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Libesphttpd intro

Libesphttpd is a HTTP server library for the ESP8266/ESP32. It supports integration in projects running under the non-os and FreeRTOS-based SDK. Its core is clean and small, but it provides an extensible architecture with plugins to handle a flash-based compressed read-only filesystem for static files, a tiny template engine, websockets, a captive portal, and more.

Requirements

  • ESP32
    • May work with other ESP32-xx variants, but not tested.
    • 🔺ESP8266 is no longer supported in this fork.
  • ESP-IDF Version 4.x
    • 🔺ESP-IDF Version 3.x is no longer supported (EOL).
  • CMAKE build system
    • 🔺legacy Make build is no longer maintained (PR welcome).

Examples

There are two example projects that integrate this code, both a non-os as well as a FreeRTOS-based example. They show how to use libesphttpd to serve files from an ESP32 and illustrate a way to make an user associate the ESP32 with an access point from a standard webbrowser on a PC or mobile phone.

Using with esp-idf (esp32)

Place the libesphttpd repository into the components directory of your esp-idf folder. This should put it at esp-idf/components/libesphttpd If it is in the correct location you should see a 'ESP-HTTPD Config' entry under 'Component config' when you run 'make menuconfig' on your esp-idf application.

SSL Support

Libesphttpd supports https under FreeRTOS via openssl/mbedtls. Server and client certificates are supported.

Enable 'ESPHTTPD_SSL_SUPPORT' during project configuration.

See the 'How to use SSL' section below.

Programming guide

Programming libesphttpd will require some knowledge of HTTP. Knowledge of the exact RFCs isn't needed, but it helps if you know the difference between a GET and a POST request, how HTTP headers work, what an mime-type is and so on. Furthermore, libesphttpd is written in the C language and uses the libraries available on the ESP-IDF. It is assumed the developer knows C and has some experience with the SDK.

Initializing libesphttpd

Initializing libesphttpd is usually done in the user_main() of your project, but it is not mandatory to place the call here. Initialization is done by the httpdInit(builtInUrls, port) call. The port is the TCP port the webserver will listen on; the builtInUrls is the CGI list. Only call the httpdInit once, calling it multiple times leads to undefined behaviour.

(As an aside: CGI actually is an abbreviation for Common Gateway Interface, which is a specification to allow external processes to interface with a non-embedded webserver. The CGI functions mentioned here have nothing to do with the CGI protocol specification; the term 'CGI' is just used as a quick handle for a function interpreting headers and generating data to send to the web client.)

The CGI list is an array of the HttpdBuiltInUrl type. Here's an example:

const HttpdBuiltInUrl builtInUrls[]={
	{"/", cgiRedirect, "/index.cgi"},
	{"/index.cgi", cgiMyFunction, NULL},
	{"*", cgiEspFsHook, NULL},
	{NULL, NULL, NULL}
};

As you can see, the array consists of a number of entries, with the last entry filled with NULLs. When the webserver gets a request, it will run down the list and try to match the URL the browser sent to the pattern specified in the first argument in the list. If a match is detected, the corresponding CGI function is called. This function gets the opportunity to handle the request, but it also can pass on handling it; if this happens, the webserver will keep going down the list to look for a CGI with a matching pattern willing to handle the request; if there is none on the list, it will generate a 404 page itself.

The patterns can also have wildcards: a * at the end of the pattern matches any text. For instance, the pattern /wifi/* will match requests for /wifi/index.cgi and /wifi/picture.jpg, but not for example /settings/wifi/. The cgiEspFsHook is used like that in the example: it will be called on any request that is not handled by the cgi functions earlier in the list.

There also is a third entry in the list. This is an optional argument for the CGI function; its purpose differs per specific function. If this is not needed, it's okay to put NULL there instead.

Sidenote: About the cgiEspFsHook call

While cgiEspFsHook isn't handled any different than any other cgi function, it may be useful to shortly elaborate what its function is. cgiEspFsHook is responsible, on most implementations, for serving up the static files that are included in the project: static HTML pages, images, Javascript code etc. Esphttpd doesn't have a built-in method to serve static files: the code responsible for doing it is plugged into it the same way as any cgi function is. This allows the developer to leave away the ability to serve static files if it isn't needed, or use a different implementation that serves e.g. files off the FAT-partition of a SD-card.

Built-in CGI functions

The webserver provides a fair amount of general-use CGI functions. Because of the structure of libesphttpd works and some linker magic in the Makefiles of the SDKs, the compiler will only include them in the output binary if they're actually used.

  • cgiRedirect (arg: URL to redirect to) This is a convenience function to redirect the browser requesting this URL to a different URL. For example, an entry like {"/google", cgiRedirect, "http://google.com"} would redirect all browsers requesting /google to the website of the search giant.

  • cgiRedirectToHostname (arg: hostname to redirect to) If the host as requested by the browser isn't the hostname in the argument, the webserver will do a redirect to the host instead. If the hostname does match, it will pass on the request.

  • cgiRedirectApClientToHostname (arg: hostname to redirect to) This does the same as cgiRedirectToHostname but only to clients connected to the SoftAP of the ESP8266/ESP32. This and the former function are used with the captive portal mode. The captive portal consists of a DNS-server (started by calling captdnsInit()) resolving all hostnames into the IP of the ESP8266/ESP32. These redirect functions can then be used to further redirect the client to the hostname of the ESP8266/ESP32.

  • Flash updating functions (OTA) - see README-flash_api

  • WiFi settings functions - see README-wifi_api.md

  • cgiWebsocket (arg: connect function) This CGI is used to set up a websocket. Websockets are described later in this document. See the example projects for an implementation that uses this function call. FreeRTOS Example

  • cgiEspFsHook (arg1: basepath or &httpdCgiEx magic, arg2:HttpdCgiExArg struct if arg1 was &httpdCgiEx) Serves files from the espfs filesystem. The espFsInit function should be called first, with as argument a pointer to the start of the espfs binary data in flash. The binary data can be both flashed separately to a free bit of SPI flash, as well as linked in with the binary. The nonos example project can be configured to do either.

    If arg1 is supplied &httpdCgiEx Magic value, then arg2 is assumed to be of type HttpdCgiExArg, which is a stuct containing extended options. This should allow for future addition of custom parameters without breaking existing cgi functions that make use of existing members of this struct. Currently available options are:

    • basepath: base directory path on filesystem (Optional, uses URL if NULL)
    • headerCb: pointer to function which supplies custom headers. (Optional, sends default headers if NULL)
    • mimetype: customize the MIMETYPE (Optional, sends default MIMETYPE if NULL)

  • cgiEspFsTemplate (arg: template function) The espfs code comes with a small but efficient template routine, which can fill a template file stored on the espfs filesystem with user-defined data.

  • cgiEspVfsGet (arg1: basepath or &httpdCgiEx magic, arg2:HttpdCgiExArg struct if arg1 was &httpdCgiEx) This is a catch-all cgi function. It takes the url passed to it, looks up the corresponding path in the filesystem and if it exists, sends the file. This simulates what a normal webserver would do with static files. If the file is not found, (or if http method is not GET) this cgi function returns NOT_FOUND, and then other cgi functions specified later in the routing table can try. See the example projects for an implementation that uses this function call. FreeRTOS Example

    The cgiArg value is the base directory path, if specified. Usage:

    • ROUTE_CGI("*", cgiEspVfsGet)
    • ROUTE_CGI_ARG("*", cgiEspVfsGet, "/base/directory/")
    • ROUTE_CGI_ARG("*", cgiEspVfsGet, ".") to use the current working directory

    Alternatively, if cgiArg is &httpdCgiEx Magic value, see section about HttpdCgiExArg in item cgiEspFsHook above.

  • cgiEspVfsUpload (arg: base filesystem path) This is a POST and PUT handler for uploading files to the VFS filesystem. See the example projects for an implementation that uses this function call. FreeRTOS Example

    Specify base directory (with trailing slash) or single file as 1st cgiArg. If http method is not PUT or POST, this cgi function returns NOT_FOUND, and then other cgi functions specified later in the routing table can try.

    Filename can be specified 3 ways, in order of priority lowest to highest:

    1. URL Path i.e. PUT http://1.2.3.4/path/newfile.txt
    2. Inside multipart/form-data (todo not supported yet)
    3. URL Parameter i.e. POST http://1.2.3.4/upload.cgi?filename=path%2Fnewfile.txt

    Usage:

    • ROUTE_CGI_ARG("*", cgiEspVfsUpload, "/base/directory/")
      • Allows creating/replacing files anywhere under "/base/directory/". Don't forget to specify trailing slash in cgiArg!
      • example: POST or PUT http://1.2.3.4/anydir/anyname.txt
    • ROUTE_CGI_ARG("/filesystem/upload.cgi", cgiEspVfsUpload, "/base/directory/")
    • ROUTE_CGI_ARG("/writeable_file.txt", cgiEspVfsUpload, "/base/directory/writeable_file.txt")

How to configure and use SSL

How to create certificates

SSL servers require certificates. Steps to use:
- Place a 'cacert.der' and 'prvtkey.der' files in your app directory.

- To create self certified certificates:
	$ openssl req -sha256 -newkey rsa:4096 -nodes -keyout key.pem -x509 -days 365 -out certificate.pem

- To generate .der certificates/keys from .pem certificates/keys:
	$ openssl x509 -outform der -in certificate.pem -out certificate.der
	$ openssl rsa -outform der -in key.pem -out key.der

Compile certificates into your binary image (option 1) OR

- Create a 'component.mk' file in your app directory and add these lines to it:
	COMPONENT_EMBED_TXTFILES := cacert.der
	COMPONENT_EMBED_TXTFILES += prvtkey.der

And use the below code to gain access to these embedded files. Note the filename with extension is used to generate the binary variables, you can modify the embedded filenames but make sure to update the _binary_xxxx_yyy_start and end entries:

extern const unsigned char cacert_der_start[] asm("_binary_cacert_der_start");
extern const unsigned char cacert_der_end[]   asm("_binary_cacert_der_end");
const size_t cacert_der_bytes = cacert_der_end - cacert_der_start;

extern const unsigned char prvtkey_der_start[] asm("_binary_prvtkey_der_start");
extern const unsigned char prvtkey_der_end[]   asm("_binary_prvtkey_der_end");
const size_t prvtkey_der_bytes = prvtkey_der_end - prvtkey_der_start;

Store / load certificates to a filesystem (option 2)

See the mkspiffs documentation for more information on creating a spiffs filesystem and loading it at runtime.

Otherwise use standard file functions, fopen/fread/fclose to read the certiricates into memory so they can be passed into libesphttpd.

Configure libesphttpd for ssl and load the server certificate and private key

    httpdFreertosSslInit(&httpdFreertosInstance); // configure libesphttpd for ssl

    // load the server certificate and private key
    httpdFreertosSslSetCertificateAndKey(&httpdFreertosInstance,
                                        cacert_der_ptr, cacert_der_size,
                                        prvtkey_der_ptr, prvtkey_der_size);

Optionally enable client certificate validation (client certificate validation is disabled by default) and load a series of client certificates

You can embed client certificates into the flash image or store them in a filesystem depending on your need.

    SslVerifySetting verifySetting = SslClientVerifyRequired;
    httpdFreertosSslSetClientValidation(&httpdFreertosInstance,
                                        verifySetting);
    httpdFreertosSslAddClientCertificate(&httpdFreertosInstance,
                                         client_certificate_ptr, client_certificate_size);

Writing a CGI function

A CGI function, in principle, is called when the HTTP headers have come in and the client is waiting for the response of the webserver. The CGI function is responsible for generating this response, including the correct headers and an appropriate body. To decide what response to generate and what other actions to take, the CGI function can inspect various information sources, like data passed as GET- or POST-arguments.

A simple CGI function may, for example, greet the user with a name given as a GET argument:

CgiStatus ICACHE_FLASH_ATTR cgiGreetUser(HttpdConnData *connData) {
	int len;			//length of user name
	char name[128];		//Temporary buffer for name
	char output[256];	//Temporary buffer for HTML output
	
	//If the browser unexpectedly closes the connection, the CGI will be called 
	//after the isConnectionClosed flag is set. We can use this to clean up any data. It's not
	//used in this simple CGI function.
	if (connData->isConnectionClosed) {
		//Connection aborted. Clean up.
		return HTTPD_CGI_DONE;
	}

	if (connData->requestType!=HTTPD_METHOD_GET) {
		//Sorry, we only accept GET requests.
		httpdStartResponse(connData, 406);  //http error code 'unacceptable'
		httpdEndHeaders(connData);
		return HTTPD_CGI_DONE;
	}

	//Look for the 'name' GET value. If found, urldecode it and return it into the 'name' var.
	len=httpdFindArg(connData->getArgs, "name", name, sizeof(name));
	if (len==-1) {
		//If the result of httpdFindArg is -1, the variable isn't found in the data.
		strcpy(name, "unknown person");
	} else {
		//If len isn't -1, the variable is found and is copied to the 'name' variable
	}
	
	//Generate the header
	//We want the header to start with HTTP code 200, which means the document is found.
	httpdStartResponse(connData, 200); 
	//We are going to send some HTML.
	httpdHeader(connData, "Content-Type", "text/html");
	//No more headers.
	httpdEndHeaders(connData);
	
	//We're going to send the HTML as two pieces: a head and a body. We could've also done
	//it in one go, but this demonstrates multiple ways of calling httpdSend.
	//Send the HTML head. Using -1 as the length will make httpdSend take the length
	//of the zero-terminated string it's passed as the amount of data to send.
	httpdSend(connData, "<html><head><title>Page</title></head>", -1)
	//Generate the HTML body. 
	len=sprintf(output, "<body><p>Hello, %s!</p></body></html>", name);
	//Send HTML body to webbrowser. We use the length as calculated by sprintf here.
	//Using -1 again would also have worked, but this is more efficient.
	httpdSend(connData, output, len);

	//All done.
	return HTTPD_CGI_DONE;
}

Putting this CGI function into the HttpdBuiltInUrl array, for example with pattern "/hello.cgi", would allow an user to request the page "http://192.168.4.1/hello.cgi?name=John+Doe" and get a document saying "Hello, John Doe!".

A word of warning: while it may look like you could forego the entire httpdStartResponse/httpdHeader/httpdEndHeader structure and send all the HTTP headers using httpdSend, this will break a few things that need to know when the headers are finished, for example the HTTP 1.1 chunked transfer mode.

The approach of parsing the arguments, building up a response and then sending it in one go is pretty simple and works just fine for small bits of data. The gotcha here is that all http data sent during the CGI function (headers and data) are temporarily stored in a buffer, which is sent to the client when the function returns. The size of this buffer is typically about 2K; if the CGI tries to send more than this, data will be lost.

The way to get around this is to send part of the data using httpdSend and then return with HTTPD_CGI_MORE instead of HTTPD_CGI_DONE. The webserver will send the partial data and will call the CGI function again so it can send another part of the data, until the CGI function finally returns with HTTPD_CGI_DONE. The CGI can store it's state in connData->cgiData, which is a freely usable pointer that will persist across all calls in the request. It is NULL on the first call, and the standard way of doing things is to allocate a pointer to a struct that stores state here. Here's an example:

typedef struct {
	char *stringPos;
} LongStringState;

static char *longString="Please assume this is a very long string, way too long to be sent"\
		"in one time because it won't fit in the send buffer in it's entirety; we have to"\
		"break up sending it in multiple parts."

CgiStatus ICACHE_FLASH_ATTR cgiSendLongString(HttpdConnData *connData) {
	LongStringState *state=connData->cgiData;
	int len;
	
	//If the browser unexpectedly closes the connection, the CGI will be called 
	//after isConnectionClosed is set to true. We can use this to clean up any data. It's pretty relevant
	//here because otherwise we may leak memory when the browser aborts the connection.
	if (connData->isConnectionClosed) {
		//Connection aborted. Clean up.
		if (state!=NULL) free(state);
		return HTTPD_CGI_DONE;
	}

	if (state==NULL) {
		//This is the first call to the CGI for this webbrowser request.
		//Allocate a state structure.
		state=malloc(sizeof(LongStringState);
		//Save the ptr in connData so we get it passed the next time as well.
		connData->cgiData=state;
		//Set initial pointer to start of string
		state->stringPos=longString;
		//We need to send the headers before sending any data. Do that now.
		httpdStartResponse(connData, 200); 
		httpdHeader(connData, "Content-Type", "text/plain");
		httpdEndHeaders(connData);
	}

	//Figure out length of string to send. We will never send more than 128 bytes in this example.
	len=strlen(state->stringPos); //Get remaining length
	if (len>128) len=128; //Never send more than 128 bytes
	
	//Send that amount of data
	httpdSend(connData, state->stringPos, len);
	//Adjust stringPos to first byte we haven't sent yet
	state->stringPos+=len;
	//See if we need to send more
	if (strlen(state->stringPos)!=0) {
		//we have more to send; let the webserver call this function again.
		return HTTPD_CGI_MORE;
	} else {
		//We're done. Clean up here as well: if the CGI function returns HTTPD_CGI_DONE, it will
		//not be called again.
		free(state);
		return HTTPD_CGI_DONE;
	}
}

In this case, the CGI is called again after each chunk of data has been sent over the socket. If you need to suspend the HTTP response and resume it asynchronously for some other reason, you may save the HttpdConnData pointer, return HTTPD_CGI_MORE, then later call httpdContinue with the saved connection pointer. For example, if you need to communicate with another device over a different connection, you could send data to that device in the initial CGI call, then return HTTPD_CGI_MORE, then, in the espconn_recv_callback for the response, you can call httpdContinue to resume the HTTP response with data retrieved from the other device.

For POST data, a similar technique is used. For small amounts of POST data (smaller than MAX_POST, typically 1024 bytes) the entire thing will be stored in connData->post->buff and is accessible in its entirely on the first call to the CGI function. For example, when using POST to send form data, if the amount of expected data is low, it is acceptable to do a call like len=httpdFindArg(connData->post->buff, "varname", buff, sizeof(buff)); to get the data for the individual form elements.

In all cases, connData->post->len will contain the length of the entirety of the POST data, while connData->post->buffLen contains the length of the data in connData->post->buff. In the case where the total POST data is larger than the POST buffer, the latter will be less than the former. In this case, the CGI function is expected to not send any headers or data out yet, but to process the incoming bit of POST data and return with HTTPD_CGI_MORE. The next call will contain the next chunk of POST data. connData->post->received will always contain the total amount of POST data received for the request, including the data passed to the CGI. When that number equals connData->post->len, it means no more POST data is expected and the CGI function is free to send out the reply headers and data for the request.

The template engine

The espfs driver comes with a tiny template engine, which allows for runtime-calculated value changes in a static html page. It can be included in the builtInUrls variable like this:

	{"/showname.tpl", cgiEspFsTemplate, tplShowName}

It requires two things. First of all, the template is needed, which specifically is a file on the espfs with the same name as the first argument of the builtInUrls value, in this case showname.tpl. It is a standard HTML file containing a number of %name% entries. For example:

<html>
<head><title>Welcome</title></head>
<body>
<h1>Welcome, %username%, to the %thing%!</h1>
</body>
</html>

When this URL is requested, the words between percent characters will invoke the tplShowName function, allowing it to output specific data. For example:

CgiStatus ICACHE_FLASH_ATTR tplShowName(HttpdConnData *connData, char *token, void **arg) {
	if (token==NULL) return HTTPD_CGI_DONE;

	if (os_strcmp(token, "username")==0) httpdSend(connData, "John Doe", -1);
	if (os_strcmp(token, "thing")==0) httpdSend(connData, "ESP8266/ESP32 webserver", -1);

	return HTTPD_CGI_DONE;
}

This will result in a page stating Welcome, John Doe, to the ESP8266/ESP32 webserver!.

Websocket functionality

ToDo: document this

Linux support

Lwip provides a POSIX interface that matches that of a Linux system. FreeRTOS primitives are also similiar to those provided under POSIX.

Running on a Linux system enables testing under a range of different conditions including different native pointer sizes (64bit vs. 32bit), as well as with different compilers. These differences can help reveal portability issues.

Linux tools such as valgrind can be used to check for memory leaks that would be much more difficult to detect on an embedded platform. Valgrind and other tools also provide ways of looking at application performance that go beyond what is typically available in an embedded environment.

See https://github.com/chmorgan/libesphttpd_linux_example for an example of how to use libesphttpd under Linux.

Licensing

libesphttpd is licensed under the MPLv2. It was originally licensed under a 'Beer-ware' license by Jeroen Domburg that was equivalent to a public domain license. Chris Morgan [email protected] initiated relicensing to MPLv2 prior to investing a number of hours into improving the library for its use in a potential commercial project. The relicensing was done after asking for and receiving the blessing from most of the projects contributors although it should be noted that the original license didn't require permission to relicense or use in any way.

The topic of licenses can be controversial. The original license was more free in that it allowed users to use the code in any way, including relicensing it to any license they chose. The MPLv2 restricts freedom in that it requires modifications to be given back to the community. This license establishes the agreement that in exchange for using this great library that users are required to give back their changes to let others benefit. This was the spirit and intention of the relicencing to the MPLv2.

While the 'Beer-ware' license text was removed to avoid license confusion the authors of this great library, especially Jeroen, deserve a beer. If you appreciate the library and you meet them in person some day please consider buying them a beer to say thanks!