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Updated: 2008/07/16


slapo-rwm - rewrite/remap overlay 




Therwmoverlay toslapd(8)performs basic DN/data rewrite and objectClass/attributeType mapping.Its usage is mostly intended to provide virtual views of existing dataeither remotely, in conjunction with the proxy backend described inslapd-ldap(5),or locally, in conjunction with the relay backend described inslapd-relay(5).

This overlay is experimental. 


An important feature of therwmoverlay is the capability to map objectClasses and attributeTypesfrom the local set (or a subset of it) to a foreign set, and vice versa.This is accomplished by means of the rwm-mapdirective.
rwm-map {attribute | objectclass} [<local name> | *] {<foreign name> | *}
Map attributeTypes and objectClasses from the foreign server todifferent values on the local slapd.The reason is that some attributes might not be part of the localslapd's schema, some attribute names might be different but serve thesame 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 attributeTypesand the objectClasses they intend to map. All in all, when remappinga remote server via back-ldap (slapd-ldap(5)) or back-meta (slapd-meta(5))their definition can be easily obtained by querying the subschemaSubentryof the remote server; the problem should not exist when remapping a local database.Note, however, that the decision whether to rewrite or not attributeTypeswith 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 appropriatelydefined as using the distinguishedName syntax.Also, note that there are DN-related syntaxes (i.e. compound types witha portion that is DN-valued), like nameAndOptionalUID,whose values are currently not rewritten. 


A basic feature of therwmoverlay is the capability to perform suffix massaging between a virtualand a real naming context by means of the rwm-suffixmassagedirective.
rwm-suffixmassage [<virtual naming context>] <real naming context>
Shortcut to implement naming context rewriting; the trailing partof the DN is rewritten from the virtual to the real naming contextin the bindDN, searchDN, searchFilterAttrDN, compareDN, compareAttrDN,addDN, addAttrDN, modifyDN, modifyAttrDN, modrDN, newSuperiorDN,deleteDN, exopPasswdDN, and from the real to the virtual naming contextin 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 thedatabase is used; this requires the rwm-suffixmassagedirective be defined after the databasesuffixdirective.Therwm-suffixmassagedirective automatically sets therwm-rewriteEnginetoON.

See the REWRITING section for details. 


A string is rewritten according to a set of rules, called a `rewritecontext'.The rules are based on POSIX (''extended'') regular expressions withsubstring 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 setof flags.

<rewrite context> ::= <rewrite rule> [...]<rewrite rule> ::= <pattern> <action> [<flags>]

The underlying concept is to build a lightweight rewrite modulefor the slapd server (initially dedicated to the LDAP backend):



An incoming string is matched against a set ofrewriteRules.Rules are made of a regex match pattern,a substitution patternand a set of actions, described by a set of optional flags.In case of match, string rewriting is performed according to thesubstitution pattern that allows to refer to substrings matched in theincoming 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 alteredby therwm-rewriteMaxPassesdirective, 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 latteralter the actions that are taken after substitution. 

Pattern Matching Flags

honors case in matching (default is case insensitive)
use POSIX ''basic'' regular expressions (default is ''extended'')
allow no more thannrecursive passes for a specific rule; does not alter the max total countof 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 toperform' error.
jumpnrules back and forth (watch for loops!).Note that `G{1}' is implicit in every rule.
ignores errors in rule; this means, in case of error, e.g. issued by amap, the error is treated as a missed match.The `unwilling to perform' is not overridden.
usesnas return code if the rule matches; the flag does not alter the recursivebehavior 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 returncode set ton;or, in other words, `@' is equivalent to `U{0}'.Positive errors are allowed, indicating the related LDAP error codesas 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 aheadboth in case of match and in case of error, while `G{2}I' means ignoreerrors, but jump two lines ahead only in case of match.

More flags (mainly Action Flags) will be added as needed. 

Pattern Matching


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/orre_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 substitutionpattern, with no limits on the nesting level.

The operators are:

sub-context invocation; <name> must be a legal, already definedrewrite context name
external command invocation; <name> must refer to a legal, alreadydefined command name (NOT IMPLEMENTED YET)
variable assignment; <name> defines a variable in the runningoperation 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 valuecan be dereferenced later by other rewrite contexts
variable dereferencing; <name> must refer to a variable that isdefined 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 valueis 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 systemavailable to the rewrite engine, as the client host name, the bind DNif any, constant parameters initialized at config time, and so on;no parameter is currently set by either back-ldaporback-meta,but constant parameters can be defined in the configuration fileby using therewriteParamdirective.

Substitution escaping has been delegated to the `$' symbol, which is used instead of `\' in string substitution patternsbecause `\' is already escaped by slapd's low level parsing routines;as a consequence, regex escaping requirestwo `\' symbols, e.g. `.*\.foo\.bar' mustbe 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 rewriteengine (think of Apache's mod_rewrite ...) with a set of rewritecontexts; when string rewriting is required, one invokes theappropriate rewrite context with the input string and obtains thenewly 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 andserver -> client rewriting.

client -> server:

(default)            if defined and no specific context                      is availablebindDN               bindsearchDN             searchsearchFilter         searchsearchFilterAttrDN   searchcompareDN            comparecompareAttrDN        compare AVAaddDN                addaddAttrDN            add AVA (DN portion of "ref" excluded)modifyDN             modifymodifyAttrDN         modify AVA (DN portion of "ref" excluded)referralAttrDN       add/modify DN portion of referrals                     (default to none)modrDN               modrdnnewSuperiorDN        modrdndeleteDN             deleteexopPasswdDN         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 prefixrwm-;for backwards compatibility with the historicalslapd-ldap(5)andslapd-meta(5)builtin rewrite/remap capabilities, the prefix may be omitted, but this practice is strongly discouraged.
rwm-rewriteEngine { on | off }
If `on', the requested rewriting is performed; if `off', norewriting takes place (an easy way to stop rewriting withoutaltering 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 nameused 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 toit 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 to define a map that transforms substring rewriting intosomething 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 thecommand `${$paramName}'.
rwm-rewriteMaxPasses <number of passes> [<number of passes per rule>]
Sets the maximum number of total rewriting passes that can beperformed in a single rewrite operation (to avoid loops).A safe default is set to 100; note that reaching this limit is stilltreated 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 limitswith the `M{n}' flag.


# set to `off' to disable rewritingrwm-rewriteEngine on# the rules the "suffixmassage" directive impliesrwm-rewriteEngine on# all dataflow from client to server referring to DNsrwm-rewriteContext defaultrwm-rewriteRule "(.+,)?<virtualnamingcontext>$" "$1<realnamingcontext>" ":"# empty filter rulerwm-rewriteContext searchFilter# all dataflow from server to clientrwm-rewriteContext searchEntryDNrwm-rewriteRule "(.+,)?<realnamingcontext>$" "$1<virtualnamingcontext>" ":"rwm-rewriteContext searchAttrDN alias searchEntryDNrwm-rewriteContext matchedDN alias searchEntryDN# misc empty rulesrwm-rewriteContext referralAttrDNrwm-rewriteContext referralDN# Everything defined here goes into the `default' context.# This rule changes the naming context of anything sent# to `dc=home,dc=net' to `dc=OpenLDAP, dc=org'rwm-rewriteRule "(.+,)?dc=home,[ ]?dc=net$"            "$1dc=OpenLDAP, dc=org"  ":"# since a pretty/normalized DN does not include spaces# after rdn separators, e.g. `,', this rule suffices:rwm-rewriteRule "(.+,)?dc=home,dc=net$"            "$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  addBlanksrwm-rewriteRule     "(.*),([^ ].*)" "$1, $2"# This one eats blanksrwm-rewriteContext  eatBlanksrwm-rewriteRule     "(.*), (.*)" "$1,$2"# Here control goes back to the default rewrite# context; rules are appended to the existing ones.# anything that gets here is piped into rule `addBlanks'rwm-rewriteContext  defaultrwm-rewriteRule     ".*" "${>addBlanks($0)}" ":"# Rewrite the search base according to `default' rules.rwm-rewriteContext  searchDN alias default# Search results with OpenLDAP DN are rewritten back with# `dc=home,dc=net' naming context, with spaces eaten.rwm-rewriteContext  searchEntryDNrwm-rewriteRule     "(.*[^ ],)?[ ]?dc=OpenLDAP,[ ]?dc=org$"                "${>eatBlanks($1)}dc=home,dc=net"    ":"# Bind with email instead of full DN: we first need# an ldap map that turns attributes into a DN (the# argument used when invoking the map is appended to # the URI and acts as the filter portion)rwm-rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"# Then we need to detect DN made up of a single email,# e.g. `'; note that the rule# in case of match stops rewriting; in case of error,# it is ignored.  In case we are mapping virtual# to real naming contexts, we also need to rewrite# regular DNs, because the definition of a bindDN# rewrite context overrides the default definition.rwm-rewriteContext bindDNrwm-rewriteRule "^mail=[^,]+@[^,]+$" "${attr2dn($0)}" ":@I"# This is a rather sophisticated example. It massages a# search filter in case who performs the search has# administrative privileges.  First we need to keep# track of the bind DN of the incoming request, which is# stored in a variable called `binddn' with session scope,# and left in place to allow regular binding:rwm-rewriteContext  bindDNrwm-rewriteRule     ".+" "${&&binddn($0)}$0" ":"# A search filter containing `uid=' is rewritten only# if an appropriate DN is bound.# To do this, in the first rule the bound DN is# dereferenced, while the filter is decomposed in a# prefix, in the value of the `uid=<arg>' AVA, and # in a suffix. A tag `<>' is appended to the DN. # If the DN refers to an entry in the `ou=admin' subtree, # the filter is rewritten OR-ing the `uid=<arg>' with# `cn=<arg>'; otherwise it is left as is. This could be# useful, for instance, to allow apache's auth_ldap-1.4# module to authenticate users with both `uid' and# `cn', but only if the request comes from a possible# `cn=Web auth,ou=admin,dc=home,dc=net' user.rwm-rewriteContext searchFilterrwm-rewriteRule "(.*\\()uid=([a-z0-9_]+)(\\).*)"  "${**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 searchEntryDNrwm-rewriteRule ".+,ou=People,dc=example,dc=com$" "$0" ":@"rwm-rewriteRule ".*" "" "#"


The following directives map the object class `groupOfNames' tothe object class `groupOfUniqueNames' and the attribute type`member' to the attribute type `uniqueMember':

map objectclass groupOfNames groupOfUniqueNamesmap attribute uniqueMember member

This presents a limited attribute set from the foreignserver:

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. 


default slapd configuration file




Pierangelo Masarati; based on back-ldap rewrite/remap featuresby Howard Chu, Pierangelo Masarati.



Pattern Matching Flags
Action Flags
Pattern Matching
Substitution Pattern Syntax
Rewrite Context
Basic Configuration Syntax
Additional Configuration Syntax

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