Wu-ftpd Upload Configuration HOWTO

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$Id: upload.configuration.HOWTO,v 1.2 2000/07/01 18:49:32 wuftpd Exp $

                           Upload Configuration
                                  HOW-TO

This document is available on-line at:
ftp://ftp.wu-ftpd.org/pub/wu-ftpd/upload.configuration.HOWTO

One of the more powerfull, yet most often misused, features of WU-FTPD is
the upload clause. Historically, the problems with the upload clause stem
from unclear documentation and poor implementation. This document is an
attempt to address these issues. The features discussed in this document
apply to WU-FTPD Version 2.6.0. If you are not running 2.6.0, you are
strongly encouraged to upgrade; it includes a number of corrections, new
features and security enhancements not available with earlier versions of
WU-FTPD.

Upload restrictions for anonymous FTP users
-------------------------------------------
For this example, we'll assume your system /etc/passwd file contains an
entry for the anonymous FTP user as follows:

ftp:*:95:95::/home/ftp:

If your /etc/passwd file does not contain an entry for the user 'ftp' your
site will not allow anonymous FTP. In addition, if the usernames 'ftp' or
'anonymous' appear in the /etc/ftpusers file, anonymous FTP will not be
allowed.

In /etc/ftpaccess, we need a class which allows anonymous access. The
following allows anonymous FTP from anywhere:

class anonftp anonymous *

To prevent anonymous FTP users attempting a Denial of Service (DoS) attack
against your system, you should create a special filesystem to receive
their uploads. This separate filesystem protects your server by limiting
the total size of all uploaded files while preventing those files from
consuming all available space on the server. For this example, mount the
filesystem on /home/ftp/incoming

By default, the server will not allow uploads from anonymous FTP users.
Just to be safe, and so we don't forget, let's add a clause saying that:

upload /home/ftp * no

What this says is, "For any user whose home directory is the anonymous FTP
area, /home/ftp, do not allow any uploads." As I said, this is the
default, but put it in anyway so you don't forget.

Now, we want to allow uploads into the incoming filesystem. We MUST add a
clause granting that privilege to anonymous users. Right now we don't want
to let anonymous users create directories. (I recommend NEVER allowing them
to do it, but I'll show you how in a bit.) We want to ensure, however,
the server is safe and cannot be used as a way-point for software pirates
(warez traders). So we'll set the directory permissions for the incoming
area to prevent anyone seeing what's there and make the area write-only for
anonymous users.

First, we need an FTP site administrator, someone who owns the files, but
isn't the root user or the anonymous user. Something like the following
/etc/passwd entry will do:

ftpadmin:*:96:96::/home/ftp:

Set the incoming area permissions and ownership to safe values. I
recommend the following:

chown ftpadmin /home/ftp/incoming
chgrp ftpadmin /home/ftp/incoming
chmod 3773 /home/ftp/incoming

Actually, ftpadmin should own more of the site, but I'm only talking about
uploads right now.

Finally, before we get into allowing uploads, one last thing. Whether you
allow on-the-fly tar'ing of directories or not, you should make sure an
end-run cannot be made and the incoming area downloaded using tar. To do
so, create the special file '.notar' in both the FTP directory and the
incoming area:

touch /home/ftp/.notar
chmod 0 /home/ftp/.notar
touch /home/ftp/incoming/.notar
chmod 0 /home/ftp/incoming/.notar

The zero-length .notar file can confuse some web clients and FTP proxies,
so let's mark it unretrievable.

noretrieve .notar

Time to allow uploads, put the following in /etc/ftpaccess:

upload /home/ftp /incoming yes ftpadmin ftpadmin 0440 nodirs

Notice the target directory for the uploads is relative to the view the
user will have during the FTP session.

What this says is, "For any user whose home directory is the anonymous FTP
area, /home/ftp, allow uploads into the directory /incoming but do not
allow the creation of new directories. Make all files uploaded owned by
the FTP administrator, mark them read-only so we don't allow them to be
downloaded." If uploaded files are to be made available for downloading,
the safest thing to do is to tell the FTP administrator to move them into a
public area and modify the permissions after validating and approving them.
I know this seems draconian but, in the long run, it's best.

Some FTP sites like to live dangerously and allow anonymous users to create
directories. I don't recommend this; it cannot be done with absolute
safety. If you insist, however, you can at least limit it to a single
directory level. For example, replace the upload clause just added with
the following:

upload /home/ftp /incoming   yes ftpadmin ftpadmin 0440 dirs 3773
upload /home/ftp /incoming/* yes ftpadmin ftpadmin 0440 nodirs

The first line allows directories to be created in the incoming area and
enforces the use of safe permissions on them. The second prevents creation
of deeper sub-directories. Notice one of the problems with allowing
directory creation is there is no way to automatically create a '.notar' in
the new directory, so a crafty user may be able to make an end-run and
download it anyway using on-the-fly tar'ing.

One last thing: since the incoming area shouldn't allow downloads, and
since it's a file system, there will be a lost+found area; you will want to
add the following clause to make SURE no downloads occur:

noretrieve /home/ftp/incoming

or, at least, add the following to prevent downloading of the lost+found
files:

noretrieve /home/ftp/incoming/lost+found

Upload restrictions for guest users
-----------------------------------
Setting up the FTP server for guest users is covered in the Guest HOWTO.
It is not my purpose here to cover how to set up for guest access. If you
have not yet done so, review the information in that document at:

ftp://ftp.fni.com/pub/wu-ftpd/guest-howto

For this example, I'll assume you have entries similar to the following in
your system /etc/passwd file:

dick:*:1010:1010::/home/users/./dick:/bin/sh
jane:*:1011:1011::/home/users/./jane:/bin/sh

By default, the WU-FTPD server will grant upload privileges to all guest
users. The example users are chroot'd to /home/users and cannot access any
area of the filesystem outside that directory structure. What we're
interested in, then, is simply protecting the areas in the chroot directory
structure we want to keep the users out of.

In a minimal installation, there will be bin, etc and dev, subdirectories
in the /home/users directory. Other files and subdirectories may exist
depending upon the requirements of your operating system. We don't want
users being able to upload into these areas. In case something happens to
the permissions on them (you did set the permissions to safe values, didn't
you?), you should deny upload privileges in your ftpaccess file. In our
case, we'll say the following:

upload /home/users/* /    no
upload /home/users/* /bin no
upload /home/users/* /etc no
upload /home/users/* /dev no

While we're at it, we'll prevent downloads with noretrieve. Don't forget
to prevent end-runs by also creating .notar files in each directory.

noretrieve /home/users/bin
noretrieve /home/users/etc
noretrieve /home/users/dev

Upload restrictions for real users
----------------------------------
First off, let me say you shouldn't have any real users in your FTP site.
Or, being more realistic, the only real user should be the site
administrator. That being said, real users should be restricted to
uploading only into specific areas. Let's start with a real user in
/etc/passwd:

ftpadmin:*:109:109::/home/users/ftpadmin:/bin/sh

Again, by default, the server will grant upload privileges everywhere, so
we have to start by revoking them and only allowing what we want to:

upload /home/users/ftpadmin *                      no
upload /home/users/ftpadmin /tmp                   yes nodirs
upload /home/users/ftpadmin /home/users/ftpadmin   yes
upload /home/users/ftpadmin /home/users/ftpadmin/* yes
upload /home/users/ftpadmin /home/ftp/incoming     yes ftpadmin ftpadmin 0440 nodirs

About matching rules
--------------------
Use extreme care when forming wildcard matching rules. It may be tempting
to say, for instance:

upload /home/users/ftpadmin /home/users/ftpadmin* yes

But, if you do, there will be unintended consequences. In the example,
we're trying to restrict upload privileges to just the ftpadmin's home
directory. Consider, though, this will match all of the following
directories:

/home/users/ftpadmin
/home/users/ftpadmin/mirrors
/home/users/ftpadministration

This last directory isn't wanted. Instead use:

upload /home/users/ftpadmin /home/users/ftpadmin   yes

to match the ftpadmin's home directory itself, then use:

upload /home/users/ftpadmin /home/users/ftpadmin/* yes

to match all subdirectories under the ftpadmin's home.

umasks for guest and real users
-------------------------------
In most cases you will want to allow guest and real users to control the
permissions on their own files and directories. As in the examples shown,
if there are no specific permissions given on upload clauses, any new files
or directories created will have all permissions set. umasks can be used
to reduce these permissions.

The daemon has a command-line option (-u) to set the default umask for all
users. Follow the -u option with an octal permissions mask. Bits in this
mask are permissions to turn off whenever the daemon creates a new file or
directory. The manpage for ftpd documents the -u option.

Often times, the global -u option is not sufficient. In the ftpaccess
file, you can control umasks by class by using the defumask clause. If no
class is given, defumask overrides the -u umask from the command line. If
the current user is a member of the named class, defumask overrides the
umask setting for this user only.

For example, assume there are several classes of users

class admin real       10.0.0.0/8 127.0.0.0/8
class local guest      10.0.0.0/8 127.0.0.0/8
class remote guest      *
class anon   anonymous *

( Notice, by the way, in this example, real users will not be allowed
access unless from the local network since they are not in any class when
coming from an outside IP address. Since the daemon gives no clue to the
remote user in this case, to outside addresses it will appear as if the
admin users do not exist on the server. The specific cause for their login
failure will appear in your system logs. )

We can control the umask by class for these users. For example, we might
say:

defumask 0377
defumask 0177 admin
defumask 0133 local remote

The first clause applies whenever another defumask clause does not match
the current user's class. This is the same as adding '-u 0377' to the
command line for the FTP daemon. In this case, the clause applies only to
anonymous users since all other classes have specific default umasks given.

The second turns off execute permissions, as well as group- and world- read
and write permissions, for all files and directories created by real users
(users in the admin class).

The last rule turns off execute permissions and group- and world-write
permissions for files and directories created by guests (in the local and
remote classes).

Remember: umasks apply to ALL files and directories created EXCEPT those
where an upload clause applies AND the upload clause gives specific
permissions. Disabling execute permissions will cause problems using newly
created directories; leaving them enabled is unsafe because all files
uploaded will have execute permission and could, therefore, be used in
attempts to break into the server.

I recommend disabling all execute permissions and instructing your users to
use the chmod command to add execute permissions to directories or to
change the umask before creating directories. This may be a bit more work
for your users, but it is safer than having a Trojan Horse program marked
executable just waiting for someone, possibly root, to try running it.

umask and chmod command restrictions
------------------------------------
As just mentioned, users have the ability to change the current umask and
modify the permissions on files and directories.

Obviously, you will want to disable this feature for anonymous users. You
may also want to control who may use these features for your guest and real
users. The defaults should be acceptable for most sites. The default
settings are equivalent to the following (which you may want to add to your
ftpaccess file so you don't forget):

chmod no anonymous
chmod yes real,guest

umask no anonymous
umask yes real,guest

If, for example, you wanted to disable these commands for guests accessing
the server from outside the local network, you could add the following:

chmod no class=remote
umask no class=remote

Be sure to insert these _before_ the 'yes' clauses. Order is important;
the daemon will apply the first matching rule it finds.   If you do
something like this, it is probably safer to rewrite the clauses to deny
everything but what you allow. For example:

chmod yes real,class=local
umask yes real,class=local
chmod no guest,anonymous
umask no guest,anonymous

Delete, overwrite, rename restrictions
--------------------------------------
The daemon also provides control over the user's ability to delete, over-
write and rename files. Again, the defaults are probably acceptable in
most cases. These are:

delete no anonymous
delete yes real,guest

rename no anonymous
rename yes real,guest

overwrite no anonymous
overwrite yes real,guest

As with the chmod and umask clauses, you can control these by class as
well. Continuing the above example, restricting these to local users only,
we could instead say:

delete    yes real,class=local
rename    yes real,class=local
overwrite yes real,class=local
delete    no guest,anonymous
rename    no guest,anonymous
overwrite no guest,anonymous

Per-class upload clauses
------------------------
Just as we can restrict the ability to change permissions, delete files,
etc., we can also define upload clauses which apply only to specific
classes of users. For instance, with the classes from the above examples,
we can revoke upload rights for remote guests.

For example, we can deny all uploads the remote guests except to their
personal tmp directories:

upload class=remote /home/users/* *      no
upload class=remote /home/users/* /*/tmp yes nodirs

Private incoming areas
----------------------
Often times, users would like to have private areas in the FTP site.
Sometimes, it is usefull to also have incoming areas in those private
areas. Examples of the permissions for private areas can be found in the
layout at ftp://ftp.wu-ftpd.org/pub/wu-ftpd/examples/ and, other than
ownership, are no different than the public incoming area, so I'll simply
present the upload clauses here.

For this example, we'll allow anonymous uploads into all private incoming
areas:

upload /home/ftp            /private/*/incoming          yes * * 0440 nodirs
upload /home/users/ftpadmin /home/ftp/private/*/incoming yes * * 0440 nodirs

The assumption here is Unix shell users have private areas in the anonymous
site. Those areas are owned by the appropriate user, and incoming files
are to be owned by that user. The wildcard match on directory allows
anonymous uploading to any private incoming directory. The wildcard for
owning user and group instructs the daemon to set the file's ownership to
that of the directory receiving it.

Don't forget, if you allow private incoming areas, they are open for
anonymous access and you should take care to ensure a DoS attempt to fill
the file system cannot take out your entire server. Create a separate
filesystem for the private incoming areas or put them inside the public
incoming area.

Differences from earlier versions
---------------------------------
This HOWTO was written for version 2.6.0 of the WU-FTPD server. Earlier
versions used different rules for the upload clause.

Some versions of the daemon required the first parameter to be the name of
the root directory for the chroot. This allowed upload control by area,
but did not provide for different rules on a per-user basis.

Some versions of the daemon required the first parameter to be lexically
identical to the user's home directory entry. This was non-obvious and the
'/./' was often forgotten.

Some versions of the daemon got totally confused, attempted to apply both
these methods at once, and ended up ignoring all your upload rules. If you
were smart, you had your permissions set properly and didn't notice.

Early versions of the VR upgrades, and all earlier versions of the daemon,
allowed file system modification as the default for all users. The current
version does not allow any modification commands (ie., upload, delete,
rename) by anonymous users unless specifically granted in the ftpaccess
file.

Early versions of the VR upgrades, and all earlier versions of the dameon,
had no method for specifying the permissions for a newly created directory.
Also, they required exact matches for the first parameter (no globbing) and
exact user and group names or numbers for ownership file files and
directories.

--

Gregory A Lundberg              WU-FTPD Development Group
1441 Elmdale Drive              lundberg@wu-ftpd.org
Kettering, OH 45409-1615 USA    1-800-809-2195