iRule Checking for No Value

[BunnyJunJun]

Hi All,

Just want to check with the fellow community members if you all have any idea on how to use iRules to check for no value?

Basically, I am crafting an iRule that search for all headers and sieving out that it starts with sec-ch. If it starts with sec-ch and there isn't any value (tried playing around and this is not an empty string), how do i use iRules to check it as an if condition?

if {$header_name eq ????}, where ???? represents no value. Do assist if possible and share your experiences 😄 

[Kevin_Stewart · Accepted solution]

Not sure I completely understand. the sec-ch-mobile header has no value and is not an empty string? I don't think it can be both of these. Try logging what the value actually is:

when HTTP_REQUEST {
    foreach x [string tolower [HTTP::header names]] {
        if { $x starts_with "sec-ch" } {
            log local0. "header($x) == |[HTTP::header $x]|"
        }
    }
}

I put the pipes in there to show if there's actually just white space. Still technically a non-empty string, not equal to "".

[Kevin_Stewart]

Something like this maybe:

when HTTP_REQUEST {
    foreach x [string tolower [HTTP::header names]] {
        if { ( $x starts_with "sec-ch" ) and ( [HTTP::header $x] eq "" ) } {
            log local0. "found empty sec-ch header"
        }
    }
}

[BunnyJunJun]

Hi Kevin,

Thanks! I did roughly the same, however, in your context, x has no value. It is not even a string. Hence, quite hard for me to further "manipulate" x.

I was thinking to concatenate it like those we do for typical coding. (i.e. var temp = "" + x, and temp will be a string with x valueless). Is this possible in terms of iRules?

[Kevin_Stewart]

x is the header name. Are you saying the header doesn't exist?

[BunnyJunJun]

Hi Kevin,

Header exist. An example of the request captured would be as such.

Header A: valueA
Header B: valueB
sec-ch-mobile: 
Header C: valueC

Given the idea of above, sec-ch-mobile has no value and it is not an empty string. As such, how do i check for x is an empty value given your sample iRules above?

[Kevin_Stewart · Accepted solution]

Not sure I completely understand. the sec-ch-mobile header has no value and is not an empty string? I don't think it can be both of these. Try logging what the value actually is:

when HTTP_REQUEST {
    foreach x [string tolower [HTTP::header names]] {
        if { $x starts_with "sec-ch" } {
            log local0. "header($x) == |[HTTP::header $x]|"
        }
    }
}

I put the pipes in there to show if there's actually just white space. Still technically a non-empty string, not equal to "".

[BunnyJunJun]

Hi Kevin, any idea how do i access the logs? I am accessing via putty and when i am in the tmsh, i tried to quit tmsh and the whole command prompt just closes.. 😞

[Kevin_Stewart · Accepted solution]

local0. goes to the LTM log:

tail -f /var/log/ltm