Forum Discussion
Festus_50639
Nimbostratus
Nimbostratuslegal comparison operator syntax
Good morning all,
I seem to have found a logic error and my first step is to rule out or confirm that my iRule is at fault.
Here is the iRule as it was used when it "broke".
===============
when HTTP_REQUEST {
set curtime [clock seconds]
set redir_time [clock format $curtime -format {%H}]
if { [HTTP::uri] contains "site/app-folder" } {
if { ($redir_time >= 21) or ($redir_time <= 2) } {
HTTP::redirect "http://host.domain/site/Common/app-unavailable.html"
}
else {
if { [HTTP::uri] contains "?" } {
HTTP::redirect "http://host.domain/site/default.aspx?[lindex [split [HTTP::uri] "?"] 1]"
}
else { HTTP::redirect "http://host.domain/site/default.aspx" }
}
}
}
===============
Here is the background:
We put this iRule in place so that we can redirect requests during a nightly period of unavailability. We were able to test in our dev and lab environments after 10:00 AM so we didn't see any errors adjusting the $redir_time values so that we were before or after the unavailability period.
Using that testing as a benchmark that the logic of the iRule was good, we put the rule in place using the actual hours of non-availability which is from 9:00 PM - 3:00 AM, hence the 21 and 2 since we are using the top of the hour as our marker.
At 9:00 PM the rule began functioning as expected redirecting requests to the "unavailable" page. At 3:00 AM requests were being directed to the default page with or without query strings as expected.
Then, at about 8:00 AM, the iRule stopped working as expected and started redirecting requests to the unavailable page.
What seems to have fixed it:
We replaced the 'or' comparison with '&&' and the rule began functioning as expected.
The million dollar questions (or at least $75 on today's market) are these:
1. Is the logic in "if { ($redir_time >= 21) or ($redir_time <= 2) }" flawed and not evaluating the hours 00 - 09 as values of 0-9? and if so, why did it work from 3:00 AM until 8:00 AM?
or
2. Do I need to search the logs on my load balancer to see what happened at 8:00 AM to change the behavior of this iRule?
Any input is appreciated as well as links to information on why my comparison statement is or is not correct.
Thanks in advance,
Kevin
3 Replies
hoolioCirrostratus
Hi Kevin,
I think the issue is that TCL interprets an integer with a leading zero as an octal:
http://phaseit.net/claird/comp.lang.tcl/fmm.htmlzero
ZERO: a numeric representation (examples: "5", "639.42", ...) which begins with '0' (example: "04") is interpreted as octal. It surprises some, when, for example, "expr 09 + 1" is a syntax error. The standard approach to this common situation is such an expression as expr [string trimleft $month 0] + 1
This is one of the two most common threads in common.lang.tcl (commenting is the other). The natives are sometimes grouchy on the subject.
I think the simple fix is to use %k in the clock format string to leave off the leading 0, instead of %H:
http://www.tcl.tk/man/tcl8.4/TclCmd/clock.htm
%k
Hour in 24-hour format, without leading zeros (0 - 23).
You can then use your original OR logic. Here is a quick test demonstrating the issue:when RULE_INIT { log local0. "=========================================================" log local0. "Test OR versus AND without leading 0 in hour" for {set redir_time 0}{$redir_time < 24}{incr redir_time}{ log local0. "\$redir_time: $redir_time" if {($redir_time >= 21) or ($redir_time <= 2)}{ log local0. "$redir_time: OR check was true. Redirect client to unavailable page." } else { log local0. "$redir_time: OR check was false. Allow client to access app." } if {($redir_time >= 21) and ($redir_time <= 2)}{ log local0. "$redir_time: AND check was true. Redirect client to unavailable page." } else { log local0. "$redir_time: AND check was false. Allow client to access app." } } log local0. "---------------------------------------------------------" log local0. "Test OR versus AND with leading 0 in hour" for {set i 0}{$i < 24}{incr i}{ set redir_time [format "%.2d" $i] log local0. "\$redir_time: $redir_time " if {($redir_time >= 21) or ($redir_time <= 2)}{ log local0. "$redir_time: OR check was true. Redirect client to unavailable page." } else { log local0. "$redir_time: OR check was false. Allow client to access app." } if {($redir_time >= 21) and ($redir_time <= 2)}{ log local0. "$redir_time: AND check was true. Redirect client to unavailable page." } else { log local0. "$redir_time: AND check was false. Allow client to access app." } } }
In the log output for the second for loop, note that the OR test is incorrect:
Rule : 08: OR check was true. Redirect client to unavailable page.
Rule : 08: AND check was false. Allow client to access app.
Rule : $redir_time: 09
Rule : 09: OR check was true. Redirect client to unavailable page.
Rule : 09: AND check was false. Allow client to access app.
Aaron- hoolioForgot to add... you can replace this:
if { [HTTP::uri] contains "?" } { HTTP::redirect "http://host.domain/site/default.aspx?[lindex [split [HTTP::uri] "?"] 1]" } else { HTTP::redirect "http://host.domain/site/default.aspx" } }
With this:HTTP::redirect "http://host.domain/site/default.aspx?[HTTP::query]"
The only difference is that if the original request didn't have a query string, the redirect location will have a trailing ?. This shouldn't have any effect on the application though.
Aaron 
Festus_50639Thanks Hoolio.
Once you mentioned the variable being treated as an octal the whole "break at 8:00" made sense. The further proof was watching the log results of the test script you provided.
Cheers,
Kevin
