TCP

Active health checks

An active health check attempts to connect to a server at a regular interval. If the connection cannot be established, then the health checks fails. The minimum configuration for a health check is the check keyword on a server line.

backend be_myapp
   server srv1 10.0.0.1:80 check
   server srv2 10.0.0.2:80 check

backend be_myapp
   server srv1 10.0.0.1:80 check port 8080
   server srv2 10.0.0.2:80 check port 8080

Change the health check interval

By default, HAProxy ALOHA sends a health check every two seconds. Change this by adding the inter parameter to the server line.

Health checks are sent once every four seconds.

backend be_myapp
   server srv1 10.0.0.1:80 check inter 4s
   server srv2 10.0.0.2:80 check inter 4s

Use any of the following time suffixes:

• us : microseconds

• ms : milliseconds

• s : seconds

• m : minutes

• h : hours

• d : days

Other parameters that affect the check interval are defined below:

Parameter	Description
inter	Sets the interval between two consecutive health checks. If not specified, the default value is 2s.
fastinter	Sets the interval between two consecutive health checks when the server is in any of the transition states: UP - transitionally DOWN or DOWN - transitionally UP. If not set, then inter is used.
downinter	Sets the interval between two consecutive health checks when the server is in the DOWN state. If not set, then inter is used.

The diagram below describes at which phase each of these settings applies.

backend be_myapp
   server srv1 10.0.0.1:80 check fall 5
   server srv2 10.0.0.2:80 check fall 5

backend be_myapp
   server srv1 10.0.0.1:80 check fall 5 rise 10
   server srv2 10.0.0.2:80 check fall 5 rise 10

Passive health checks

A passive health check monitors live traffic for failed TCP connections. Passive checks will detect errors returning from any part of your proxied service, but they require active traffic to monitor.

backend servers
   server server1 192.168.0.10:80 check  observe layer4  error-limit 10  on-error mark-down

Active health checks (Direct Server Return)

The LB Layer 7 tab configures an HAProxy reverse proxy. You can also load balance TCP by using the LB Layer 4 tab, which instead configures a network router using Linux Virtual Server (LVS). While an HAProxy reverse proxy is simpler, you may prefer to use the LB Layer 4 tab (LVS) when you want to load balance TCP in combination with Direct Server Return.

In that case, the director section supports checking a connection to a TCP port.

director exchange 10.0.0.9:443 TCP
   balance roundrobin                               # load balancing algorithm
   mode gateway                                     # forwarding mode

   option tcpcheck
   check port 21 interval 5s timeout 1s             # advanced check parameters
   server exchange1 10.0.0.13:443 weight 10 check   # server exchange1
   server exchange2 10.0.0.14:443 weight 10 check   # server exchange2

1. Add a check parameter to each server line for which you would like to enable health checking.

2. Add option tcpcheck in the same section. It take no arguments.

3. Add the line check to the section to configure other options. It uses the following syntax:

   check { [timeout <seconds>] [interval <seconds>] [source <ip>] [port <port>]
           [rise <count>] [fall <count>] [inhibit] }

   timeout <seconds>	Period after which an attempt without a response from the server is considered as failed (default: half of interval)
   interval <seconds>	Interval between two checks, in seconds (default: 10 seconds)
   source <ip>	Source IP to use when performing the check
   port <port>	Forces the destination port (default: real server port, if it exists)
   rise <count>	A server will be considered as operational after <count> consecutive successful health checks. The default is 1.
   fall <count	A server will be considered as dead after <count> consecutive unsuccessful health checks. The default is 1.
   inhibit	If a server is down, its weight is passed to 0 but not deleted. Established connection are not broken but new connections are dispatched on the other servers.