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Piotr_Lewandows's avatar
Piotr_Lewandows
Icon for Altostratus rankAltostratus
May 29, 2017

iRule TCL Error and Fallback Host

Hi,

 

Just another finding that surprised me a bit :-)

 

Scenario:

 

  • HTTP profile with Fallback Host set (Fallback on Error Codes can be empty or contain codes)
  • Single member pool
  • iRule causing TCL error

I was under impression that Fallback Host is used under two conditions:

 

  • Member returned one of codes defined in Fallback on Error Codes
  • There is no active member left in the pool

It turns out that Fallback Host is as well used when iRule TCL Error is generated - instead of sending RST to client. Then BIG-IP sends FIN-ACK to client to close connection.

 

Piotr

 

application delivery
devops
fallback host
irule
iRules
LTM
tcl error

2 Replies

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  • Anesh's avatar
    Anesh
    Icon for Cirrostratus rankCirrostratus
    May 30, 2017

    Please read answer posted by F5 in this thread

     

    "Anything that raises an abort above the HTTP filter in the chain will trigger a HTTP fallback. This includes things like errors in iRules, Out of Memory situations, misconfiguration, and of course the proxy failing to connect"

     

  • JG's avatar
    JG
    Icon for Cumulonimbus rankCumulonimbus
    May 30, 2017

    From K6510: BIG-IP system fallback host behavior when a host is down:

    When a Fallback Host is defined, and the connection attempt to the selected load balancing target has failed by the time the BIG-IP LTM system is ready to send the request, the BIG-IP LTM system sends the specified HTTP redirect in the following situations:

   - A pool member has not been chosen (the system failed to select a pool or a pool member, or no pool members were available)
   - A pool member is unreachable (when no route to the target exists)
   - A pool member is unresponsive (resets a connection request)
   - A pool member closes an established connection before the request is sent

    .