Apache Use Stored MD5 as ETAG

get_etag.pl

#!/usr/bin/perl

use strict;

$| = 1; # Turn off I/O buffering

sub sanitize {
    my $fname = $_[0];
    $fname =~ s#[^-_a-zA-Z0-9./]*##g;
    $fname =~ s#/\.\.+/##g;
    $fname =~ s%/+$%%;
    return $fname;
}

while (<STDIN>) {
    local $/ = undef;  # slurp mode. Makes chomp ineffective.

    # Remove whitespace. Input has a newline.
    $_ =~ s/^\s+|\s+$//g;
    # Verify that input appears sane or reject
    if (sanitize($_) ne $_) {
        print "\n";
        next;
    }

    my $fname = $_ . '.md5';
    open(my $fh, $fname) or print "\n" and next;

    binmode $fh;
    my $content = <$fh>;
    close $fh;

    # Remove any extraneous whitespace
    $content =~ s/\s//sg;
    # Verify that content is no longer than an md5 string.
    if (length($content) > 32) {
        print "\n";
        next;
    }

    print $content . "\n";
}

site-snippet.conf

<VirtualHost *:443>
    # ....
    
    # This looks for a static file *.md5 along side the requested
    # file and, if found, returns the .md5 file contents as the etag.
    # On request, the if-none-match value is compared with the
    # value from the .md5 file contents.
    #
    # 1. Does not handle multiple etags in an array.
    # 2. Uses a separator of | in check condition, which is actually
    #    a valid etag character. Should use a control character to
    #    avoid possible conflict.
    # 3. Not tested but likely not compatible with etags that include
    #    embedded newlines.
    # 4. In VirtualHost context, REQUEST_FILENAME is REQUEST_URI.
    #    This likely means must assume completely unsanitized.
    #
    # Reference https://serverfault.com/a/693079/425597 for use
    # of RewriteMap with RewriteCond
    
    RewriteMap getmd5 "prg:/opt/bin/get_etag.pl"
    RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME}.md5 -s
    RewriteRule / - [E=ETAG:"${getmd5:%{DOCUMENT_ROOT}%{REQUEST_FILENAME}}"]
    Header always set ETag %{ETAG}e env=ETAG

    RewriteMap getmd5 "prg:/opt/bin/get_etag.pl"
    RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME}.md5 -s
    RewriteCond "%{HTTP:If-None-Match}|${getmd5:%{DOCUMENT_ROOT}%{REQUEST_FILENAME}}" ^(?:W/)?"([^|]+)"\|\1$
    RewriteRule ^(.*)$ $1 [R=304,L]    
</VirtualHost>