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Mar 31 2010

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webDAV in lighttpd on debian

I back up all my critical files to one of my slugs using rsync over ssh (and just because I am really cautious I back that slug up to another NAS). Most of the files I care about are the obvious photos of friends and family. I guess that most people these days will have large collections of jpeg files on their hard disks whereas previous generations (including myself I might add) would have old shoe boxes filled with photographs.

The old shoe box approach has much to recommend it. Not least the fact that anyone can open that box and browse the photo collection without having to think about user ids or passwords, or having to search for some way of reading the medium holding the photograph. I sometimes worry about how future generations’ lives will be subtly impoverished by the loss of the serendipity of discovery of said old shoe box in the attic. Somehow the idea of the discovery of a box of old DVDs doesn’t feel as if it will give the discoverer the immediate sense of delight which can follow from opening a long forgotten photo album. Old photographs feel “real” to me in a way that digital images will never do. In fact the same problem affects other media these days. I am old enough (and sentimental enough) to have a stash of letters from friends and family past. These days I communicate almost exclusively via email and SMS. And I feel that I have lost some part of me when I lose some old messages in the transfer from one ‘phone to another or from one computing environment to another.

In order to preserve some of the more important photographs in my collection I print them and store them in old fashioned albums. But that still leaves me with a huge number of (often similar) photographs in digital form on my PC’s disk. As I said, I back those up regularly, but my wife pointed out to me that only I really know where those photos are, and moreover, they are on media which are password protected. What happens if I fall under a bus tomorrow? She can’t open the shoebox.

Now given that all the photos are on a networked device, it is trivially easy to give her acccess to those photos from her PC. But I don’t like samba, and NFS feels like overkill when all she wants is read-only access to a directory full of jpegs. The slug is already running a web server and her gnome desktop conveniently offers the capability to connect to a remote server over a variety of different protocols, including the rather simple option of WebDAV. So all I had to do was configure lighty on the slug to give her that access.

Here’s how:-

If not already installed, then run aptitude to install “lighttpd-mod-webdav”. If you want to use basic authenticated access then it may also be useful to install “apache2-utils” which will give you the apache htpasswd utility. The htpasswd authentication function uses unix crypt and is pretty weak, but we are really only concerned here with limiting browsing of the directory to local users on the local network, and if someone has access to my htpasswd file then I’ve got bigger problems than just worrying about them browsing my photos. There are other authentication mechanisms we can use in lighty if we really care – although I would argue that if you really want to protect access to a network resource you shouldn’t be providing web access in the first place.

To enable the webdav and auth modules, you can simply run “lighty-enable-mod webdav” and “lighty-enable-mod auth” (which actually just create symlinks from the relevant files in /etc/lighttpd/conf-available to /etc/lighttpd/conf-enabled directories) or you can activate the modules directly in /etc/lighttpd/lighttpd.conf or the appropriate virtual-host configuration file with the directives:

server.modules += ( “mod_auth” )
server.modules += ( “mod_webdav” )

lighty is pretty flexible and doesn’t really care where you activate the modules. The advantage of using the “lighty-enable-mod” approach however, is that it allows you to quickly change by running “lighty-disable-mod whatever” at a future date. The symlink will then be removed and so long as you remember to restart lighty, the module activation will cease.

Now to enable the webdav access to the directory in question we need to configure the virtual host along the following lines:

$HTTP["host"] == “slug” {
# turn off directory listing (assuming that it is on by default elsewhere)
server.dir-listing = “disable”

# turn on webdav on the directory we wish to share
$HTTP["url"] =~ “^/photos($|/)” {
webdav.activate = “enable”
webdav.is-readonly = “enable”
webdav.sqlite-db-name = “/var/run/lighttpd/lighttpd.webdav_lock.db”
auth.backend = “htpasswd”
auth.backend.htpasswd.userfile = “/etc/lighttpd/htpasswd”
auth.require = ( “” => ( “method” => “basic”,
“realm” => “photos”,
“require” => “valid-user” ) )
}

}

Note that the above configuration will allow any named user listed in the file /etc/lighttpd/htpasswd read only access to the “/photos” directory on the virtual host called “slug” if they can sucessfully authenticate. Note also, however, that because directory listing is turned off, it will not be possible for that user to access this directory with a web browser (which would be possible if listing were allowed). Happily however, the gnome desktop (“Connect to server” mechanism) will still permit authenticated access and once connected will provide full read only access to all files and subdirectories of “/photos” in the file browser (which is what we want). The “auth.backend” directive tells lighty that we are using the apache htpasswd authentication mechanism and the file can be found at the location specified (make sure this is outside webspace). the “auth.require” directives specify the authentication method (bear in mind that “basic” is clear text, albeit base64 encoded, so authenticatiion credentials can be trivially sniffed off the network); the “realm” is a string which will be displayed in the dialogue box presented to the user (though it has additional functions if digest authentication is used); “require” specifies which authenticated users are allowed access. This can be useful if you have a single htpasswd file for multiple virtual hosts (or directories) and you wish to limit access to certain users.

Passing authentication credentials in clear over a hostile network is not smart, so I would not expose this sort of configuration to the wider internet. However, the webDAV protocol supports ssl encryption so it would be easy to secure the above configuration by changing the setup to use lighty configured for ssl. I choose not to here because of the overhead that would impose on the slug when passing large photographic images – and I trust my home network……

Now given that we have specified a password file called “htpasswd” we need to create that file in the “/etc/lighttpd” configuration directory thusly:

httpasswd -cm /etc/lighttpd/htpasswd user

The -c switch to htpasswd creates the file if it does not already exist. When adding any subsequent users you should omit this switch or the file will be recreated (and thus overwritten). The “user” name adds the named user to the file. You will be prompted for the password for that user after running the command. As I mentioned above, by default the password will only be encrypted by crypt, the -m switch used forces htpasswd to use (the stronger) MD5 hash. Note that htpasswd also gives the option of SHA (-s switch) but this is not supported by lighty in htpasswd authentication. Make sure that the passwd file is owned by root and is not writeable by non root users (“chown root:root htpasswd; chmod 644 htpasswd” if necessary).

The above configuration also assumes that the files you wish to share are actually in the directory called “photos” in the web root of the virtual host called “slug”. In my case this is not true because I have all my backup files outside the webspace on the slug (for fairly obvious reasons). In order to share the files we simply need to provide a symlink to the relevant directory outside webspace.

Now whenever the backup is updated, my wife has access to the latest copy of all the photos in the shoebox. But let’s hope I don’t fall under a bus just yet.

Permanent link to this article: http://baldric.net/2010/03/31/webdav-in-lighttpd-on-debian/