- NAME
- SIMPLE NONBLOCKING QUIC SERVER EXAMPLE
  - Creating the SSL_CTX and SSL objects
  - Server loop
  - Final clean up
- SEE ALSO
- COPYRIGHT

# NAME

ossl-guide-quic-server-non-block - OpenSSL Guide: Writing a simple nonblocking QUIC server

# SIMPLE NONBLOCKING QUIC SERVER EXAMPLE

This page presents various source code samples demonstrating how to write a simple, non-concurrent, QUIC "echo" server application which accepts one client connection at a time, echoing input from the client back to the same client. Once the current client disconnects, the next client connection is accepted.

The server only accepts `http/1.0` and `hq-interop` ALPN's and doesn't actually implement HTTP but only does a simple echo. This is non-standard and will not be supported by real world servers. This is for demonstration purposes only.

There are various methods to test this server: quic-client-block.c and quic-client-non-block.c will send a basic HTTP/1.0 request, which the server will echo back. You can also test this server by running `openssl s_client -connect localhost:4443 -4 -quic -alpn http/1.0` and entering text that will be echoed back by the server.

Both the listening socket and connected socket are "nonblocking". However, we use select() to make the listening socket block when it cannot read/write. Rather than stopping and waiting, your application may need to go and do other tasks whilst the SSL object is unable to read/write. For example: updating a GUI or performing operations on some other connection or stream.

The complete source code for this example nonblocking QUIC server is available in the demos/guide directory of the OpenSSL source distribution in the file quic-server-non-block.c. It is also available online at https://github.com/openssl/openssl/blob/master/demos/guide/quic-server-non-block.c.

We assume that you already have OpenSSL installed on your system; that you already have some fundamental understanding of OpenSSL concepts and QUIC (see ossl-guide-libraries-introduction(7) and ossl-guide-quic-introduction(7)); and that you know how to write and build C code and link it against the libcrypto and libssl libraries that are provided by OpenSSL. It also assumes that you have a basic understanding of UDP/IP and sockets.

## Creating the SSL_CTX and SSL objects

The first step is to create an SSL_CTX object for our server. We use the SSL_CTX_new(3) function for this purpose. We pass as an argument the return value of the function OSSL_QUIC_server_method(3). You should use this method whenever you are writing a QUIC server.

    /*
     * An SSL_CTX holds shared configuration information for multiple
     * subsequent per-client SSL connections. We specifically load a QUIC
     * server method here.
     */
    ctx = SSL_CTX_new(OSSL_QUIC_server_method());
    if (ctx == NULL)
        goto err;

Servers need a private key and certificate. Intermediate issuer CA certificates are often required, and both the server (end-entity or EE) certificate and the issuer ("chain") certificates are most easily configured in a single "chain file". Below we load such a chain file (the EE certificate must appear first), and then load the corresponding private key, checking that it matches the server certificate. No checks are performed to check the integrity of the chain (CA signatures or certificate expiration dates, for example), but we do verify the consistency of the private key with the corresponding certificate.

    /*
     * Load the server's certificate *chain* file (PEM format), which includes
     * not only the leaf (end-entity) server certificate, but also any
     * intermediate issuer-CA certificates.  The leaf certificate must be the
     * first certificate in the file.
     *
     * In advanced use-cases this can be called multiple times, once per public
     * key algorithm for which the server has a corresponding certificate.
     * However, the corresponding private key (see below) must be loaded first,
     * *before* moving on to the next chain file.
     */
    if (SSL_CTX_use_certificate_chain_file(ctx, cert_path) <= 0) {
        fprintf(stderr, "couldn't load certificate file: %s\n", cert_path);
        goto err;
    }

    /*
     * Load the corresponding private key, this also checks that the private
     * key matches the just loaded end-entity certificate.  It does not check
     * whether the certificate chain is valid, the certificates could be
     * expired, or may otherwise fail to form a chain that a client can
     * validate.
     */
    if (SSL_CTX_use_PrivateKey_file(ctx, key_path, SSL_FILETYPE_PEM) <= 0) {
        fprintf(stderr, "couldn't load key file: %s\n", key_path);
        goto err;
    }

Most servers, including this one, do not solicit client certificates. We therefore do not need a "trust store" and allow the handshake to complete even when the client does not present a certificate. Note: Even if a client did present a trusted certificate, for it to be useful, the server application would still need custom code to use the verified identity to grant nondefault access to that particular client. Some servers grant access to all clients with certificates from a private CA, this then requires processing of certificate revocation lists to deauthorise a client. It is often simpler and more secure to instead keep a list of authorised public keys.

Though this is the default setting, we explicitly call the SSL_CTX_set_verify(3) function and pass the SSL_VERIFY_NONE value to it. The final argument to this function is a callback that you can optionally supply to override the default handling for certificate verification. Most applications do not need to do this so this can safely be set to NULL to get the default handling.

    /*
     * Clients rarely employ certificate-based authentication, and so we don't
     * require "mutual" TLS authentication (indeed there's no way to know
     * whether or how the client authenticated the server, so the term "mutual"
     * is potentially misleading).
     *
     * Since we're not soliciting or processing client certificates, we don't
     * need to configure a trusted-certificate store, so no call to
     * SSL_CTX_set_default_verify_paths() is needed.  The server's own
     * certificate chain is assumed valid.
     */
    SSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, NULL);

QUIC also dictates using Application-Layer Protocol Negotiation (ALPN) to select an application protocol. We use SSL_CTX_set_alpn_select_cb(3) for this purpose. We can pass a callback which will be called for each connection to select an ALPN the server considers acceptable.

    /* Setup ALPN negotiation callback to decide which ALPN is accepted. */
    SSL_CTX_set_alpn_select_cb(ctx, select_alpn, NULL);

In this case, we only accept "http/1.0" and "hq-interop".

    /*
    * ALPN strings for TLS handshake. Only 'http/1.0' and 'hq-interop'
    * are accepted.
    */
    static const unsigned char alpn_ossltest[] = {
        8,  'h', 't', 't', 'p', '/', '1', '.', '0',
        10, 'h', 'q', '-', 'i', 'n', 't', 'e', 'r', 'o', 'p',
    };

    static int select_alpn(SSL *ssl, const unsigned char **out,
                           unsigned char *out_len, const unsigned char *in,
                           unsigned int in_len, void *arg)
    {
        if (SSL_select_next_proto((unsigned char **)out, out_len, alpn_ossltest,
                                  sizeof(alpn_ossltest), in,
                                  in_len) == OPENSSL_NPN_NEGOTIATED)
            return SSL_TLSEXT_ERR_OK;
        return SSL_TLSEXT_ERR_ALERT_FATAL;
    }

That is all the setup that we need to do for the SSL_CTX. Next, we create a UDP socket and bind to it on localhost.

    /* Retrieve the file descriptor for a new UDP socket */
    if ((fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
        fprintf(stderr, "cannot create socket");
        return -1;
    }

    sa.sin_family = AF_INET;
    sa.sin_port = htons(port);

    /* Bind to the new UDP socket on localhost */
    if (bind(fd, (const struct sockaddr *)&sa, sizeof(sa)) < 0) {
        fprintf(stderr, "cannot bind to %u\n", port);
        BIO_closesocket(fd);
        return -1;
    }

    /* Set port to nonblocking mode */
    if (BIO_socket_nbio(fd, 1) <= 0) {
        fprintf(stderr, "Unable to set port to nonblocking mode");
        BIO_closesocket(fd);
        return -1;
    }

To run the QUIC server, we create an SSL_LISTENER to listen for incoming connections. We provide it with the bound UDP port to then explicitly begin listening for new connections.

    /* Create a new QUIC listener */
    if ((listener = SSL_new_listener(ctx, 0)) == NULL)
        goto err;

    /* Provide the listener with our UDP socket. */
    if (!SSL_set_fd(listener, fd))
        goto err;

    /* Set the listener mode to nonblocking, which is inherited by
     * child objects.
     */
    if (!SSL_set_blocking_mode(listener, 0))
        goto err;

    /*
     * Begin listening. Note that is not usually needed as SSL_accept_connection
     * will implicitly start listening. It is only needed if a server wishes to
     * ensure it has started to accept incoming connections but does not wish to
     * actually call SSL_accept_connection yet.
     */
    if (!SSL_listen(listener))
        goto err;

## Server loop

The server now enters a "forever" loop, handling one client connection at a time. Before each connection, we clear the OpenSSL error stack so that any error reports are related to just the new connection.

    /* Pristine error stack for each new connection */
    ERR_clear_error();

We then wait until a connection is ready for reading. It uses the select function to wait until the socket is either readable or writable, depending on what the SSL connection requires.

We then accept a new connection in which the handshake will have already occurred. However, since we are in nonblocking mode, SSL_accept_connection(3) will return immediately. Therefore, we use a helper function to essentially block until a connection is established.

    printf("Waiting for connection\n");
    while ((conn = SSL_accept_connection(listener, 0)) == NULL) {
        wait_for_activity(listener);
    }
    printf("Accepted new connection\n");

The helper function wait_for_activity uses select() to block until the file descriptor belonging to the passed SSL object is readable. As mentioned earlier, a more real-world application would likely use this time to perform other tasks.

    /* Initialize the fd_set structure */
    FD_ZERO(&read_fd);
    FD_ZERO(&write_fd);

    /*
     * Determine if we would like to write to the socket, read from it, or both.
     */
    if (SSL_net_write_desired(ssl))
        FD_SET(sock, &write_fd);
    if (SSL_net_read_desired(ssl))
        FD_SET(sock, &read_fd);

    /*
     * Find out when OpenSSL would next like to be called, regardless of
     * whether the state of the underlying socket has changed or not.
     */
    if (SSL_get_event_timeout(ssl, &tv, &isinfinite) && !isinfinite)
        tvp = &tv;

    /*
     * Wait until the socket is writeable or readable. We use select here
     * for the sake of simplicity and portability, but you could equally use
     * poll/epoll or similar functions
     *
     * NOTE: For the purposes of this demonstration code this effectively
     * makes this demo block until it has something more useful to do. In a
     * real application you probably want to go and do other work here (e.g.
     * update a GUI, or service other connections).
     *
     * Let's say for example that you want to update the progress counter on
     * a GUI every 100ms. One way to do that would be to use the timeout in
     * the last parameter to "select" below. If the tvp value is greater
     * than 100ms then use 100ms instead. Then, when select returns, you
     * check if it did so because of activity on the file descriptors or
     * because of the timeout. If the 100ms GUI timeout has expired but the
     * tvp timeout has not then go and update the GUI and then restart the
     * "select" (with updated timeouts).
     */

    select(sock + 1, &read_fd, &write_fd, NULL, tvp);

With the handshake complete, the server reads all the client input.

    /* Read from client until the client sends a end of stream packet */
    while (!eof) {
        ret = SSL_read_ex(conn, buf + total_read, sizeof(buf) - total_read,
                          &nread);
        total_read += nread;
        if (total_read >= 8192) {
            fprintf(stderr, "Could not fit all data into buffer\n");
            goto err;
        }
        switch (handle_io_failure(conn, ret)) {
        case 1:
            continue; /* Retry */
        case 0:
            /* Reached end of stream */
            if (!SSL_has_pending(conn))
                eof = 1;
            break;
        default:
            fprintf(stderr, "Failed reading remaining data\n");
            goto err;
        }
    }

Finally, we echo the received data back to the client. We can use SSL_write_ex2(3) to pass in a special flag SSL_WRITE_FLAG_CONCLUDE that will send a FIN packet once the write has successfully finished writing all the data to the peer.

    /* Echo client input */
    while (!SSL_write_ex2(conn, buf,
                          total_read,
                          SSL_WRITE_FLAG_CONCLUDE, &total_written)) {
        if (handle_io_failure(conn, 0) == 1)
            continue;
        fprintf(stderr, "Failed to write data\n");
        goto err;
    }

We then shut down the connection with SSL_shutdown(3), which may need to be called multiple times to ensure the connection is shutdown completely.

    /*
     * Shut down the connection. We may need to call this multiple times
     * to ensure the connection is shutdown completely.
     */
    while ((ret = SSL_shutdown(conn)) != 1) {
        if (ret < 0 && handle_io_failure(conn, ret) == 1)
            continue; /* Retry */
    }

Finally, we free the SSL connection, and the server is now ready to accept the next client connection.

    SSL_free(conn);

## Final clean up

If the server somehow manages to break out of the infinite loop and be ready to exit, it would deallocate the constructed SSL.

    SSL_free(listener);

And in the main function, it would deallocate the constructed SSL_CTX.

    SSL_CTX_free(ctx);
    BIO_closesocket(fd);

# SEE ALSO

ossl-guide-introduction(7), ossl-guide-libraries-introduction(7), ossl-guide-libssl-introduction(7), ossl-guide-quic-introduction(7), ossl-guide-quic-client-non-block(7), ossl-guide-quic-client-block(7), ossl-guide-tls-server-block(7), ossl-guide-quic-server-block(7)

# COPYRIGHT

Copyright 2024-2025 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.