Question

I have a script i use that checks an IP address stored within my hosts.allow file against what IP is mapped to my dyndns hostname so i can log into my servers once i’ve synced my current IP to that hostname. For some reason though the script seems to cause really intermittent issues.

within my hosts.allow file i have a section like this:

#SOme.gotdns.com
sshd : 192.168.0.1
#EOme.gotdns.com

#SOme2.gotdns.com
sshd : 192.168.0.2
#EOme2.gotdns.com

I have a script running on a cron (every minute) that looks like this:

#!/usr/bin/php
<?php
$hosts = array('me.gotdns.com','me2.gotdns.com');
foreach($hosts as $host)
{
        $ip = gethostbyname($host);
        $replaceWith = "#SO".$host."\nsshd : ".$ip."\n#EO".$host;
        $filename = '/etc/hosts.allow';
        $handle = fopen($filename,'r');
        $contents = fread($handle, filesize($filename));
        fclose($handle);
        if (preg_match('/#SO'.$host.'(.*?)#EO'.$host.'/si', $contents, $regs))
        {
                $result = $regs[0];
        }
        if($result != $replaceWith)
        {
                $newcontents = str_replace($result,$replaceWith,$contents);
                $handle = fopen($filename,'w');
                if (fwrite($handle, $newcontents) === FALSE) {
                }
                fclose($handle);
        }
}
?>

The problem i have is that intermittently characters are being dropped (i assume during the replace) that causes future updates to fail as it inserts something like:

#SOme.gotdns.com
sshd : 192.168.0.1
#EOme.gotdn

note the missing “s.com”

This of course means i lose access to the server, any ideas why this would be happening?

Thanks.

Answer 1

I would say that in order to do this safely, you should acquire an exclusive lock on the file at the beginning of the script, read it all into memory once, modify it in memory, then write it back to the file at the end. This would also be considerably more efficient in terms of disk I/O.

You should also alter the cron job to run less frequently. It is likely that the reason you currently have this problem is because two processes are running at the same time – by locking the file, if this is the case, you risk having the processes stack up waiting to acquire a lock. Setting it for every 5 minutes should be good enough – your IP shouldn’t change that often!

So do this (FIXED):

#!/usr/bin/php
<?php

  // Settings
  $hosts = array(
    'me.gotdns.com',
    'me2.gotdns.com'
  );
  $filename = '/etc/hosts.allow';

  // No time limit (shouldn't be necessary with CLI, but just in case)
  set_time_limit(0);

  // Open the file in read/write mode and lock it
  // flock() should block until it gets a lock
  if ((!$handle = fopen($filename, 'r+')) || !flock($handle, LOCK_EX)) exit(1);

  // Read the file
  if (($contents = fread($handle, filesize($filename)) === FALSE) exit(1);

  // Will be set to true if we actually make any changes to the file
  $changed = FALSE;

  // Loop hosts list
  foreach ($hosts as $host) {

    // Get current IP address of host
    if (($ip = gethostbyname($host)) == $host) continue;

    // Find the entry in the file
    $replaceWith = "#SO{$host}\nsshd : {$ip}\n#EO{$host}";
    if (preg_match("/#SO{$host}(.*?)#EO{$host}/si", $contents, $regs)) {
      // Only do this if there was a match - otherise risk overwriting previous
      // entries because you didn't reset the value of $result
      if ($regs[0] != $replaceWith) {
        $changed = TRUE;
        $contents = str_replace($regs[0], $replaceWith, $contents);
      }
    }

  }

  // We'll only change the contents of the file if the data changed
  if ($changed) {
    ftruncate($handle, 0); // Zero the length of the file
    rewind($handle); // start writing from the beginning
    fwrite($handle, $contents); // write the new data
  }

  flock($handle, LOCK_UN); // Unlock
  fclose($handle); // close