HTTP Health Monitor


This article covers the specific configuration for this monitor type.

The HTTP health monitor may only be applied to a pool whose virtual service has an HTTP application profile attached. Health monitoring of HTTPS is covered in the SSL Attributes in HTTPS Health Monitor section at the end of this article.

General Monitor Configuration

  • Send Interval: Frequency at which the health monitor initiates a server check, in seconds.
  • Receive Timeout: Maximum amount of time before the server must return a valid response to the health monitor, in seconds.
  • Successful Checks: Number of consecutive health checks that must succeed before Avi Vantage marks a down server as being back up.
  • Failed Checks: Number of consecutive health checks that must fail before Avi Vantage marks an up server as being down.

HTTP Specific Configuration

The following options are available for the HTTP health monitor:

  • Client Request Data: Send an HTTP request to the web server. Avi Vantage does not validate the request, as different servers may support unique request syntax.
    • Method: Any method may be used, though GET, POST and HEAD are the most common for monitoring. If no method is defined, Avi Vantage will use GET.
      • GET /index.htm
      • POST /upload.asp HTTP/1.0\r\nHost:\r\nContent-Length: 10\r\nABCDE12345
    • Path: The path may include the URI and query, such as /index.htm?user=test. If no path is specified, Avi Vantage will use /
    • Version: The HTTP version can be 1.0 or 1.1. If no version is specified, Avi Vantage will use 1.0.
    • Host: If no Host header is specified, Avi Vantage includes a host header populated with the server’s name. HTTP 1.1 servers expect a Host header to be included in the request.
    • Carriage Return: By default, Avi Vantage will add a carriage return line feed to the end of the send string in the form of \r\n. For HTTP 1.0, and additional \r\n may be required. For send strings that require multiple lines of data, such as headers, the carriage returns may need to be added, such as in the POST example above.
  • Response Code: A successful HTTP monitor requires either the Response Code, the Server Response Data, or both fields to be populated. The Response Code expects the server to return a response code within the specified range. For a GET request, a server should usually return a 200, 301 or 302. For a HEAD request, the server will typically return a 304. A response code by itself does not validate the server’s response content, just the status.
  • Response Size: The maximum http server response which will be parsed for response string match can be configured using this knob. This knob can be configured within [2k, 16k] range. By default its value is 2k.
  • Server Response Data: Enter a snippet of content from the server’s HTTP response by copying and pasting text from either the source HTML or the web page of the server. Avi Vantage inspects raw HTML data and not rendered web pages. For example, Avi Vantage does not follow HTTP redirects and will compare the redirect response with the defined Server Response string, while a browser will show the redirected page. The Server Response content is matched against the first 2KB of data returned from the server, including both headers and content/body. The Server Response Data can also be used to search for a specific response code, such as 200 OK. When both Response Code and Server Response Data are populated, both must be true for the health check to pass.
  • Health Monitor Port: Specify a port that should be used for the health check. When this setting is blank, the default port configured for the server will be used. When it is specified, clients may be directed to a different port than what is monitored. For instance, a server at HTTP port 80 may have two health monitors attached, one for HTTP default port, and a second for HTTPS specifically on port 443. If both health monitors pass, the server can receive traffic on HTTP port 80. This ensures clients can input items in their shopping cart and later purchase those items over SSL on 443.

Server Maintenance Mode

Custom server response can be used to mark a server as disabled. During this time, health checks will continue, and servers operate the same as if manually disabled, which means existing client flows are allow to continue, but new flows are sent to other available servers. Once a server stops responding with the maintenance string it will be brought online, being marked up or down as it normally would based on the server response data.

This feature allows an application owner to gracefully bleed connections from a server prior to taking the server offline without the requirement to log into Avi Vantage to first place the server in disabled state.

  • Maintenance Response Code: If the defined HTTP response code is seen in the server response, place the server in maintenance mode. Multiple response codes may be used via comma separation.
  • Maintenance Server Response Data: If the defined string is seen in the server response, place the server in maintenance mode.

Example Health Check

Sample HTTP health monitor config:


Sample HTTP health check send string:

GET /health/local HTTP/1.0
User-Agent: avi/1.0
Accept: */*

Sample server response:

HTTP/1.0 200 OK
Server: Apache-Coyote/1.1
Cache-Control: no-cache, no-store
Pragma: no-cache
Content-Type: text/plain
Content-Length: 15
Date: Fri, 20 May 2016 18:23:05 GMT
Connection: close

Health Check Ok

The server response includes both the Response Code: 200, and the Server Response Data: Health Check ok. Therefore this server will be marked up. Notice that Avi automatically includes additional headers in the send string, including User-Agent, Host, and Accept to ensure the server receives a fully formed request.

SSL Attributes in HTTPS Health Monitor

Prior to Avi Vantage release 17.1, the HTTPS health monitor used the pool SSL settings to verify the health of the servers. No SSL configuration was allowed in a health monitor. With 17.1, support has been added for configuration of SSL attributes under https_monitor mode of health monitor type HTTPS.

The use-case of this requirement is to allow SSL encrypted traffic to pass to servers without decrypting in the load balancer (the SE). Since the traffic is still SSL / HTTPS, we still are expected to conduct a relevant health monitor.

Health Monitor Configuration

Below is the CLI sequence by which to enable this feature:

[admin:my-cntrlr]: > configure healthmonitor System-HTTPS
[admin:my-cntrlr]: healthmonitor> https_monitor
[admin:my-cntrlr]: healthmonitor:https_monitor> ssl_attributes
[admin:my-cntrlr]: healthmonitor:https_monitor:ssl_attributes> ssl_profile_ref System-Standard
[admin:my-cntrlr]: healthmonitor:https_monitor:ssl_attributes> ssl_key_and_certificate_ref System-Default-Cert
[admin:my-cntrlr]: healthmonitor:https_monitor:ssl_attributes> pki_profile_ref System-pki-profile.

Behavior Change

The SSL settings on health monitor are always considered if provided. If SSL settings for the health monitor are not provided, the health monitor falls back to using pool SSL settings. An HTTPS health monitor needs SSL settings on either the health monitor config itself or in the pool config. If is absent in both, Avi Vantage reports an error.

Upgrade Impact

Upgrade happens smoothly and needs no manual configuration. Upgrading from releases prior to 17.1 causes the HTTPS health monitor to use pool SSL settings. If a new SSL config is added to the health monitor, it will be placed into effect.

Configuring POST Method

Starting with Avi Vantage version 20.1.1, you can configure POST method.

The following is the configuration example:

[admin:ctrl2]: > configure healthmonitor HTTP-HM-POST
[admin:ctrl2]: healthmonitor> type health_monitor_http
[admin:ctrl2]: healthmonitor> http_monitor
[admin:ctrl2]: healthmonitor:http_monitor> http_request "POST /echo_post_header_body HTTP/1.0"
[admin:ctrl2]: healthmonitor:http_monitor> http_request_body "This is a test for HTTP POST method"
[admin:ctrl2]: healthmonitor:http_monitor> http_response "test for HTTP"
[admin:ctrl2]: healthmonitor:http_monitor> http_response_code http_2xx
[admin:ctrl2]: healthmonitor:http_monitor> save
[admin:ctrl2]: healthmonitor> save

Note: When exact_request is configured, http_request_body will be appended to the end of http_request.

For further details on authenticating health monitors refer to Authenticating HTTP/HTTPS Health Monitor user guide.