SSL_set_bio(ssl, client_bio, client_bio);
|
|
.Ve
|
|
.PP
|
|
And now we're ready to attempt the \s-1SSL\s0 handshake. With a blocking socket
|
|
OpenSSL will perform all the read and write operations required to complete the
|
|
handshake (or detect and report a failure) before returning.
|
|
.PP
|
|
.Vb 7
|
|
\& /* Attempt an SSL handshake with the client */
|
|
\& if (SSL_accept(ssl) <= 0) {
|
|
\& ERR_print_errors_fp(stderr);
|
|
\& warnx("Error performing SSL handshake with client");
|
|
\& SSL_free(ssl);
|
|
\& continue;
|
|
\& }
|
|
.Ve
|
|
.PP
|
|
With the handshake complete, the server loops echoing client input back to the
|
|
client:
|
|
.PP
|
|
.Vb 9
|
|
\& while (SSL_read_ex(ssl, buf, sizeof(buf), &nread) > 0) {
|
|
\& if (SSL_write_ex(ssl, buf, nread, &nwritten) > 0 &&
|
|
\& nwritten == nread) {
|
|
\& total += nwritten;
|
|
\& continue;
|
|
\& }
|
|
\& warnx("Error echoing client input");
|
|
\& break;
|
|
\& }
|
|
.Ve
|
|
.PP
|
|
Once the client closes its connection, we report the number of bytes sent to
|
|
\&\fBstderr\fR and free the \s-1SSL\s0 handle, which also frees the \fBclient_bio\fR and
|
|
closes the underlying socket.
|
|
.PP
|
|
.Vb 2
|
|
\& fprintf(stderr, "Client connection closed, %zu bytes sent\en", total);
|
|
\& SSL_free(ssl);
|
|
.Ve
|
|
.PP
|
|
The server is now ready to accept the next client connection.
|
|
.SS "Final clean up"
|
|
.IX Subsection "Final clean up"
|
|
If the server could somehow manage to break out of the infinite loop, and
|
|
be ready to exit, it would first deallocate the constructed \fB\s-1SSL_CTX\s0\fR.
|
|
.PP
|
|
.Vb 5
|
|
\& /*
|
|
\& * Unreachable placeholder cleanup code, the above loop runs forever.
|
|
\& */
|
|
\& SSL_CTX_free(ctx);
|
|
\& return EXIT_SUCCESS;
|
|
.Ve
|
|
.SH "SEE ALSO"
|
|
.IX Header "SEE ALSO"
|
|
\&\fBossl\-guide\-introduction\fR\|(7), \fBossl\-guide\-libraries\-introduction\fR\|(7),
|
|
\&\fBossl\-guide\-libssl\-introduction\fR\|(7), \fBossl\-guide\-tls\-introduction\fR\|(7),
|
|
\&\fBossl\-guide\-tls\-client\-non\-block\fR\|(7), \fBossl\-guide\-quic\-client\-block\fR\|(7)
|
|
.SH "COPYRIGHT"
|
|
.IX Header "COPYRIGHT"
|
|
Copyright 2024 The OpenSSL Project Authors. All Rights Reserved.
|
|
.PP
|
|
Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
|
|
this file except in compliance with the License. You can obtain a copy
|
|
in the file \s-1LICENSE\s0 in the source distribution or at
|
|
<https://www.openssl.org/source/license.html>.
|