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KingMeow_3883's avatar
KingMeow_3883
Icon for Altostratus rankAltostratus
Apr 01, 2015

Calling commands by string / reflection

Hi,

Is there any way to call iRule commands by string? For example, can I turn the string "starts_with" into a variable which calls the command 'starts_with'?. I'm trying to make an iRule that interpret strings in a datagroup into commands efficiently, rather than using a bunch of switch statements.

To elaborate, say my iRule has just read a datagroup value, performed a split procedure on it, and returned a list with the values "HTTP::URI" , "starts_with" and "/blah" and then stored them into the variables $a , $b and $c respectively.

I want to be able to do something like 

if {[$a $b $c]}
and have it be interpreted as though it was 
'if {"[HTTP::URI] starts_with /blah"}
`.

in c this is called 'reflection', is there an iRule equivalent? http://www.dotnetperls.com/methodinfo-invoke

Thanks.

application delivery
call by name
devops
irule
iRules
meta info
reflection

4 Replies

  • Hannes_Rapp's avatar
    Hannes_Rapp
    Icon for Nimbostratus rankNimbostratus
    Apr 01, 2015

    Hi,

    As far as I'm aware, no. The TCL 

    exec
    command which could otherwise be used for such purpose is excluded from the available iRule commands. Can you perhaps explain a bit what you'd like to do?

    Do you want to select a specific pool or initiate a redirect, based on HTTP URI value? It also appears you want to manage all updates via an LTM data-group instead of iRule. Please give a few samples of condition/action in iRule format.

    Regards,

  • Arie's avatar
    Arie
    Icon for Altostratus rankAltostratus
    Apr 01, 2015

    To a degree you can use the 

    eval
    command for this. Be very careful with this, though - it can easily lead to injection attack vulnerabilities.

    Documentation: eval

  • StephanManthey's avatar
    StephanManthey
    Icon for Nacreous rankNacreous
    Apr 01, 2015

    Hi KingMeow,

    I haven´t found a way to concat a comparison as a condition.

    Using 
    eval
    works fine with commands as demonstrated in this post.

    If you find a solution to this, please let us know.

    As Hannes already wrote, there are probably alternative solutions to meet your requirements from functional perspective.

    Thanks & +1 for the interesting question,

    Stephan
  • KingMeow_3883's avatar
    KingMeow_3883
    Icon for Altostratus rankAltostratus
    Apr 03, 2015

    Hi Stephan,

    I can confirm you're correct for 11.4.1 at least. I haven't built my iRule exactly but fiddled did a lot of testing. I can turn attributes like 

    HTTP::uri
    into variables just by set 
    x "HTTP::uri"
    . then I can get the command HTTP::uri by 
    $x
    . Alternatively if I do 
    [$x]
    it returns the value of the HTTP uri.

    I even did something like 

    when HTTP_REQUEST {
    set a HTTP::header
    set b "replace"
    set c "host"
    set d "new.host.header.value"
    $a $b $c $d
}

    And the last line actually replaced the host header.

    I haven't been able to turn things like "starts_with" into variables though - every approach has resulted in a run time error but I think I've gotten around it by using 

    matches_glob
    for all string matching and leaving it up to the user to enter the correct wildcards into the datagroup that feeds into the iRule.

    I think this is pretty much solved but I'll build my iRule when I get a chance again and repost in a couple of days with an update.

    I'm quite sure this is going to cut my iRule down to a third of its size at least. Thanks for your help again!