HTTP Server

Overview

Zephyr provides an HTTP server library, which allows to register HTTP services and HTTP resources associated with those services. The server creates a listening socket for every registered service, and handles incoming client connections. It’s possible to communicate over a plain TCP socket (HTTP) or a TLS socket (HTTPS). Both, HTTP/1.1 (RFC 2616) and HTTP/2 (RFC 9113) protocol versions are supported.

The server operation is generally transparent for the application, running in a background thread. The application can control the server activity with respective API functions.

Certain resource types (for example dynamic resource) provide resource-specific application callbacks, allowing the server to interact with the application (for instance provide resource content, or process request payload).

Currently, the following resource types are supported:

Zephyr provides a sample demonstrating HTTP(s) server operation and various resource types usage. See HTTP Server for more information.

Server Setup

A few prerequisites are needed in order to enable HTTP server functionality in the application.

First of all, the HTTP server has to be enabled in applications configuration file with CONFIG_HTTP_SERVER Kconfig option:

prj.conf
CONFIG_HTTP_SERVER=y

All HTTP services and HTTP resources are placed in a dedicated linker section. The linker section for services is predefined locally, however the application is responsible for defining linker sections for resources associated with respective services. Linker section names for resources should be prefixed with http_resource_desc_, appended with the service name.

Linker sections for resources should be defined in a linker file. For example, for a service named my_service, the linker section shall be defined as follows:

sections-rom.ld
#include <zephyr/linker/iterable_sections.h>

ITERABLE_SECTION_ROM(http_resource_desc_my_service, Z_LINK_ITERABLE_SUBALIGN)

Finally, the linker file and linker section have to be added to your application using CMake:

CMakeLists.txt
zephyr_linker_sources(SECTIONS sections-rom.ld)
zephyr_linker_section(NAME http_resource_desc_my_service
                      KVMA RAM_REGION GROUP RODATA_REGION
                      SUBALIGN ${CONFIG_LINKER_ITERABLE_SUBALIGN})

Note

You need to define a separate linker section for each HTTP service registered in the system.

Sample Usage

Services

The application needs to define an HTTP service (or multiple services), with the same name as used for the linker section with HTTP_SERVICE_DEFINE macro:

#include <zephyr/net/http/service.h>

static uint16_t http_service_port = 80;

HTTP_SERVICE_DEFINE(my_service, "0.0.0.0", &http_service_port, 1, 10, NULL);

Alternatively, an HTTPS service can be defined with HTTPS_SERVICE_DEFINE:

#include <zephyr/net/http/service.h>
#include <zephyr/net/tls_credentials.h>

#define HTTP_SERVER_CERTIFICATE_TAG 1

static uint16_t https_service_port = 443;
static const sec_tag_t sec_tag_list[] = {
    HTTP_SERVER_CERTIFICATE_TAG,
};

HTTPS_SERVICE_DEFINE(my_service, "0.0.0.0", &https_service_port, 1, 10,
                     NULL, sec_tag_list, sizeof(sec_tag_list));

Note

HTTPS services rely on TLS credentials being registered in the system. See TLS credentials subsystem for information on how to configure TLS credentials in the system.

Once HTTP(s) service is defined, resources can be registered for it with HTTP_RESOURCE_DEFINE macro.

Application can enable resource wildcard support by enabling CONFIG_HTTP_SERVER_RESOURCE_WILDCARD option. When this option is set, then it is possible to match several incoming HTTP requests with just one resource handler. The fnmatch() POSIX API function is used to match the pattern in the URL paths.

Example:

HTTP_RESOURCE_DEFINE(my_resource, my_service, "/foo*", &resource_detail);

This would match all URLs that start with a string foo. See POSIX.2 chapter 2.13 for pattern matching syntax description.

Static resources

Static resource content is defined build-time and is immutable. The following example shows how gzip compressed webpage can be defined as a static resource in the application:

static const uint8_t index_html_gz[] = {
    #include "index.html.gz.inc"
};

struct http_resource_detail_static index_html_gz_resource_detail = {
    .common = {
        .type = HTTP_RESOURCE_TYPE_STATIC,
        .bitmask_of_supported_http_methods = BIT(HTTP_GET),
        .content_encoding = "gzip",
    },
    .static_data = index_html_gz,
    .static_data_len = sizeof(index_html_gz),
};

HTTP_RESOURCE_DEFINE(index_html_gz_resource, my_service, "/",
                     &index_html_gz_resource_detail);

The resource content and content encoding is application specific. For the above example, a gzip compressed webpage can be generated during build, by adding the following code to the application’s CMakeLists.txt file:

CMakeLists.txt
set(gen_dir ${ZEPHYR_BINARY_DIR}/include/generated/)
set(source_file_index src/index.html)
generate_inc_file_for_target(app ${source_file_index} ${gen_dir}/index.html.gz.inc --gzip)

where src/index.html is the location of the webpage to be compressed.

Static filesystem resources

Static filesystem resource content is defined build-time and is immutable. The following example shows how the path can be defined as a static resource in the application:

struct http_resource_detail_static_fs static_fs_resource_detail = {
    .common = {
        .type                              = HTTP_RESOURCE_TYPE_STATIC_FS,
        .bitmask_of_supported_http_methods = BIT(HTTP_GET),
    },
    .fs_path = "/lfs1/www",
};

HTTP_RESOURCE_DEFINE(static_fs_resource, my_service, "*", &static_fs_resource_detail);

All files located in /lfs1/www are made available to the client. If a file is gzipped, .gz must be appended to the file name (e.g. index.html.gz), then the server delivers index.html.gz when the client requests index.html and adds gzip content-encoding to the HTTP header.

The content type is evaluated based on the file extension. The server supports .html, .js, .css, .jpg, .png and .svg. More content types can be provided with the HTTP_SERVER_CONTENT_TYPE macro. All other files are provided with the content type text/html.

HTTP_SERVER_CONTENT_TYPE(json, "application/json")

Dynamic resources

For dynamic resource, a resource callback is registered to exchange data between the server and the application.

The following example code shows how to register a dynamic resource with a simple resource handler, which echoes received data back to the client:

static int dyn_handler(struct http_client_ctx *client, enum http_data_status status,
                       const struct http_request_ctx *request_ctx,
                       struct http_response_ctx *response_ctx, void *user_data)
{
#define MAX_TEMP_PRINT_LEN 32
    static char print_str[MAX_TEMP_PRINT_LEN];
    enum http_method method = client->method;
    static size_t processed;

    __ASSERT_NO_MSG(buffer != NULL);

    if (status == HTTP_SERVER_DATA_ABORTED) {
        LOG_DBG("Transaction aborted after %zd bytes.", processed);
        processed = 0;
        return 0;
    }

    processed += request_ctx->data_len;

    snprintf(print_str, sizeof(print_str), "%s received (%zd bytes)",
             http_method_str(method), request_ctx->data_len);
    LOG_HEXDUMP_DBG(request_ctx->data, request_ctx->data_len, print_str);

    if (status == HTTP_SERVER_DATA_FINAL) {
        LOG_DBG("All data received (%zd bytes).", processed);
        processed = 0;
    }

    /* Echo data back to client */
    response_ctx->body = request_ctx->data;
    response_ctx->body_len = request_ctx->data_len;
    response_ctx->final_chunk = (status == HTTP_SERVER_DATA_FINAL);

    return 0;
}

struct http_resource_detail_dynamic dyn_resource_detail = {
    .common = {
        .type = HTTP_RESOURCE_TYPE_DYNAMIC,
        .bitmask_of_supported_http_methods =
            BIT(HTTP_GET) | BIT(HTTP_POST),
    },
    .cb = dyn_handler,
    .user_data = NULL,
};

HTTP_RESOURCE_DEFINE(dyn_resource, my_service, "/dynamic",
                     &dyn_resource_detail);

The resource callback may be called multiple times for a single request, hence the application should be able to keep track of the received data progress.

The status field informs the application about the progress in passing request payload from the server to the application. As long as the status reports HTTP_SERVER_DATA_MORE, the application should expect more data to be provided in a consecutive callback calls. Once all request payload has been passed to the application, the server reports HTTP_SERVER_DATA_FINAL status. In case of communication errors during request processing (for example client closed the connection before complete payload has been received), the server reports HTTP_SERVER_DATA_ABORTED. Either of the two events indicate that the application shall reset any progress recorded for the resource, and await a new request to come. The server guarantees that the resource can only be accessed by single client at a time.

The request_ctx parameter is used to pass request data to the application:

  • The data and data_len fields pass request data to the application.

  • The headers, header_count and headers_status fields pass request headers to the application, if CONFIG_HTTP_SERVER_CAPTURE_HEADERS is enabled.

The response_ctx field is used by the application to pass response data to the HTTP server:

  • The status field allows the application to send an HTTP response code. If not populated, the response code will be 200 by default.

  • The headers and header_count fields can be used for the application to send any arbitrary HTTP headers. If not populated, only Transfer-Encoding and Content-Type are sent by default. The callback may override the Content-Type if desired.

  • The body and body_len fields are used to send body data.

  • The final_chunk field is used to indicate that the application has no more response data to send.

Headers and/or response codes may only be sent in the first populated response_ctx, after which only further body data is allowed in subsequent callbacks.

The server will call the resource callback until it provided all request data to the application, and the application reports there is no more data to include in the reply.

Websocket resources

Websocket resources register an application callback, which is called when a Websocket connection upgrade takes place. The callback is provided with a socket descriptor corresponding to the underlying TCP/TLS connection. Once called, the application takes full control over the socket, i. e. is responsible to release it when done.

static int ws_socket;
static uint8_t ws_recv_buffer[1024];

int ws_setup(int sock, void *user_data)
{
    ws_socket = sock;
    return 0;
}

struct http_resource_detail_websocket ws_resource_detail = {
    .common = {
        .type = HTTP_RESOURCE_TYPE_WEBSOCKET,
        /* We need HTTP/1.1 Get method for upgrading */
        .bitmask_of_supported_http_methods = BIT(HTTP_GET),
    },
    .cb = ws_setup,
    .data_buffer = ws_recv_buffer,
    .data_buffer_len = sizeof(ws_recv_buffer),
    .user_data = NULL, /* Fill this for any user specific data */
};

HTTP_RESOURCE_DEFINE(ws_resource, my_service, "/", &ws_resource_detail);

The above minimalistic example shows how to register a Websocket resource with a simple callback, used only to store the socket descriptor provided. Further processing of the Websocket connection is application-specific, hence outside of scope of this guide. See HTTP Server for an example Websocket-based echo service implementation.

Accessing request headers

The application can register an interest in any specific HTTP request headers. These headers are then stored for each incoming request, and can be accessed from within a dynamic resource callback. Request headers are only included in the first callback for a given request, and are not passed to any subsequent callbacks.

This feature must first be enabled with CONFIG_HTTP_SERVER_CAPTURE_HEADERS Kconfig option.

Then the application can register headers to be captured, and read the values from within the dynamic resource callback:

HTTP_SERVER_REGISTER_HEADER_CAPTURE(capture_user_agent, "User-Agent");

static int dyn_handler(struct http_client_ctx *client, enum http_data_status status,
                       uint8_t *buffer, size_t len, void *user_data)
{
    size_t header_count = client->header_capture_ctx.count;
    const struct http_header *headers = client->header_capture_ctx.headers;

    LOG_INF("Captured %d headers with request", header_count);

    for (uint32_t i = 0; i < header_count; i++) {
        LOG_INF("Header: '%s: %s'", headers[i].name, headers[i].value);
    }

    return 0;
}

API Reference

HTTP service API

Related code samples

  • HTTP ServerImplement an HTTP(s) Server demonstrating various resource types.
  • Prometheus SampleImplement a Prometheus Metric Server demonstrating various metric types.
HTTP server API

Related code samples

  • HTTP ServerImplement an HTTP(s) Server demonstrating various resource types.
  • Prometheus SampleImplement a Prometheus Metric Server demonstrating various metric types.