913 lines
35 KiB
XML
913 lines
35 KiB
XML
<?xml version="1.0" encoding="ISO8859-1" standalone="no"?>
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<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V4.2-Based Extension//EN"
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"../../../share/sgml/freebsd42.dtd" [
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<!ENTITY % entities PUBLIC "-//FreeBSD//ENTITIES DocBook FreeBSD Entity Set//EN" "../../share/sgml/entities.ent">
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%entities;
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]>
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<article lang="en">
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<articleinfo>
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<title>LDAP Authentication</title>
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<authorgroup>
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<author>
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<firstname>Toby</firstname>
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<surname>Burress</surname>
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<affiliation>
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<address><email>kurin@causa-sui.net</email></address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2007</year>
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<year>2008</year>
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<holder>The FreeBSD Documentation Project</holder>
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</copyright>
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<legalnotice id="trademarks" role="trademarks">
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&tm-attrib.freebsd;
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&tm-attrib.general;
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</legalnotice>
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<pubdate>$FreeBSD$</pubdate>
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<releaseinfo>$FreeBSD$</releaseinfo>
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<abstract>
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<para>This document is intended as a guide for the configuration
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of an LDAP server (principally an <application>OpenLDAP</application>
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server) for authentication on &os;. This is useful for situations
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where many servers need the same user accounts, for example as a
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replacement for <application>NIS</application>.</para>
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</abstract>
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</articleinfo>
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<sect1 id="preface">
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<title>Preface</title>
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<para>This document is intended to give the reader enough of an
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understanding of LDAP to configure an LDAP server. This document will
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attempt to provide an
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explanation of <filename role="package">net/nss_ldap</filename>
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and <filename role="package">security/pam_ldap</filename> for use with
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client machines services for use with the LDAP server.</para>
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<para>When finished, the reader should be able to configure and
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deploy a &os; server that can host an LDAP directory, and to
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configure and deploy a &os; server which can authenticate against
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an LDAP directory.</para>
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<para>This article is not intended to be an exhaustive account of
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the security, robustness, or best practice considerations for
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configuring LDAP or the other services discussed herein. While the author
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takes care to do everything correctly, he does not
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address security issues beyond a general scope. This article should be
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considered to lay the theoretical groundwork only, and any actual
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implementation should be accompanied by careful requirement
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analysis.</para>
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</sect1>
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<sect1 id="ldap">
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<title>Configuring LDAP</title>
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<para>LDAP stands for <quote>Lightweight Directory Access
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Protocol</quote> and is a subset of the X.500 Directory Access
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Protocol. Its most recent specifications are in <ulink
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url="http://www.ietf.org/rfc/rfc4510.txt">RFC4510</ulink> and
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friends. Essentially it is a database that expects to be read from
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more often than it is written to.</para>
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<para>The LDAP server <ulink
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url="http://www.openldap.org/">OpenLDAP</ulink> will be used in the
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examples in this document; while the principles here should be
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generally applicable to many different servers, most of the
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concrete administration is
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<application>OpenLDAP</application>-specific. There are several
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server versions in ports, for example <filename
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role="package">net/openldap24-server</filename>. Client servers
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will need the corresponding <filename
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role="package">net/openldap24-client</filename> libraries.</para>
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<para>There are (basically) two areas of the LDAP service which need
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configuration. The first is setting up a server to receive
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connections properly, and the second is adding entries to the
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server's directory so that &os; tools know how to interact with it.</para>
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<sect2 id="ldap-connect">
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<title>Setting Up the Server for Connections</title>
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<note>
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<para>This section is specific to
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<application>OpenLDAP</application>. If you are using another
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server, you will need to consult that server's
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documentation.</para>
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</note>
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<sect3 id="ldap-connect-install">
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<title>Installing <application>OpenLDAP</application></title>
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<para>First, install <application>OpenLDAP</application>:</para>
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<example id="oldap-install">
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<title>Installing <application>OpenLDAP</application></title>
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<screen>&prompt.root; <userinput>cd /usr/ports/net/openldap24-server</userinput>
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&prompt.root; make install clean</screen>
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</example>
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<para>This installs the <command>slapd</command> and
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<command>slurpd</command> binaries, along with the required
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<application>OpenLDAP</application> libraries.</para>
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</sect3>
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<sect3 id="ldap-connect-config">
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<title>Configuring <application>OpenLDAP</application></title>
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<para>Next we must configure
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<application>OpenLDAP</application>.</para>
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<para>You will want to require encryption in your
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connections to the LDAP server; otherwise your users' passwords
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will be transferred in plain text, which is considered
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insecure. The tools we will be using support two very similar kinds
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of encryption, SSL and TLS.</para>
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<para>TLS stands for <quote>Transportation Layer Security</quote>.
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Services that employ TLS tend to connect on the
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<emphasis>same</emphasis> ports as the same services without
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TLS; thus an SMTP server which supports TLS will listen for
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connections on port 25, and an LDAP server will listen on 389.</para>
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<para>SSL stands for <quote>Secure Sockets Layer</quote>, and
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services that implement SSL do <emphasis>not</emphasis> listen on
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the same ports as their non-SSL counterparts. Thus SMTPS listens
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on port 465 (not 45), HTTPS listens on 443, and LDAPS on
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636.</para>
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<para>The reason SSL uses a different port than TLS is because a
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TLS connection begins as plain text, and switches to encrypted
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traffic after the <literal>STARTTLS</literal> directive. SSL
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connections are encrypted from the beginning. Other than that
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there are no substantial differences between the two.</para>
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<note>
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<para>We will adjust
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<application>OpenLDAP</application> to use TLS, as SSL is
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considered deprecated.</para>
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</note>
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<para>Once <application>OpenLDAP</application> is installed via
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ports, the following configuration parameters in
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<filename>/usr/local/etc/openldap/slapd.conf</filename> will
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enable TLS:</para>
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<programlisting>security ssf=128
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TLSCertificateFile /path/to/your/cert.crt
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TLSCertificateKeyFile /path/to/your/cert.key
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TLSCACertificateFile /path/to/your/cacert.crt</programlisting>
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<para>Here, <literal>ssf=128</literal> tells
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<application>OpenLDAP</application> to require 128-bit
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encryption for all connections, both search and update. This
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parameter may be configured based on the security needs of your
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site, but rarely you need to weaken it, as most LDAP client
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libraries support strong encryption.</para>
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<para>The <filename>cert.crt</filename>,
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<filename>cert.key</filename>, and
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<filename>cacert.crt</filename> files are necessary for clients
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to authenticate <emphasis>you</emphasis> as the valid LDAP
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server. If you simply want a server that runs, you can create a
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self-signed certificate with OpenSSL:</para>
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<example id="genrsa">
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<title>Generating an RSA key</title>
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<screen>&prompt.user; <userinput>openssl genrsa -out cert.key 1024</userinput>
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Generating RSA private key, 1024 bit long modulus
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....................++++++
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...++++++
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e is 65537 (0x10001)
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&prompt.user; <userinput>openssl req -new -key cert.key -out cert.csr</userinput></screen>
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</example>
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<para>At this point you should be prompted for some values. You
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may enter whatever values you like; however, it is important the
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<quote>Common Name</quote> value be the fully qualified domain
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name of the <application>OpenLDAP</application> server.
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In our case, and the examples here, the server is
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<replaceable>server.example.org</replaceable>.
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Incorrectly setting this value will cause clients to fail when
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making connections. This can the
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cause of great frustration, so ensure that you follow these
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steps closely.</para>
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<para>Finally, the certificate signing request needs to be
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signed:</para>
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<example id="self-sign">
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<title>Self-signing the certificate</title>
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<screen>&prompt.user; <userinput>openssl x509 -req -in cert.csr -days 365 -signkey cert.key -out cert.crt</userinput>
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Signature ok
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subject=/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd
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Getting Private key</screen>
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</example>
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<para>This will create a self-signed certificate that can be
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used for the directives in <filename>slapd.conf</filename>,
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where <filename>cert.crt</filename> and
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<filename>cacert.crt</filename> are the same file. If you are
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going to use many <application>OpenLDAP</application> servers
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(for replication via <literal>slurpd</literal>) you will want to
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see <xref linkend="ssl-ca"/> to generate a CA key and use it to
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sign individual server certificates.</para>
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<para>Once this is done, put the following in
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<filename>/etc/rc.conf</filename>:</para>
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<programlisting>slapd_enable="YES"</programlisting>
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<para>Then run <userinput>/usr/local/etc/rc.d/slapd
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start</userinput>. This should start
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<application>OpenLDAP</application>. Confirm that it is
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listening on 389 with</para>
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<screen>&prompt.user; <userinput>sockstat -4 -p 389</userinput>
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ldap slapd 3261 7 tcp4 *:389 *:*</screen>
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</sect3>
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<sect3 id="ldap-connect-client">
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<title>Configuring the Client</title>
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<para>Install the <filename
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role="package">net/openldap24-client</filename> port for the
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<application>OpenLDAP</application> libraries. The client
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machines will always have <application>OpenLDAP</application>
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libraries since that is all <filename
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role="package">security/pam_ldap</filename> and <filename
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role="package">net/nss_ldap</filename> support, at least for the
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moment.</para>
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<para>The configuration file for the
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<application>OpenLDAP</application> libraries is
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<filename>/usr/local/etc/openldap/ldap.conf</filename>. Edit
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this file to contain the following values:</para>
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<programlisting>base dc=example,dc=org
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uri ldap://server.example.org/
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ssl start_tls
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tls_cacert /path/to/your/cacert.crt</programlisting>
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<note>
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<para>It is important that your clients have access to
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<filename>cacert.crt</filename>, otherwise they will not be
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able to connect.</para>
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</note>
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<note>
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<para>There are two files called
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<filename>ldap.conf</filename>. The first is this file, which
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is for the <application>OpenLDAP</application> libraries and
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defines how to talk to the server. The second is
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<filename>/usr/local/etc/ldap.conf</filename>, and is for
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<application>pam_ldap</application>.</para>
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</note>
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<para>At this point you should be able to run
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<userinput>ldapsearch -Z</userinput> on the client machine;
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<option>-Z</option> means <quote>use TLS</quote>. If you
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encounter an error, then something is configured wrong; most
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likely it is your certificates. Use &man.openssl.1;'s
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<command>s_client</command> and <command>s_server</command> to
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ensure you have them configured and signed properly.</para>
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</sect3>
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</sect2>
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<sect2 id="ldap-database">
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<title>Entries in the Database</title>
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<para>Authentication against an LDAP directory is generally
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accomplished by attempting to bind to the directory as the connecting user.
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This is done by establishing a <quote>simple</quote>
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bind on the directory with the user name supplied. If there is an
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entry with the <literal>uid</literal> equal to the user name and
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that entry's <literal>userPassword</literal> attribute matches the
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password supplied, then the bind is successful.</para>
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<para>The first thing we have to do is figure out is where in the
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directory our users will live.</para>
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<para>The base entry for our database is
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<literal>dc=example,dc=org</literal>. The default location for
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users that most clients seem to expect is something like
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<literal>ou=people,<replaceable>base</replaceable></literal>, so
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that is what will be used here. However keep in mind that this is
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configurable.</para>
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<para>So the ldif entry for the <literal>people</literal>
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organizational unit will look like:</para>
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<programlisting>dn: ou=people,dc=example,dc=org
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objectClass: top
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objectClass: organizationalUnit
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ou: people</programlisting>
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<para>All users will be created as subentries of this
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organizational unit.</para>
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<para>Some thought might be given to the object class your users
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will belong to. Most tools by default will use
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<literal>people</literal>, which is fine if you simply want to
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provide entries against which to authenticate. However, if you
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are going to store user information in the LDAP database as well,
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you will probably want to use <literal>inetOrgPerson</literal>,
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which has many useful attributes. In either case, the relevant
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schemas need to be loaded in
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<filename>slapd.conf</filename>.</para>
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<para>For this example we will use the <literal>person</literal>
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object class. If you are using <literal>inetOrgPerson</literal>,
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the steps are basically identical, except that the
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<literal>sn</literal> attribute is required.</para>
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<para>To add a user <literal>testuser</literal>, the ldif would
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be:</para>
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<programlisting>dn: uid=tuser,ou=people,dc=example,dc=org
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objectClass: person
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objectClass: posixAccount
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objectClass: shadowAccount
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objectClass: top
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uidNumber: 10000
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gidNumber: 10000
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homeDirectory: /home/tuser
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loginShell: /bin/csh
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uid: tuser
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cn: tuser</programlisting>
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<para>I start my LDAP users' UIDs at 10000 to avoid collisions with
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system accounts; you can configure whatever number you wish here,
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as long as it's less than 65536.</para>
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<para>We also need group entries. They are as configurable as
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user entries, but we will use the defaults below:</para>
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<programlisting>dn: ou=groups,dc=example,dc=org
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objectClass: top
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objectClass: organizationalUnit
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ou: groups
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dn: cn=tuser,ou=groups,dc=example,dc=org
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objectClass: posixGroup
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objectClass: top
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gidNumber: 10000
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cn: tuser</programlisting>
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<para>To enter these into your database, you can use
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<command>slapadd</command> or <command>ldapadd</command>
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on a file containing these entries. Alternatively, you can use
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<filename role="package">sysutils/ldapvi</filename>.</para>
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<para>The <command>ldapsearch</command> utility on the client machine
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should now return these entries. If it does, your database is
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properly configured to be used as an LDAP authentication server.</para>
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</sect2>
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</sect1>
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<sect1 id="client">
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<title>Client Configuration</title>
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<para>The client should already have
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<application>OpenLDAP</application> libraries from <xref
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linkend="ldap-connect-client"/>, but if you are installing several
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client machines you will need to install <filename
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role="package">net/openldap24-client</filename> on each of
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them.</para>
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<para>&os; requires two ports to be installed to authenticate
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against an LDAP server, <filename
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role="package">security/pam_ldap</filename> and <filename
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role="package">net/nss_ldap</filename>.</para>
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<sect2 id="client-auth">
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<title>Authentication</title>
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<para><filename role="package">security/pam_ldap</filename> is
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configured via <filename>/usr/local/etc/ldap.conf</filename>.</para>
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<note>
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<para>This is a <emphasis>different file</emphasis> than the
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<application>OpenLDAP</application> library functions'
|
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configuration file,
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<filename>/usr/local/etc/openldap/ldap.conf</filename>; however,
|
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it takes many of the same options; in fact it is a superset of
|
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that file. For the rest of this section, references to
|
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<filename>ldap.conf</filename> will mean
|
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<filename>/usr/local/etc/ldap.conf</filename>.</para>
|
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</note>
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<para>Thus, we will want to copy all of our original
|
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configuration parameters from
|
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<filename>openldap/ldap.conf</filename> to the new
|
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<filename>ldap.conf</filename>. Once this is done, we want to
|
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tell <filename role="package">security/pam_ldap</filename> what to
|
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look for on the directory server.</para>
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<para>We are identifying our users with the <literal>uid</literal>
|
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attribute. To configure this (though it is the default), set the
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<literal>pam_login_attribute</literal> directive in
|
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<filename>ldap.conf</filename>:</para>
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<example id="set-pam-login-attr">
|
|
<title>Setting <literal>pam_login_attribute</literal></title>
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|
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<programlisting>pam_login_attribute uid</programlisting>
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</example>
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<para>With this set, <filename
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role="package">security/pam_ldap</filename> will search the entire
|
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LDAP directory under <literal>base</literal> for the value
|
|
<literal>uid=<replaceable>username</replaceable></literal>. If it
|
|
finds one and only one entry, it will attempt to bind as that user
|
|
with the password it was given. If it binds correctly, then it
|
|
will allow access. Otherwise it will fail.</para>
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|
|
<sect3 id="client-auth-pam">
|
|
<title>PAM</title>
|
|
|
|
<para>PAM, which stands for <quote>Pluggable Authentication
|
|
Modules</quote>, is the method by which &os; authenticates most
|
|
of its sessions. To tell &os; we wish to use an LDAP server, we
|
|
will have to add a line to the appropriate PAM file.</para>
|
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|
|
<para>Most of the time the appropriate PAM file is
|
|
<filename>/etc/pam.d/sshd</filename>, if you want to use
|
|
<application>SSH</application> (remember to set the relevant
|
|
options in <filename>/etc/ssh/sshd_config</filename>, otherwise
|
|
<application>SSH</application> will not use PAM).</para>
|
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|
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<para>To use PAM for authentication, add the line</para>
|
|
|
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<programlisting>auth sufficient /usr/local/lib/pam_ldap.so no_warn</programlisting>
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|
|
|
<para>Exactly where this line shows up in the file and which
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options appear in the fourth column determine the exact behavior
|
|
of the authentication mechanism; see &man.pam.d.5;</para>
|
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|
|
<para>With this configuration you should be able to authenticate
|
|
a user against an LDAP directory.
|
|
<application>PAM</application> will perform a bind with your
|
|
credentials, and if successful will tell
|
|
<application>SSH</application> to allow access.</para>
|
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|
|
<para>However it is not a good idea to allow
|
|
<emphasis>every</emphasis> user in the directory into
|
|
<emphasis>every</emphasis> client machine. With the
|
|
current configuration, all that a user needs to log into a
|
|
machine is an LDAP entry. Fortunately there are a few ways to
|
|
restrict user access.</para>
|
|
|
|
<para><filename>ldap.conf</filename> supports a
|
|
<literal>pam_groupdn</literal> directive; every account that
|
|
connects to this machine needs to be a member of the group
|
|
specified here. For example, if you have</para>
|
|
|
|
<programlisting>pam_groupdn cn=servername,ou=accessgroups,dc=example,dc=org</programlisting>
|
|
|
|
<para>in <filename>ldap.conf</filename>, then only members of
|
|
that group will be able to log in. There are a few things to
|
|
bear in mind, however.</para>
|
|
|
|
<para>Members of this group are specified in one or more
|
|
<literal>memberUid</literal> attributes, and each attribute must
|
|
have the full distinguished name of the member. So
|
|
<literal>memberUid: someuser</literal> will not work; it must
|
|
be:</para>
|
|
|
|
<programlisting>memberUid: uid=someuser,ou=people,dc=example,dc=org</programlisting>
|
|
|
|
<para>Additionally, this directive is not checked in PAM during
|
|
authentication, it is checked during account management, so you
|
|
will need a second line in your PAM files under
|
|
<literal>account</literal>. This will require, in turn,
|
|
<emphasis>every</emphasis> user to be listed in the group, which
|
|
is not necessarily what we want. To avoid blocking users that
|
|
are not in LDAP, you should enable the
|
|
<literal>ignore_unknown_user</literal> attribute. Finally, you
|
|
should set the <literal>ignore_authinfo_unavail</literal> option
|
|
so that you are not locked out of every computer when the LDAP
|
|
server is unavailable.</para>
|
|
|
|
<para>Your <filename>pam.d/sshd</filename> might then end up
|
|
looking like this:</para>
|
|
|
|
<example id="pam">
|
|
<title>Sample <filename>pam.d/sshd</filename></title>
|
|
|
|
<programlisting>auth required pam_nologin.so no_warn
|
|
auth sufficient pam_opie.so no_warn no_fake_prompts
|
|
auth requisite pam_opieaccess.so no_warn allow_local
|
|
auth sufficient /usr/local/lib/pam_ldap.so no_warn
|
|
auth required pam_unix.so no_warn try_first_pass
|
|
|
|
account required pam_login_access.so
|
|
account required /usr/local/lib/pam_ldap.so no_warn ignore_authinfo_unavail ignore_unknown_user</programlisting>
|
|
</example>
|
|
|
|
<note>
|
|
<para>Since we are adding these lines specifically to
|
|
<filename>pam.d/sshd</filename>, this will only have an effect
|
|
on <application>SSH</application> sessions. LDAP users will
|
|
be unable to log in at the console. To change this behavior,
|
|
examine the other files in <filename>/etc/pam.d</filename> and
|
|
modify them accordingly.</para>
|
|
</note>
|
|
</sect3>
|
|
</sect2>
|
|
|
|
<sect2 id="client-nss">
|
|
<title>Name Service Switch</title>
|
|
|
|
<para><application>NSS</application> is the service that maps
|
|
attributes to names. So, for example, if a file is owned by user
|
|
<literal>1001</literal>, an application will query
|
|
<application>NSS</application> for the name of
|
|
<literal>1001</literal>, and it might get <literal>bob</literal>
|
|
or <literal>ted</literal> or whatever the user's name is.</para>
|
|
|
|
<para>Now that our user information is kept in LDAP, we need to
|
|
tell <application>NSS</application> to look there when
|
|
queried.</para>
|
|
|
|
<para>The <filename role="package">net/nss_ldap</filename> port
|
|
does this. It uses the same configuration file as <filename
|
|
role="package">security/pam_ldap</filename>, and should not need
|
|
any extra parameters once it is installed. Instead, what is left
|
|
is simply to edit <filename>/etc/nsswitch.conf</filename> to take
|
|
advantage of the directory. Simply replace the following
|
|
lines:</para>
|
|
|
|
<programlisting>group: compat
|
|
passwd: compat</programlisting>
|
|
|
|
<para>with</para>
|
|
|
|
<programlisting>group: files ldap
|
|
passwd: files ldap</programlisting>
|
|
|
|
<para>This will allow you to map usernames to UIDs and UIDs to
|
|
usernames.</para>
|
|
|
|
<para>Congratulations! You should now have working LDAP
|
|
authentication.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="caveats">
|
|
<title>Caveats</title>
|
|
|
|
<para>Unfortunately, as of the time this was written &os; did not
|
|
support changing user passwords with &man.passwd.1;. Because of
|
|
this, most administrators are left to implement a solution
|
|
themselves. I provide some examples here. Note that if you write
|
|
your own password change script, there are some security issues
|
|
you should be made aware of; see <xref
|
|
linkend="security-passwd"/></para>
|
|
|
|
<example id="chpw-shell">
|
|
<title>Shell script for changing passwords</title>
|
|
|
|
<programlisting><![CDATA[#!/bin/sh
|
|
|
|
stty -echo
|
|
read -p "Old Password: " oldp; echo
|
|
read -p "New Password: " np1; echo
|
|
read -p "Retype New Password: " np2; echo
|
|
stty echo
|
|
|
|
if [ "$np1" != "$np2" ]; then
|
|
echo "Passwords do not match."
|
|
exit 1
|
|
fi
|
|
|
|
ldappasswd -D uid="$USER",ou=people,dc=example,dc=org \
|
|
-w "$oldp" \
|
|
-a "$oldp" \
|
|
-s "$np1"]]></programlisting>
|
|
</example>
|
|
|
|
<caution>
|
|
<para>This script does hardly any error checking, but more
|
|
important it is very cavalier about how it stores your
|
|
passwords. If you do anything like this, at least adjust
|
|
the <literal>security.bsd.see_other_uids</literal>
|
|
sysctl value:</para>
|
|
|
|
<screen>&prompt.root; <userinput>sysctl security.bsd.see_other_uids=0</userinput>.</screen>
|
|
</caution>
|
|
|
|
<para>A more flexible (and probably more secure) approach can be
|
|
used by writing a custom program, or even a web interface. The
|
|
following is part of a <application>Ruby</application> library
|
|
that can change LDAP passwords. It sees use both on the command
|
|
line, and on the web.</para>
|
|
|
|
<example id="chpw-ruby">
|
|
<title>Ruby script for changing passwords</title>
|
|
|
|
<programlisting><![CDATA[require 'ldap'
|
|
require 'base64'
|
|
require 'digest'
|
|
require 'password' # ruby-password
|
|
|
|
ldap_server = "ldap.example.org"
|
|
luser = "uid=#{ENV['USER']},ou=people,dc=example,dc=org"
|
|
|
|
# get the new password, check it, and create a salted hash from it
|
|
def get_password
|
|
pwd1 = Password.get("New Password: ")
|
|
pwd2 = Password.get("Retype New Password: ")
|
|
|
|
raise if pwd1 != pwd2
|
|
pwd1.check # check password strength
|
|
|
|
salt = rand.to_s.gsub(/0\./, '')
|
|
pass = pwd1.to_s
|
|
hash = "{SSHA}"+Base64.encode64(Digest::SHA1.digest("#{pass}#{salt}")+salt).chomp!
|
|
return hash
|
|
end
|
|
|
|
oldp = Password.get("Old Password: ")
|
|
newp = get_password
|
|
|
|
# We'll just replace it. That we can bind proves that we either know
|
|
# the old password or are an admin.
|
|
|
|
replace = LDAP::Mod.new(LDAP::LDAP_MOD_REPLACE | LDAP::LDAP_MOD_BVALUES,
|
|
"userPassword",
|
|
[newp])
|
|
|
|
conn = LDAP::SSLConn.new(ldap_server, 389, true)
|
|
conn.set_option(LDAP::LDAP_OPT_PROTOCOL_VERSION, 3)
|
|
conn.bind(luser, oldp)
|
|
conn.modify(luser, [replace])]]></programlisting>
|
|
</example>
|
|
|
|
<para>Although not guaranteed to be free of security holes (the
|
|
password is kept in memory, for example) this is cleaner and more
|
|
flexible than a simple <command>sh</command> script.</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="secure">
|
|
<title>Security Considerations</title>
|
|
|
|
<para>Now that your machines (and possibly other services) are
|
|
authenticating against your LDAP server, this server needs to be
|
|
protected at least as well as
|
|
<filename>/etc/master.passwd</filename> would be on a regular
|
|
server, and possibly even more so since a broken or cracked LDAP
|
|
server would break every client service.</para>
|
|
|
|
<para>Remember, this section is not exhaustive. You should
|
|
continually review your configuration and procedures for
|
|
improvements.</para>
|
|
|
|
<sect2 id="secure-readonly">
|
|
<title>Setting attributes read-only</title>
|
|
|
|
<para>Several attributes in LDAP should be read-only. If left
|
|
writable by the user, for example, a user could change his
|
|
<literal>uidNumber</literal> attribute to <literal>0</literal> and
|
|
get <username>root</username> access!</para>
|
|
|
|
<para>To begin with, the <literal>userPassword</literal> attribute
|
|
should not be world-readable. By default, anyone who can connect
|
|
to the LDAP server can read this attribute. To disable this, put
|
|
the following in <filename>slapd.conf</filename>:</para>
|
|
|
|
<example id="hide-userpass">
|
|
<title>Hide passwords</title>
|
|
|
|
<programlisting>access to dn.subtree="ou=people,dc=example,dc=org"
|
|
attrs=userPassword
|
|
by self write
|
|
by anonymous auth
|
|
by * none
|
|
|
|
access to *
|
|
by self write
|
|
by * read</programlisting>
|
|
</example>
|
|
|
|
<para>This will disallow reading of the
|
|
<literal>userPassword</literal> attribute, while still allowing
|
|
users to change their own passwords.</para>
|
|
|
|
<para>Additionally, you'll want to keep users from changing some
|
|
of their own attributes. By default, users can change any
|
|
attribute (except for those which the LDAP schemas themselves deny
|
|
changes), such as <literal>uidNumber</literal>. To close this
|
|
hole, modify the above to</para>
|
|
|
|
<example id="attrib-readonly">
|
|
<title>Read-only attributes</title>
|
|
|
|
<programlisting>access to dn.subtree="ou=people,dc=example,dc=org"
|
|
attrs=userPassword
|
|
by self write
|
|
by anonymous auth
|
|
by * none
|
|
|
|
access to attrs=homeDirectory,uidNumber,gidNumber
|
|
by * read
|
|
|
|
access to *
|
|
by self write
|
|
by * read</programlisting>
|
|
</example>
|
|
|
|
<para>This will stop users from being able to masquerade as other
|
|
users.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="secure-root">
|
|
<title><username>Root</username> account definition</title>
|
|
|
|
<para>Often the <username>root</username> or manager account for
|
|
the LDAP service will be defined in the configuration file.
|
|
<application>OpenLDAP</application> supports this, for example,
|
|
and it works, but it can lead to trouble if
|
|
<filename>slapd.conf</filename> is compromised. It may be better
|
|
to use this only to bootstrap yourself into LDAP, and then define
|
|
a <username>root</username> account there.</para>
|
|
|
|
<para>Even better is to define accounts that have limited
|
|
permissions, and omit a <username>root</username> account entirely.
|
|
For example, users to can add or remove user accounts are added to
|
|
one group, but they cannot themselves change the membership of
|
|
this group. Such a security policy would help mitigate the effects
|
|
of a leaked password.</para>
|
|
|
|
<sect3 id="manager-acct">
|
|
<title>Creating a management group</title>
|
|
|
|
<para>Say you want your IT department to be able to change home
|
|
directories for users, but you don't want all of them to be able
|
|
to add or remove users. The way to do this is to add a group
|
|
for these admins:</para>
|
|
|
|
<example id="manager-acct-dn">
|
|
<title>Creating a management group</title>
|
|
|
|
<programlisting>dn: cn=homemanagement,dc=example,dc=org
|
|
objectClass: top
|
|
objectClass: posixGroup
|
|
cn: homemanagement
|
|
gidNumber: 121 # required for posixGroup
|
|
memberUid: uid=tuser,ou=people,dc=example,dc=org
|
|
memberUid: uid=user2,ou=people,dc=example,dc=org</programlisting>
|
|
</example>
|
|
|
|
<para>And then change the permissions attributes in
|
|
<filename>slapd.conf</filename>:</para>
|
|
|
|
<example id="management-acct-acl">
|
|
<title>ACLs for a home directory management group</title>
|
|
|
|
<programlisting>access to dn.subtree="ou=people,dc=example,dc=org"
|
|
attr=homeDirectory
|
|
by dn="cn=homemanagement,dc=example,dc=org"
|
|
dnattr=memberUid write</programlisting>
|
|
</example>
|
|
|
|
<para>Now <username>tuser</username> and <username>user2</username>
|
|
can change other users' home directories.</para>
|
|
|
|
<para>In this example we've given a subset of administrative
|
|
power to certain users without giving them power in other
|
|
domains. The idea is that soon no single user account has the
|
|
power of a <username>root</username> account, but every power
|
|
root had is had by at least one user. The <username>root</username>
|
|
account then becomes unnecessary and can be removed.</para>
|
|
</sect3>
|
|
</sect2>
|
|
|
|
<sect2 id="security-passwd">
|
|
<title>Password storage</title>
|
|
|
|
<para>By default <application>OpenLDAP</application> will store
|
|
the value of the <literal>userPassword</literal> attribute as it
|
|
stores any other data: in the clear. Most of the time it is base
|
|
64 encoded, which provides enough protection to keep an honest
|
|
administrator from knowing your password, but little else.</para>
|
|
|
|
<para>It is a good idea, then, to store passwords in a more secure
|
|
format, such as SSHA (salted SHA). This is done by whatever
|
|
program you use to change users' passwords.</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<appendix id="useful">
|
|
<title>Useful Aids</title>
|
|
|
|
<para>There are a few other programs that might be useful,
|
|
particularly if you have many users and do not want to configure
|
|
everything manually.</para>
|
|
|
|
<para><filename role="package">security/pam_mkhomedir</filename> is
|
|
a PAM module that always succeeds; its purpose is to create home
|
|
directories for users which do not have them. If you have dozens of
|
|
client servers and hundreds of users, it is much easier to use this
|
|
and set up skeleton directories than to prepare every home
|
|
directory.</para>
|
|
|
|
<para><filename role="package">sysutils/cpu</filename> is a
|
|
&man.pw.8;-like utility that can be used to manage users in the LDAP
|
|
directory. You can call it directly, or wrap scripts around it. It
|
|
can handle both TLS (with the <option>-x</option> flag) and
|
|
SSL (directly).</para>
|
|
|
|
<para><filename role="package">sysutils/ldapvi</filename> is a great
|
|
utility for editing LDAP values in an LDIF-like syntax. The
|
|
directory (or subsection of the directory) is presented in the
|
|
editor chosen by the <envar>EDITOR</envar> environment variable.
|
|
This makes it easy to enable large-scale changes in the directory
|
|
without having to write a custom tool.</para>
|
|
|
|
<para><filename role="package">security/openssh-portable</filename>
|
|
has the ability to contact an LDAP server to verify
|
|
<application>SSH</application> keys. This is extremely nice if you
|
|
have many servers and do not want to copy your public keys across
|
|
all of them.</para>
|
|
</appendix>
|
|
|
|
<appendix id="ssl-ca">
|
|
<title><application>OpenSSL</application> Certificates For LDAP</title>
|
|
|
|
<para>If you are hosting two or more LDAP servers, you will probably
|
|
not want to use self-signed certificates, since each client will
|
|
have to be configured to work with each certificate. While this is
|
|
possible, it is not nearly as simple as creating your own
|
|
certificate authority, and signing your servers' certificates with
|
|
that.</para>
|
|
|
|
<para>The steps here are presented as they are with very little
|
|
attempt at explaining what is going on—further explanation
|
|
can be found in &man.openssl.1; and its friends.</para>
|
|
|
|
<para>To create a certificate authority, we simply need a
|
|
self-signed certificate and key. The steps for this again
|
|
are</para>
|
|
|
|
<example id="make-cert">
|
|
<title>Creating a certificate</title>
|
|
|
|
<screen>&prompt.user; <userinput>openssl genrsa -out root.key 1024</userinput>
|
|
&prompt.user; <userinput>openssl req -new -key root.key -out root.csr</userinput>
|
|
&prompt.user; <userinput>openssl x509 -req -days 1024 -in root.csr -signkey root.key -out root.crt</userinput></screen>
|
|
</example>
|
|
|
|
<para>These will be your root CA key and certificate. You will
|
|
probably want to encrypt the key and store it in a cool, dry place;
|
|
anyone with access to it can masquerade as one of your LDAP
|
|
servers.</para>
|
|
|
|
<para>Next, using the first two steps above create a key
|
|
<filename>ldap-server-one.key</filename> and certificate signing
|
|
request <filename>ldap-server-one.csr</filename>. Once you sign the
|
|
signing request with <filename>root.key</filename>, you will be able
|
|
to use <filename>ldap-server-one.*</filename> on your LDAP
|
|
servers.</para>
|
|
|
|
<note>
|
|
<para>Do not forget to use the fully qualified domain name for the
|
|
<quote>common name</quote> attribute when generating the
|
|
certificate signing request; otherwise clients will reject a
|
|
connection with you, and it can be very tricky to diagnose.</para>
|
|
</note>
|
|
|
|
<para>To sign the key, use <option>-CA</option> and
|
|
<option>-CAkey</option> instead of
|
|
<option>-signkey</option>:</para>
|
|
|
|
<example id="ca-sign">
|
|
<title>Signing as a certificate authority</title>
|
|
|
|
<screen>&prompt.user; <userinput>openssl x509 -req -days 1024 \
|
|
-in ldap-server-one.csr -CA root.crt -CAkey root.key \
|
|
-out ldap-server-one.crt</userinput></screen>
|
|
</example>
|
|
|
|
<para>The resulting file will be the certificate that you can use on
|
|
your LDAP servers.</para>
|
|
|
|
<para>Finally, for clients to trust all your servers, distribute
|
|
<filename>root.crt</filename> (the <emphasis>certificate</emphasis>,
|
|
not the key!) to each client, and specify it in the
|
|
<literal>TLSCACertificateFile</literal> directive in
|
|
<filename>ldap.conf</filename>.</para>
|
|
</appendix>
|
|
</article>
|