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NoamRotter's avatar
NoamRotter
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Apr 26, 2020

Extract value from json

Hi, I have a webapp which accepts a POST with JSON payload. the JSON has only one key:value I need to write to log the value of the key. For example: This is the payload of the JSON: {      ...
application delivery
iRules
LTM
  • PeteWhite's avatar
    PeteWhite
    Apr 29, 2020

    Great! HTTP::payload must be able to retrieve it because it comes in a single packet so it doesn't need to accumulate. Note that if the payload was larger ( eg 1K ) then this might fail and you'd have to use HTTP::collect.

    We can use regexp in multiple ways to retrieve the substring. Try this:

    when HTTP_REQUEST {
  if { [HTTP::method] == "POST"} {
    log local0. ">>> Method is POST<<<"
    if {  [ regexp -- {"token": "(.+)"} [HTTP::payload] a b ] } {
        log local0.debug "Token Found a:$a b:$b"
    }
  }
}

    $a should be the whole of the line, $b should be only the token part.

    Take a look here for more detail on how to use regexp:

    PS the other way we could have done it is this:

    set token [lindex [ regexp -inline -- {"token": "(.+)"} [HTTP::payload] ] 1]
PeteWhite's avatar
PeteWhite
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when HTTP_REQUEST {
  if { [HTTP::method] == "POST"} {
    HTTP::collect
  }
}
when HTTP_REQUEST_DATA {
  set token [ regexp -inline -- {"token": "(.+)"} [HTTP::payload] ]
  log local0.debug "Token: $token" 
}

Try the above - I haven't tested it out but it looks ok

NoamRotter's avatar
NoamRotter
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Apr 29, 2020

Hi Pete,

Thank you for your answer.

I tested your iRule.

When using postman to post the JSON,

the postman keeps saying "Sending Request" until I hit the cancel button

I have added 2 logs line:

when HTTP_REQUEST {
  if { [HTTP::method] == "POST"} {
  log local0. ">>> Method is POST<<<"
	HTTP::collect
    log local0. ">>> HTTP Collected<<<"
  }
}
when HTTP_REQUEST_DATA {
  set token [ regexp -inline -- {"token": "(.+)"} [HTTP::payload] ]
  log local0.debug "Token: $token" 
}

In the ltm log I see:

Apr 29 08:27:43 bigip1 info tmm2[18399]: Rule /Common/test-iRule <HTTP_REQUEST>: >>> Method is POST<<<

Apr 29 08:27:43 bigip1 info tmm2[18399]: Rule /Common/test-iRule <HTTP_REQUEST>: >>> HTTP Collected<<<

When I disable the HTTP::collect line

Postman sends the request and finishes. Does not keep saying "Sending request"

But, the when HTTP_REQUEST_DATA part is not executed.

I moved these lines to HTTP_REQUEST part:

set token [ regexp -inline -- {"token": "(.+)"} [HTTP::payload] ]

log local0.debug "Token: $token"

like this:

when HTTP_REQUEST {
  if { [HTTP::method] == "POST"} {
    log local0. ">>> Method is POST<<<"
    set token [ regexp -inline -- {"token": "(.+)"} [HTTP::payload] ]
    log local0.debug "Token: $token" 
  }
}

And now I get in the ltm log:

Apr 29 08:33:45 bigip1 debug tmm2[18399]: Rule /Common/test-iRule <HTTP_REQUEST>: Token: {"token": "01593317-66f9-4e8a-8467-6f4652e680e3"} 01593317-66f9-4e8a-8467-6f4652e680e3

And I expected to get:

Apr 29 08:33:45 bigip1 debug tmm2[18399]: Rule /Common/test-iRule <HTTP_REQUEST>: Token: 01593317-66f9-4e8a-8467-6f4652e680e3

Thank you for your time.

Noam.