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<center><h2><b>ProFTPD: Logins and Authentication</b></h2></center>
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<p>
Logging into <code>proftpd</code> and being successfully authenticated by
the server involves a lot of different modules and different checks.  This
document aims to discuss the sort of checks and configuration involved, and
hopefully provide a better idea of how <code>proftpd</code> authenticates users.

<p><b>PAM</b><br>
PAM, which stands for <b>P</b>luggable <b>A</b>uthentication <b>M</b>odules,
is an API intended to make it easy to replace the old Unix-style DES password
hashes stored in <code>/etc/passwd</code> with a flexible system that allows
system administrators to use MD5 checksums, SQL tables, LDAP servers,
RADIUS servers, <i>etc</i> in place of that password check.  However, what
PAM does <b>not</b> provide is the rest of the user account information in
<code>/etc/passwd</code>, <i>i.e.</i> the user's UID and GID, home directory,
and shell.  This means that PAM <b>cannot</b> be used as a drop-in replacement
for user information stored in <code>/etc/passwd</code>. <a href="http://www.gnu.org/manual/glibc-2.2.5/html_node/Name-Service-Switch.html#Name%20Service%20Switch">NSS</a> (<b>N</b>ame <b>S</b>ervice <b>S</b>witch) modules, supported by
some operating systems, are a complementary API to PAM which can be used to
supply the rest of this user information.  <code>proftpd</code> uses the normal
<code>libc</code> functions for looking up user information, and those
<code>libc</code> functions typically read <code>/etc/passwd</code>.  NSS is
an abstraction layer within some <code>libc</code> implementations that causes
those functions to read other sources rather than <code>/etc/passwd</code>.
Regardless of NSS support, <code>proftpd</code> has support for
"<a href="#virtual">virtual</a>" users via its authentication
<a href="#modules">modules</a>.

<p>
When configuring <code>proftpd</code>, the <code>configure</code> script
will automatically try to determine whether your operating system supports PAM.
If it does, the <code>mod_auth_pam</code> module will automatically be compiled
into your <code>proftpd</code>.  If you explicitly do not want PAM support,
you can use the <code>--disable-auth-pam</code> configure option to disable
this automatic detection.  The point of using PAM is that it can provide an
extra authentication step during a login.  By "authentication", I
mean that PAM answers a yes/no question: "Is this user who they say they
are?".  PAM modules are configured either in <code>/etc/pam.conf</code> or
<code>/etc/pam.d/</code>, depending on your operating system.  However,
many of the PAM modules provided by vendors are not designed to work well
with some of the authentication <a href="#modules">modules</a> supported by
<code>proftpd</code>.  If PAM is not a necessity for you, and you plan to
use one of the authentication modules (other than <code>mod_auth_unix</code>),
then you need do nothing.  By default, proftpd uses PAM as an <i>additional</i>
check during logins, but if that check fails, the login may still succeed.
If you <i>do</i> need the PAM check to be authoritative, then you need to
use the <code>AuthOrder</code> directive, <i>e.g.</i>:
<pre>
  AuthOrder mod_auth_pam.c* ...
</pre>
To disable use of PAM entirely, use:
<pre>
  <IfModule mod_auth_pam.c>
    AuthPAM off
  </IfModule>
</pre>

<p><a name="directives"><b>Configuration Directives</b><br>
There are several configuration directives that can cause login problems.
The most common one is <a href="../modules/mod_auth.html#RequireValidShell"><code>RequireValidShell</code></a>, so common that it is a
<a href="http://www.proftpd.org/docs/faq/faq_full.html#AEN267">FAQ</a>.
If <code>proftpd</code> does not actually <i>use</i> the shell configured for
a user, why does it check to see if the shell is valid by looking in
<code>/etc/shells</code>?  Certain other FTP servers (<i>e.g.</i>
<code>wu-ftpd</code>, <code>pure-ftpd</code>) do check for invalid shells and
deny logins based on this criterion; <code>proftpd</code> follows this pattern
so as not to surprise too many system administrators.  Use of invalid shells
is a common sysadmin trick for denying shell-based login access (<i>e.g.</i>
<code>ssh</code> logins); many sites use other means, however, and so use of
the <code>RequireValidShell</code> directive is also frequently seen.

<p>
Another reason why a client cannot login might be if the login user is
<code>root</code> (or has a UID of zero, and hence has root privileges).
Logging in as <code>root</code> is dangerous, and should be avoided if
possible.  If you do find it absolutely necessary to login as <code>root</code>,
<i>please</i> use <a href="TLS.html">SSL/TLS</a>, or at least <a href="SSH.html">tunnel</a> your FTP connection using SSH.  The <a href="../modules/mod_auth.html#RootLogin"><code>RootLogin</code></a> configuration directive is needed
in your <code>proftpd.conf</code> in order for <code>proftpd</code> to
explicitly allow root logins.

<p>
One uncommon obstacle that you might encounter to allowing a user to login is
the possibility that that user is listed in an <code>/etc/ftpusers</code>
file.  This is another legacy check, courtesy of <code>wu-ftpd</code>.
Any user that is listed in <code>/etc/ftpusers</code> is <b>not</b> allowed
to login via FTP.  A little backwards from what might be expected from the
file name, I agree.  <code>proftpd</code> was made to similarly honor any <code>/etc/ftpusers</code> file by default in order to ease the pain for sites
migrating from <code>wu-ftpd</code> to <code>proftpd</code>. Disabling
<code>proftpd</code>'s check for this file is as simple as using the
<a href="../modules/mod_auth.html#UseFtpUsers"><code>UseFtpUsers</code></a>
configuration directive, like so:
<pre>
  UseFtpUsers off
</pre>
in your <code>proftpd.conf</code> file.

<p>
The <a href="../modules/mod_auth_unix.html#PersistentPasswd"><code>PersistentPasswd</code></a> configuration directive can
be necessary in some environments, particularly those that use NIS/YP,
NSS modules, or (in the case of Mac OSX) the <code>netinfo</code> service.
In order to be able to lookup and map UIDs and GIDs to names, as when
listing directories and files, <code>proftpd</code> tries to keep the
<code>/etc/passwd</code> file open.  This is particularly relevant if the
<code>DefaultRoot</code> directive is in effect, for once <code>chroot</code>ed,
<code>proftpd</code> cannot open <code>/etc/passwd</code>.  However, services
such as NIS, NSS, and <code>netinfo</code> function very differently while
providing a file-like interface, and they do not function properly if
<code>proftpd</code> keeps them open.  Using:
<pre>
  PersistentPasswd off
</pre>
in your <code>proftpd.conf</code> should cause name lookups to work
properly if you use NIS, NSS, or <code>netinfo</code>.

<p>
If you feel your logins are slow, then you might be encountering another
<a href="http://www.proftpd.org/docs/faq/faq_full.html#AEN519">FAQ</a>.
The timeouts when performing RFC931 <code>ident</code> lookups, and
DNS reverse resolutions, add a noticeable delay to a login.

<p><a name="anonymous"><b>Anonymous Logins</b><br>
Anonymous logins are allowed by defining an <a href="http://www.proftpd.org/docs/modules/mod_core.html#Anonymous"><code><Anonymous></code></a>
section, or <i>context</i>, in your <code>proftpd.conf</code>.  No
<code><Anonymous></code> contexts mean that <code>proftpd</code> will
not allow anonymous logins.  As the documentation describes,
<code>proftpd</code> knows to treat a given login name (given to the server by
the client via the <code>USER</code> FTP command) by seeing if the login
name is the same as the <code>User</code> name in an
<code><Anonymous></code> context.  For example:
<pre>
  <Anonymous /var/ftp/anon/dave>
    User dave
    Group ftpanon
    ...
  </Anonymous>
</pre>
would cause any client logging in as <code>dave</code> to be treated as an
anonymous login, and to be handled using the <code><Anonymous></code>
context above.  This structure allows for multiple login names to be treated
as anonymous logins, and for each anonymous login to have its own specific
anonymous configuration.  Some administrators use <code><Anonymous></code>
contexts to define "virtual" users directly in their
<code>proftpd.conf</code>, but this practice is <b>discouraged</b>.  Virtual
user accounts are discussed next.

<p><a name="homedir"><b>Resolving <code>~</code></b><br>
The <code>DefaultRoot</code> directive is commonly used to restrict or
"<a href="Chroot.html">jail</a>" users into specific directories,
usually their individual home directories.  This is done via:
<pre>
  DefaultRoot ~
</pre>
where the tilde (<code>~</code>) is expanded to the home directory of the
logging in user.  Now, when <code>proftpd</code> is resolving the tilde,
it switches to the privileges of the logging-in user and attempts to resolve
the home directory.  This ensures that the user will, once restricted to
that directory, will have the ability to see files and move around.  So
if using the tilde does not appear to be working in your configuration,
double-check that the permissions on the home directory of the user in
question at least allow that user to change into the directory (which requires
execute permission on the home directory).  If <code>proftpd</code> finds
that the permissions are too restrictive, an error message like:
<pre>
  chroot("~"): No such file or directory
</pre>
will be logged.

<p><a name="virtual"><b>Virtual Users</b><br>
One question that often arises is "How do I create a proftpd user?"
<code>proftpd</code> uses your system's <code>/etc/passwd</code> file by
default, and so <code>proftpd</code> users are the same as your system users.
"Virtual" users, sometimes described as FTP-only user accounts, are
users that can login to <code>proftpd</code>, but who are separate from the
normal system users, and who do not have entries in <code>/etc/passwd</code>.
<code>proftpd</code> does not care how or where user information is defined.
The daemon is designed with an abstraction layer on top of user information
sources, and that abstraction is responsible for supplying that data that is
<b>required</b> for every user: a name, a UID, a GID, a home directory, and a
shell.  A user's shell will not be used except in
<code>RequireValidShell</code> checks, but it must still present.  The code
that is responsible for supplying this data, reading it from whatever storage
format is supported, lies in <code>proftpd</code>'s various configurable
authentication modules.

<p><a name="modules"><b>Authentication Modules</b><br>
<code>proftpd</code> uses authentication modules for accessing user account
information.  These modules implement an API that that daemon uses to
lookup account information by name or by ID, to authenticate a user using
the provided password, and to resolve names to IDs or IDs to names.  The
following authentication modules are all provided with <code>proftpd</code>:
<ul>
  <li><code>mod_auth_unix</code><br>
    Handles normal authentication via <code>/etc/passwd</code>, <code>/etc/group</code>
  </li>

  <p>
  <li><a href="../modules/mod_auth_file.html"><code>mod_auth_file</code></a><br>
    Handles the <code>AuthUserFile</code> and <code>AuthGroupFile</code> directives, for storing user account information in <a href="AuthFiles.html">other files</a>
  </li>

  <p>
  <li><a href="../contrib/mod_ldap.html"><code>mod_ldap</code></a><br>
    Handles user account information stored in LDAP directories
  </li>

  <p>
  <li><a href="../contrib/mod_radius.html"><code>mod_radius</code></a><br>
    Handles user account information provided by RADIUS servers
  </li>

  <p>
  <li><a href="../contrib/mod_sql.html"><code>mod_sql</code></a><br>
    Handles user account information stored in <a href="SQL.html">SQL tables</a>
  </li>
</ul>
Note that <code>mod_auth_pam</code> is not on this list because it cannot
provide the necessary user account information.  It can be used to supplement
other auth modules by adding its PAM checks, however.

<p><a name="order">
Since <code>proftpd</code> supports multiple authentication modules at the
same time, how does it know which authentication module to use?  What if you
want to tell <code>proftpd</code> which modules to check, and in which order?
What if you want some authentication modules to be used in one
<code><VirtualHost></code>, and different authentication modules in
another?

<p>
By default, <code>proftpd</code> will ask every configured authentication
module about a given user, until it finds an authentication module that knows
about that user, or until an authentication module signals an unrecoverable
error.  The order in which these modules are asked depends on the order of
modules in the <code>--with-modules</code> option used when configuring
<code>proftpd</code>.

<p>
Some modules can be figured to not "play nice" and allow other
authentication modules a chance at providing user information.  That is, some
modules can be "authoritative", and if that module does not know
about the user, it will signal an error and prevent <code>proftpd</code>
from asking other modules.  <code>mod_auth_pam</code>'s
<code>AuthPAMAuthoritative</code> directive, and the <code>*</code> syntax
in the <code>SQLAuthenticate</code> directive of <code>mod_sql</code>, are
examples of this authoritativeness.  In general, it is best to avoid using
such mechanisms, and to use the
<a href="../modules/mod_core.html#AuthOrder"><code>AuthOrder</code></a> configuration directive instead.

<p>
The following illustrates a situation where <code>AuthOrder</code> is
useful.  The default build of <code>proftpd</code> has two authentication
modules included: <code>mod_auth_file</code> and <code>mod_auth_unix</code>.
<code>proftpd</code> will consult <b>both</b> modules when authenticating a
user: first <code>mod_auth_file</code>, then <code>mod_auth_unix</code>.
(<i>Note</i>: versions of <code>proftpd</code> before 1.2.8rc1 would only
support either <code>AuthUserFile</code> or <code>/etc/passwd</code>, but not
both at the same time.)  If any authentication module can authenticate a user,
then authentication succeeds. This holds true of other authentication modules
like <code>mod_ldap</code>, <code>mod_sql</code>, <code>mod_radius</code>,
<i>etc</i>.

<p>
However, if you only want <code>proftpd</code> to use your
<code>AuthUserFile</code> and no other authentication modules, then you would
use the <code>AuthOrder</code> directive like this:
<pre>
  AuthOrder mod_auth_file.c
</pre>
Or, if you use <code>mod_sql</code> and wanted <code>proftpd</code> to check
your SQL tables first, and then default to system users:
<pre>
  AuthOrder mod_sql.c mod_auth_unix.c
</pre>
Note that the <code>mod_auth.c</code> module should <b>never</b> be used in an
<code>AuthOrder</code> directive.

<p><a name="FAQ"></a>
<b>Frequently Asked Questions</b><br>

<p><a name="LogPassword">
<font color=red>Question</font>: How can I configure <code>proftpd</code> to
log/show the password typed by the user?<br>
<font color=blue>Answer</font>:  You cannot.  Period.  The <code>proftpd</code>
code goes out of its way to ensure that the password is never logged.

<p>
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© Copyright 2017 The ProFTPD Project<br>
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