Jan 30 2017

variable substitution – redux

Back in October last year, I posted a note about the usage of variable substitution in lighttpd’s configuration files. In fact I got that post very slightly wrong (now corrected) in that I showed the test I applied in the file as: “$HTTP[“remoteip”] !~ “12.34.56.78″”. (Note the “!~” when I should have used “!=”). This works, in that it would limit access, but it is subtly wrong because it does not limit access in quite the way I intended. I only noticed this when I later came to change the variable assignment to allow access from three separate IP addresses (on which more later) rather than just one.

The “!~” operator is a perl style regular expression “not” match whilst the “!=” operator is the more strict string not equal match. This matters. My construct using the perl regex not wouldn’t actually just limit access solely to remote address 12.34.56.78 but would also allow in addresses of the form n12n.n34n.n56n.n78n where “n” is any other valid numeral (or none). So for example, my construct would have allowed in connections from 125.134.56.178 or 212.34.156.78 or 121.34.156.78 etc. That is not what I wanted at all.

The (correct) assignment and test now looks like this:

var.IP = “12\.34\.56\.78|23\.45\.67\.78|34\.56\.78\.90”

$HTTP[“remoteip”] !~ var.IP {
$HTTP[“url”] =~ “^/wp-admin/” {
url.access-deny = (“”)
}

Which says, allow connections from address 12.34.56.78 or 23.45.67.89 or 34.56.78.90 but no others.

For reference, the BNF like notation used in the basic configuration for lighty is given on the redmine wiki.

Permanent link to this article: http://baldric.net/2017/01/30/variable-substitution-redux/