Forum Discussion

Blue_whale's avatar
Blue_whale
Icon for Cirrocumulus rankCirrocumulus
Jun 17, 2021

https health monitor configured with only send strings but no receive strings - what it will check ?

Hi Team ,

 

If we have https health monitor configured with only send strings but no receive strings in it , what checks does it perform .

can you please explain with below configuration as an example .

 

Pool Member : 10.10.10.10 : 443

 

https Monitor

 

Send string :  GET /health\r\n

Receive string : ( not configured ) 

application delivery
BIG-IP
health monitors
http
https
TMSH
  • crodriguez's avatar
    crodriguez
    Jun 18, 2021

    Per F5 documentation:

     

    "Content check monitors are designed to determine whether a service is available and whether the server is serving the appropriate content."

     

    Also in the same article, "You can also use the default null Receive String value [""] [in a content check monitor]. In this case, any content retrieved is considered a match." (Emphasis mine.)

     

    In other words, without a receive string, a content check monitor satisfies only its first purpose - "to determine whether a service is available." To satisfy the second purpose of a content check monitor - "to determine if the server is serving the appropriate content" - you must include a Receive String with the appropriate content. This is by design, as F5 cannot possibly know what constitutes appropriate content for your application. We provide the monitor shell; you customize it for your application delivery needs.

  • Blue_whale's avatar
    Blue_whale
    Icon for Cirrocumulus rankCirrocumulus
    Jun 18, 2021

    ok so in that case the default https monitor in the F5 also not having receive string.So it's of no use right ? it is same as using tcp-443 monitor ? is that right .

  • crodriguez's avatar
    crodriguez
    Ret. Employee
    Jun 17, 2021

    A content check monitor with no receive string is relatively powerless to assess application delivery health. In the case of your HTTPS monitor, for example, the system will establish a secure connection to the pool member, send the HTTP 0.9 GET request you provided in the Send String, and any response - including a 404 Not Found or other HTTP 500 error - is considered a successful response and the monitor will mark the pool member as available. As such, your monitor is not much more effective than a service check monitor and certainly not (necessarily) indicative of application delivery health.

     

    Conventional wisdom is to use an appropriate receive string in a content check monitor.

    • Blue_whale's avatar
      Blue_whale
      Icon for Cirrocumulus rankCirrocumulus
      Jun 18, 2021

      ok so in that case the default https monitor in the F5 also not having receive string.So it's of no use right ? it is same as using tcp-443 monitor ? is that right .

      • crodriguez's avatar
        crodriguez
        Ret. Employee
        Jun 18, 2021

        Per F5 documentation:

         

        "Content check monitors are designed to determine whether a service is available and whether the server is serving the appropriate content."

         

        Also in the same article, "You can also use the default null Receive String value [""] [in a content check monitor]. In this case, any content retrieved is considered a match." (Emphasis mine.)

         

        In other words, without a receive string, a content check monitor satisfies only its first purpose - "to determine whether a service is available." To satisfy the second purpose of a content check monitor - "to determine if the server is serving the appropriate content" - you must include a Receive String with the appropriate content. This is by design, as F5 cannot possibly know what constitutes appropriate content for your application. We provide the monitor shell; you customize it for your application delivery needs.