mirror of https://github.com/openssl/openssl.git
282 lines
11 KiB
Plaintext
282 lines
11 KiB
Plaintext
=pod
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=begin comment
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NB: Changes to the source code samples in this file should also be reflected in
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demos/guide/quic-server-block.c
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=end comment
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=head1 NAME
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ossl-guide-quic-server-block
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- OpenSSL Guide: Writing a simple blocking QUIC server
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=head1 SIMPLE BLOCKING QUIC 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, QUIC "echo" 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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The server only accepts HTTP/1.0 requests, which is non-standard and will not
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be supported by real world servers. This is for demonstration purposes only.
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Both the accepting socket and client connections are "blocking". 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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The complete source code for this example blocking QUIC server is available in
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the B<demos/guide> directory of the OpenSSL source distribution in the file
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B<quic-server-block.c>. It is also available online at
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L<https://github.com/openssl/openssl/blob/master/demos/guide/quic-server-block.c>.
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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 QUIC (see
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L<ossl-guide-libraries-introduction(7)> and L<ossl-guide-quic-introduction(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 UDP/IP and sockets.
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=head2 Creating the SSL_CTX and SSL objects
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The first step is to create an B<SSL_CTX> object for our server. We use the
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L<SSL_CTX_new(3)> function for this purpose. We pass as an argument the return
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value of the function L<OSSL_QUIC_server_method(3)>. You should use this method
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whenever you are writing a QUIC server.
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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. We specifically load a QUIC
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* server method here.
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*/
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ctx = SSL_CTX_new(OSSL_QUIC_server_method());
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if (ctx == NULL)
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goto err;
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Servers need a private key and certificate. Intermediate issuer CA
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certificates are often required, and both the server (end-entity or EE)
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certificate and the issuer ("chain") certificates are most easily configured in
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a single "chain file". Below we load such a chain file (the EE 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 (CA signatures or certificate expiration dates, for
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example), but we do verify the consistency of the private key with the
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corresponding certificate.
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/*
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* Load the server's 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, cert_path) <= 0) {
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fprintf(stderr, "couldn't load certificate file: %s\n", cert_path);
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goto err;
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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
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* validate.
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*/
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if (SSL_CTX_use_PrivateKey_file(ctx, key_path, SSL_FILETYPE_PEM) <= 0) {
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fprintf(stderr, "couldn't load key file: %s\n", key_path);
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goto err;
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}
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Most servers, including this one, do not solicit client certificates. We
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therefore do not need a "trust store" 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 certificate, 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 CA, 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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Though this is the default setting, we explicitly call the
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L<SSL_CTX_set_verify(3)> function and pass the B<SSL_VERIFY_NONE> 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 NULL to get
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the default handling.
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/*
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* Clients rarely employ certificate-based authentication, and so we don't
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* require "mutual" TLS authentication (indeed there's 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're not soliciting or processing client certificates, we don't
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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's 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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QUIC also dictates using Application-Layer Protocol Negotiation (ALPN) to select
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an application protocol. We use L<SSL_CTX_set_alpn_select_cb(3)> for this
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purpose. We can pass a callback which will be called for each connection to
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select an ALPN the server considers acceptable.
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/* Setup ALPN negotiation callback to decide which ALPN is accepted. */
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SSL_CTX_set_alpn_select_cb(ctx, select_alpn, NULL);
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In this case, we only accept "http/1.0" and "hq-interop".
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/*
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* ALPN strings for TLS handshake. Only 'http/1.0' and 'hq-interop'
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* are accepted.
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*/
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static const unsigned char alpn_ossltest[] = {
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8, 'h', 't', 't', 'p', '/', '1', '.', '0',
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10, 'h', 'q', '-', 'i', 'n', 't', 'e', 'r', 'o', 'p',
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};
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static int select_alpn(SSL *ssl, const unsigned char **out,
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unsigned char *out_len, const unsigned char *in,
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unsigned int in_len, void *arg)
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{
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if (SSL_select_next_proto((unsigned char **)out, out_len, alpn_ossltest,
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sizeof(alpn_ossltest), in,
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in_len) == OPENSSL_NPN_NEGOTIATED)
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return SSL_TLSEXT_ERR_OK;
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return SSL_TLSEXT_ERR_ALERT_FATAL;
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}
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That is all the setup that we need to do for the B<SSL_CTX>. Next, we create a
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UDP socket and bind to it on localhost.
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/* Retrieve the file descriptor for a new UDP socket */
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if ((fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
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fprintf(stderr, "cannot create socket");
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goto err;
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}
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sa.sin_family = AF_INET;
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sa.sin_port = htons(port);
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/* Bind to the new UDP socket on localhost */
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if (bind(fd, (const struct sockaddr *)&sa, sizeof(sa)) < 0) {
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fprintf(stderr, "cannot bind to %u\n", port);
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BIO_closesocket(fd);
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goto err;
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}
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To run the QUIC server, we create an B<SSL_LISTENER> to listen for incoming
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connections. We provide it with the bound UDP port to then explicitly begin
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listening for new connections.
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/*
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* Create a new QUIC listener. Listeners, and other QUIC objects, default
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* to operating in blocking mode. The configured behaviour is inherited by
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* child objects.
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*/
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if ((listener = SSL_new_listener(ctx, 0)) == NULL) {
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goto err;
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}
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/* Provide the listener with our UDP socket. */
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if (!SSL_set_fd(listener, fd))
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goto err;
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/* Begin listening. */
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if (!SSL_listen(listener))
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goto err;
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=head2 Server loop
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The server now enters a "forever" 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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/* Pristine error stack for each new connection */
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ERR_clear_error();
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At this point, the server blocks to accept the next client.
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L<SSL_accept_connection(3)> will return an accepted connection within a fresh
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SSL, in which the handshake will have already occurred.
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/* Block while waiting for a client connection */
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conn = SSL_accept_connection(listener, 0);
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if (conn == NULL) {
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fprintf(stderr, "error while accepting connection\n");
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goto err;
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}
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With the handshake complete, the server echoes client input back to the client
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in a loop.
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while (SSL_read_ex(conn, buf, sizeof(buf), &nread) > 0) {
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if (SSL_write_ex(conn, buf, nread, &nwritten) > 0 &&
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nwritten == nread) {
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continue;
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}
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fprintf(stderr, "Error echoing client input");
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break;
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}
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Once the client closes its connection, we signal the end of the stream by using
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L<SSL_stream_conclude(3)>. This will send a final Finished packet to the
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client.
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/* Signal the end of the stream. */
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if (SSL_stream_conclude(conn, 0) != 1) {
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fprintf(stderr, "Unable to conclude stream\n");
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SSL_free(conn);
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goto err;
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}
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We then shut down the connection with L<SSL_shutdown_ex(3)>, which may need
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to be called multiple times to ensure the connection is shutdown completely.
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while (SSL_shutdown_ex(conn, 0, &shutdown_args,
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sizeof(SSL_SHUTDOWN_EX_ARGS)) != 1) {
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fprintf(stderr, "Re-attempting SSL shutdown\n");
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}
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Finally, we free the SSL connection, and the server is now ready to accept the
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next client connection.
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SSL_free(conn);
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=head2 Final clean up
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If the server somehow manages to break out of the infinite loop and
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be ready to exit, it would deallocate the constructed B<SSL>.
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SSL_free(listener);
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And in the main function, it would deallocate the constructed B<SSL_CTX>.
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SSL_CTX_free(ctx);
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BIO_closesocket(fd);
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res = EXIT_SUCCESS;
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return res;
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=head1 SEE ALSO
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L<ossl-guide-introduction(7)>, L<ossl-guide-libraries-introduction(7)>,
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L<ossl-guide-libssl-introduction(7)>, L<ossl-guide-quic-introduction(7)>,
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L<ossl-guide-quic-client-non-block(7)>, L<ossl-guide-quic-client-block(7)>,
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L<ossl-guide-tls-server-block(7)>, L<ossl-guide-quic-server-non-block(7)>
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=head1 COPYRIGHT
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Copyright 2024-2025 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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