SLAPD-META(5)SLAPD-META(5)NAMEslapd-meta - metadirectory backend
SYNOPSIS
ETCDIR/slapd.conf
DESCRIPTION
The meta backend to slapd(8) performs basic LDAP proxying
with respect to a set of remote LDAP servers, called "tar-
gets". The information contained in these servers can be
presented as belonging to a single Directory Information
Tree (DIT).
A basic knowledge of the functionality of the
slapd-ldap(5) backend is recommended. This backend has
been designed as an enhancement of the ldap backend. The
two backends share many features (actually they also share
portions of code). While the ldap backend is intended to
proxy operations directed to a single server, the meta
backend is mainly intended for proxying of multiple
servers and possibly naming context masquerading. These
features, although useful in many scenarios, may result in
excessive overhead for some applications, so its use
should be carefully considered. In the examples section,
some typical scenarios will be discussed.
EXAMPLES
There are examples in various places in this document, as
well as in the slapd/back-meta/data/ directory in the
OpenLDAP source tree.
CONFIGURATION
These slapd.conf options apply to the META backend
database. That is, they must follow a "database meta"
line and come before any subsequent "backend" or
"database" lines. Other database options are described in
the slapd.conf(5) manual page.
Note: as with the ldap backend, operational attributes
related to entry creation/modification should not be used,
as they would be passed to the target servers, generating
an error. Moreover, it makes little sense to use such
attributes in proxying, as the proxy server doesn't actu-
ally store data, so it should have no knowledge of such
attributes. While code to strip the modification
attributes has been put in place (and #ifdef'd), it
implies unmotivated overhead. So it is strongly recom-
mended to set
lastmod off
for every ldap and meta backend.
SPECIAL CONFIGURATION DIRECTIVES
Target configuration starts with the "uri" directive. All
the configuration directives that are not specific to tar-
gets should be defined first for clarity, including those
that are common to all backends. They are:
default-target none
This directive forces the backend to reject all
those operations that must resolve to a single tar-
get in case none or multiple targets are selected.
They include: add, delete, modify, modrdn; compare
is not included, as well as bind since, as they
don't alter entries, in case of multiple matches an
attempt is made to perform the operation on any
candidate target, with the constraint that at most
one must succeed. This directive can also be used
when processing targets to mark a specific target
as default.
dncache-ttl {forever|disabled|<ttl>}
This directive sets the time-to-live of the DN
cache. This caches the target that holds a given
DN to speed up target selection in case multiple
targets would result from an uncached search; for-
ever means cache never expires; disabled means no
DN caching; otherwise a valid ( > 0 ) ttl in sec-
onds is required.
TARGET SPECIFICATION
Target specification starts with a "uri" directive:
uri <protocol>://[<host>[:<port>]]/<naming context>
The "server" directive that was allowed in the LDAP
backend (although deprecated) has been discarded in
the Meta backend. The <protocol> part can be any-
thing ldap_initialize(3) accepts
({ldap|ldaps|ldapi} and variants); <host> and
<port> may be omitted, defaulting to whatever is
set in /etc/ldap.conf. The <naming context> part
is mandatory. It must end with one of the naming
contexts defined for the backend, e.g.:
suffix "dc=foo,dc=com"
uri "ldap://x.foo.com/dc=x,dc=foo,dc=com"
The <naming context> part doesn't need to be unique across
the targets; it may also match one of the values of the
"suffix" directive. Multiple URIs may be defined in a
single argument. The URIs must be separated by TABs (e.g.
'\t'), and the additional URIs must have no <naming con-
text> part. This causes the underlying library to contact
the first server of the list that responds.
default-target [<target>]
The "default-target" directive can also be used
during target specification. With no arguments it
marks the current target as the default. The
optional number marks target <target> as the
default one, starting from 1. Target <target> must
be defined.
binddn <administrative DN for access control purposes>
This directive, as in the LDAP backend, allows to
define the DN that is used to query the target
server for acl checking; it should have read access
on the target server to attributes used on the
proxy for acl checking. There is no risk of giving
away such values; they are only used to check per-
missions.
bindpw <password for access control purposes>
This directive sets the password for acl checking
in conjunction with the above mentioned "binddn"
directive.
rebind-as-user
If this option is given, the client's bind
credentials are remembered for rebinds when chasing
referrals.
pseudorootdn <substitute DN in case of rootdn bind>
This directive, if present, sets the DN that will
be substituted to the bind DN if a bind with the
backend's "rootdn" succeeds. The true "rootdn" of
the target server ought not be used; an arbitrary
administrative DN should used instead.
pseudorootpw <substitute password in case of rootdn bind>
This directive sets the credential that will be
used in case a bind with the backend's "rootdn"
succeeds, and the bind is propagated to the target
using the "pseudorootdn" DN.
Note: cleartext credentials must be supplied here; as a
consequence, using the pseudorootdn/pseudorootpw direc-
tives is inherently unsafe.
rewrite* ...
The rewrite options are described in the "REWRIT-
ING" section.
suffixmassage <virtual naming context> <real naming con-
text>
All the directives starting with "rewrite" refer to
the rewrite engine that has been added to slapd.
The "suffixmassage" directive was introduced in the
LDAP backend to allow suffix massaging while proxy-
ing. It has been obsoleted by the rewriting tools.
However, both for backward compatibility and for
ease of configuration when simple suffix massage is
required, it has been preserved. It wraps the
basic rewriting instructions that perform suffix
massaging.
Note: this also fixes a flaw in suffix massaging, which
operated on (case insensitive) DNs instead of normalized
DNs, so "dc=foo, dc=com" would not match "dc=foo,dc=com".
See the "REWRITING" section.
map {attribute|objectclass} [<local name>|*] {<foreign
name>|*}
This maps object classes and attributes as in the
LDAP backend. See slapd-ldap(5).
SCENARIOS
A powerful (and in some sense dangerous) rewrite engine
has been added to both the LDAP and Meta backends. While
the former can gain limited beneficial effects from
rewriting stuff, the latter can become an amazingly power-
ful tool.
Consider a couple of scenarios first.
1) Two directory servers share two levels of naming con-
text; say "dc=a,dc=foo,dc=com" and "dc=b,dc=foo,dc=com".
Then, an unambiguous Meta database can be configured as:
database meta
suffix "dc=foo,dc=com"
uri "ldap://a.foo.com/dc=a,dc=foo,dc=com"
uri "ldap://b.foo.com/dc=b,dc=foo,dc=com"
Operations directed to a specific target can be easily
resolved because there are no ambiguities. The only oper-
ation that may resolve to multiple targets is a search
with base "dc=foo,dc=com" and scope at least "one", which
results in spawning two searches to the targets.
2a) Two directory servers don't share any portion of nam-
ing context, but they'd present as a single DIT [Caveat:
uniqueness of (massaged) entries among the two servers is
assumed; integrity checks risk to incur in excessive over-
head and have not been implemented]. Say we have
"dc=bar,dc=org" and "o=Foo,c=US", and we'd like them to
appear as branches of "dc=foo,dc=com", say
"dc=a,dc=foo,dc=com" and "dc=b,dc=foo,dc=com". Then we
need to configure our Meta backend as:
database meta
suffix "dc=foo,dc=com"
uri "ldap://a.bar.com/dc=a,dc=foo,dc=com"
suffixmassage "dc=a,dc=foo,dc=com" "dc=bar,dc=org"
uri "ldap://b.foo.com/dc=b,dc=foo,dc=com"
suffixmassage "dc=b,dc=foo,dc=com" "o=Foo,c=US"
Again, operations can be resolved without ambiguity,
although some rewriting is required. Notice that the vir-
tual naming context of each target is a branch of the
database's naming context; it is rewritten back and forth
when operations are performed towards the target servers.
What "back and forth" means will be clarified later.
When a search with base "dc=foo,dc=com" is attempted, if
the scope is "base" it fails with "no such object"; in
fact, the common root of the two targets (prior to massag-
ing) does not exist. If the scope is "one", both targets
are contacted with the base replaced by each target's
base; the scope is derated to "base". In general, a scope
"one" search is honored, and the scope is derated, only
when the incoming base is at most one level lower of a
target's naming context (prior to massaging).
Finally, if the scope is "sub" the incoming base is
replaced by each target's unmassaged naming context, and
the scope is not altered.
2b) Consider the above reported scenario with the two
servers sharing the same naming context:
database meta
suffix "dc=foo,dc=com"
uri "ldap://a.bar.com/dc=foo,dc=com"
suffixmassage "dc=foo,dc=com" "dc=bar,dc=org"
uri "ldap://b.foo.com/dc=foo,dc=com"
suffixmassage "dc=foo,dc=com" "o=Foo,c=US"
All the previous considerations hold, except that now
there is no way to unambiguously resolve a DN. In this
case, all the operations that require an unambiguous tar-
get selection will fail unless the DN is already cached or
a default target has been set. Practical configurations
may result as a combination of all the above scenarios.
ACLs
Note on ACLs: at present you may add whatever ACL rule you
desire to to the Meta (and LDAP) backends. However, the
meaning of an ACL on a proxy may require some considera-
tions. Two philosophies may be considered:
a) the remote server dictates the permissions; the proxy
simply passes back what it gets from the remote server.
b) the remote server unveils "everything"; the proxy is
responsible for protecting data from unauthorized access.
Of course the latter sounds unreasonable, but it is not.
It is possible to imagine scenarios in which a remote host
discloses data that can be considered "public" inside an
intranet, and a proxy that connects it to the internet may
impose additional constraints. To this purpose, the proxy
should be able to comply with all the ACL matching crite-
ria that the server supports. This has been achieved with
regard to all the criteria supported by slapd except a
special subtle case (please drop me a note if you can find
other exceptions: <ando@openldap.org>). The rule
access to dn="<dn>" attr=<attr>
by dnattr=<dnattr> read
by * none
cannot be matched iff the attribute that is being
requested, <attr>, is NOT <dnattr>, and the attribute that
determines membership, <dnattr>, has not been requested
(e.g. in a search)
In fact this ACL is resolved by slapd using the portion of
entry it retrieved from the remote server without requir-
ing any further intervention of the backend, so, if the
<dnattr> attribute has not been fetched, the match cannot
be assessed because the attribute is not present, not
because no value matches the requirement!
Note on ACLs and attribute mapping: ACLs are applied to
the mapped attributes; for instance, if the attribute
locally known as "foo" is mapped to "bar" on a remote
server, then local ACLs apply to attribute "foo" and are
totally unaware of its remote name. The remote server
will check permissions for "bar", and the local server
will possibly enforce additional restrictions to "foo".
REWRITING
A string is rewritten according to a set of rules, called
a `rewrite context'. The rules are based on Regular
Expressions (POSIX regex) with substring matching; exten-
sions are planned to allow basic variable substitution and
map resolution of substrings. The behavior of pattern
matching/substitution can be altered by a set of flags.
The underlying concept is to build a lightweight rewrite
module for the slapd server (initially dedicated to the
LDAP backend).
Passes
An incoming string is matched agains a set of rules.
Rules are made of a match pattern, a substitution pattern
and a set of actions. In case of match a string rewriting
is performed according to the substitution pattern that
allows to refer to substrings matched in the incoming
string. The actions, if any, are finally performed. The
substitution pattern allows map resolution of substrings.
A map is a generic object that maps a substitution pattern
to a value.
Pattern Matching Flags
`C' honors case in matching (default is case insensi-
tive)
`R' use POSIX Basic Regular Expressions (default is
Extended)
Action Flags
`:' apply the rule once only (default is recursive)
`@' stop applying rules in case of match.
`#' stop current operation if the rule matches, and
issue an `unwilling to perform' error.
`G{n}' jump n rules back and forth (watch for loops!).
Note that `G{1}' is implicit in every rule.
`I' ignores errors in rule; this means, in case of
error, e.g. issued by a map, the error is treated
as a missed match. The `unwilling to perform' is
not overridden.
The ordering of the flags is significant. For instance:
`IG{2}' means ignore errors and jump two lines ahead both
in case of match and in case of error, while `G{2}I' means
ignore errors, but jump thwo lines ahead only in case of
match.
More flags (mainly Action Flags) will be added as needed.
Pattern matching:
See regex(7).
Substitution Pattern Syntax:
Everything starting with `%' requires substitution;
the only obvious exception is `%%', which is left as is;
the basic substitution is `%d', where `d' is a digit; 0
means the whole string, while 1-9 is a submatch, as dis-
cussed in regex(7);
a `%' followed by a `{' invokes an advanced substitution.
The pattern is:
`%' `{' [ <op> ] <name> `(' <substitution> `)' `}'
where <name> must be a legal name for the map, i.e.
<name> ::= [a-z][a-z0-9]* (case insensitive)
<op> ::= `>' `|' `&' `&&' `*' `**' `$'
and <substitution> must be a legal substitution pattern,
with no limits on the nesting level.
The operators are:
> sub context invocation; <name> must be a legal,
already defined rewrite context name
| external command invocation; <name> must refer to a
legal, already defined command name (NOT IMPL.)
& variable assignment; <name> defines a variable in
the running operation structure which can be deref-
erenced later; operator & assigns a variable in the
rewrite context scope; operator && assigns a vari-
able that scopes the entire session, e.g. its value
can be derefenced later by other rewrite contexts
* variable dereferencing; <name> must refer to a
variable that is defined and assigned for the run-
ning operation; operator * dereferences a variable
scoping the rewrite context; operator ** derefer-
ences a variable scoping the whole session, e.g.
the value is passed across rewrite contexts
$ parameter dereferencing; <name> must refer to an
existing parameter; the idea is to make some run-
time parameters set by the system available to the
rewrite engine, as the client host name, the bind
DN if any, constant parameters initialized at con-
fig time, and so on; no parameter is currently set
by either back-ldap or back-meta, but constant
parameters can be defined in the configuration file
by using the rewriteParam directive.
Substitution escaping has been delegated to the `%' sym-
bol, which is used instead of `\' in string substitution
patterns because `\' is already escaped by slapd's low
level parsing routines; as a consequence, regex(7) escap-
ing requires two `\' symbols, e.g. `.*\.foo\.bar' must be
written as `.*\\.foo\\.bar'.
Rewrite context:
A rewrite context is a set of rules which are applied in
sequence. The basic idea is to have an application ini-
tialize a rewrite engine (think of Apache's mod_rewrite
...) with a set of rewrite contexts; when string rewriting
is required, one invokes the appropriate rewrite context
with the input string and obtains the newly rewritten one
if no errors occur.
Each basic server operation is associated to a rewrite
context; they are divided in two main groups: client ->
server and server -> client rewriting.
client -> server:
(default) if defined and no specific context
is available
bindDn bind
searchBase search
searchFilter search
compareDn compare
addDn add
modifyDn modify
modrDn modrdn
newSuperiorDn modrdn
deleteDn delete
server -> client:
searchResult search (only if defined; no default;
acts on DN and DN-syntax attributes
of search results)
matchedDn all ops (only if defined; no default;
NOT IMPL. except in search)
Basic configuration syntax
rewriteEngine { on | off }
If `on', the requested rewriting is performed; if
`off', no rewriting takes place (an easy way to
stop rewriting without altering too much the con-
figuration file).
rewriteContext <context name> [ alias <aliased context
name> ]
<Context name> is the name that identifies the con-
text, i.e. the name used by the application to
refer to the set of rules it contains. It is used
also to reference sub contexts in string rewriting.
A context may aliase another one. In this case the
alias context contains no rule, and any reference
to it will result in accessing the aliased one.
rewriteRule <regex pattern> <substitution pattern> [
<flags> ]
Determines how a tring can be rewritten if a pat-
tern is matched. Examples are reported below.
Additional configuration syntax:
rewriteMap <map name> <map type> [ <map attrs> ]
Allows to define a map that transforms substring
rewriting into something else. The map is refer-
enced inside the substitution pattern of a rule.
rewriteParam <param name> <param value>
Sets a value with global scope, that can be deref-
erenced by the command `%{$paramName}'.
rewriteMaxPasses <number of passes>
Sets the maximum number of total rewriting passes
that can be performed in a single rewrite operation
(to avoid loops).
Configuration examples:
# set to `off' to disable rewriting
rewriteEngine on
# Everything defined here goes into the `default' context.
# This rule changes the naming context of anything sent
# to `dc=home,dc=net' to `dc=OpenLDAP, dc=org'
rewriteRule "(.*)dc=home,[ ]?dc=net"
"%1dc=OpenLDAP, dc=org" ":"
# since a pretty/normalized DN does not include spaces
# after rdn separators, e.g. `,', this rule suffices:
rewriteRule "(.*)dc=home,dc=net"
"%1dc=OpenLDAP,dc=org" ":"
# Start a new context (ends input of the previous one).
# This rule adds blanks between DN parts if not present.
rewriteContext addBlanks
rewriteRule "(.*),([^ ].*)" "%1, %2"
# This one eats blanks
rewriteContext eatBlanks
rewriteRule "(.*),[ ](.*)" "%1,%2"
# Here control goes back to the default rewrite
# context; rules are appended to the existing ones.
# anything that gets here is piped into rule `addBlanks'
rewriteContext default
rewriteRule ".*" "%{>addBlanks(%0)}" ":"
# Rewrite the search base according to `default' rules.
rewriteContext searchBase alias default
# Search results with OpenLDAP DN are rewritten back with
# `dc=home,dc=net' naming context, with spaces eaten.
rewriteContext searchResult
rewriteRule "(.*[^ ]?)[ ]?dc=OpenLDAP,[ ]?dc=org"
"%{>eatBlanks(%1)}dc=home,dc=net" ":"
# Bind with email instead of full DN: we first need
# an ldap map that turns attributes into a DN (the
# argument used when invoking the map is appended to
# the URI and acts as the filter portion)
rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"
# Then we need to detect DN made up of a single email,
# e.g. `mail=someone@example.com'; note that the rule
# in case of match stops rewriting; in case of error,
# it is ignored. In case we are mapping virtual
# to real naming contexts, we also need to rewrite
# regular DNs, because the definition of a bindDn
# rewrite context overrides the default definition.
rewriteContext bindDn
rewriteRule "^mail=[^,]+@[^,]+$" "%{attr2dn(%0)}" "@I"
# This is a rather sophisticated example. It massages a
# search filter in case who performs the search has
# administrative privileges. First we need to keep
# track of the bind DN of the incoming request, which is
# stored in a variable called `binddn' with session scope,
# and left in place to allow regular binding:
rewriteContext bindDn
rewriteRule ".+" "%{&&binddn(%0)}%0" ":"
# A search filter containing `uid=' is rewritten only
# if an appropriate DN is bound.
# To do this, in the first rule the bound DN is
# dereferenced, while the filter is decomposed in a
# prefix, in the value of the `uid=<arg>' AVA, and
# in a suffix. A tag `<>' is appended to the DN.
# If the DN refers to an entry in the `ou=admin' subtree,
# the filter is rewritten OR-ing the `uid=<arg>' with
# `cn=<arg>'; otherwise it is left as is. This could be
# useful, for instance, to allow apache's auth_ldap-1.4
# module to authenticate users with both `uid' and
# `cn', but only if the request comes from a possible
# `cn=Web auth,ou=admin,dc=home,dc=net' user.
rewriteContext searchFilter
rewriteRule "(.*\\()uid=([a-z0-9_]+)(\\).*)"
"%{**binddn}<>%{&prefix(%1)}%{&arg(%2)}%{&suffix(%3)}"
":I"
rewriteRule "[^,]+,ou=admin,dc=home,dc=net"
"%{*prefix}|(uid=%{*arg})(cn=%{*arg})%{*suffix}" "@I"
rewriteRule ".*<>" "%{*prefix}uid=%{*arg}%{*suffix}" ":"
LDAP Proxy resolution (a possible evolution of slapd-ldap(5)):
In case the rewritten DN is an LDAP URI, the operation is
initiated towards the host[:port] indicated in the uri, if
it does not refer to the local server. E.g.:
rewriteRule '^cn=root,.*' '%0' 'G{3}'
rewriteRule '^cn=[a-l].*' 'ldap://ldap1.my.org/%0' '@'
rewriteRule '^cn=[m-z].*' 'ldap://ldap2.my.org/%0' '@'
rewriteRule '.*' 'ldap://ldap3.my.org/%0' '@'
(Rule 1 is simply there to illustrate the `G{n}' action;
it could have been written:
rewriteRule '^cn=root,.*' 'ldap://ldap3.my.org/%0' '@'
with the advantage of saving one rewrite pass ...)
FILES
ETCDIR/slapd.conf
default slapd configuration file
SEE ALSOslapd.conf(5), slapd-ldap(5), slapd(8), regex(7).
OpenLDAP LDVERSION RELEASEDATE SLAPD-META(5)
