Description:
------------
I have a script that uses php’s curl interface to connect to a web server and download data. I don’t put a timeout on the request because it could take a long time to respond. However, sometimes the TCP connection gets stale. The underlying sockets never disconnect because the connection never gets reset because the network connection is stable (this is on a VPS). TCP offers KeepAlive which allows for detecting a dead connection by sending data outside of the application visible stream to validate that the other host still sees the connection as valid. Thus, sometimes the connection dies without resetting the socket (the remote system closes it uncleanly) and the script on my VPS hangs for months until I notice it is stuck and manually kill it.

libcurl offers a way to enable TCP KeepAlive on its connections: https://curl.haxx.se/libcurl/c/CURLOPT_TCP_KEEPALIVE.html . I don’t see this option documented at either http://php.net/manual/en/curl.constants.php or http://php.net/manual/en/function.curl-setopt.php . I would like to be able to turn on that option from my PHP scripts or have PHP just enable it by default.

For the test script, start the script and then execute something like “iptables -I INPUT 1 -s httpbin.org -j DROP” before the 10 second delay expires. Note that the script hangs forever. If you have a recent enough version of libcurl, the CLI will instead have this behavior after a couple minutes (because the CLI curl command turns on CURLOPT_TCP_KEEPALIVE by default):

ohnobinki@ohnopublishing ~ $ http_proxy= curl http://httpbin.org/delay/10
curl: (56) Recv failure: Connection timed out


When done testing, use “iptables -D INPUT 1” to clean up the rule.

Test script:
---------------
#!/usr/bin/env php
<?php
$curl = curl_init('http://httpbin.org/delay/10');
curl_setopt($curl, CURLOPT_TCP_KEEPALIVE, TRUE); // I get a warning I shouldn’t here about undefined constant
/*
 * Within 10 seconds of this line being reached, configure your
 * gateway to start dropping all packets from the connection,
 * including connection reset packets without letting the client
 * computer reset its interface (turning off WiFi or unplugging
 * Ethernet will reset the connection and *not* trigger this
 * condition). If TCP KeepAlive is enabled, the call should terminate
 * when the OS’s keepalive timers expire. Otherwise, the command will
 * hang forever.
 */
curl_exec($curl);


Expected result:
----------------
The script exit. The curl_exec() call would return an error and nothing would be printed on screen.

Actual result:
--------------
Script hangs forever with following output:

Notice: Use of undefined constant CURLOPT_TCP_KEEPALIVE - assumed 'CURLOPT_TCP_KEEPALIVE' in /home/ohnobinki/php_curl_tcp_keepalive.php on line 4
PHP Warning:  curl_setopt() expects parameter 2 to be long, string given in /home/ohnobinki/php_curl_tcp_keepalive.php on line 4

Warning: curl_setopt() expects parameter 2 to be long, string given in /home/ohnobinki/php_curl_tcp_keepalive.php on line 4