Tutorial: HTTP/2 server ========================= In this tutorial, we are going to write a single-threaded, event-based HTTP/2 web server, which supports HTTPS only. It can handle concurrent multiple requests, but only the GET method is supported. The complete source code, `libevent-server.c`_, is attached at the end of this page. The source also resides in the examples directory in the archive or repository. This simple server takes 3 arguments: The port number to listen on, the path to your SSL/TLS private key file, and the path to your certificate file. The synopsis is:: $ libevent-server PORT /path/to/server.key /path/to/server.crt We use libevent in this tutorial to handle networking I/O. Please note that nghttp2 itself does not depend on libevent. The server starts with some libevent and OpenSSL setup in the ``main()`` and ``run()`` functions. This setup isn't specific to nghttp2, but one thing you should look at is setup of the NPN callback. The NPN callback is used by the server to advertise which application protocols the server supports to a client. In this example program, when creating the ``SSL_CTX`` object, we store the application protocol name in the wire format of NPN in a statically allocated buffer. This is safe because we only create one ``SSL_CTX`` object in the program's entire lifetime. If you are following TLS related RFC, you know that NPN is not the standardized way to negotiate HTTP/2. NPN itself is not even published as RFC. The standard way to negotiate HTTP/2 is ALPN, Application-Layer Protocol Negotiation Extension, defined in `RFC 7301 `_. The one caveat of ALPN is that OpenSSL >= 1.0.2 is required. We use macro to enable/disable ALPN support depending on OpenSSL version. In ALPN, client sends the list of supported application protocols, and server selects one of them. We provide the callback for it:: static unsigned char next_proto_list[256]; static size_t next_proto_list_len; static int next_proto_cb(SSL *s _U_, const unsigned char **data, unsigned int *len, void *arg _U_) { *data = next_proto_list; *len = (unsigned int)next_proto_list_len; return SSL_TLSEXT_ERR_OK; } #if OPENSSL_VERSION_NUMBER >= 0x10002000L static int alpn_select_proto_cb(SSL *ssl _U_, const unsigned char **out, unsigned char *outlen, const unsigned char *in, unsigned int inlen, void *arg _U_) { int rv; rv = nghttp2_select_next_protocol((unsigned char **)out, outlen, in, inlen); if (rv != 1) { return SSL_TLSEXT_ERR_NOACK; } return SSL_TLSEXT_ERR_OK; } #endif // OPENSSL_VERSION_NUMBER >= 0x10002000L static SSL_CTX *create_ssl_ctx(const char *key_file, const char *cert_file) { SSL_CTX *ssl_ctx; EC_KEY *ecdh; ssl_ctx = SSL_CTX_new(SSLv23_server_method()); ... next_proto_list[0] = NGHTTP2_PROTO_VERSION_ID_LEN; memcpy(&next_proto_list[1], NGHTTP2_PROTO_VERSION_ID, NGHTTP2_PROTO_VERSION_ID_LEN); next_proto_list_len = 1 + NGHTTP2_PROTO_VERSION_ID_LEN; SSL_CTX_set_next_protos_advertised_cb(ssl_ctx, next_proto_cb, NULL); #if OPENSSL_VERSION_NUMBER >= 0x10002000L SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, NULL); #endif // OPENSSL_VERSION_NUMBER >= 0x10002000L return ssl_ctx; } The wire format of NPN is a sequence of length prefixed strings, with exactly one byte used to specify the length of each protocol identifier. In this tutorial, we advertise the specific HTTP/2 protocol version the current nghttp2 library supports, which is exported in the identifier :macro:`NGHTTP2_PROTO_VERSION_ID`. The ``next_proto_cb()`` function is the server-side NPN callback. In the OpenSSL implementation, we just assign the pointer to the NPN buffers we filled in earlier. The NPN callback function is set to the ``SSL_CTX`` object using ``SSL_CTX_set_next_protos_advertised_cb()``. In ``alpn_select_proto_cb()``, we use `nghttp2_select_next_protocol()` to select application protocol. The `nghttp2_select_next_protocol()` returns 1 only if it selected h2 (ALPN identifier for HTTP/2), and out parameters were assigned accordingly. Next, let's take a look at the main structures used by the example application: We use the ``app_context`` structure to store application-wide data:: struct app_context { SSL_CTX *ssl_ctx; struct event_base *evbase; }; We use the ``http2_session_data`` structure to store session-level (which corresponds to one HTTP/2 connection) data:: typedef struct http2_session_data { struct http2_stream_data root; struct bufferevent *bev; app_context *app_ctx; nghttp2_session *session; char *client_addr; } http2_session_data; We use the ``http2_stream_data`` structure to store stream-level data:: typedef struct http2_stream_data { struct http2_stream_data *prev, *next; char *request_path; int32_t stream_id; int fd; } http2_stream_data; A single HTTP/2 session can have multiple streams. To manage them, we use a doubly linked list: The first element of this list is pointed to by the ``root->next`` in ``http2_session_data``. Initially, ``root->next`` is ``NULL``. libevent's bufferevent structure is used to perform network I/O, with the pointer to the bufferevent stored in the ``http2_session_data`` structure. Note that the bufferevent object is kept in ``http2_session_data`` and not in ``http2_stream_data``. This is because ``http2_stream_data`` is just a logical stream multiplexed over the single connection managed by the bufferevent in ``http2_session_data``. We first create a listener object to accept incoming connections. libevent's ``struct evconnlistener`` is used for this purpose:: static void start_listen(struct event_base *evbase, const char *service, app_context *app_ctx) { int rv; struct addrinfo hints; struct addrinfo *res, *rp; memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; hints.ai_socktype = SOCK_STREAM; hints.ai_flags = AI_PASSIVE; #ifdef AI_ADDRCONFIG hints.ai_flags |= AI_ADDRCONFIG; #endif /* AI_ADDRCONFIG */ rv = getaddrinfo(NULL, service, &hints, &res); if (rv != 0) { errx(1, NULL); } for (rp = res; rp; rp = rp->ai_next) { struct evconnlistener *listener; listener = evconnlistener_new_bind( evbase, acceptcb, app_ctx, LEV_OPT_CLOSE_ON_FREE | LEV_OPT_REUSEABLE, 16, rp->ai_addr, (int)rp->ai_addrlen); if (listener) { freeaddrinfo(res); return; } } errx(1, "Could not start listener"); } We specify the ``acceptcb`` callback, which is called when a new connection is accepted:: static void acceptcb(struct evconnlistener *listener _U_, int fd, struct sockaddr *addr, int addrlen, void *arg) { app_context *app_ctx = (app_context *)arg; http2_session_data *session_data; session_data = create_http2_session_data(app_ctx, fd, addr, addrlen); bufferevent_setcb(session_data->bev, readcb, writecb, eventcb, session_data); } Here we create the ``http2_session_data`` object. The connection's bufferevent is initialized at the same time. We specify three callbacks for the bufferevent: ``readcb``, ``writecb``, and ``eventcb``. The ``eventcb()`` callback is invoked by the libevent event loop when an event (e.g. connection has been established, timeout, etc.) occurs on the underlying network socket:: static void eventcb(struct bufferevent *bev _U_, short events, void *ptr) { http2_session_data *session_data = (http2_session_data *)ptr; if (events & BEV_EVENT_CONNECTED) { const unsigned char *alpn = NULL; unsigned int alpnlen = 0; SSL *ssl; fprintf(stderr, "%s connected\n", session_data->client_addr); ssl = bufferevent_openssl_get_ssl(session_data->bev); SSL_get0_next_proto_negotiated(ssl, &alpn, &alpnlen); #if OPENSSL_VERSION_NUMBER >= 0x10002000L if (alpn == NULL) { SSL_get0_alpn_selected(ssl, &alpn, &alpnlen); } #endif // OPENSSL_VERSION_NUMBER >= 0x10002000L if (alpn == NULL || alpnlen != 2 || memcmp("h2", alpn, 2) != 0) { fprintf(stderr, "%s h2 is not negotiated\n", session_data->client_addr); delete_http2_session_data(session_data); return; } initialize_nghttp2_session(session_data); if (send_server_connection_header(session_data) != 0 || session_send(session_data) != 0) { delete_http2_session_data(session_data); return; } return; } if (events & BEV_EVENT_EOF) { fprintf(stderr, "%s EOF\n", session_data->client_addr); } else if (events & BEV_EVENT_ERROR) { fprintf(stderr, "%s network error\n", session_data->client_addr); } else if (events & BEV_EVENT_TIMEOUT) { fprintf(stderr, "%s timeout\n", session_data->client_addr); } delete_http2_session_data(session_data); } Here we validate that HTTP/2 is negotiated, and if not, drop connection. For the ``BEV_EVENT_EOF``, ``BEV_EVENT_ERROR``, and ``BEV_EVENT_TIMEOUT`` events, we just simply tear down the connection. The ``delete_http2_session_data()`` function destroys the ``http2_session_data`` object and its associated bufferevent member. As a result, the underlying connection is closed. The ``BEV_EVENT_CONNECTED`` event is invoked when SSL/TLS handshake has completed successfully. After this we are ready to begin communicating via HTTP/2. The ``initialize_nghttp2_session()`` function initializes the nghttp2 session object and several callbacks:: static void initialize_nghttp2_session(http2_session_data *session_data) { nghttp2_session_callbacks *callbacks; nghttp2_session_callbacks_new(&callbacks); nghttp2_session_callbacks_set_send_callback(callbacks, send_callback); nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks, on_frame_recv_callback); nghttp2_session_callbacks_set_on_stream_close_callback( callbacks, on_stream_close_callback); nghttp2_session_callbacks_set_on_header_callback(callbacks, on_header_callback); nghttp2_session_callbacks_set_on_begin_headers_callback( callbacks, on_begin_headers_callback); nghttp2_session_server_new(&session_data->session, callbacks, session_data); nghttp2_session_callbacks_del(callbacks); } Since we are creating a server, we use `nghttp2_session_server_new()` to initialize the nghttp2 session object. We also setup 5 callbacks for the nghttp2 session, these are explained later. The server now begins by sending the server connection preface, which always consists of a SETTINGS frame. ``send_server_connection_header()`` configures and submits it:: static int send_server_connection_header(http2_session_data *session_data) { nghttp2_settings_entry iv[1] = { {NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100}}; int rv; rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv, ARRLEN(iv)); if (rv != 0) { warnx("Fatal error: %s", nghttp2_strerror(rv)); return -1; } return 0; } In the example SETTINGS frame we've set SETTINGS_MAX_CONCURRENT_STREAMS to 100. `nghttp2_submit_settings()` is used to queue the frame for transmission, but note it only queues the frame for transmission, and doesn't actually send it. All functions in the ``nghttp2_submit_*()`` family have this property. To actually send the frame, `nghttp2_session_send()` should be used, as described later. Since bufferevent may buffer more than the first 24 bytes from the client, we have to process them here since libevent won't invoke callback functions for this pending data. To process the received data, we call the ``session_recv()`` function:: static int session_recv(http2_session_data *session_data) { ssize_t readlen; struct evbuffer *input = bufferevent_get_input(session_data->bev); size_t datalen = evbuffer_get_length(input); unsigned char *data = evbuffer_pullup(input, -1); readlen = nghttp2_session_mem_recv(session_data->session, data, datalen); if (readlen < 0) { warnx("Fatal error: %s", nghttp2_strerror((int)readlen)); return -1; } if (evbuffer_drain(input, (size_t)readlen) != 0) { warnx("Fatal error: evbuffer_drain failed"); return -1; } if (session_send(session_data) != 0) { return -1; } return 0; } In this function, we feed all unprocessed but already received data to the nghttp2 session object using the `nghttp2_session_mem_recv()` function. The `nghttp2_session_mem_recv()` function processes the data and may both invoke the previously setup callbacks and also queue outgoing frames. To send any pending outgoing frames, we immediately call ``session_send()``. The ``session_send()`` function is defined as follows:: static int session_send(http2_session_data *session_data) { int rv; rv = nghttp2_session_send(session_data->session); if (rv != 0) { warnx("Fatal error: %s", nghttp2_strerror(rv)); return -1; } return 0; } The `nghttp2_session_send()` function serializes the frame into wire format and calls the ``send_callback()``, which is of type :type:`nghttp2_send_callback`. The ``send_callback()`` is defined as follows:: static ssize_t send_callback(nghttp2_session *session _U_, const uint8_t *data, size_t length, int flags _U_, void *user_data) { http2_session_data *session_data = (http2_session_data *)user_data; struct bufferevent *bev = session_data->bev; /* Avoid excessive buffering in server side. */ if (evbuffer_get_length(bufferevent_get_output(session_data->bev)) >= OUTPUT_WOULDBLOCK_THRESHOLD) { return NGHTTP2_ERR_WOULDBLOCK; } bufferevent_write(bev, data, length); return (ssize_t)length; } Since we use bufferevent to abstract network I/O, we just write the data to the bufferevent object. Note that `nghttp2_session_send()` continues to write all frames queued so far. If we were writing the data to a non-blocking socket directly using the ``write()`` system call in the ``send_callback()``, we'd soon receive an ``EAGAIN`` or ``EWOULDBLOCK`` error since sockets have a limited send buffer. If that happens, it's possible to return :macro:`NGHTTP2_ERR_WOULDBLOCK` to signal the nghttp2 library to stop sending further data. But here, when writing to the bufferevent, we have to regulate the amount data to buffered ourselves to avoid using huge amounts of memory. To achieve this, we check the size of the output buffer and if it reaches more than or equal to ``OUTPUT_WOULDBLOCK_THRESHOLD`` bytes, we stop writing data and return :macro:`NGHTTP2_ERR_WOULDBLOCK`. The next bufferevent callback is ``readcb()``, which is invoked when data is available to read in the bufferevent input buffer:: static void readcb(struct bufferevent *bev _U_, void *ptr) { http2_session_data *session_data = (http2_session_data *)ptr; if (session_recv(session_data) != 0) { delete_http2_session_data(session_data); return; } } In this function, we just call ``session_recv()`` to process incoming data. The third bufferevent callback is ``writecb()``, which is invoked when all data in the bufferevent output buffer has been sent:: static void writecb(struct bufferevent *bev, void *ptr) { http2_session_data *session_data = (http2_session_data *)ptr; if (evbuffer_get_length(bufferevent_get_output(bev)) > 0) { return; } if (nghttp2_session_want_read(session_data->session) == 0 && nghttp2_session_want_write(session_data->session) == 0) { delete_http2_session_data(session_data); return; } if (session_send(session_data) != 0) { delete_http2_session_data(session_data); return; } } First we check whether we should drop the connection or not. The nghttp2 session object keeps track of reception and transmission of GOAWAY frames and other error conditions as well. Using this information, the nghttp2 session object can state whether the connection should be dropped or not. More specifically, if both `nghttp2_session_want_read()` and `nghttp2_session_want_write()` return 0, the connection is no-longer required and can be closed. Since we are using bufferevent and its deferred callback option, the bufferevent output buffer may still contain pending data when the ``writecb()`` is called. To handle this, we check whether the output buffer is empty or not. If all of these conditions are met, we drop connection. Otherwise, we call ``session_send()`` to process the pending output data. Remember that in ``send_callback()``, we must not write all data to bufferevent to avoid excessive buffering. We continue processing pending data when the output buffer becomes empty. We have already described the nghttp2 callback ``send_callback()``. Let's learn about the remaining nghttp2 callbacks setup in ``initialize_nghttp2_setup()`` function. The ``on_begin_headers_callback()`` function is invoked when the reception of a header block in HEADERS or PUSH_PROMISE frame is started:: static int on_begin_headers_callback(nghttp2_session *session, const nghttp2_frame *frame, void *user_data) { http2_session_data *session_data = (http2_session_data *)user_data; http2_stream_data *stream_data; if (frame->hd.type != NGHTTP2_HEADERS || frame->headers.cat != NGHTTP2_HCAT_REQUEST) { return 0; } stream_data = create_http2_stream_data(session_data, frame->hd.stream_id); nghttp2_session_set_stream_user_data(session, frame->hd.stream_id, stream_data); return 0; } We are only interested in the HEADERS frame in this function. Since the HEADERS frame has several roles in the HTTP/2 protocol, we check that it is a request HEADERS, which opens new stream. If the frame is a request HEADERS, we create a ``http2_stream_data`` object to store the stream related data. We associate the created ``http2_stream_data`` object with the stream in the nghttp2 session object using `nghttp2_set_stream_user_data()`. The ``http2_stream_data`` object can later be easily retrieved from the stream, without searching through the doubly linked list. In this example server, we want to serve files relative to the current working directory in which the program was invoked. Each header name/value pair is emitted via ``on_header_callback`` function, which is called after ``on_begin_headers_callback()``:: static int on_header_callback(nghttp2_session *session, const nghttp2_frame *frame, const uint8_t *name, size_t namelen, const uint8_t *value, size_t valuelen, uint8_t flags _U_, void *user_data _U_) { http2_stream_data *stream_data; const char PATH[] = ":path"; switch (frame->hd.type) { case NGHTTP2_HEADERS: if (frame->headers.cat != NGHTTP2_HCAT_REQUEST) { break; } stream_data = nghttp2_session_get_stream_user_data(session, frame->hd.stream_id); if (!stream_data || stream_data->request_path) { break; } if (namelen == sizeof(PATH) - 1 && memcmp(PATH, name, namelen) == 0) { size_t j; for (j = 0; j < valuelen && value[j] != '?'; ++j) ; stream_data->request_path = percent_decode(value, j); } break; } return 0; } We search for the ``:path`` header field among the request headers and store the requested path in the ``http2_stream_data`` object. In this example program, we ignore the ``:method`` header field and always treat the request as a GET request. The ``on_frame_recv_callback()`` function is invoked when a frame is fully received:: static int on_frame_recv_callback(nghttp2_session *session, const nghttp2_frame *frame, void *user_data) { http2_session_data *session_data = (http2_session_data *)user_data; http2_stream_data *stream_data; switch (frame->hd.type) { case NGHTTP2_DATA: case NGHTTP2_HEADERS: /* Check that the client request has finished */ if (frame->hd.flags & NGHTTP2_FLAG_END_STREAM) { stream_data = nghttp2_session_get_stream_user_data(session, frame->hd.stream_id); /* For DATA and HEADERS frame, this callback may be called after on_stream_close_callback. Check that stream still alive. */ if (!stream_data) { return 0; } return on_request_recv(session, session_data, stream_data); } break; default: break; } return 0; } First we retrieve the ``http2_stream_data`` object associated with the stream in ``on_begin_headers_callback()`` using `nghttp2_session_get_stream_user_data()`. If the requested path cannot be served for some reason (e.g. file is not found), we send a 404 response using ``error_reply()``. Otherwise, we open the requested file and send its content. We send the header field ``:status`` as a single response header. Sending the file content is performed by the ``send_response()`` function:: static int send_response(nghttp2_session *session, int32_t stream_id, nghttp2_nv *nva, size_t nvlen, int fd) { int rv; nghttp2_data_provider data_prd; data_prd.source.fd = fd; data_prd.read_callback = file_read_callback; rv = nghttp2_submit_response(session, stream_id, nva, nvlen, &data_prd); if (rv != 0) { warnx("Fatal error: %s", nghttp2_strerror(rv)); return -1; } return 0; } nghttp2 uses the :type:`nghttp2_data_provider` structure to send the entity body to the remote peer. The ``source`` member of this structure is a union, which can be either a void pointer or an int (which is intended to be used as file descriptor). In this example server, we use it as a file descriptor. We also set the ``file_read_callback()`` callback function to read the contents of the file:: static ssize_t file_read_callback(nghttp2_session *session _U_, int32_t stream_id _U_, uint8_t *buf, size_t length, uint32_t *data_flags, nghttp2_data_source *source, void *user_data _U_) { int fd = source->fd; ssize_t r; while ((r = read(fd, buf, length)) == -1 && errno == EINTR) ; if (r == -1) { return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE; } if (r == 0) { *data_flags |= NGHTTP2_DATA_FLAG_EOF; } return r; } If an error occurs while reading the file, we return :macro:`NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`. This tells the library to send RST_STREAM to the stream. When all data has been read, the :macro:`NGHTTP2_DATA_FLAG_EOF` flag is set to signal nghttp2 that we have finished reading the file. The `nghttp2_submit_response()` function is used to send the response to the remote peer. The ``on_stream_close_callback()`` function is invoked when the stream is about to close:: static int on_stream_close_callback(nghttp2_session *session, int32_t stream_id, uint32_t error_code _U_, void *user_data) { http2_session_data *session_data = (http2_session_data *)user_data; http2_stream_data *stream_data; stream_data = nghttp2_session_get_stream_user_data(session, stream_id); if (!stream_data) { return 0; } remove_stream(session_data, stream_data); delete_http2_stream_data(stream_data); return 0; } Lastly, we destroy the ``http2_stream_data`` object in this function, since the stream is about to close and we no longer need the object.