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Postfix LDAP Howto

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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
  * Configuring LDAP lookups
  * Example: aliases
  * Example: virtual domains/addresses
  * Example: expanding LDAP groups
  * Other uses of LDAP lookups
  * Notes and things to think about
  * Feedback
  * Credits

Building Postfix with LDAP support

These instructions assume that you build Postfix from source code as described
in the INSTALL document. Some modification may be required if you build Postfix
from a vendor-specific source package.

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 OpenLDAP (https://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_LDAP="-L/usr/local/lib -lldap -L/usr/local/lib -llber"

If your LDAP shared library is in a directory that the RUN-TIME linker does not
know about, add a "-Wl,-R,/path/to/directory" option after "-lldap".

Postfix versions before 3.0 use AUXLIBS instead of AUXLIBS_LDAP. With Postfix
3.0 and later, the old AUXLIBS variable still supports building a statically-
loaded LDAP database client, but only the new AUXLIBS_LDAP variable supports
building a dynamically-loaded or statically-loaded LDAP database client.

    Failure to use the AUXLIBS_LDAP variable will defeat the purpose of dynamic
    database client loading. Every Postfix executable file will have LDAP
    database library dependencies. And that was exactly what dynamic database
    client loading was meant to avoid.

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_LDAP="-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.

If you are using OpenLDAP, and the libraries were built with SASL support, you
can add -DUSE_LDAP_SASL to the CCARGS to enable SASL support. For example:

         CCARGS="-I/usr/local/include -DHAS_LDAP -DUSE_LDAP_SASL"

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 or STARTTLS, and LDAP SASL. 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.example.com
    search_base = dc=example, 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.example.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 "@fake.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".

Example: expanding LDAP groups

LDAP is frequently used to store group member information. There are a number
of ways of handling LDAP groups. We will show a few examples in order of
increasing complexity, but owing to the number of independent variables, we can
only present a tiny portion of the solution space. We show how to:

 1. query groups as lists of addresses;

 2. query groups as lists of user objects containing addresses;

 3. forward special lists unexpanded to a separate list server, for moderation
    or other processing;

 4. handle complex schemas by controlling expansion and by treating leaf nodes
    specially, using features that are new in Postfix 2.4.

The example LDAP entries and implied schema below show two group entries
("agroup" and "bgroup") and four user entries ("auser", "buser", "cuser" and
"duser"). The group "agroup" has the users "auser" (1) and "buser" (2) as
members via DN references in the multi-valued attribute "memberdn", and direct
email addresses of two external users "auser@example.org" (3) and
"buser@example.org" (4) stored in the multi-valued attribute "memberaddr". The
same is true of "bgroup" and "cuser"/"duser" (6)/(7)/(8)/(9), but "bgroup" also
has a "maildrop" attribute of "bgroup@mlm.example.com" (5):

         dn: cn=agroup, dc=example, dc=com
         objectclass: top
         objectclass: ldapgroup
         cn: agroup
         mail: agroup@example.com
    1 -> memberdn: uid=auser, dc=example, dc=com
    2 -> memberdn: uid=buser, dc=example, dc=com
    3 -> memberaddr: auser@example.org
    4 -> memberaddr: buser@example.org

         dn: cn=bgroup, dc=example, dc=com
         objectclass: top
         objectclass: ldapgroup
         cn: bgroup
         mail: bgroup@example.com
    5 -> maildrop: bgroup@mlm.example.com
    6 -> memberdn: uid=cuser, dc=example, dc=com
    7 -> memberdn: uid=duser, dc=example, dc=com
    8 -> memberaddr: cuser@example.org
    9 -> memberaddr: duser@example.org

         dn: uid=auser, dc=example, dc=com
         objectclass: top
         objectclass: ldapuser
         uid: auser
    10 -> mail: auser@example.com
    11 -> maildrop: auser@mailhub.example.com

         dn: uid=buser, dc=example, dc=com
         objectclass: top
         objectclass: ldapuser
         uid: buser
    12 -> mail: buser@example.com
    13 -> maildrop: buser@mailhub.example.com

         dn: uid=cuser, dc=example, dc=com
         objectclass: top
         objectclass: ldapuser
         uid: cuser
    14 -> mail: cuser@example.com

         dn: uid=duser, dc=example, dc=com
         objectclass: top
         objectclass: ldapuser
         uid: duser
    15 -> mail: duser@example.com

Our first use case ignores the "memberdn" attributes, and assumes that groups
hold only direct "memberaddr" strings as in (3), (4), (8) and (9). The goal is
to map the group address to the list of constituent "memberaddr" values. This
is simple, ignoring the various connection related settings (hosts, ports, bind
settings, timeouts, ...) we have:

        simple.cf:
            ...
            search_base = dc=example, dc=com
            query_filter = mail=%s
            result_attribute = memberaddr
        $ postmap -q agroup@example.com ldap:/etc/postfix/simple.cf \
            auser@example.org,buser@example.org

We search "dc=example, dc=com". The "mail" attribute is used in the
query_filter to locate the right group, the "result_attribute" setting
described in ldap_table(5) is used to specify that "memberaddr" values from the
matching group are to be returned as a comma separated list. Always check
tables using postmap(1) with the "-q" option, before deploying them into
production use in main.cf.

Our second use case instead expands "memberdn" attributes (1), (2), (6) and
(7), follows the DN references and returns the "maildrop" of the referenced
user entries. Here we use the "special_result_attribute" setting from
ldap_table(5) to designate the "memberdn" attribute as holding DNs of the
desired member entries. The "result_attribute" setting selects which attributes
are returned from the selected DNs. It is important to choose a result
attribute that is not also present in the group object, because result
attributes are collected from both the group and the member DNs. In this case
we choose "maildrop" and assume for the moment that groups never have a
"maildrop" (the "bgroup" "maildrop" attribute is for a different use case). The
returned data for "auser" and "buser" is from items (11) and (13) in the
example data.

        special.cf:
            ...
            search_base = dc=example, dc=com
            query_filter = mail=%s
            result_attribute = maildrop
            special_result_attribute = memberdn
        $ postmap -q agroup@example.com ldap:/etc/postfix/special.cf \
            auser@mailhub.example.com,buser@mailhub.example.com

Note: if the desired member object result attribute is always also present in
the group, you get surprising results: the expansion also returns the address
of the group. This is a known limitation of Postfix releases prior to 2.4, and
is addressed in the new with Postfix 2.4 "leaf_result_attribute" feature
described in ldap_table(5).

Our third use case has some groups that are expanded immediately, and other
groups that are forwarded to a dedicated mailing list manager host for delayed
expansion. This uses two LDAP tables, one for users and forwarded groups and a
second for groups that can be expanded immediately. It is assumed that groups
that require forwarding are never nested members of groups that are directly
expanded.

        no_expand.cf:
            ...
            search_base = dc=example, dc=com
            query_filter = mail=%s
            result_attribute = maildrop
        expand.cf
            ...
            search_base = dc=example, dc=com
            query_filter = mail=%s
            result_attribute = maildrop
            special_result_attribute = memberdn
        $ postmap -q auser@example.com \
            ldap:/etc/postfix/no_expand.cf ldap:/etc/postfix/expand.cf \
            auser@mailhub.example.com
        $ postmap -q agroup@example.com \
            ldap:/etc/postfix/no_expand.cf ldap:/etc/postfix/expand.cf \
            auser@mailhub.example.com,buser@mailhub.example.com
        $ postmap -q bgroup@example.com \
            ldap:/etc/postfix/no_expand.cf ldap:/etc/postfix/expand.cf \
            bgroup@mlm.example.com

Non-group objects and groups with delayed expansion (those that have a maildrop
attribute) are rewritten to a single maildrop value. Groups that don't have a
maildrop are expanded as the second use case. This admits a more elegant
solution with Postfix 2.4 and later.

Our final use case is the same as the third, but this time uses new features in
Postfix 2.4. We now are able to use just one LDAP table and no longer need to
assume that forwarded groups are never nested inside expanded groups.

        fancy.cf:
            ...
            search_base = dc=example, dc=com
            query_filter = mail=%s
            result_attribute = memberaddr
            special_result_attribute = memberdn
            terminal_result_attribute = maildrop
            leaf_result_attribute = mail
        $ postmap -q auser@example.com ldap:/etc/postfix/fancy.cf \
            auser@mailhub.example.com
        $ postmap -q cuser@example.com ldap:/etc/postfix/fancy.cf \
            cuser@example.com
        $ postmap -q agroup@example.com ldap:/etc/postfix/fancy.cf \

    auser@mailhub.example.com,buser@mailhub.example.com,auser@example.org,buser@example.org
        $ postmap -q bgroup@example.com ldap:/etc/postfix/fancy.cf \
            bgroup@mlm.example.com

Above, delayed expansion is enabled via "terminal_result_attribute", which, if
present, is used as the sole result and all other expansion is suppressed.
Otherwise, the "leaf_result_attribute" is only returned for leaf objects that
don't have a "special_result_attribute" (non-groups), while the
"result_attribute" (direct member address of groups) is returned at every level
of recursive expansion, not just the leaf nodes. This fancy example illustrates
all the features of Postfix 2.4 group expansion.

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@example.com" instead of
"userid@example.com".

Notes and things to think about

  * The bits of schema and attribute names used in this document are just
    examples. There's nothing special about them, other than that some are the
    defaults in the LDAP configuration parameters. You can use whatever schema
    you like, and configure Postfix accordingly.

  * You probably want to make sure that mailacceptinggeneralids are unique, and
    that not just anyone can specify theirs as postmaster or root, say.

  * An entry can have an arbitrary number of mailacceptinggeneralids or
    maildrops. Maildrops can also be comma-separated lists of addresses. They
    will all be found and returned by the lookups. For example, you could
    define an entry intended for use as a mailing list that looks like this
    (Warning! Schema made up just for this example):

        dn: cn=Accounting Staff List, dc=example, dc=com
        cn: Accounting Staff List
        o: example.com
        objectclass: maillist
        mailacceptinggeneralid: accountingstaff
        mailacceptinggeneralid: accounting-staff
        maildrop: mylist-owner
        maildrop: an-accountant
        maildrop: some-other-accountant
        maildrop: this, that, theother

  * If you use an LDAP map for lookups other than aliases, you may have to make
    sure the lookup makes sense. In the case of virtual lookups, maildrops
    other than mail addresses are pretty useless, because Postfix can't know
    how to set the ownership for program or file delivery. Your query_filter
    should probably look something like this:

        query_filter = (&(mailacceptinggeneralid=%s)(!(|(maildrop="*|*")
        (maildrop="*:*")(maildrop="*/*"))))

  * And for that matter, even for aliases, you may not want users to be able to
    specify their maildrops as programs, includes, etc. This might be
    particularly pertinent on a "sealed" server where they don't have local
    UNIX accounts, but exist only in LDAP and Cyrus. You might allow the fun
    stuff only for directory entries owned by an administrative account, so
    that if the object had a program as its maildrop and weren't owned by
    "cn=root" it wouldn't be returned as a valid local user. This will require
    some thought on your part to implement safely, considering the
    ramifications of this type of delivery. You may decide it's not worth the
    bother to allow any of that nonsense in LDAP lookups, ban it in the
    query_filter, and keep things like majordomo lists in local alias
    databases.

        query_filter = (&(mailacceptinggeneralid=%s)(!(|(maildrop="*|*")
        (maildrop="*:*")(maildrop="*/*"))(owner=cn=root, dc=your, dc=com)))

  * LDAP lookups are slower than local DB or DBM lookups. For most sites they
    won't be a bottleneck, but it's a good idea to know how to tune your
    directory service.

  * Multiple LDAP maps share the same LDAP connection if they differ only in
    their query related parameters: base, scope, query_filter, and so on. To
    take advantage of this, avoid spurious differences in the definitions of
    LDAP maps: host selection order, version, bind, tls parameters, ... should
    be the same for multiple maps whenever possible.

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

  * Manuel Guesdon: Spotted a bug with the timeout attribute.
  * John Hensley: Multiple LDAP sources with more configurable attributes.
  * Carsten Hoeger: Search scope handling.
  * LaMont Jones: Domain restriction, URL and DN searches, multiple result
    attributes.
  * Mike Mattice: Alias dereferencing control.
  * Hery Rakotoarisoa: Patches for LDAPv3 updating.
  * Prabhat K Singh: Wrote the initial Postfix LDAP lookups and connection
    caching.
  * Keith Stevenson: RFC 2254 escaping in queries.
  * Samuel Tardieu: Noticed that searches could include wildcards, prompting
    the work on RFC 2254 escaping in queries. Spotted a bug in binding.
  * Sami Haahtinen: Referral chasing and v3 support.
  * Victor Duchovni: ldap_bind() timeout. With fixes from LaMont Jones:
    OpenLDAP cache deprecation. Limits on recursion, expansion and search
    results size. LDAP connection sharing for maps differing only in the query
    parameters.
  * Liviu Daia: Support for SSL/STARTTLS. Support for storing map definitions
    in external files (ldap:/path/ldap.cf) needed to securely store passwords
    for plain auth.
  * Liviu Daia revised the configuration interface and added the main.cf
    configuration feature.
  * Liviu Daia with further refinements from Jose Luis Tallon and Victor
    Duchovni developed the common query, result_format, domain and
    expansion_limit interface for LDAP, MySQL and PosgreSQL.
  * Gunnar Wrobel provided a first implementation of a feature to limit LDAP
    search results to leaf nodes only. Victor generalized this into the Postfix
    2.4 "leaf_result_attribute" feature.
  * Quanah Gibson-Mount contributed support for advanced LDAP SASL mechanisms,
    beyond the password-based LDAP "simple" bind.

And of course Wietse.

