SLAPD-METASection: File Formats (5)Updated: 2004/06/10 |
SLAPD-METASection: File Formats (5)Updated: 2004/06/10 |
A basic knowledge of the functionality of the 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 ldap backend is intended to proxy operations directed to a single server, the 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.
Note: as with the 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
suffix "dc=foo,dc=com" uri "ldap://x.foo.com/dc=x,dc=foo,dc=com"
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. Multiple URIs may be defined in a single argument. The URIs must be separated by TABs (e.g. '\t'), and the additional URIs must have no <naming context> part. This causes the underlying library to contact the first server of the list that responds.
Note: cleartext credentials must be supplied here; as a consequence, using the pseudorootdn/pseudorootpw directives is inherently unsafe.
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".
See the "REWRITING" section.
Consider a couple of scenarios first.
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:
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"
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.
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:
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"
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.
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).
Finally, if the scope is "sub" the incoming base is replaced by each target's unmassaged naming context, and the scope is not altered.
2b) Consider the above reported scenario with the two servers sharing the same naming context:
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"
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. Practical configurations may result as a combination of all the above scenarios.
a) the remote server dictates the permissions; the proxy simply passes back what it gets from the remote server.
b) the remote server unveils "everything"; the proxy is responsible for protecting data from unauthorized access.
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
access to dn="<dn>" attr=<attr>
by dnattr=<dnattr> read
by * none
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)
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!
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".
The underlying concept is to build a lightweight rewrite module for the slapd server (initially dedicated to the LDAP backend).
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.
the only obvious exception is `%%', which is left as is;
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);
a `%' followed by a `{' invokes an advanced substitution. The pattern is:
where <name> must be a legal name for the map, i.e.
<name> ::= [a-z][a-z0-9]* (case insensitive) <op> ::= `>' `|' `&' `&&' `*' `**' `$'
and <substitution> must be a legal substitution pattern, with no limits on the nesting level.
The operators are:
Substitution escaping has been delegated to the `%' symbol, which is used instead of `\' in string substitution patterns because `\' is already escaped by slapd's low level parsing routines; as a consequence, regex(7) escaping requires two `\' symbols, e.g. `.*\.foo\.bar' must be written as `.*\\.foo\\.bar'.
Each basic server operation is associated to a rewrite context; they are divided in two main groups: client -> server and server -> client rewriting.
client -> server:
(default) if defined and no specific context
is available
bindDN bind
searchBase search
searchFilter search
searchFilterAttrDN search
compareDN compare
compareAttrDN compare AVA
addDN add
addAttrDN add AVA
modifyDN modify
modifyAttrDN modify AVA
modrDN modrdn
newSuperiorDN modrdn
deleteDN delete
exopPasswdDN passwd exop DN if proxy
server -> client:
searchResult search (only if defined; no default;
acts on DN and DN-syntax attributes
of search results)
searchAttrDN search AVA
matchedDN all ops (only if applicable)
# set to `off' to disable rewriting
rewriteEngine on
# the rules the "suffixmassage" directive implies
rewriteEngine on
# all dataflow from client to server referring to DNs
rewriteContext default
rewriteRule "(.*)<virtualnamingcontext>$" "%1<realnamingcontext>" ":"
# empty filter rule
rewriteContext searchFilter
# all dataflow from server to client
rewriteContext searchResult
rewriteRule "(.*)<realnamingcontext>$" "%1<virtualnamingcontext>" ":"
rewriteContext searchAttrDN alias searchResult
rewriteContext matchedDN alias searchResult
# Everything defined here goes into the `default' context.
# This rule changes the naming context of anything sent
# to `dc=home,dc=net' to `dc=OpenLDAP, dc=org'
rewriteRule "(.*)dc=home,[ ]?dc=net"
"%1dc=OpenLDAP, dc=org" ":"
# since a pretty/normalized DN does not include spaces
# after rdn separators, e.g. `,', this rule suffices:
rewriteRule "(.*)dc=home,dc=net"
"%1dc=OpenLDAP,dc=org" ":"
# Start a new context (ends input of the previous one).
# This rule adds blanks between DN parts if not present.
rewriteContext addBlanks
rewriteRule "(.*),([^ ].*)" "%1, %2"
# This one eats blanks
rewriteContext eatBlanks
rewriteRule "(.*),[ ](.*)" "%1,%2"
# Here control goes back to the default rewrite
# context; rules are appended to the existing ones.
# anything that gets here is piped into rule `addBlanks'
rewriteContext default
rewriteRule ".*" "%{>addBlanks(%0)}" ":"
# Rewrite the search base according to `default' rules.
rewriteContext searchBase alias default
# Search results with OpenLDAP DN are rewritten back with
# `dc=home,dc=net' naming context, with spaces eaten.
rewriteContext searchResult
rewriteRule "(.*[^ ]?)[ ]?dc=OpenLDAP,[ ]?dc=org"
"%{>eatBlanks(%1)}dc=home,dc=net" ":"
# Bind with email instead of full DN: we first need
# an ldap map that turns attributes into a DN (the
# argument used when invoking the map is appended to
# the URI and acts as the filter portion)
rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"
# Then we need to detect DN made up of a single email,
# e.g. `mail=someone@example.com'; note that the rule
# in case of match stops rewriting; in case of error,
# it is ignored. In case we are mapping virtual
# to real naming contexts, we also need to rewrite
# regular DNs, because the definition of a bindDn
# rewrite context overrides the default definition.
rewriteContext bindDN
rewriteRule "^mail=[^,]+@[^,]+$" "%{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:
rewriteContext bindDN
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.
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}" ":"
# 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.
rewriteContext searchResult
rewriteRule ".*,ou=People,dc=example,dc=com" "%0" ":@"
rewriteRule ".*" "" "#"
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' ':@'
(Rule 1 is simply there to illustrate the `G{n}' action; it could have been written:
rewriteRule '^cn=root,.*' 'ldap://ldap3.my.org/%0' ':@'
with the advantage of saving one rewrite pass ...)
A template is defined by a filter string and an index identifying a set of attributes. The template string for a query can be obtained by removing assertion values from the RFC 2254 representation of its search filter. A query belongs to a template if its template string and set of projected attributes correspond to a cacheable template. Examples of template strings are (mail=), (|(sn=)(cn=)), (&(sn=)(givenName=)).
The following cache specific directives can be used to configure the proxy cache:
The following adds a template with filter string (&sn=)(givenName=)) and attributes mail, postaladdress, telephonenumber and a TTL of 1 hour.
proxyattrset 0 mail postaladdress telephonenumber proxytemplate (&(sn=)(givenName=)) 0 3600
Directives for configuring the underlying database must also be given, as shown here:
directory /var/tmp/cache cachesize 100
Any valid directives for the chosen database type may be used.