Provided by: slapd_2.5.18+dfsg-0ubuntu0.22.04.2_amd64 
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 objectClass/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 objectClasses 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 attributeTypes with distinguishedName syntax, requires the
knowledge of the attributeType 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 mapping from remote to local, first
the attributeType is mapped, and then the DN is rewritten. As such, it is important that
the local attributeType 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-normalize-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-relay(5), allows one to create virtual views of databases. A
distinguishing 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 subschemaSubentry DN (usually "cn=subschema"), would
prevent clients from reading 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 rewrite contexts. If no <virtual naming context> is
given, the first suffix of the database is used; this requires the
rwm-suffixmassage directive be defined after the database suffix directive. 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 expressions with substring matching; basic
variable substitution and map resolution of substrings is allowed by specific mechanisms
detailed in the following. The behavior of pattern matching/substitution can be altered
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 actions, described by a set of optional
flags. In case of match, string rewriting is performed according to the substitution
pattern that allows one to refer to substrings matched in the incoming string. The
actions, if any, are finally performed. Each rule is executed recursively, unless altered
by specific action flags; see "Action Flags" for details. A default limit on the
recursion level is set, and can be altered by the rwm-rewriteMaxPasses directive, as
detailed in the "Additional 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 after substitution.
Pattern Matching Flags
`C' honors case in matching (default is case insensitive)
`R' use POSIX ''basic'' regular expressions (default is ''extended'')
`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 `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.
`U{n}' uses n as return code if the rule matches; the flag does not alter 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, already 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 && assigns 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 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 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.
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 altering 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 result 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 substitution 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 operation 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 registered at runtime.
Supported maps are:
LDAP <URI> [bindwhen=<when>] [version=<version>] [binddn=<DN>] [credentials=<cred>]
The LDAP map expands a value by performing a simple LDAP search. Its configuration
is based on a mandatory URI, whose attrs portion 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 established. It can take
the values now, later, and everytime, respectively 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 anonymous 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 portion 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 object 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-relay(5), slapd(8),
regex(7), re_format(7).
AUTHOR
Pierangelo Masarati; based on back-ldap rewrite/remap features by Howard Chu, Pierangelo
Masarati.