Apache Rewrite Tricks

It is possible to use Apache’s Rewrite Module features to perform a number of useful tricks that can make people’s lives much easier.

Short URLs

Making short URLs for common destinations can simplify making printed media as well as shortening or simplifying what people need to type. These are also easy to add and require minimal maintenance, and generally can be implemented with a single line addition to your eg_vhost.conf file.

# My Account - http://host.ext/myaccount -> My Account Page
RewriteRule ^/myaccount https://%{HTTP_HOST}/eg/opac/myopac/main [R]

# ISBN Search - http://host.ext/search/isbn/<ISBN NUMBER> -> Search Page
RewriteRule ^/search/isbn/(.*) /eg/opac/results?_special=1&qtype=identifier|isbn&query=$1 [R]

Domain Based Content with RewriteMaps

One creative use of Rewrite features is domain-based configuration in a single eg_vhost.conf file. Regardless of how many VirtualHost blocks use the configuration you don’t need to duplicate things for minor changes, and can in fact use wildcard VirtualHost blocks to serve multiple subdomains.

For the wildcard blocks you will want to use a ServerAlias directive, and for SSL VirtualHost blocks ensure you have a wildcard SSL certificate.

ServerAlias *.example.com

For actually changing things based on the domain, or subdomain, you can use RewriteMaps. Each RewriteMap is generally a lookup table of some kind. In the following examples we will generally use text files, though database lookups and external programs are also possible.

Note that in the examples below we generally store things in Environment Variables. From within Template Toolkit templates you can access environment variables with the ENV object.

Template Toolkit ENV example, link library name/url if set
[% IF ENV.eglibname && ENV.egliburl %]<a href="[% ENV.egliburl %]">[% ENV.eglibname %]</a>[% END %]

The first lookup to do is a domain to identifier, allowing us to re-use identifiers for multiple domains. In addition we can also supply a default identifier, for when the domain isn’t present in the lookup table.

Apache Config
# This internal map allows us to lowercase our hostname, removing case issues in our lookup table
# If you prefer uppercase you can use "uppercase int:toupper" instead.
RewriteMap lowercase int:tolower
# This provides a hostname lookup
RewriteMap eglibid txt:/openils/conf/libid.txt
# This stores the identifier in a variable (eglibid) for later use
# In this case CONS is the default value for when the lookup table has no entry
RewriteRule . - [E=eglibid:${eglibid:${lowercase:%{HTTP_HOST}}|CONS}]
Contents of libid.txt File
# Comments can be included
# Multiple hostnames for Branch 1
branch1.example.com BRANCH1
branch1.example.net BRANCH1
# Branches 2 and 3 don't have alternate hostnames
branch2.example.com BRANCH2
branch3.example.com BRANCH3

Once we have identifiers we can look up other information, when appropriate. For example, say we want to look up library names and URLs:

Apache Config
# Library Name Lookup - Note we provide no default in this case.
RewriteMap eglibname txt:/openils/conf/libname.txt
RewriteRule . - [E=eglibname:${eglibname:%{ENV:eglibid}}]
# Library URL Lookup - Also with no default.
RewriteMap egliburl txt:/openils/conf/liburl.txt
RewriteRule . - [E=egliburl:${egliburl:%{ENV:eglibid}}]
Contents of libname.txt File
# Note that we cannot have spaces in the "value", so instead &#32; is used. &nbsp; is also an option.
BRANCH1 Branch&#32;One
BRANCH2 Branch&#32;Two
BRANCH3 Branch&#32;Three
CONS Example&#32;Consortium&#32;Name
Contents of liburl.txt File
BRANCH1 http://branch1.example.org
BRANCH2 http://branch2.example.org
BRANCH3 http://branch3.example.org
CONS http://example.org

Or, perhaps set the "physical location" variable for default search/display library:

Apache Config
# Lookup "physical location" IDs
RewriteMap eglibphysloc txt:/openils/conf/libphysloc.txt
# Note: physical_loc is a variable used in the TTOPAC and should not be re-named
RewriteRule . - [E=physical_loc:${eglibphysloc:%{ENV:eglibid}}]
Contents of libphysloc.txt File

Going further, you could also replace files to be downloaded, such as images or stylesheets, on the fly:

Apache Config
# Check if a file exists based on eglibid and the requested file name
# Say, BRANCH1/opac/images/main_logo.png
RewriteCond %{DOCUMENT_ROOT}/%{ENV:eglibid}%{REQUEST_URI} -f
# Serve up the eglibid version of the file instead
RewriteRule (.*) /%{ENG:eglibid}$1

Note that template files themselves cannot be replaced in that manner.