Health checking is the ability to probe a server to ensure the service is up and running. This is one of the root features of any load-balancer.

One can probe servers and services at a different layer of the OSI model:

  • ARP check (not available in HAProxy)

  • ICMP (ping) check (not available in HAProxy)

  • TCP (handshake) check

  • Application (HTTP, MySql, SMTP, POP, etc…) check

The most representative of the application status of the check is, the best.

This means that the best way to check a service is to “speak” the protocol itself. Unfortunately, it is impossible to write one check per protocol, there are too many protocols and some of them are proprietary and/or binary. That’s why HAProxy 1.5 now embeds a new health checking method called “tcp-check“. It is a very simple and basic “send/expect” probing method where HAProxy can send arbitrary strings and match string or regex comparisons on server responses. Many send and expect can be executed in a row to determine the status of a server.

I’ve already explained how to check Redis server and how to balance traffic only to the Redis master server of a cluster.

Today’s article introduces a binary protocol widely deployed: fastcgi used by php-fpm.

Fastcgi Binary Ping/Pong Health Check

Fastcgi is a binary protocol. It means data on the network are not readable by humans, like HTTP or SMTP. Php-fpm relies on this protocol to treat PHP code. It is common to use HAProxy to load-balance many php-fpm servers for resiliency and scalability.

Php-Fpm configuration

Enable a dedicated url to probe and setup the response in your php-fpm configuration:

ping.path = /ping
ping.response = pong

HAProxy Health Checking for Php-Fpm

Add the following tcp-check sequence in your php-fpm backend to probe the /ping url and ensure the server answers a “pong“.

The comment at the end of the line describes the php-cgi protocol fields.

option tcp-check
 # FCGI_BEGIN_REQUEST
 tcp-check send-binary   01 # version
 tcp-check send-binary   01 # FCGI_BEGIN_REQUEST
 tcp-check send-binary 0001 # request id
 tcp-check send-binary 0008 # content length
 tcp-check send-binary   00 # padding length
 tcp-check send-binary   00 #
 tcp-check send-binary 0001 # FCGI responder
 tcp-check send-binary 0000 # flags
 tcp-check send-binary 0000 #
 tcp-check send-binary 0000 #
 # FCGI_PARAMS
 tcp-check send-binary   01 # version
 tcp-check send-binary   04 # FCGI_PARAMS
 tcp-check send-binary 0001 # request id
 tcp-check send-binary 0045 # content length
 tcp-check send-binary   03 # padding length: padding for content % 8 = 0
 tcp-check send-binary   00 #
 tcp-check send-binary 0e03524551554553545f4d4554484f44474554 # REQUEST_METHOD = GET
 tcp-check send-binary 0b055343524950545f4e414d452f70696e67   # SCRIPT_NAME = /ping
 tcp-check send-binary 0f055343524950545f46494c454e414d452f70696e67 # SCRIPT_FILENAME = /ping
 tcp-check send-binary 040455534552524F4F54 # USER = ROOT
 tcp-check send-binary 000000 # padding
 # FCGI_PARAMS
 tcp-check send-binary   01 # version
 tcp-check send-binary   04 # FCGI_PARAMS
 tcp-check send-binary 0001 # request id
 tcp-check send-binary 0000 # content length
 tcp-check send-binary   00 # padding length: padding for content % 8 = 0
 tcp-check send-binary   00 #
 tcp-check expect binary 706f6e67 # pong

Note that the whole send string could be written on a single line.

Any protocol?

If someday, you have to do the same type of configuration on a protocol nobody knows, simply capture network traffic of a “hello” sequence using tcpdump.

Then send the tcp payload captured using the tcp-check send command and configure the appropriate expectations.

And it will work!

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