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Guillaume_Rouss's avatar
Guillaume_Rouss
Icon for Nimbostratus rankNimbostratus
Aug 12, 2019

Formating issue: default value for undefined variable

Hello.

We're using an irule to log http access to a remote syslog server. I know there is also a dedicated profile for this, but we prefer to use a consistent implementation for all our logging needs, even those not covered by a profile. In order to ease parsing on SIEM side, I'm emulating standard Combined Log format, which use a dash symbol as a default value when a field is empty, for instance when there is no user name, ie:

foo.domain.tld 127.0.0.1 - - [12/Aug/2019:17:41:02 +0200]

I tried to use a ternary operator directly while formatting the message, ie:

set message  [format \
    "%s - %s \[%s\]..." \
    ... \
    [[info exists [HTTP::username]] ? [HTTP::username] : '-' ] \
]

However, this syntax triggers a TCL error:

invalid command name "0"  while executing "[info exists [HTTP::username]] ? [HTTP::username] : '-' "

I searched TCL documentation for conditional expansion, such as in bash, or default value in the format string, without success.

The only workaround I found was to explicitly define a variable:

if { [info exists [HTTP::username]] } {
    set user [HTTP::username]
} else {
    set user -
}

This is quite cumbersome, as I have another variable with the same formating issue (the size header in the response).

Is there any other way to achieve the same result in a more compact way ?

application delivery
iRules
LTM
  • Lee_Sutcliffe's avatar
    Lee_Sutcliffe
    Aug 13, 2019

    You need to make sure you use the expr syntax when using a ternary operator within a variable.

    For example:

    % set colour [expr {"1" ? "red" : "blue"}]
red
% set colour [expr {"0" ? "red" : "blue"}]
blue

    So looking at your example, you'd need to change it to be something like this.. (note that I have changed the from single to double quotation marks for the hyphen)

    set message  [format \
    "%s - %s \[%s\]..." \
    ... \
    [expr {[info exists [HTTP::username]] ? [HTTP::username] : "-" }]
]

  • Lee_Sutcliffe's avatar
    Lee_Sutcliffe
    Icon for Nacreous rankNacreous
    Aug 13, 2019

    You need to make sure you use the expr syntax when using a ternary operator within a variable.

    For example:

    % set colour [expr {"1" ? "red" : "blue"}]
red
% set colour [expr {"0" ? "red" : "blue"}]
blue

    So looking at your example, you'd need to change it to be something like this.. (note that I have changed the from single to double quotation marks for the hyphen)

    set message  [format \
    "%s - %s \[%s\]..." \
    ... \
    [expr {[info exists [HTTP::username]] ? [HTTP::username] : "-" }]
]