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Postfix LDAP Howto


LDAP Support in Postfix

Postfix can use an LDAP directory as a source for any of its lookups: aliases(5), virtual(5), canonical(5), etc. This allows you to keep information for your mail service in a replicated network database with fine-grained access controls. By not storing it locally on the mail server, the administrators can maintain it from anywhere, and the users can control whatever bits of it you think appropriate. You can have multiple mail servers using the same information, without the hassle and delay of having to copy it to each.

Topics covered in this document:

Building Postfix with LDAP support

Note 1: Postfix no longer supports the LDAP version 1 interface.

Note 2: to use LDAP with Debian GNU/Linux's Postfix, all you need is to install the postfix-ldap package and you're done. There is no need to recompile Postfix.

You need to have LDAP libraries and include files installed somewhere on your system, and you need to configure the Postfix Makefiles accordingly.

For example, to build the OpenLDAP libraries for use with Postfix (i.e. LDAP client code only), you could use the following command:

% ./configure  --without-kerberos --without-cyrus-sasl --without-tls \
    --without-threads --disable-slapd --disable-slurpd \
    --disable-debug --disable-shared

If you're using the libraries from the UM distribution (http://www.umich.edu/~dirsvcs/ldap/ldap.html) or OpenLDAP (http://www.openldap.org), something like this in the top level of your Postfix source tree should work:

% make tidy
% make makefiles CCARGS="-I/usr/local/include -DHAS_LDAP" \
    AUXLIBS="-L/usr/local/lib -lldap -L/usr/local/lib -llber"

On Solaris 2.x you may have to specify run-time link information, otherwise ld.so will not find some of the shared libraries:

% make tidy
% make makefiles CCARGS="-I/usr/local/include -DHAS_LDAP" \
    AUXLIBS="-L/usr/local/lib -R/usr/local/lib -lldap \
            -L/usr/local/lib -R/usr/local/lib -llber"

The 'make tidy' command is needed only if you have previously built Postfix without LDAP support.

Instead of '/usr/local' specify the actual locations of your LDAP include files and libraries. Be sure to not mix LDAP include files and LDAP libraries of different versions!!

If your LDAP libraries were built with Kerberos support, you'll also need to include your Kerberos libraries in this line. Note that the KTH Kerberos IV libraries might conflict with Postfix's lib/libdns.a, which defines dns_lookup. If that happens, you'll probably want to link with LDAP libraries that lack Kerberos support just to build Postfix, as it doesn't support Kerberos binds to the LDAP server anyway. Sorry about the bother.

If you're using one of the Netscape LDAP SDKs, you'll need to change the AUXLIBS line to point to libldap10.so or libldapssl30.so or whatever you have, and you may need to use the appropriate linker option (e.g. '-R') so the executables can find it at runtime.

Configuring LDAP lookups

In order to use LDAP lookups, define an LDAP source as a table lookup in main.cf, for example:

alias_maps = hash:/etc/aliases, ldap:/etc/postfix/ldap-aliases.cf

The file /etc/postfix/ldap-aliases.cf can specify a great number of parameters, including parameters that enable LDAP SSL and STARTTLS. For a complete description, see the ldap_table(5) manual page.

Example: local(8) aliases

Here's a basic example for using LDAP to look up local(8) aliases. Assume that in main.cf, you have:

alias_maps = hash:/etc/aliases, ldap:/etc/postfix/ldap-aliases.cf

and in ldap:/etc/postfix/ldap-aliases.cf you have:

server_host = ldap.my.com
search_base = dc=my, dc=com

Upon receiving mail for a local address "ldapuser" that isn't found in the /etc/aliases database, Postfix will search the LDAP server listening at port 389 on ldap.my.com. It will bind anonymously, search for any directory entries whose mailacceptinggeneralid attribute is "ldapuser", read the "maildrop" attributes of those found, and build a list of their maildrops, which will be treated as RFC822 addresses to which the message will be delivered.

Example: virtual domains/addresses

If you want to keep information for virtual lookups in your directory, it's only a little more complicated. First, you need to make sure Postfix knows about the virtual domain. An easy way to do that is to add the domain to the mailacceptinggeneralid attribute of some entry in the directory. Next, you'll want to make sure all of your virtual recipient's mailacceptinggeneralid attributes are fully qualified with their virtual domains. Finally, if you want to designate a directory entry as the default user for a virtual domain, just give it an additional mailacceptinggeneralid (or the equivalent in your directory) of "@virtual.dom". That's right, no user part. If you don't want a catchall user, omit this step and mail to unknown users in the domain will simply bounce.

In summary, you might have a catchall user for a virtual domain that looks like this:

     dn: cn=defaultrecipient, dc=fake, dc=dom
     objectclass: top
     objectclass: virtualaccount
     cn: defaultrecipient
     owner: uid=root, dc=someserver, dc=isp, dc=dom
1 -> mailacceptinggeneralid: fake.dom
2 -> mailacceptinggeneralid: @fake.dom
3 -> maildrop: realuser@real.dom         

1: Postfix knows fake.dom is a valid virtual domain when it looks for this and gets something (the maildrop) back.

2: This causes any mail for unknown users in fake.dom to go to this entry ...

3: ... and then to its maildrop.

Normal users might simply have one mailacceptinggeneralid and maildrop, e.g. "normaluser@fake.dom" and "normaluser@real.dom".

Other uses of LDAP lookups

Other common uses for LDAP lookups include rewriting senders and recipients with Postfix's canonical lookups, for example in order to make mail leaving your site appear to be coming from "First.Last@site.dom" instead of "userid@site.dom".

Notes and things to think about

Feedback

If you have questions, send them to postfix-users@postfix.org. Please include relevant information about your Postfix setup: LDAP-related output from postconf, which LDAP libraries you built with, and which directory server you're using. If your question involves your directory contents, please include the applicable bits of some directory entries.

Credits

And of course Wietse.