#include <string.h>
  #include <stdio.h>
  #include <unistd.h>
  #include <netdb.h>
  
  int main(int argc, char ** argv)
  {
          struct hostent * he;
          struct sockaddr_in sa = { AF_INET };
          unsigned int port;
          int foo, sk;
  
          if (argc != 3)
                  return 1; /* Syntax error */
  
          if (sscanf(argv[2], "%u%n", &port, &foo) != 1 ||
              strlen(argv[2]) != foo ||
              port > 65535)
                  return 1; /* Syntax error */
  
          he = gethostbyname(argv[1]);
          if (! he)
                  return 2; /* DNS error,OS error or syntax error */
  
          sa.sin_addr.s_addr = *(unsigned long*)he->h_addr;
          sa.sin_port = htons(port);
  
          sk = socket(AF_INET, SOCK_STREAM, 0);
          if (sk < 0)
                  return 3; /* OS error */
  
          if (connect(sk, (void*)&sa, sizeof sa) < 0)
                  return 4; /* TCP error */
  
          return 0;
  }

  gcc tcp-check.c -o tcp-check

  ./tcp-check yahoo.com 80; echo $?

The trick will be just limiting it to simple functionality. Easy for these types of apps to get out of hand.

About ten years ago I wanted to write a windows app that allowed text to appear as a window -- no rectangular frame, menu, or close button. Just text. The text was the window, and there was nothing else but the text.

I figured out how to do it, and ended up writing something that had a properties page that looked like it could launch Trident Missiles. Fun project, but definitely overkill for what I needed.

Nagios will do the job but it's total overkill. Ideally a simple solution such as:

    ./monitor http://localhost:someport/ 60 "/etc/init.d/someservice restart" 

And that's exactly why you don't roll these things on your own unless you have a very good reason. Making sure a cronjob doesn't stack up reliably is non-trivial and becomes extremely hairy when the network gets involved. Many people think "I'll just use a lockfile" - think again.

Since for monitoring scripts this kind of reliability is the whole point, I strongly suggest to use something like monit where someone else has already worked out all the little corner cases.

I still think a smart wrapper around check_tcp is the way to do this. Perhaps not out of cron, it can instead be a long running process in a sleep/fork/exec(check_tcp)/waitpid loop.

I use it in a medium to high volume production environment [2], and it has done a very good job at getting our lighttpd and nginx back up on the air whenever we experience peaks above the normal workload.

[1] http://god.rubyforge.org/

[2] http://video.nrkbeta.no/

http://haproxy.1wt.eu/