Purpose

To show how to set up and use TCP Keep-Alive with php.

Most PHP HowTo-s and examples over internet have problems:

• only turn on TCP Keep-Alive, with letting Keep-Alive parameters by default
• doing it wrong way

How TCP Keep-Alive works

There are 4 options for Keep-Alive:

• SOL_SOCKET - SO_KEEPALIVE - enables TCP Keep-Alive
• SOL_TCP - TCP_KEEPIDLE - timeout between data-packets, when hit Keep-Alive probe should be sent
• SOL_TCP - TCP_KEEPINTVL - timout between sending probes
• SOL_TCP - TCP_KEEPCNT - number of "TCP Keep-Alive" packets without an answer ("TCP Keep-Alive ACK") to consider socket as "broken pipe"

Most systems (OSes) have following defaults:

Option	Default value	Description
SO_KEEPALIVE	0	TCP Keep-Alive is disabled
TCP_KEEPIDLE	7200	2 hours
TCP_KEEPINTVL	75	75 seconds
TCP_KEEPCNT	8	8 probes before drop connection

Gotchas

Option numbers

Unfortunately PHP doesn't have TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT constants. Since most OSes can handle TCP Keep-Alive it is the matter of time to find correct numbers.

Found values

Option	Linux	Darwin	BSD
TCP_KEEPIDLE	4	16	256
TCP_KEEPINTVL	5	257	512
TCP_KEEPCNT	6	258	1024

Linux: Linux tcp.h Darwin: Darwin XNU tcp.h BSD: FreeBSD tcp.h

Streams

Streams is a very powerful way to handle tcp/tcp+ssl connections, but there no way to set socket option (setsockopt) for a stream.

sockets extension have function socket_import_stream which helps to get a socket for a given stream.

SSL/TLS Streams

There is no way to get socket of a stream (socket_import_stream) for established connection.

Instead of trying to "get inside into tcp+ssl stream", we can establish TCP connection, set up TCP Keep-Alive and enable SSL/TLS encryption.

Examples

General

    socket_set_option($socket, SOL_SOCKET, SO_KEEPALIVE, 1);

    socket_set_option($socket, SOL_TCP, TCP_KEEPIDLE,  $timeout);
    socket_set_option($socket, SOL_TCP, TCP_KEEPINTVL, $interval);
    socket_set_option($socket, SOL_TCP, TCP_KEEPCNT,   $probes);

Note: this example can't work - constants are missing.

Predefined constants and some OS-detection could be found at keepalive.php.

For all examples you can use tcpdump/wireshark to see TCP Keep-Alive packets, even for SSL/TLS connections.

All examples use one ip:port - 127.0.0.1:9898.

Plain TCP

Directory tcp contains:

• socket-client - echo client, just socket_* functions
• stream-client - echo client, stream_socket_client + socket_import_stream
• stream-server - echo server, stream_socket_server + socket_import_stream

As a common server you can use netcat: nc -kl 9898

As a common client you also can use netcat: nc 127.0.0.1 9898

SSL/TLS

Directory ssl contains:

• dummy-client - regular stream_* echo client
• dummy-server - regular stream_* echo server
• keepalive-client - keep-alive stream_* echo client
• keepalive-server - keep-alive stream_* echo server
• ssl - just generator for CA-certificate

As a common client (without peer validation) you also can use openssl: openssl s_client -connect 127.0.0.1:9898