SLAPO-RWM(5) File Formats Manual SLAPO-RWM(5)
NAME
slapo-rwm - rewrite/remap overlay to slapd
SYNOPSIS
/etc/ldap/slapd.conf
DESCRIPTION
The rwm overlay to slapd(8) performs basic DN/data rewrite and object-
Class/attributeType mapping. Its usage is mostly intended to provide
virtual views of existing data either remotely, in conjunction with the
proxy backend described in slapd-ldap(5), or locally, in conjunction
with the relay backend described in slapd-relay(5).
This overlay is experimental.
MAPPING
An important feature of the rwm overlay is the capability to map ob-
jectClasses and attributeTypes from the local set (or a subset of it)
to a foreign set, and vice versa. This is accomplished by means of the
rwm-map directive.
rwm-map {attribute | objectclass} [<local name> | *] {<foreign name> |
*}
Map attributeTypes and objectClasses from the foreign server to
different values on the local slapd. The reason is that some
attributes might not be part of the local slapd's schema, some
attribute names might be different but serve the same purpose,
etc. If local or foreign name is `*', the name is preserved.
If local name is omitted, the foreign name is removed. Unmapped
names are preserved if both local and foreign name are `*', and
removed if local name is omitted and foreign name is `*'.
The local objectClasses and attributeTypes must be defined in the local
schema; the foreign ones do not have to, but users are encouraged to
explicitly define the remote attributeTypes and the objectClasses they
intend to map. All in all, when remapping a remote server via back-
ldap (slapd-ldap(5)) or back-meta (slapd-meta(5)) their definition can
be easily obtained by querying the subschemaSubentry of the remote
server; the problem should not exist when remapping a local database.
Note, however, that the decision whether to rewrite or not attribute-
Types with distinguishedName syntax, requires the knowledge of the at-
tributeType syntax. See the REWRITING section for details.
Note that when mapping DN-valued attributes from local to remote, first
the DN is rewritten, and then the attributeType is mapped; while map-
ping from remote to local, first the attributeType is mapped, and then
the DN is rewritten. As such, it is important that the local at-
tributeType is appropriately defined as using the distinguishedName
syntax. Also, note that there are DN-related syntaxes (i.e. compound
types with a portion that is DN-valued), like nameAndOptionalUID, whose
values are currently not rewritten.
If the foreign type of an attribute mapping is not defined on the local
server, it might be desirable to have the attribute values normalized
after the mapping process. Not normalizing the values can lead to wrong
results, when the rwm overlay is used together with e.g. the pcache
overlay. This normalization can be enabled by means of the rwm-normal-
ize-mapped-attrs directive.
rwm-normalize-mapped-attrs {yes|no}
Set this to "yes", if the rwm overlay should try to normalize
the values of attributes that are mapped from an attribute type
that is unknown to the local server. The default value of this
setting is "no".
rwm-drop-unrequested-attrs {yes|no}
Set this to "yes", if the rwm overlay should drop attributes
that are not explicitly requested by a search operation. When
this is set to "no", the rwm overlay will leave all attributes
in place, so that subsequent modules can further manipulate
them. In any case, unrequested attributes will be omitted from
search results by the frontend, when the search entry response
package is encoded. The default value of this setting is "yes".
SUFFIX MASSAGING
A basic feature of the rwm overlay is the capability to perform suffix
massaging between a virtual and a real naming context by means of the
rwm-suffixmassage directive. This, in conjunction with proxy backends,
slapd-ldap(5) and slapd-meta(5), or with the relay backend, slapd-re-
lay(5), allows one to create virtual views of databases. A distin-
guishing feature of this overlay is that, when instantiated before any
database, it can modify the DN of requests before database selection.
For this reason, rules that rewrite the empty DN ("") or the subschema-
Subentry DN (usually "cn=subschema"), would prevent clients from read-
ing the root DSE or the DSA's schema.
rwm-suffixmassage [<virtual naming context>] <real naming context>
Shortcut to implement naming context rewriting; the trailing
part of the DN is rewritten from the virtual to the real naming
context in the bindDN, searchDN, searchFilterAttrDN, compareDN,
compareAttrDN, addDN, addAttrDN, modifyDN, modifyAttrDN, modrDN,
newSuperiorDN, deleteDN, exopPasswdDN, and from the real to the
virtual naming context in the searchEntryDN, searchAttrDN and
matchedDN rewrite contexts. By default no rewriting occurs for
the searchFilter and for the referralAttrDN and referralDN re-
write contexts. If no <virtual naming context> is given, the
first suffix of the database is used; this requires the rwm-suf-
fixmassage directive be defined after the database suffix direc-
tive. The rwm-suffixmassage directive automatically sets the
rwm-rewriteEngine to ON.
See the REWRITING section for details.
REWRITING
A string is rewritten according to a set of rules, called a `rewrite
context'. The rules are based on POSIX (''extended'') regular expres-
sions with substring matching; basic variable substitution and map res-
olution of substrings is allowed by specific mechanisms detailed in the
following. The behavior of pattern matching/substitution can be al-
tered by a set of flags.
<rewrite context> ::= <rewrite rule> [...]
<rewrite rule> ::= <pattern> <action> [<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 against a set of rewriteRules. Rules are
made of a regex match pattern, a substitution pattern and a set of ac-
tions, described by a set of optional flags. In case of match, string
rewriting is performed according to the substitution pattern that al-
lows one to refer to substrings matched in the incoming string. The
actions, if any, are finally performed. Each rule is executed recur-
sively, unless altered by specific action flags; see "Action Flags" for
details. A default limit on the recursion level is set, and can be al-
tered by the rwm-rewriteMaxPasses directive, as detailed in the "Addi-
tional Configuration Syntax" section. The substitution pattern allows
map resolution of substrings. A map is a generic object that maps a
substitution pattern to a value. The flags are divided in "Pattern
Matching Flags" and "Action Flags"; the former alter the regex match
pattern behavior, while the latter alter the actions that are taken af-
ter substitution.
Pattern Matching Flags
`C' honors case in matching (default is case insensitive)
`R' use POSIX ''basic'' regular expressions (default is ''ex-
tended'')
`M{n}' allow no more than n recursive passes for a specific rule; does
not alter the max total count of passes, so it can only enforce
a stricter limit for a specific rule.
Action Flags
`:' apply the rule once only (default is recursive)
`@' stop applying rules in case of match; the current rule is still
applied recursively; combine with `:' to apply the current rule
only once and then stop.
`#' stop current operation if the rule matches, and issue an `un-
willing 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. is-
sued by a map, the error is treated as a missed match. The `un-
willing to perform' is not overridden.
`U{n}' uses n as return code if the rule matches; the flag does not al-
ter the recursive behavior of the rule, so, to have it performed
only once, it must be used in combination with `:', e.g.
`:U{32}' returns the value `32' (indicating noSuchObject) after
exactly one execution of the rule, if the pattern matches. As a
consequence, its behavior is equivalent to `@', with the return
code set to n; or, in other words, `@' is equivalent to `U{0}'.
Positive errors are allowed, indicating the related LDAP error
codes as specified in draft-ietf-ldapbis-protocol.
The ordering of the flags can be 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 two lines
ahead only in case of match.
More flags (mainly Action Flags) will be added as needed.
Pattern Matching
See regex(7) and/or re_format(7).
Substitution Pattern Syntax
Everything starting with `$' requires substitution;
the only obvious exception is `$$', which is turned into a single `$';
the basic substitution is `$<d>', where `<d>' is a digit; 0 means the
whole string, while 1-9 is a submatch, as discussed in regex(7) and/or
re_format(7).
a `$' followed by a `{' invokes an advanced substitution. The pattern
is:
`$' `{' [ <operator> ] <name> `(' <substitution> `)' `}'
where <name> must be a legal name for the map, i.e.
<name> ::= [a-z][a-z0-9]* (case insensitive)
<operator> ::= `>' `|' `&' `&&' `*' `**' `$'
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, al-
ready defined command name (NOT IMPLEMENTED YET)
& 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 && as-
signs a variable that scopes the entire session, e.g. its value
can be dereferenced 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 param-
eter; 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 al-
ready escaped by slapd's low level parsing routines; as a consequence,
regex 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 con-
text 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
searchDN search
searchFilter search
searchFilterAttrDN search
compareDN compare
compareAttrDN compare AVA
addDN add
addAttrDN add AVA (DN portion of "ref" excluded)
modifyDN modify
modifyAttrDN modify AVA (DN portion of "ref" excluded)
referralAttrDN add/modify DN portion of referrals
(default to none)
renameDN modrdn (the old DN)
newSuperiorDN modrdn (the new parent DN, if any)
newRDN modrdn (the new relative DN)
deleteDN delete
exopPasswdDN password modify extended operation DN
server -> client:
searchEntryDN search (only if defined; no default;
acts on DN of search entries)
searchAttrDN search AVA (only if defined; defaults
to searchEntryDN; acts on DN-syntax
attributes of search results)
matchedDN all ops (only if applicable; defaults
to searchEntryDN)
referralDN all ops (only if applicable; defaults
to none)
Basic Configuration Syntax
All rewrite/remap directives start with the prefix rwm-
rwm-rewriteEngine { on | off }
If `on', the requested rewriting is performed; if `off', no
rewriting takes place (an easy way to stop rewriting without al-
tering too much the configuration file).
rwm-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 alias another one. In this case the
alias context contains no rule, and any reference to it will re-
sult in accessing the aliased one.
rwm-rewriteRule <regex match pattern> <substitution pattern> [ <flags>
]
Determines how a string can be rewritten if a pattern is
matched. Examples are reported below.
Additional Configuration Syntax
rwm-rewriteMap <map type> <map name> [ <map attrs> ]
Allows one to define a map that transforms substring rewriting
into something else. The map is referenced inside the substitu-
tion pattern of a rule.
rwm-rewriteParam <param name> <param value>
Sets a value with global scope, that can be dereferenced by the
command `${$paramName}'.
rwm-rewriteMaxPasses <number of passes> [<number of passes per rule>]
Sets the maximum number of total rewriting passes that can be
performed in a single rewrite operation (to avoid loops). A
safe default is set to 100; note that reaching this limit is
still treated as a success; recursive invocation of rules is
simply interrupted. The count applies to the rewriting opera-
tion as a whole, not to any single rule; an optional per-rule
limit can be set. This limit is overridden by setting specific
per-rule limits with the `M{n}' flag.
MAPS
Currently, few maps are builtin but additional map types may be regis-
tered at runtime.
Supported maps are:
LDAP <URI> [bindwhen=<when>] [version=<version>] [binddn=<DN>] [creden-
tials=<cred>]
The LDAP map expands a value by performing a simple LDAP search.
Its configuration is based on a mandatory URI, whose attrs por-
tion must contain exactly one attribute (use entryDN to fetch
the DN of an entry). If a multi-valued attribute is used, only
the first value is considered.
The parameter bindwhen determines when the connection is estab-
lished. It can take the values now, later, and everytime, re-
spectively indicating that the connection should be created at
startup, when required, or any time it is used. In the former
two cases, the connection is cached, while in the latter a fresh
new one is used all times. This is the default.
The parameters binddn and credentials represent the DN and the
password that is used to perform an authenticated simple bind
before performing the search operation; if not given, an anony-
mous connection is used.
The parameter version can be 2 or 3 to indicate the protocol
version that must be used. The default is 3.
slapd <URI>
The slapd map expands a value by performing an internal LDAP
search. Its configuration is based on a mandatory URI, which
must begin with ldap:/// (i.e., it must be an LDAP URI and it
must not specify a host). As with the LDAP map, the attrs por-
tion must contain exactly one attribute, and if a multi-valued
attribute is used, only the first value is considered.
REWRITE CONFIGURATION EXAMPLES
# set to `off' to disable rewriting
rwm-rewriteEngine on
# the rules the "suffixmassage" directive implies
rwm-rewriteEngine on
# all dataflow from client to server referring to DNs
rwm-rewriteContext default
rwm-rewriteRule "(.+,)?<virtualnamingcontext>$" "$1<realnamingcontext>" ":"
# empty filter rule
rwm-rewriteContext searchFilter
# all dataflow from server to client
rwm-rewriteContext searchEntryDN
rwm-rewriteRule "(.+,)?<realnamingcontext>$" "$1<virtualnamingcontext>" ":"
rwm-rewriteContext searchAttrDN alias searchEntryDN
rwm-rewriteContext matchedDN alias searchEntryDN
# misc empty rules
rwm-rewriteContext referralAttrDN
rwm-rewriteContext referralDN
# Everything defined here goes into the `default' context.
# This rule changes the naming context of anything sent
# to `dc=home,dc=net' to `dc=OpenLDAP, dc=org'
rwm-rewriteRule "(.+,)?dc=home,[ ]?dc=net$"
"$1dc=OpenLDAP, dc=org" ":"
# since a pretty/normalized DN does not include spaces
# after rdn separators, e.g. `,', this rule suffices:
rwm-rewriteRule "(.+,)?dc=home,dc=net$"
"$1dc=OpenLDAP,dc=org" ":"
# Start a new context (ends input of the previous one).
# This rule adds blanks between DN parts if not present.
rwm-rewriteContext addBlanks
rwm-rewriteRule "(.*),([^ ].*)" "$1, $2"
# This one eats blanks
rwm-rewriteContext eatBlanks
rwm-rewriteRule "(.*), (.*)" "$1,$2"
# Here control goes back to the default rewrite
# context; rules are appended to the existing ones.
# anything that gets here is piped into rule `addBlanks'
rwm-rewriteContext default
rwm-rewriteRule ".*" "${>addBlanks($0)}" ":"
# Rewrite the search base according to `default' rules.
rwm-rewriteContext searchDN alias default
# Search results with OpenLDAP DN are rewritten back with
# `dc=home,dc=net' naming context, with spaces eaten.
rwm-rewriteContext searchEntryDN
rwm-rewriteRule "(.*[^ ],)?[ ]?dc=OpenLDAP,[ ]?dc=org$"
"${>eatBlanks($1)}dc=home,dc=net" ":"
# Bind with email instead of full DN: we first need
# an ldap map that turns attributes into a DN (the
# argument used when invoking the map is appended to
# the URI and acts as the filter portion)
rwm-rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"
# Then we need to detect DN made up of a single email,
# e.g. `mail=someone@example.com'; note that the rule
# in case of match stops rewriting; in case of error,
# it is ignored. In case we are mapping virtual
# to real naming contexts, we also need to rewrite
# regular DNs, because the definition of a bindDN
# rewrite context overrides the default definition.
rwm-rewriteContext bindDN
rwm-rewriteRule "^mail=[^,]+@[^,]+$" "${attr2dn($0)}" ":@I"
# This is a rather sophisticated example. It massages a
# search filter in case who performs the search has
# administrative privileges. First we need to keep
# track of the bind DN of the incoming request, which is
# stored in a variable called `binddn' with session scope,
# and left in place to allow regular binding:
rwm-rewriteContext bindDN
rwm-rewriteRule ".+" "${&&binddn($0)}$0" ":"
# A search filter containing `uid=' is rewritten only
# if an appropriate DN is bound.
# To do this, in the first rule the bound DN is
# dereferenced, while the filter is decomposed in a
# prefix, in the value of the `uid=<arg>' AVA, and
# in a suffix. A tag `<>' is appended to the DN.
# If the DN refers to an entry in the `ou=admin' subtree,
# the filter is rewritten OR-ing the `uid=<arg>' with
# `cn=<arg>'; otherwise it is left as is. This could be
# useful, for instance, to allow apache's auth_ldap-1.4
# module to authenticate users with both `uid' and
# `cn', but only if the request comes from a possible
# `cn=Web auth,ou=admin,dc=home,dc=net' user.
rwm-rewriteContext searchFilter
rwm-rewriteRule "(.*\\()uid=([a-z0-9_]+)(\\).*)"
"${**binddn}<>${&prefix($1)}${&arg($2)}${&suffix($3)}"
":I"
rwm-rewriteRule "^[^,]+,ou=admin,dc=home,dc=net$"
"${*prefix}|(uid=${*arg})(cn=${*arg})${*suffix}" ":@I"
rwm-rewriteRule ".*<>$" "${*prefix}uid=${*arg}${*suffix}" ":"
# This example shows how to strip unwanted DN-valued
# attribute values from a search result; the first rule
# matches DN values below "ou=People,dc=example,dc=com";
# in case of match the rewriting exits successfully.
# The second rule matches everything else and causes
# the value to be rejected.
rwm-rewriteContext searchEntryDN
rwm-rewriteRule ".+,ou=People,dc=example,dc=com$" "$0" ":@"
rwm-rewriteRule ".*" "" "#"
MAPPING EXAMPLES
The following directives map the object class `groupOfNames' to the ob-
ject class `groupOfUniqueNames' and the attribute type `member' to the
attribute type `uniqueMember':
map objectclass groupOfNames groupOfUniqueNames
map attribute uniqueMember member
This presents a limited attribute set from the foreign server:
map attribute cn *
map attribute sn *
map attribute manager *
map attribute description *
map attribute *
These lines map cn, sn, manager, and description to themselves, and any
other attribute gets "removed" from the object before it is sent to the
client (or sent up to the LDAP server). This is obviously a simplistic
example, but you get the point.
FILES
/etc/ldap/slapd.conf
default slapd configuration file
SEE ALSO
slapd.conf(5), slapd-config(5), slapd-ldap(5), slapd-meta(5), slapd-re-
lay(5), slapd(8), regex(7), re_format(7).
AUTHOR
Pierangelo Masarati; based on back-ldap rewrite/remap features by
Howard Chu, Pierangelo Masarati.
OpenLDAP 2.5.13+dfsg-5 2022/07/14 SLAPO-RWM(5)
Generated by dwww version 1.15 on Sat Aug 30 14:47:29 CEST 2025.