Forum Discussion

Eljay's avatar
Eljay
Icon for Cirrus rankCirrus
Aug 12, 2021

Bypassing ASM on HTTP response

Is it possible to prevent ASM from blocking responses when there's a specific HTTP header present in the HTTP response? Let's say we block responses with HTTP status code 500 by default in our securi...
ASM Advanced WAF
devops
security
  • spalande's avatar
    spalande
    Aug 13, 2021

    Try with an iRule. use appropriate asm policy name.

    Use logging to see if condition is getting triggered and then it can be disabled. 

    when HTTP_RESPONSE  {
ASM::enable "/common/asm_policy"
if { ([HTTP::status] == 500) and ([HTTP::header value Content-Type] eq "application/problem-handled-return-to-client") }{
log local0.info "disable asm"
ASM::disable
return
   }
}
spalande's avatar
spalande
Icon for Nacreous rankNacreous

ASM::disable is valid command in HTTP_RESPONSE event.

​

If you want to use generic one, you can remove ASM::enable command at the start of the iRule and test it ASM gets enabled in the response for every and all subsequent events​ after a match. But F5 recommends to enable it before disabling for specific traffic.

Eljay's avatar
Eljay
Icon for Cirrus rankCirrus
Aug 19, 2021

Something isn't working as expected. I use this simple code which I cut and pasted from https://clouddocs.f5.com/api/irules/HTTP_RESPONSE.html and made only a small change inside the IF-statement :

when HTTP_RESPONSE {
 
  if { [HTTP::status] contains "500"} {
     log local0.debug "HTTP_RESPONSE - err: 500"
  }
 
}

But when I make a request that triggers a 500 status code then I get this message in the LTM log:

 

<date removed> <host removed> err tmm[14816]: 01220001:3: TCL error: /Common/asm-override <HTTP_RESPONSE> - Can't call after responding - ERR_NOT_SUPPORTED (line 1)   invoked from within "HTTP::status"

 

We are using v15.1.2.1 with an Eng Hotfix.

 

  • Yiğit_Uslu's avatar
    Yiğit_Uslu
    Icon for Cirrus rankCirrus
    Aug 22, 2021

    Can't call after responding means most probably an event overlap is occurring. Checkout if there is another rule or policy attached to VS with HTTP_RESPONSE event.

    If so, combine both HTTP_RESPONSE event content in one iRule.

     

    Hope this helps

  • Eljay's avatar
    Eljay
    Icon for Cirrus rankCirrus
    Aug 23, 2021

     Thank you! When I browsed through the other iRules, then I found an iRule that was executed before this one. Thanks a lot! 🙂