slapd-meta(5)
SLAPD-META(5) OpenLDAP 2.1.12 SLAPD-META(5)
NAME
slapd-meta - metadirectory backend
SYNOPSIS
/etc/openldap/slapd.conf
DESCRIPTION
The meta backend to slapd(8) performs basic LDAP proxying
with respect to a set of remote LDAP servers, called
"targets". 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 actually
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 recommended to set
lastmod off
for every ldap and meta backend.
SPECIAL CONFIGURATION DIRECTIVES
Target configuration starts with the "uri" directive. All
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the configuration directives that are not specific to
targets 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 target 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; forever means cache
never expires; disabled means no DN caching; otherwise
a valid ( > 0 ) ttl in seconds 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 anything
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.
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>
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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 permissions.
bindpw <password for access control purposes>
This directive sets the password for acl checking in
conjunction with the above mentioned "binddn"
directive.
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 directives
is inherently unsafe.
rewrite* ...
The rewrite options are described in the "REWRITING"
section.
suffixmassage <virtual naming context> <real naming context>
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 proxying. 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>|*}
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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 powerful tool.
Consider a couple of scenarios first.
1) Two directory servers share two levels of naming context;
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
operation 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 naming
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
overhead 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
virtual 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.
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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 massaging) 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 target 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
considerations. 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 criteria
that the server supports. This has been achieved with
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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 requiring
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;
extensions 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 insensitive)
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`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
discussed 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.
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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 dereferenced
later; operator & assigns a variable in the rewrite
context scope; operator && assigns a variable 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 running operation;
operator * dereferences a variable scoping the rewrite
context; operator ** dereferences 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 config 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 `%' symbol,
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) escaping 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
initialize 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.
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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 configuration
file).
rewriteContext <context name> [ alias <aliased context name>
]
<Context name> is the name that identifies the context,
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 pattern 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 referenced
inside the substitution pattern of a rule.
rewriteParam <param name> <param value>
Sets a value with global scope, that can be
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dereferenced 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)
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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' '@'
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(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
/etc/openldap/slapd.conf
default slapd configuration file
SEE ALSO
slapd.conf(5), slapd-ldap(5), slapd(8), regex(7).
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