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Code Editor : Timestamps.html

<!DOCTYPE html>
<html>
<head>
<title>ProFTPD: Timestamps</title>
</head>

<hr>
<center><h2>ProFTPD: Timestamps</h2></center>
<hr>

There have been various reports that timestamps displayed in various
ProFTPD log files, such as an <code>ExtendedLog</code> or
<code>TransferLog</code>, or even in directory listings, are not as expected.
Other applications (e.g. <code>sshd</code>) on the same machine do
not exhibit the same problem. So is <code>proftpd</code> having issues
with timestamps?

If the timestamps in question are those displayed in directory listings, then
you need to check your <a href="../modules/mod_core.html#TimesGMT"><code>TimesGMT</code></a> configuration. Otherwise,
depending on the logs at which you are looking, the timestamps will be correct
until the user logs in. After that, the timestamps are wrong. This
is one clue. The other clue is that these wrong timestamps go away when
the <code>DefaultRoot</code> directive is removed from the
<code>proftpd.conf</code> file.

The answer turns out to be the <code>chroot(2)</code> system call, which
<code>proftpd</code> uses to restrict users to certain directories, usually
their home directory. There are two configuration directives which tell
<code>proftpd</code> to use the <code>chroot(2)</code> system call:
<pre>
 &lt;Anonymous&gt;
 DefaultRoot 
</pre>
The system <code>libc</code> library is responsible for generating times,
based on timezone settings and such. The GNU libc library (called
<code>glibc</code> for short), used on all Linux systems changed in later
versions, and now makes some assumptions. Specifically, <code>glibc</code>
assumes the location of the system timezone files. But once a process
has been chrooted, those assumptions break. The fallback behavior of
<code>glibc</code>, when it cannot find the system timezone files, is to
use GMT.

The good news is that the above behavior does not happen if the
<code>TZ</code> environment variable is set explicitly, to the required
timezone. That is, if <code>glibc</code> needs to detect the timezone and
it finds that <code>TZ</code> is set, then <code>glibc</code> will use that
<code>TZ</code> setting, rather than falling back to GMT.

Affected Library Functions 
Once a process has chrooted, the following <code>libc</code> function
calls will have possibly-bad side effects, with regard to timezone detection:
<code>localtime(3)</code>, <code>ctime(3)</code>, and <code>mktime(3)</code>.
As documented in the man pages for these functions, each of them will
set the <code>tzname</code> global variable to the &quot;current&quot;
timezone. This &quot;current&quot; timezone is what changes, once the
process has become chrooted. (For more information on the <code>tzname</code>
global variable, read the <code>tzset(3)</code> man page.)

Solutions 
Recent releases of ProFTPD attempt to work around these issues by
preserving the <code>tzname</code> global variable after calling
<code>localtime(3)</code>, and by automatting setting the <code>TZ</code>
environment variable, is not already set, before calling <code>chroot(2)</code>.
These measures are defensive at best, though.

The best way to deal with this issue, which is especially prominent on systems
running <code>glibc-2.3</code> or later, is to set the <code>TZ</code>
environment variable explicitly, before starting <code>proftpd</code>. This
can be done in an <code>init</code> script for <code>proftpd</code>, for
example.

You can also use the <code>SetEnv</code> configuration directive within the
<code>proftpd.conf</code> file to set the <code>TZ</code> environment variable,
e.g.:
<pre>
 # Set the TZ environment variable to be Pacific Standard Time (PST)
 SetEnv TZ PST
</pre>
Obviously you will need to set the timezone to whatever is appropriate for
your site. For example, some systems expect different values for the
<code>TZ</code> environment variable, e.g.:
<pre>
 SetEnv TZ :/etc/localtime
</pre>
Note: Using a filesystem location such as ":/etc/localtime" for the
<code>TZ</code> environment variable can still result in log timestamp issues,
e.g. when the <code>MFMT</code>/<code>MFF</code> FTP commands are used.
The best solution is to always specify the timezone name, rather than a file,
whenever possible.

These are provided as examples; please consult your system documentation
for the proper syntax to use for the <code>TZ</code> value.

<a name="FAQ">
Frequently Asked Questions

Question: Why is the timestamp on my newly uploaded file wrong? 
Answer: If by "wrong" you mean "different from
the timestamp of that file on my client machine", then that is the expected
behavior.

The modification timestamp on a file is not part of the data uploaded
over the data connection when uploading a file. Timestamps like that are
metadata for that file, not part of the bytes of the file itself.
FTP does not supporting sending of timestamps as part of the upload transfer
process. See below for a common follow-up question.

Question: Does ProFTPD support changing the timestamp on a file using the <code>MDTM</code> command? That's what my client does. 
Answer: No.

Why not? For several reasons:
<ul>
 <li>The <code>MDTM</code> command is not one of the commands
 mandated by <a href="http://www.faqs.org/rfcs/rfc959.html">RFC 959</a>.
 </li>
 
 <li>Some FTP clients use <code>MDTM</code> for retrieving the
 modification time, others try to use it to set the modification
 time, and still others do both.
 </li>

<li>Among those FTP clients that do use <code>MDTM</code>, there is no
 consistent format of the timestamp used.
 </li>
</ul>
In short, it is a royal mess. ProFTPD supports <code>MDTM</code> for
retrieving the modification time, in GMT. Period.

If you want to set the modification time, you can use the
<code>MFMT</code> command supported by the <code>mod_facts</code> module,
or the <code>mod_site_misc</code> module's
<a href="../contrib/mod_site_misc.html#SITE_UTIME"><code>SITE UTIME</code></a>
command.

Question: I thought that the <code>TimesGMT</code> directive was affected the timestamps that proftpd uses? 
Answer: No. The <a href="../modules/mod_core.html#TimesGMT"><code>TimesGMT</code></a> directive only affects the timestamps as
displayed to FTP clients in directory listings; it does not
affect the timestamps used in log files.

</body>
</html>

${this.title}

Code Editor : Timestamps.html