slapo-rwm — rewrite/remap overlay to slapd
ETCDIR/slapd.conf
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.
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.
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.
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".
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 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.
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.
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):
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 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.
`C'honors case in matching (default is case insensitive)
`R'use POSIX ''basic'' regular expressions (default is ''extended'')
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.
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.
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.
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.
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'.
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) modrDN modrdn newSuperiorDN modrdn 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)
All rewrite/remap directives start with the prefix
rwm-; for backwards
compatibility with the historical slapd-ldap(5) and slapd-meta(5) builtin
rewrite/remap capabilities, the prefix may be omitted, but
this practice is strongly discouraged.
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).
<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.
Determines how a string can be rewritten if a pattern is matched. Examples are reported below.
Allows to define a map that transforms substring rewriting into something else. The map is referenced inside the substitution pattern of a rule.
Sets a value with global scope, that can be dereferenced by the command `${$paramName}'.
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.
Currently, few maps are builtin and there are no provisions for developers to register new map types at runtime.
Supported maps are:
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.
# 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 ".*" "" "#"
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.
slapd.conf(5), slapd−ldap(5), slapd−meta(5), slapd−relay(5), slapd(8), regex(7), re_format(7).