PHP fopen() Tricks

The fopen() function in PHP allows one to read a remote file, or a web page. I recently added a feature in the logmon module to read and parse a remote syslog event definition page by its URL in the Cisco web space, such as this one documenting Cisco NX-OS events.

Here are two tricks that I have learned implementing the above feature. In my case, the server on which my code is run is on a subnet not NATted from the internal network to the outside, which means that I can not directly access an external website. There are two ways to work around this issue:

1. Use an IP address that is NATted or accessible from the outside;
2. Go through a proxy server.

Multi-Homed Server

Re-addressing a server may be a bit more involved especially in networking where IP addresses do get directly used a lot. However, a server may have more than one IP addresses. So if we add an IP address that is NATted or public, we can go back to work. To use a different IP address on the server for fopen(), one needs to use the forth parameter to fopen(): the $context.

Here is a little snippet of PHP code specifying source interface for fopen():

$srcip = '192.168.20.30';  // Assuming this is a NATted IP address
$context = stream_context_create
(
array
(
 'socket' => array
 (
  'bindto' => $srcip,
 )
)
);
($fn = fopen($url, "r", false, $context)) || die("Can not open '$url'\n\n");

while (!feof($fn)) {
echo fgetss($fn);
}

Proxy

Another way out of this problem is to use a proxy server that is accessible from the internal network. We do have a few of those deployed. To use one of them, we only need to make a simple change in how the $context is created in the example above:

$context = stream_context_create
(
array
(
 'http' => array
 (
  'proxy' => 'tcp://192.168.10.10:80' // The proxy server address and port
 ),
)
);