openldap/doc/man/man5/slapd-meta.5

.TH SLAPD_META 5 "28 April 2002" "OpenLDAP LDVERSION"
.\" Copyright 1998-2002 The OpenLDAP Foundation, All Rights Reserved.
.\" Copying restrictions apply.  See the COPYRIGHT file.
.\" Copyright 2001, Pierangelo Masarati, All rights reserved. <ando@sys-net.it>
.\" $OpenLDAP$
.\"
.\" Portions of this document should probably be moved to slapd-ldap(5)
.\" and maybe manual pages for librewrite.
.\"
.SH NAME
slapd_meta \- metadirectory backend
.SH SYNOPSIS
ETCDIR/slapd.conf
.SH DESCRIPTION
The
.B meta
backend to
.BR 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).
.LP
A basic knowledge of the functionality of the
.BR 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
.B ldap
backend is intended to proxy operations directed to a single server, the
.B 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.
.SH EXAMPLES
There are examples various places in this document, as well as in the
slapd/back-meta/data/ directory in the OpenLDAP source tree.
.SH CONFIGURATION
The
.BR slapd.conf (5)
options in this category apply to the META backend database.
That is, they must follow a "database meta" line and come before any
subsequent "backend" or "database" lines.
.LP
Note: as with the
.B 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
.LP
.nf
	lastmod		off
.fi
.LP
for every
.B ldap
and
.B meta
backend.
.SH "SPECIAL CONFIGURATION DIRECTIVES"
Target configuration starts with the "uri" directive.
All 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:
.TP
.B 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 on must succeed.
This directive can also be used when processing targets to mark a
specific target as default.
.TP
.B 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.
.SH "TARGET SPECIFICATION"
Target specification starts with a "uri" directive:
.TP
.B 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 (correct me!?!).
The <naming context> part is mandatory.
It must end with one of the naming contexts defined for the backend,
e.g.:
.LP
.nf
  suffix "dc=foo,dc=com"
  uri    "ldap://x.foo.com/dc=x,dc=foo,dc=com"
.fi
.LP
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.
.TP
.B 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.
.TP
.B binddn <administrative dn for ac 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 permissions.
.TP
.B bindpw <password for ac purposes>
This directive sets the password for acl checking in conjunction
with the above mentioned "binddn" directive.
.TP
.B 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.
.TP
.B 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.
.TP
.B rewrite* ...
The rewrite options are described in the "REWRITING" section.
.TP
.B 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.
.LP
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".
.LP
See the "REWRITING" section.
.TP
.B map {objectClass|attribute} {<source>|*} [<dest>|*]
This maps object classes and attributes as in the LDAP backend.
See
.BR slapd-ldap (5).
.SH "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.
.LP
Consider a couple of scenarios first.
.LP
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:
.LP
.nf
  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"
.fi
.LP
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.
.LP
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:
.LP
.nf
  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"
.fi
.LP
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.
.LP
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).
.LP
Finally, if the scope is "sub" the incoming base is replaced
by each target's unmassaged naming context, and the scope
is not altered.
.LP
2b) Consider the above reported scenario with the two servers
sharing the same naming context:
.LP
.nf
  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"
.fi
.LP
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.
.SH 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:
.LP
a) the remote server dictates the permissions; the proxy simply passes
back what it gets from the remote server.
.LP
b) the remote server unveils "everything"; the proxy is responsible
for protecting data from unauthorized access.
.LP
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 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
.LP
.nf
  access to dn="<dn>" attr=<attr>
	 by dnattr=<dnattr> read
	 by * none
.fi
.LP
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)
.LP
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!
.LP
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".
.\"
.\" If this section is moved, also update the reference in
.\" libraries/librewrite/RATIONALE.
.\"
.SH 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.
.LP
The underlying concept is to build a lightweight rewrite module
for the slapd server (initially dedicated to the LDAP backend).
.SH 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.
.SH "Pattern Matching Flags"
.TP
.B `C'
honors case in matching (default is case insensitive)
.TP
.B `R'
use POSIX Basic Regular Expressions (default is Extended)
.SH "Action Flags"
.TP
.B `:'
apply the rule once only (default is recursive)
.TP
.B `@'
stop applying rules in case of match.
.TP
.B `#'
stop current operation if the rule matches, and issue an `unwilling to
perform' error.
.TP
.B `G{n}'
jump n rules back and forth (watch for loops!).
Note that `G{1}' is implicit in every rule.
.TP
.B `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.
.LP
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.
.LP
More flags (mainly Action Flags) will be added as needed.
.SH "Pattern matching:"
See
.BR regex (7).
.SH "String Substitution:"
The string substitution happens according to a substitution pattern.
.TP
.B -
substring substitution is allowed with the syntax `\\d' where `d' is a
digit ranging 0-9 (0 is the full match).
I see that 0-9 digit expansion is a widely accepted practise; however
there is no technical reason to use such a strict limit.
A syntax of the form `\\{ddd}' should be fine if there is any need to
use a higher number of possible submatches.
.TP
.B -
variable substitution will be allowed (at least when I figure out
which kind of variable could be proficiently substituted)
.TP
.B -
map lookup will be allowed (map lookup of substring matches in gdbm,
ldap(!), math(?) and so on maps `a la sendmail'.
.TP
.B -
subroutine invocation will make it possible to rewrite a submatch in
terms of the output of another rewriteContext.
.Sh "Old syntax:"
.TP
.B `\\' {0-9} [ `{' <name> [ `(' <args> `)' ] `}' ]
where <name> is the name of a built-in map, and <args> are optional
arguments to the map, if the map <name> requires them.
The following experimental maps have been implemented:
.TP
.B \\n{xpasswd}
maps the n-th substring match as uid to the gecos field in
/etc/passwd;
.TP
.B \\n{xfile(/absolute/path)}
maps the n-th substring match to a `key value' style plain text file.
.TP
.B \\n{xldap(ldap://url/with?%0?in?filter)
maps the n-th substring match to an attribute retrieved by means of an
LDAP url with substitution of %0 in the filter (NOT IMPL.)
.SH "New scheme:"
everything starting with `\\' requires substitution;
.LP
the only obvious exception is `\\\\', which is left as is;
.LP
the basic substitution is `\\d', where `d' is a digit;
0 means the whole string, while 1-9 is a submatch;
.LP
in the outdated schema, the digit may be optionally
followed by a `{', which means pipe the submatch into
the map described by the string up to the following `}';
.LP
the output of the map is used instead of the submatch;
.LP
in the new schema, a `\\' followed by a `{' invokes an
advanced substitution scheme.
The pattern is:
.LP
.nf
  `\\' `{' [{ <op> }] <name> `(' <substitution schema> `)' `}'
.fi
.LP
where <name> must be a legal name for the map, i.e.
.LP
.nf
  <name> ::= [a-z][a-z0-9]* (case insensitive)
  <op> ::= `>' `|' `&' `&&' `*' `**' `$'
.fi
.LP
and <substitution schema> must be a legal substitution
schema, with no limits on the nesting level.
.LP
The operators are:
.TP
.B >
sub context invocation; <name> must be a legal, already defined
rewrite context name
.TP
.B |
external command invocation; <name> must refer to a legal, already
defined command name (NOT IMPL.)
.TP
.B &
variable assignment; <name> defines a variable in the running
operation structure which can be dereferenced later (NOT IMPL.)
.TP
.B *
variable dereferencing; <name> must refer to a variable that is
defined and assigned for the running operation (NOT IMPL.)
.TP
.B $
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 (NOT
IMPL.)
.LP
Note: as the slapd parsing routines escape backslashes ('\\'),
a double backslash is required inside substitution patterns.
To overcome the resulting heavy notation, the substitution escaping
has been delegated to the `%' symbol, which should be used
instead of `\\' in string substitution patterns.
The symbol can be altered at will by redefining the related macro in
"rewrite-int.h".
In the current snapshot, all the `\\' on the left side of each rule
(the regex pattern) must be converted in `\\\\'; all the `\\' on the
right side of the rule (the substitution pattern) must be turned into
`%'.
In the following examples, the original (more readable) syntax is
used.
.SH "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.
.LP
An interesting application, in the LDAP backend or in slapd itself,
could associate each basic server operation to a rewrite context
(most of them possibly aliasing the default one).
Then, DN rewriting could take place at any invocation of a backend
operation.
.LP
client -> server:
.LP
.nf
     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
.fi
.LP
server -> client:
.LP
.nf
     searchResult    search (only if defined; no default)
.fi
.LP
.SH "Basic configuration syntax"
.TP
.B 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).
.TP
.B 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.
.TP
.B rewriteRule <regex pattern> <substitution pattern> [ <flags> ]
Determines how a tring can be rewritten if a pattern is matched.
Examples are reported below.
.SH "Additional configuration syntax:"
.TP
.B 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.
.TP
.B rewriteParam <param name> <param value>
Sets a value with global scope, that can be dereferenced by the
command `\\{$paramName}'.
.TP
.B rewriteMaxPasses <number of passes>
Sets the maximum number of total rewriting passes taht can be
performed in a signle rewriting operation (to avoid loops).
.SH "Configuration examples:"
.nf
     # 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"  ":"

     # Start a new context (ends input of the previous one).
     # This rule adds blancs between dn parts if not present.
     rewriteContext  addBlancs
     rewriteRule     "(.*),([^ ].*)" "\\1, \\2"

     # This one eats blancs
     rewriteContext  eatBlancs
     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 `addBlancs'
     rewriteContext  default
     rewriteRule     ".*" "\\{>addBlancs(\\0)}" ":"

     # Anything with `uid=username' is looked up in
     # /etc/passwd for gecos (I know it's nearly useless,
     # but it is there just to test something fancy!). Note
     # the `I' flag that leaves `uid=username' in place if
     # `username' does not have a valid account, and the
     # `:' that forces the rule to be processed exactly once.
     rewriteContext  uid2Gecos
     rewriteRule     "(.*)uid=([a-z0-9]+),(.+)"
                     "\\1cn=\\2{xpasswd},\\3"      "I:"

     # Finally, in a bind, if one uses a `uid=username' dn,
     # it is rewritten in `cn=name surname' if possible.
     rewriteContext  bindDn
     rewriteRule     ".*" "\\{>addBlancs(\\{>uid2Gecos(\\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"
                     "\\{>eatBlancs(\\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
     # filter is provided by the substitution string):
     rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"

     # Then we need to detect emails; 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(\\1)}" "@I"

     # This is a rather sophisticate 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:
     rewriteContext  bindDn
     rewriteRule     ".+" "\\{**&binddn(\\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, the argument of the `uid=', 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
     # `dn: 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}"
.fi
.SH "LDAP Proxy resolution (a possible evolution of slapd-ldap(5):"
In case the rewritten dn is an LDAP URL, the operation is initiated
towards the host[:port] indicated in the url, if it does not refer
to the local server.
E.g.:
.LP
.nf
  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\' \'@\'
.fi
.LP
(Rule 1 is simply there to illustrate the `G{n}' action; it could have
been written:
.LP
.nf
  rewriteRule \'^cn=root,.*\' \'ldap://ldap3.my.org/\\0\' \'@\'
.fi
.LP
with the advantage of saving one rewrite pass ...)
.SH "SEE ALSO"
.BR slapd.conf (5),
.BR slapd (8),
.BR regex (7).