Handle query strings using socket programming

[c00get]

Hi, I need to make a daemon which listens to port 81 for messages like http://localhost/message?command=move&dir=left
So far I made a daemon which serves as a simple stream server: I set up a socket to listen to a non-reserved port (like 9999), but I don't know how to read the query strings.

Linux distro: Kubuntu 9.04
Language: C

[S6k9GNh0]

Any code?

[PanGalactic]

Why not use Apache and create a CGI program in C?

[Galik]

If you type the address of your demon program into a browser with the query string, then your demon should receive something like this through the socket:


GET /message?key1=value1&key2=value2 HTTP/1.1
Host: localhost
Connection: keep-alive
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/4.0.253.0 Safari/532.5
Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-GB,en-US;q=0.8,en;q=0.6
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3



That is because the browser takes the URL that includes the query string and writes it to the socket it connects to using the HTTP protocol.

So your demon needs to read in each line from the socket and the first line (that begins with "GET" and ends with "HTTP/..." will contain the raw query string.

[c00get]

PanGalactic wrote:

Why not use Apache and create a CGI program in C?

I can't use Apache, because I have to use this program on an embedded system running OpenWRT. The resources are limited.


Thanks Galic, I'll try that.

computerquip wrote:

Any code?

#include <stdlib.h>
#include <syslog.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>

#define PORT		81
#define MAXBUF		64
#define SLEEP_TIME 	3
#define COUNT_MAX	3


int main(void)
{
	pid_t pid, sid;
	int status;

	/* Fork off the parent process */
	pid = fork();
	if (pid < 0)
	{
		syslog(LOG_ERR, "fork(): %m");
		exit(EXIT_FAILURE);
	}
	
    /* If we got a good PID, then
	we can exit the parent process. */
	if (pid > 0)
	{
		exit(EXIT_SUCCESS);
	}

	/* Change the file mode mask */
	umask(0);

	/* Create a new SID for the child process */
	sid = setsid();
	if (sid < 0)
	{
		syslog(LOG_ERR, "setsid(): %m");
		exit(EXIT_FAILURE);
	}

	/* Change the current working directory */
	if ((chdir("/")) < 0)
	{
		syslog(LOG_ERR, "chdir(): %m");
		exit(EXIT_FAILURE);
	}

	/* Close out the standard file descriptors */
	status = close(STDIN_FILENO);
	if (status < 0)
	{
		syslog(LOG_ERR, "close(STDIN_FILENO): %m");
		exit(EXIT_FAILURE);
	}
	
	status = close(STDOUT_FILENO);
	if (status < 0)
	{
		syslog(LOG_ERR, "close(STDOUT_FILENO): %m");
		exit(EXIT_FAILURE);
	}
	
	status = close(STDERR_FILENO);
	if (status < 0)
	{
		syslog(LOG_ERR, "close(STDERR_FILENO): %m");
		exit(EXIT_FAILURE);
	}

	//---------------------------------------------------------------------------
	
	int sockfd, clientfd;
	struct sockaddr_in self, client_addr;
	int addrlen = sizeof(client_addr);
	char buffer[MAXBUF], command[8], param[8];
	int count;

	/* Create streaming socket */
	sockfd = socket(AF_INET, SOCK_STREAM, 0);
	if (sockfd < 0)
	{
		syslog(LOG_ERR, "socket(): %m");
		exit(EXIT_FAILURE);
	}

	/* Initialize address/port structure */
	bzero(&self, sizeof(self));
	self.sin_family = AF_INET;
	self.sin_port = htons(PORT);
	self.sin_addr.s_addr = INADDR_ANY;

	/* Assign a port number to the socket */
	status = bind(sockfd, (struct sockaddr*) &self, sizeof(self));
	if (status < 0)
	{
		syslog(LOG_ERR, "bind(): %m");
		exit(EXIT_FAILURE);
	}
	
	/* Make it a "listening socket" */
	status = listen(sockfd, 20);
	if (status < 0)
	{
		syslog(LOG_ERR, "listen(): %m");
		exit(EXIT_FAILURE);
	}
	
	/* The Big Loop */
	while (1)
	{
		/* Accept a connection (creating a data pipe) */
		count = COUNT_MAX;
		do {
			clientfd = accept(sockfd, (struct sockaddr*) &client_addr, &addrlen);
			if (clientfd < 0)
			{
				syslog(LOG_ERR, "accept(): %m");
				count--;
				sleep(SLEEP_TIME); // Wait
			}
		} while (status < 0 && count > 0);
		if (count <= 0)
			exit(EXIT_FAILURE);

		/* Receive message */
		status = recv(clientfd, buffer, MAXBUF, 0);
		if (status < 0)
			syslog(LOG_ERR, "recv(): %m");
		else if (status > 0)
		{

		}

		/* Close data connection */
		count = COUNT_MAX;
		do {
			status = close(clientfd);
			if (status < 0)
			{
				syslog(LOG_ERR, "close(clientfd): %m");
				count--;
				sleep(SLEEP_TIME); // Wait
			}
		} while (status < 0 && count > 0);
		if (count <= 0)
			exit(EXIT_FAILURE);
	}

	/* Clean up */
	count = COUNT_MAX;
	do {
		status = close(sockfd);
		if (status < 0)
		{
			syslog(LOG_ERR, "close(sockfd): %m");
			count--;
			sleep(SLEEP_TIME); // Wait
		}
	} while (status < 0 && count > 0);
	if (count <= 0)
		exit(EXIT_FAILURE);

	/* Everything went well */
	exit(EXIT_SUCCESS);
}

[PanGalactic]

Cool. I have an OpenWRT router. So, let me ask it this way: "Why not use BusyBox's httpd and create a CGI program in C?"

[c00get]

PanGalactic wrote:

Why not use BusyBox's httpd and create a CGI program in C?

Because I didn't know of its existence. I'm new to programming, not only in Linux, but also in network related stuff, so there are many things I don't know.

Thanks for your help, now it's time to get my hands dirty.

[c00get]

I finally managed to get the query string. I wasn't reading/parsing the buffer correctly because I didn't know the format. Thank you for your help.