We want to create a homogeneous and consistent URL layout across all WWW servers on an Intranet web cluster, i.e., all URLs (by definition server-local and thus server-dependent!) become server independent! What we want is to give the WWW namespace a single consistent layout: no URL should refer to any particular target server. The cluster itself should connect users automatically to a physical target host as needed, invisibly.

First, the knowledge of the target servers comes from (distributed) external maps which contain information on where our users, groups, and entities reside. They have the form:

user1  server_of_user1
user2  server_of_user2
:      :

We put them into files map.xxx-to-host. Second we need to instruct all servers to redirect URLs of the forms:

/u/user/anypath
/g/group/anypath
/e/entity/anypath

to

http://physical-host/u/user/anypath
http://physical-host/g/group/anypath
http://physical-host/e/entity/anypath

when any URL path need not be valid on every server. The following ruleset does this for us with the help of the map files (assuming that server0 is a default server which will be used if a user has no entry in the map):

RewriteEngine on

RewriteMap      user-to-host   txt:/path/to/map.user-to-host
RewriteMap     group-to-host   txt:/path/to/map.group-to-host
RewriteMap    entity-to-host   txt:/path/to/map.entity-to-host

RewriteRule   ^/u/([^/]+)/?(.*)   http://${user-to-host:$1|server0}/u/$1/$2
RewriteRule   ^/g/([^/]+)/?(.*)  http://${group-to-host:$1|server0}/g/$1/$2
RewriteRule   ^/e/([^/]+)/?(.*) http://${entity-to-host:$1|server0}/e/$1/$2

RewriteRule   ^/([uge])/([^/]+)/?$          /$1/$2/.www/
RewriteRule   ^/([uge])/([^/]+)/([^.]+.+)   /$1/$2/.www/$3\

Some sites with thousands of users use a structured homedir layout, i.e. each homedir is in a subdirectory which begins (for instance) with the first character of the username. So, /~foo/anypath is /home/f/foo/.www/anypath while /~bar/anypath is /home/b/bar/.www/anypath.

We use the following ruleset to expand the tilde URLs into the above layout.

RewriteEngine on
RewriteRule   ^/~(([a-z])[a-z0-9]+)(.*)  /home/$2/$1/.www$3

This really is a hardcore example: a killer application which heavily uses per-directory RewriteRules to get a smooth look and feel on the Web while its data structure is never touched or adjusted. Background: net.sw is my archive of freely available Unix software packages, which I started to collect in 1992. It is both my hobby and job to do this, because while I'm studying computer science I have also worked for many years as a system and network administrator in my spare time. Every week I need some sort of software so I created a deep hierarchy of directories where I stored the packages:

drwxrwxr-x   2 netsw  users    512 Aug  3 18:39 Audio/
drwxrwxr-x   2 netsw  users    512 Jul  9 14:37 Benchmark/
drwxrwxr-x  12 netsw  users    512 Jul  9 00:34 Crypto/
drwxrwxr-x   5 netsw  users    512 Jul  9 00:41 Database/
drwxrwxr-x   4 netsw  users    512 Jul 30 19:25 Dicts/
drwxrwxr-x  10 netsw  users    512 Jul  9 01:54 Graphic/
drwxrwxr-x   5 netsw  users    512 Jul  9 01:58 Hackers/
drwxrwxr-x   8 netsw  users    512 Jul  9 03:19 InfoSys/
drwxrwxr-x   3 netsw  users    512 Jul  9 03:21 Math/
drwxrwxr-x   3 netsw  users    512 Jul  9 03:24 Misc/
drwxrwxr-x   9 netsw  users    512 Aug  1 16:33 Network/
drwxrwxr-x   2 netsw  users    512 Jul  9 05:53 Office/
drwxrwxr-x   7 netsw  users    512 Jul  9 09:24 SoftEng/
drwxrwxr-x   7 netsw  users    512 Jul  9 12:17 System/
drwxrwxr-x  12 netsw  users    512 Aug  3 20:15 Typesetting/
drwxrwxr-x  10 netsw  users    512 Jul  9 14:08 X11/

In July 1996 I decided to make this archive public to the world via a nice Web interface. "Nice" means that I wanted to offer an interface where you can browse directly through the archive hierarchy. And "nice" means that I didn't want to change anything inside this hierarchy - not even by putting some CGI scripts at the top of it. Why? Because the above structure should later be accessible via FTP as well, and I didn't want any Web or CGI stuff mixed in there.

The solution has two parts: The first is a set of CGI scripts which create all the pages at all directory levels on-the-fly. I put them under /e/netsw/.www/ as follows:

-rw-r--r--   1 netsw  users    1318 Aug  1 18:10 .wwwacl
drwxr-xr-x  18 netsw  users     512 Aug  5 15:51 DATA/
-rw-rw-rw-   1 netsw  users  372982 Aug  5 16:35 LOGFILE
-rw-r--r--   1 netsw  users     659 Aug  4 09:27 TODO
-rw-r--r--   1 netsw  users    5697 Aug  1 18:01 netsw-about.html
-rwxr-xr-x   1 netsw  users     579 Aug  2 10:33 netsw-access.pl
-rwxr-xr-x   1 netsw  users    1532 Aug  1 17:35 netsw-changes.cgi
-rwxr-xr-x   1 netsw  users    2866 Aug  5 14:49 netsw-home.cgi
drwxr-xr-x   2 netsw  users     512 Jul  8 23:47 netsw-img/
-rwxr-xr-x   1 netsw  users   24050 Aug  5 15:49 netsw-lsdir.cgi
-rwxr-xr-x   1 netsw  users    1589 Aug  3 18:43 netsw-search.cgi
-rwxr-xr-x   1 netsw  users    1885 Aug  1 17:41 netsw-tree.cgi
-rw-r--r--   1 netsw  users     234 Jul 30 16:35 netsw-unlimit.lst

The DATA/ subdirectory holds the above directory structure, i.e. the real net.sw stuff, and gets automatically updated via rdist from time to time. The second part of the problem remains: how to link these two structures together into one smooth-looking URL tree? We want to hide the DATA/ directory from the user while running the appropriate CGI scripts for the various URLs. Here is the solution: first I put the following into the per-directory configuration file in the DocumentRoot of the server to rewrite the public URL path /net.sw/ to the internal path /e/netsw:

RewriteRule  ^net.sw$       net.sw/        [R]
RewriteRule  ^net.sw/(.*)$  e/netsw/$1

The first rule is for requests which miss the trailing slash! The second rule does the real thing. And then comes the killer configuration which stays in the per-directory config file /e/netsw/.www/.wwwacl:

Options       ExecCGI FollowSymLinks Includes MultiViews

RewriteEngine on

#  we are reached via /net.sw/ prefix
RewriteBase   /net.sw/

#  first we rewrite the root dir to
#  the handling cgi script
RewriteRule   ^$                       netsw-home.cgi     [L]
RewriteRule   ^index\.html$            netsw-home.cgi     [L]

#  strip out the subdirs when
#  the browser requests us from perdir pages
RewriteRule   ^.+/(netsw-[^/]+/.+)$    $1                 [L]

#  and now break the rewriting for local files
RewriteRule   ^netsw-home\.cgi.*       -                  [L]
RewriteRule   ^netsw-changes\.cgi.*    -                  [L]
RewriteRule   ^netsw-search\.cgi.*     -                  [L]
RewriteRule   ^netsw-tree\.cgi$        -                  [L]
RewriteRule   ^netsw-about\.html$      -                  [L]
RewriteRule   ^netsw-img/.*$           -                  [L]

#  anything else is a subdir which gets handled
#  by another cgi script
RewriteRule   !^netsw-lsdir\.cgi.*     -                  [C]
RewriteRule   (.*)                     netsw-lsdir.cgi/$1

Some hints for interpretation:

1. Notice the L (last) flag and no substitution field ('-') in the fourth part
2. Notice the ! (not) character and the C (chain) flag at the first rule in the last part
3. Notice the catch-all pattern in the last rule

A typical FAQ about URL rewriting is how to redirect failing requests on webserver A to webserver B. Usually this is done via ErrorDocument CGI scripts in Perl, but there is also a mod_rewrite solution. But note that this performs more poorly than using an ErrorDocument CGI script!

The first solution has the best performance but less flexibility, and is less safe:

RewriteEngine on
RewriteCond   /your/docroot/%{REQUEST_FILENAME} !-f
RewriteRule   ^(.+)                             http://webserverB.dom/$1

The problem here is that this will only work for pages inside the DocumentRoot. While you can add more Conditions (for instance to also handle homedirs, etc.) there is a better variant:

RewriteEngine on
RewriteCond   %{REQUEST_URI} !-U
RewriteRule   ^(.+)          http://webserverB.dom/$1

This uses the URL look-ahead feature of mod_rewrite. The result is that this will work for all types of URLs and is safe. But it does have a performance impact on the web server, because for every request there is one more internal subrequest. So, if your web server runs on a powerful CPU, use this one. If it is a slow machine, use the first approach or better an ErrorDocument CGI script.

Do you know the great CPAN (Comprehensive Perl Archive Network) under http://www.perl.com/CPAN? CPAN automatically redirects browsers to one of many FTP servers around the world (generally one near the requesting client); each server carries a full CPAN mirror. This is effectively an FTP access multiplexing service. CPAN runs via CGI scripts, but how could a similar approach be implemented via mod_rewrite?

First we notice that as of version 3.0.0, mod_rewrite can also use the "ftp:" scheme on redirects. And second, the location approximation can be done by a RewriteMap over the top-level domain of the client. With a tricky chained ruleset we can use this top-level domain as a key to our multiplexing map.

RewriteEngine on
RewriteMap    multiplex                txt:/path/to/map.cxan
RewriteRule   ^/CxAN/(.*)              %{REMOTE_HOST}::$1                 [C]
RewriteRule   ^.+\.([a-zA-Z]+)::(.*)$  ${multiplex:$1|ftp.default.dom}$2  [R,L]

##
##  map.cxan -- Multiplexing Map for CxAN
##

de        ftp://ftp.cxan.de/CxAN/
uk        ftp://ftp.cxan.uk/CxAN/
com       ftp://ftp.cxan.com/CxAN/
 :
##EOF##

At least for important top-level pages it is sometimes necessary to provide the optimum of browser dependent content, i.e., one has to provide one version for current browsers, a different version for the Lynx and text-mode browsers, and another for other browsers.

We cannot use content negotiation because the browsers do not provide their type in that form. Instead we have to act on the HTTP header "User-Agent". The following config does the following: If the HTTP header "User-Agent" begins with "Mozilla/3", the page foo.html is rewritten to foo.NS.html and the rewriting stops. If the browser is "Lynx" or "Mozilla" of version 1 or 2, the URL becomes foo.20.html. All other browsers receive page foo.32.html. This is done with the following ruleset:

RewriteCond %{HTTP_USER_AGENT}  ^Mozilla/3.*
RewriteRule ^foo\.html$         foo.NS.html          [L]

RewriteCond %{HTTP_USER_AGENT}  ^Lynx/.*         [OR]
RewriteCond %{HTTP_USER_AGENT}  ^Mozilla/[12].*
RewriteRule ^foo\.html$         foo.20.html          [L]

RewriteRule ^foo\.html$         foo.32.html          [L]

Assume there are nice web pages on remote hosts we want to bring into our namespace. For FTP servers we would use the mirror program which actually maintains an explicit up-to-date copy of the remote data on the local machine. For a web server we could use the program webcopy which runs via HTTP. But both techniques have a major drawback: The local copy is always only as up-to-date as the last time we ran the program. It would be much better if the mirror was not a static one we have to establish explicitly. Instead we want a dynamic mirror with data which gets updated automatically as needed on the remote host(s).

To provide this feature we map the remote web page or even the complete remote web area to our namespace by the use of the Proxy Throughput feature (flag [P]):

RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^hotsheet/(.*)$  http://www.tstimpreso.com/hotsheet/$1  [P]

RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^usa-news\.html$   http://www.quux-corp.com/news/index.html  [P]

This is a tricky way of virtually running a corporate (external) Internet web server (www.quux-corp.dom), while actually keeping and maintaining its data on an (internal) Intranet web server (www2.quux-corp.dom) which is protected by a firewall. The trick is that the external web server retrieves the requested data on-the-fly from the internal one.

First, we must make sure that our firewall still protects the internal web server and only the external web server is allowed to retrieve data from it. On a packet-filtering firewall, for instance, we could configure a firewall ruleset like the following:

ALLOW Host www.quux-corp.dom Port >1024 --> Host www2.quux-corp.dom Port 80
DENY  Host *                 Port *     --> Host www2.quux-corp.dom Port 80

Just adjust it to your actual configuration syntax. Now we can establish the mod_rewrite rules which request the missing data in the background through the proxy throughput feature:

RewriteRule ^/~([^/]+)/?(.*)          /home/$1/.www/$2
RewriteCond %{REQUEST_FILENAME}       !-f
RewriteCond %{REQUEST_FILENAME}       !-d
RewriteRule ^/home/([^/]+)/.www/?(.*) http://www2.quux-corp.dom/~$1/pub/$2 [P]

Suppose we want to load balance the traffic to www.foo.com over www[0-5].foo.com (a total of 6 servers). How can this be done?

There are many possible solutions for this problem. We will first discuss a common DNS-based method, and then one based on mod_rewrite:

1. DNS Round-Robin

   The simplest method for load-balancing is to use DNS round-robin. Here you just configure www[0-9].foo.com as usual in your DNS with A (address) records, e.g.,

   www0   IN  A       1.2.3.1
   www1   IN  A       1.2.3.2
   www2   IN  A       1.2.3.3
   www3   IN  A       1.2.3.4
   www4   IN  A       1.2.3.5
   www5   IN  A       1.2.3.6
   

   Then you additionally add the following entries:

   www   IN  A       1.2.3.1
   www   IN  A       1.2.3.2
   www   IN  A       1.2.3.3
   www   IN  A       1.2.3.4
   www   IN  A       1.2.3.5
   

   Now when www.foo.com gets resolved, BIND gives out www0-www5 - but in a permutated (rotated) order every time. This way the clients are spread over the various servers. But notice that this is not a perfect load balancing scheme, because DNS resolutions are cached by clients and other nameservers, so once a client has resolved www.foo.com to a particular wwwN.foo.com, all its subsequent requests will continue to go to the same IP (and thus a single server), rather than being distributed across the other available servers. But the overall result is okay because the requests are collectively spread over the various web servers.

2. DNS Load-Balancing

   A sophisticated DNS-based method for load-balancing is to use the program lbnamed which can be found at http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html. It is a Perl 5 program which, in conjunction with auxilliary tools, provides real load-balancing via DNS.

3. Proxy Throughput Round-Robin

   In this variant we use mod_rewrite and its proxy throughput feature. First we dedicate www0.foo.com to be actually www.foo.com by using a single

   www    IN  CNAME   www0.foo.com.
   

   entry in the DNS. Then we convert www0.foo.com to a proxy-only server, i.e., we configure this machine so all arriving URLs are simply passed through its internal proxy to one of the 5 other servers (www1-www5). To accomplish this we first establish a ruleset which contacts a load balancing script lb.pl for all URLs.

   RewriteEngine on
   RewriteMap    lb      prg:/path/to/lb.pl
   RewriteRule   ^/(.+)$ ${lb:$1}           [P,L]
   

   Then we write lb.pl:

   #!/path/to/perl
   ##
   ##  lb.pl -- load balancing script
   ##
   
   $| = 1;
   
   $name   = "www";     # the hostname base
   $first  = 1;         # the first server (not 0 here, because 0 is myself)
   $last   = 5;         # the last server in the round-robin
   $domain = "foo.dom"; # the domainname
   
   $cnt = 0;
   while (<STDIN>) {
       $cnt = (($cnt+1) % ($last+1-$first));
       $server = sprintf("%s%d.%s", $name, $cnt+$first, $domain);
       print "http://$server/$_";
   }
   
   ##EOF##
   

   A last notice: Why is this useful? Seems like www0.foo.com still is overloaded? The answer is yes, it is overloaded, but with plain proxy throughput requests, only! All SSI, CGI, ePerl, etc. processing is handled done on the other machines. For a complicated site, this may work well. The biggest risk here is that www0 is now a single point of failure -- if it crashes, the other servers are inaccessible.
4. Dedicated Load Balancers

   There are more sophisticated solutions, as well. Cisco, F5, and several other companies sell hardware load balancers (typically used in pairs for redundancy), which offer sophisticated load balancing and auto-failover features. There are software packages which offer similar features on commodity hardware, as well. If you have enough money or need, check these out. The lb-l mailing list is a good place to research.

On the net there are many nifty CGI programs. But their usage is usually boring, so a lot of webmasters don't use them. Even Apache's Action handler feature for MIME-types is only appropriate when the CGI programs don't need special URLs (actually PATH_INFO and QUERY_STRINGS) as their input. First, let us configure a new file type with extension .scgi (for secure CGI) which will be processed by the popular cgiwrap program. The problem here is that for instance if we use a Homogeneous URL Layout (see above) a file inside the user homedirs might have a URL like /u/user/foo/bar.scgi, but cgiwrap needs URLs in the form /~user/foo/bar.scgi/. The following rule solves the problem:

RewriteRule ^/[uge]/([^/]+)/\.www/(.+)\.scgi(.*) ...
... /internal/cgi/user/cgiwrap/~$1/$2.scgi$3  [NS,T=application/x-http-cgi]

Or assume we have some more nifty programs: wwwlog (which displays the access.log for a URL subtree) and wwwidx (which runs Glimpse on a URL subtree). We have to provide the URL area to these programs so they know which area they are really working with. But usually this is complicated, because they may still be requested by the alternate URL form, i.e., typically we would run the swwidx program from within /u/user/foo/ via hyperlink to

/internal/cgi/user/swwidx?i=/u/user/foo/

which is ugly, because we have to hard-code both the location of the area and the location of the CGI inside the hyperlink. When we have to reorganize, we spend a lot of time changing the various hyperlinks.

The solution here is to provide a special new URL format which automatically leads to the proper CGI invocation. We configure the following:

RewriteRule   ^/([uge])/([^/]+)(/?.*)/\*  /internal/cgi/user/wwwidx?i=/$1/$2$3/
RewriteRule   ^/([uge])/([^/]+)(/?.*):log /internal/cgi/user/wwwlog?f=/$1/$2$3

Now the hyperlink to search at /u/user/foo/ reads only

HREF="*"

which internally gets automatically transformed to

/internal/cgi/user/wwwidx?i=/u/user/foo/

The same approach leads to an invocation for the access log CGI program when the hyperlink :log gets used.

Here comes a really esoteric feature: Dynamically generated but statically served pages, i.e., pages should be delivered as pure static pages (read from the filesystem and just passed through), but they have to be generated dynamically by the web server if missing. This way you can have CGI-generated pages which are statically served unless an admin (or a cron job) removes the static contents. Then the contents gets refreshed.

RewriteCond %{REQUEST_FILENAME}   !-s
RewriteRule ^page\.html$          page.cgi   [T=application/x-httpd-cgi,L]

Here a request for page.html leads to an internal run of a corresponding page.cgi if page.html is missing or has filesize null. The trick here is that page.cgi is a CGI script which (additionally to its STDOUT) writes its output to the file page.html. Once it has completed, the server sends out page.html. When the webmaster wants to force a refresh of the contents, he just removes page.html (typically from cron).

Wouldn't it be nice, while creating a complex web page, if the web browser would automatically refresh the page every time we save a new version from within our editor? Impossible?

No! We just combine the MIME multipart feature, the web server NPH feature, and the URL manipulation power of mod_rewrite. First, we establish a new URL feature: Adding just :refresh to any URL causes the 'page' to be refreshed every time it is updated on the filesystem.

RewriteRule   ^(/[uge]/[^/]+/?.*):refresh  /internal/cgi/apache/nph-refresh?f=$1

Now when we reference the URL

/u/foo/bar/page.html:refresh

this leads to the internal invocation of the URL

/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html

The only missing part is the NPH-CGI script. Although one would usually say "left as an exercise to the reader" ;-) I will provide this, too.

#!/sw/bin/perl
##
##  nph-refresh -- NPH/CGI script for auto refreshing pages
##  Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;

#   split the QUERY_STRING variable
@pairs = split(/&/, $ENV{'QUERY_STRING'});
foreach $pair (@pairs) {
    ($name, $value) = split(/=/, $pair);
    $name =~ tr/A-Z/a-z/;
    $name = 'QS_' . $name;
    $value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
    eval "\$$name = \"$value\"";
}
$QS_s = 1 if ($QS_s eq '');
$QS_n = 3600 if ($QS_n eq '');
if ($QS_f eq '') {
    print "HTTP/1.0 200 OK\n";
    print "Content-type: text/html\n\n";
    print "<b>ERROR</b>: No file given\n";
    exit(0);
}
if (! -f $QS_f) {
    print "HTTP/1.0 200 OK\n";
    print "Content-type: text/html\n\n";
    print "<b>ERROR</b>: File $QS_f not found\n";
    exit(0);
}

sub print_http_headers_multipart_begin {
    print "HTTP/1.0 200 OK\n";
    $bound = "ThisRandomString12345";
    print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
    &print_http_headers_multipart_next;
}

sub print_http_headers_multipart_next {
    print "\n--$bound\n";
}

sub print_http_headers_multipart_end {
    print "\n--$bound--\n";
}

sub displayhtml {
    local($buffer) = @_;
    $len = length($buffer);
    print "Content-type: text/html\n";
    print "Content-length: $len\n\n";
    print $buffer;
}

sub readfile {
    local($file) = @_;
    local(*FP, $size, $buffer, $bytes);
    ($x, $x, $x, $x, $x, $x, $x, $size) = stat($file);
    $size = sprintf("%d", $size);
    open(FP, "<$file");
    $bytes = sysread(FP, $buffer, $size);
    close(FP);
    return $buffer;
}

$buffer = &readfile($QS_f);
&print_http_headers_multipart_begin;
&displayhtml($buffer);

sub mystat {
    local($file) = $_[0];
    local($time);

    ($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
    return $mtime;
}

$mtimeL = &mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n < $QS_n; $n++) {
    while (1) {
        $mtime = &mystat($QS_f);
        if ($mtime ne $mtimeL) {
            $mtimeL = $mtime;
            sleep(2);
            $buffer = &readfile($QS_f);
            &print_http_headers_multipart_next;
            &displayhtml($buffer);
            sleep(5);
            $mtimeL = &mystat($QS_f);
            last;
        }
        sleep($QS_s);
    }
}

&print_http_headers_multipart_end;

exit(0);

##EOF##

The VirtualHost feature of Apache is nice and works great when you just have a few dozen virtual hosts. But when you are an ISP and have hundreds of virtual hosts, this feature is suboptimal.

To provide this feature we map the remote web page or even the complete remote web area to our namespace using the Proxy Throughput feature (flag [P]):

##
##  vhost.map
##
www.vhost1.dom:80  /path/to/docroot/vhost1
www.vhost2.dom:80  /path/to/docroot/vhost2
     :
www.vhostN.dom:80  /path/to/docroot/vhostN

##
##  httpd.conf
##
    :
#   use the canonical hostname on redirects, etc.
UseCanonicalName on

    :
#   add the virtual host in front of the CLF-format
CustomLog  /path/to/access_log  "%{VHOST}e %h %l %u %t \"%r\" %>s %b"
    :

#   enable the rewriting engine in the main server
RewriteEngine on

#   define two maps: one for fixing the URL and one which defines
#   the available virtual hosts with their corresponding
#   DocumentRoot.
RewriteMap    lowercase    int:tolower
RewriteMap    vhost        txt:/path/to/vhost.map

#   Now do the actual virtual host mapping
#   via a huge and complicated single rule:
#
#   1. make sure we don't map for common locations
RewriteCond   %{REQUEST_URI}  !^/commonurl1/.*
RewriteCond   %{REQUEST_URI}  !^/commonurl2/.*
    :
RewriteCond   %{REQUEST_URI}  !^/commonurlN/.*
#
#   2. make sure we have a Host header, because
#      currently our approach only supports
#      virtual hosting through this header
RewriteCond   %{HTTP_HOST}  !^$
#
#   3. lowercase the hostname
RewriteCond   ${lowercase:%{HTTP_HOST}|NONE}  ^(.+)$
#
#   4. lookup this hostname in vhost.map and
#      remember it only when it is a path
#      (and not "NONE" from above)
RewriteCond   ${vhost:%1}  ^(/.*)$
#
#   5. finally we can map the URL to its docroot location
#      and remember the virtual host for logging purposes
RewriteRule   ^/(.*)$   %1/$1  [E=VHOST:${lowercase:%{HTTP_HOST}}]
    :

How can we forbid a list of externally configured hosts from using our server?

For Apache >= 1.3b6:

RewriteEngine on
RewriteMap    hosts-deny  txt:/path/to/hosts.deny
RewriteCond   ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND} !=NOT-FOUND [OR]
RewriteCond   ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule   ^/.*  -  [F]

For Apache <= 1.3b6:

RewriteEngine on
RewriteMap    hosts-deny  txt:/path/to/hosts.deny
RewriteRule   ^/(.*)$ ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND}/$1
RewriteRule   !^NOT-FOUND/.* - [F]
RewriteRule   ^NOT-FOUND/(.*)$ ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND}/$1
RewriteRule   !^NOT-FOUND/.* - [F]
RewriteRule   ^NOT-FOUND/(.*)$ /$1

##
##  hosts.deny
##
##  ATTENTION! This is a map, not a list, even when we treat it as such.
##             mod_rewrite parses it for key/value pairs, so at least a
##             dummy value "-" must be present for each entry.
##

193.102.180.41 -
bsdti1.sdm.de  -
192.76.162.40  -

How can we forbid a certain host or even a user of a special host from using the Apache proxy?

We first have to make sure mod_rewrite is below(!) mod_proxy in the Configuration file when compiling the Apache web server. This way it gets called before mod_proxy. Then we configure the following for a host-dependent deny...

RewriteCond %{REMOTE_HOST} ^badhost\.mydomain\.com$
RewriteRule !^http://[^/.]\.mydomain.com.*  - [F]

...and this one for a user@host-dependent deny:

RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST}  ^badguy@badhost\.mydomain\.com$
RewriteRule !^http://[^/.]\.mydomain.com.*  - [F]

Sometimes very special authentication is needed, for instance authentication which checks for a set of explicitly configured users. Only these should receive access and without explicit prompting (which would occur when using Basic Auth via mod_auth_basic).

We use a list of rewrite conditions to exclude all except our friends:

RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} !^friend1@client1.quux-corp\.com$
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} !^friend2@client2.quux-corp\.com$
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} !^friend3@client3.quux-corp\.com$
RewriteRule ^/~quux/only-for-friends/      -                                 [F]

How can we program a flexible URL Deflector which acts on the "Referer" HTTP header and can be configured with as many referring pages as we like?

Use the following really tricky ruleset...

RewriteMap  deflector txt:/path/to/deflector.map

RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}} ^-$
RewriteRule ^.* %{HTTP_REFERER} [R,L]

RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^.* ${deflector:%{HTTP_REFERER}} [R,L]

... in conjunction with a corresponding rewrite map:

##
##  deflector.map
##

http://www.badguys.com/bad/index.html    -
http://www.badguys.com/bad/index2.html   -
http://www.badguys.com/bad/index3.html   http://somewhere.com/

This automatically redirects the request back to the referring page (when "-" is used as the value in the map) or to a specific URL (when an URL is specified in the map as the second argument).