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Edit File: Umask.html

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<title>ProFTPD mini-HOWTO - Umask</title>
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<hr>
<center><h2>Umask</h2></center>
<hr>

ProFTPD's <a href="../modules/mod_core.html#Umask"><code>Umask</code></a>
configuration directive is used to set the file permission bits on newly
created files and directories. However, the way in which <code>Umask</code>
is to be used is not entirely straightforward.

<code>Umask</code> is used to set the value that <code>proftpd</code> will
use when calling <code>umask(2)</code>. The <code>umask(2)</code> function
works something like this:
<pre>
 new file mode = base-mode - umask
</pre>
(Technically, the operation is <code>base-mode & ~umask</code>).
Thus, with a base-mode of <code>0666</code>, and a umask of
<code>0022</code>, the permissions on the newly created file will be
<code>0644</code> (e.g. <code>rw-r--r--</code>).

A quick review of permission bits:
<pre>
 4 is read permission (r)
 2 is write permission (w)
 1 is execute permission (x)
</pre>
The first digit of a mode (<code>0750</code>, for example) is used to
specify some special bits (e.g. set-user-ID, set-group-ID, and the
&quot;sticky bit&quot;). The second digit, the <code>7</code> in this
example, specifies the user owner permissions, and is a sum of the above
permission bits: <code>7 = 4 + 2 + 1</code> (e.g. <code>rwx</code>).
Group owner permissions are specified by the third bit, <code>5</code>:
<code>5 = 4 + 1</code> (e.g. <code>r-x</code>). And finally, other
or world permissions are specified using the last bit, which in the
example is <code>0</code> (no permissions, e.g. <code>---</code>).

Here are some concrete examples to help illustrate things:

<table border=1>
 <tr>
 <td>Mode</td>
 <td>Label</td>
 <td>Description</td>
 </tr>

<tr>
 <td>&nbsp;&nbsp;<code>0777</code>&nbsp;&nbsp;</td>
 <td>&nbsp;&nbsp;<code>rwxrwxrwx</code>&nbsp;&nbsp;</td>
 <td>read/write/execute permissions for user owner, group owner, and other</td>
 </tr>

<tr>
 <td>&nbsp;&nbsp;<code>0666</code>&nbsp;&nbsp;</td>
 <td>&nbsp;&nbsp;<code>rw-rw-rw-</code>&nbsp;&nbsp;</td>
 <td>read/write permissions for user owner, group owner, and other</td>
 </tr>

<tr>
 <td>&nbsp;&nbsp;<code>0755</code>&nbsp;&nbsp;</td>
 <td>&nbsp;&nbsp;<code>rwxr-xr-x</code>&nbsp;&nbsp;</td>
 <td>read/write/execute permissions for user owner, read/execute permissions for group owner and other</td>
 </tr>

<tr>
 <td>&nbsp;&nbsp;<code>0750</code>&nbsp;&nbsp;</td>
 <td>&nbsp;&nbsp;<code>rwxr-x---</code>&nbsp;&nbsp;</td>
 <td>read/write/execute permissions for user owner, read permission for group owner, no permissions for other</td>
 </tr>

<tr>
 <td>&nbsp;&nbsp;<code>0644</code>&nbsp;&nbsp;</td>
 <td>&nbsp;&nbsp;<code>rw-r--r--</code>&nbsp;&nbsp;</td>
 <td>read/write permissions for user owner, read permission for group owner and other</td>
 </tr>

<tr>
 <td>&nbsp;&nbsp;<code>0511</code>&nbsp;&nbsp;</td>
 <td>&nbsp;&nbsp;<code>r-x--x--x</code>&nbsp;&nbsp;</td>
 <td>read/execute permissions for user owner, execute permission for group owner and other</td>
 </tr>
</table>

The <code>proftpd</code> daemon always starts with a base-mode of
<code>0666</code> when creating files. Note that <code>Umask</code> can only
be used to &quot;take away&quot; permissions granted by the base-mode;
it cannot be used to add permissions that are not there. This means that
files uploaded to a <code>proftpd</code> server will never have the execute
permission enabled by default, since the <code>0666</code> base-mode
does not have any execute bits enabled). This is a conscious security design
decision. For directories, a different base-mode of <code>0777</code>
is used. The umask used for directories can be configured using the
optional second parameter to the <code>Umask</code> directive; if this second
parameter is not used, the umask used for created directories will
default to the same umask as used for files.

If it is necessary to make uploaded files executable, the
<code>SITE CHMOD</code> FTP command can be used:
<pre>
 SITE CHMOD mode file
</pre>
Use of this command can be restricted using a &quot;command&quot; of
<code>SITE_CHMOD</code> in a <code>&lt;Limit&gt;</code> section. For
example, this section of a <code>proftpd.conf</code> file:
<pre>
 &lt;Limit SITE_CHMOD&gt;
 AllowUser ftpadmin
 DenyAll
 &lt;/Limit&gt;
</pre>
will deny everyone except user <code>ftpadmin</code> from being able to
use the <code>SITE CHMOD</code> command to change the permissions on files
via FTP. Note that this construction is recommended instead of using the
deprecated (as of <code>proftpd-1.2.2rc2</code>) <code>AllowChmod</code>
configuration directive.

Examples of Using the <code>Umask</code> Directive 
You have just installed <code>proftpd</code>, and now need to figure out what
permissions file/directories created on your FTP server should have. As
a conscientious FTP server administrator, you want files/directories to
have the minimum necessary permissions (rather than letting users have access
to files/directories that they do not need).

If only the user who creates the files and directories should have full
access, e.g. so they can read and write their own files, then you might
use:
<pre>
 # Only the user can see their own files/directories
 Umask 0066 0077
</pre>
With this configuration, a newly uploaded file would have <code>0600</code>
(<code>rw-------</code>) permissions:
<pre>
 0600 = 0666 - 0066
</pre>
and a newly created directory would have <code>0700</code>
(<code>rwx------</code>) permissions:
<pre>
 0700 = 0777 - 0077
</pre>

Another common case is where you have many users who are uploading files
for sharing with other users. So you want the files to be readable by
everyone, but only the user who uploaded the file should have permission for
writing/changing the file. For this, you might use:
<pre>
 # Only the user can change their own files
 Umask 0022
</pre>
With this configuration, a newly uploaded file would have <code>0644</code>
(<code>rw-r--r--</code>) permissions:
<pre>
 0644 = 0666 - 0022
</pre>
and a newly created directory would have <code>0755</code>
(<code>rwxr-xr-x</code>) permissions:
<pre>
 0755 = 0777 - 0022
</pre>

<a name="FAQ"></a>
Frequently Asked Questions

<a name="UmaskExecutePermission">
Question: How can I configure <code>proftpd</code> so
that I can upload a file with <code>770</code> permissions? 
Answer: Short answer: you can't. Too many FTP servers,
in the past, would allow users to upload executable files. Hackers would
use this capability, and then exploit a flaw in one of the servers on that
machine to execute the crafted file they just uploaded. Thus ProFTPD does
not allow uploading of files with execute permissions.

The workaround, as mentioned above, is to allow the client to use the
<code>SITE CHMOD</code> command to change the permissions on the file to
have the execute permissions.

<a name="UmaskDeletePermission">
Question: I have a <code>Umask</code> value of
<code>0066</code>, so that only I have read/write permissions on my files. But other users can delete my files! Is this a <code>proftpd</code> bug? 
Answer: No. The permission for deleting a file is
not governed by the write permission on the deleted file; it is
controlled by the write permission on the directory containing the file.

If you think of a directory as a "table of contents", with entries for each
of the files in that directory, then deleting a file means deleting the entry
for that file from the "table of contents", which is a write on the directory
(not on the deleted file).

Let's assume that your files were in a directory whose permissions were
<code>0777</code> (<code>rwxrwxrwx</code>). This means that everyone has
write permissions in that directory. It also means that everyone can
delete files from that directory.

Now let's assume that your files instead were in a directory whose permissions
where <code>0755</code> (<code>rwxr-xr-x</code>). This means that only the
user owner of the directory can delete files from that directory, and no one
else.

For directories which contain files from different users, one of the
little-known (and very useful) permissions to have for the directory is
<code>1777</code> (<code>rwxrwxrwt</code>). The leading 1 (and <code>t</code>)
indicates the "sticky bit". This obscure bit is little used these days,
except in this useful configuration. When the sticky bit is set on
a directory (making it a "sticky directory"), normal users may not delete or
rename files of other users in that directory. Because of this property,
"sticky directories" are quite useful as shared directories (e.g.
<code>/tmp</code>).

<hr>
Last Updated: $Date: 2012-03-24 18:41:57 $ 
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