SLAPD-LDAP(5)

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version 2.7.2 · 24 versions
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✨ SLAPD-LDAP(5) ✨ — File Formats Manual 🦋

💖 NAME 💖

  • slapd-ldap — the LDAP backend to slapd. 💅

🌸 SYNOPSIS 🌸

/etc/openldap/slapd.conf

🎀 DESCRIPTION 🎀

Ok bestie, listen up: the LDAP backend for slapd(8) is not an actual database — it's a proxy that forwards incoming requests to another LDAP server. 💫 While it's processing requests, it will also chase referrals, so referrals get fully processed instead of being handed back to the slapd client.

Sessions that explicitly Bind to the back-ldap database always create their own private connection to the remote LDAP server. Anonymous sessions will share a single anonymous connection to the remote server. For sessions bound through other mechanisms, all sessions with the same DN share the same connection. This connection pooling strategy can enhance the proxy's efficiency by reducing the overhead of repeatedly making/breaking multiple connections. 🔥

The ldap database can also act as an information service, i.e. the identity of locally authenticated clients is asserted to the remote server, possibly in some modified form. For this purpose, the proxy binds to the remote server with some administrative identity, and, if required, authorizes the asserted identity. See the idassert-* rules below. The administrative identity of the proxy, on the remote server, must be allowed to authorize by means of appropriate authzTo rules; see slapd.conf(5) for details.

The proxy instance of slapd(8) must contain schema information for the attributes and objectClasses used in filters, request DNs and request-related data in general. It should also contain schema information for the data returned by the proxied server. It is the responsibility of the proxy administrator to keep the schema of the proxy lined up with that of the proxied server. 👑

Note: When looping back to the same instance of slapd(8), each connection requires a new thread; as a consequence, the slapd(8) threads parameter may need some tuning. In those cases, one may consider using slapd-relay(5) instead, which performs the relayed operation internally and thus reuses the same connection. 🧵

💅 CONFIGURATION 💅

These slapd.conf options apply to the LDAP backend database. That is, they must follow a "database ldap" line and come before any subsequent "backend" or "database" lines. Other database options are described in the slapd.conf(5) manual page.

Note: In early versions of back-ldap it was recommended to always set
lastmod  off

for ldap and meta databases. This was required because operational attributes related to entry creation and modification should not be proxied, as they could be mistakenly written to the target server(s), generating an error. The current implementation automatically sets lastmod to off, so its use is redundant and should be omitted. 💀

  • uri <ldapurl> — LDAP server to use. Multiple URIs can be set in a single ldapurl argument, resulting in the underlying library automatically calling the first server of the list that responds, e.g.
uri "ldap://host/ ldap://backup-host/"

The URI list is space- or comma-separated. Whenever the server that responds is not the first one in the list, the list is rearranged and the responsive server is moved to the head, so that it will be first contacted the next time a connection needs to be created. ✨

  • acl-bind — allows one to define the parameters of the authentication method that is internally used by the proxy to collect info related to access control, and whenever an operation occurs with the identity of the rootdn of the LDAP proxy database. The identity defined by this directive, according to the properties associated to the authentication method, is supposed to have read access on the target server to attributes used on the proxy for ACL checking.
       acl-bind bindmethod=simple|sasl [binddn=<simple DN>]
              [credentials=<simple password>] [saslmech=<SASL mech>]
              [secprops=<properties>] [realm=<realm>] [authcId=<authentication
              ID>] [authzId=<authorization ID>] [starttls=no|yes|critical]
              [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>]
              [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand]
              [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>]
              [tls_ecname=<names>] [tls_protocol_min=<major>[.<minor>]]
              [tls_crlcheck=none|peer|all]

There is no risk of giving away such values; they are only used to check permissions. The default is to use simple bind, with empty binddn and credentials, which means that the related operations will be performed anonymously. If not set, and if idassert-bind is defined, this latter identity is used instead. See idassert-bind for details. 👀

The connection between the proxy database and the remote server associated to this identity is cached regardless of the lifespan of the client-proxy connection that first established it.

This identity is not implicitly used by the proxy when the client connects anonymously. The idassert-bind feature, instead, in some cases can be crafted to implement that behavior, which is intrinsically unsafe and should be used with extreme care. 🚨

The TLS settings default to the same as the main slapd TLS settings, except for tls_reqcert which defaults to "demand", and tls_reqsan which defaults to "allow".

  • cancel {ABANDON|ignore|exop[-discover]} — defines how to handle operation cancellation. By default, abandon is invoked, so the operation is abandoned immediately. If set to ignore, no action is taken and any further response is ignored; this may result in further response messages to be queued for that connection, so it is recommended that long lasting connections are timed out either by idle-timeout or conn-ttl, so that resources eventually get released. If set to exop, a cancel operation (RFC 3909) is issued, resulting in the cancellation of the current operation; the cancel operation waits for remote server response, so its use may not be recommended. If set to exop-discover, support of the cancel extended operation is detected by reading the remote server's root DSE. 💀
  • chase-referrals {YES|no} — enable/disable automatic referral chasing, which is delegated to the underlying libldap, with rebinding eventually performed if the rebind-as-user directive is used. The default is to chase referrals. 🌈
  • conn-pool-max <int> — this directive defines the maximum size of the privileged connections pool. 💅
  • conn-ttl <time> — this directive causes a cached connection to be dropped after a given ttl, regardless of being idle or not. If a client connection outlives the remote connection, the client will receive LDAP_UNAVAILABLE when it executes the next operation. 💖
  • idassert-authzFrom <authz-regexp> — if defined, selects what local identities are authorized to exploit the identity assertion feature. The string <authz-regexp> mostly follows the rules defined for the authzFrom attribute. See slapd.conf(5), section related to authz-policy, for details on the syntax of this field. This parameter differs from the documented behavior in relation to the meaning of *, which in this case allows anonymous rather than denies. 🤫
  • idassert-bind — allows one to define the parameters of the authentication method that is internally used by the proxy to authorize connections that are authenticated by other databases. Direct binds are always proxied without any idassert handling.
       idassert-bind bindmethod=none|simple|sasl [binddn=<simple DN>]
              [credentials=<simple password>] [saslmech=<SASL mech>]
              [secprops=<properties>] [realm=<realm>] [authcId=<authentication
              ID>] [authzId=<authorization ID>] [authz={native|proxyauthz}]
              [mode=<mode>] [flags=<flags>] [starttls=no|yes|critical]
              [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>]
              [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand]
              [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>]
              [tls_ecname=<names>] [tls_protocol_min=<version>]
              [tls_crlcheck=none|peer|all]

The identity defined by this directive, according to the properties associated to the authentication method, is supposed to have auth access on the target server to attributes used on the proxy for authentication and authorization, and to be allowed to authorize the users. This requires to have proxyAuthz privileges on a wide set of DNs, e.g. authzTo=dn.subtree:"", and the remote server to have authz-policy set to to or both. See slapd.conf(5) for details on these statements and for remarks and drawbacks about their usage. The supported bindmethods are

none|simple|sasl

where none is the default, i.e. no identity assertion is performed. 🚫

The authz parameter is used to instruct the SASL bind to exploit native SASL authorization, if available; since connections are cached, this should only be used when authorizing with a fixed identity (e.g. by means of the authzDN or authzID parameters). Otherwise, the default proxyauthz is used, i.e. the proxyAuthz control (Proxied Authorization, RFC 4370) is added to all operations. 🔐

The supported modes are:

<mode> := {legacy|anonymous|none|self}

If <mode> is not present, and authzId is given, the proxy always authorizes that identity. <authorization ID> can be

u:<user>

[dn:]<DN>

The former is supposed to be expanded by the remote server according to the authz rules; see slapd.conf(5) for details. In the latter case, whether or not the dn: prefix is present, the string must pass DN validation and normalization. 💖

The default mode is legacy, bestie, and lowkey what that means is your proxy is gonna either do a simple bind as the authcDN or a SASL bind as the authcID, and then assert the client's identity whenever the client isn't anonymous. ✨

Every other mode is like, ok so the proxy is always gonna either simple-bind as the authcDN or SASL-bind as the authcID — unless idassert-authzFrom rules say nah (details below), and in that case the operation just Fails 💀. After that, it'll assert some other identity depending on <mode>. The other identity assertion modes are:

  • anonymous — the empty identity gets asserted
  • self — the client's identity gets asserted
  • none — no proxyAuthz control gets used at all, so the authcDN or authcID identity is what gets asserted

For every mode that needs the proxyAuthz control, the remote server side has to have the proxy identity carrying appropriate authzTo permissions, OR the asserted identities themselves need appropriate authzFrom permissions. But ngl, the ID assertion feature is mostly useful when the asserted identities don't even exist on the remote server. iykyk 🫶

Flags can be

override,[non-]prescriptive,proxy-authz-[non-]critical,dn-{authzid|whoami}
  • When the override flag is used, identity assertion happens even when the database is authorizing for the identity of the client. Like, after binding with the provided identity and thus authenticating it, the proxy does the identity assertion using the configured identity and authentication method. 👑
  • When the prescriptive flag is used (this is the default btw), operations fail with inappropriateAuthentication for those identities whose assertion isn't allowed by the idassert-authzFrom patterns. If the non-prescriptive flag is used instead, operations get performed anonymously for those identities. 🙈
  • When the proxy-authz-non-critical flag is used (default again, bestie), the proxyAuthz control is not marked as critical — which is literally a violation of RFC 4370 😭. Use proxy-authz-critical instead, it's recommended, fr fr.
  • When the dn-authzid flag is used, RFC 3829 LDAP Authorization Identity Controls is used to retrieve the identity associated to the SASL identity. When the dn-whoami flag is used, the RFC 4532 LDAP Who am I? Operation is performed after the bind, for the exact same purpose. slay 🎀

The TLS settings default to the same as the main slapd TLS settings, EXCEPT tls_reqcert which defaults to "demand", and tls_reqsan which defaults to "allow".

The identity associated to this directive is also used for privileged operations whenever idassert-bind is defined and acl-bind is not. See acl-bind for details. 💅

  • idassert-passthru <authz-regexp> — if it's defined, this selects which local identities bypass the identity assertion feature. Those identities need to be known by the remote host. The string <authz-regexp> follows the rules defined for the authzFrom attribute. See slapd.conf(5), section related to authz-policy, for the deets on the syntax of this field. 🌸
  • idle-timeout <time> — this directive causes a cached connection to be dropped after it's been idle for the specified time. So like, if a client connection outlives the remote connection, the client will get LDAP_UNAVAILABLE when it runs the next operation. 💀
  • keepalive <idle>:<probes>:<interval> — the keepalive parameter sets the values of idle, probes, and interval used to check whether a socket is alive. Quick rundown: idle = the number of seconds a connection needs to stay idle before TCP starts sending keepalive probes; probes = the max number of keepalive probes TCP should send before dropping the connection; interval = the interval in seconds between individual keepalive probes. Only some systems support customizing these values — otherwise the keepalive parameter is just ignored and system-wide settings are used. 🙃
  • tcp-user-timeout <milliseconds> — if non-zero, this corresponds to the TCP_USER_TIMEOUT set on the target connections, overriding the operating system setting. Again, only some systems support customizing this parameter, it's ignored otherwise and system-wide settings are used. 🦋
  • network-timeout <time> — sets the network timeout value after which poll(2)/select(2) following a connect(2) returns in case of no activity. The value is in seconds, and it can be specified as for idle-timeout. ⏰
  • norefs <NO|yes> — if yes, do not return search reference responses. By default they are returned, unless the request is LDAPv2. ✨
  • omit-unknown-schema <NO|yes> — if yes, do not return objectClasses or attributes that aren't known to the local server. The default is to return all schema elements. 🍓
  • noundeffilter <NO|yes> — if yes, return success instead of searching when a filter is undefined or contains undefined portions. By default, the search is propagated after replacing undefined portions with (!(objectClass=*)), which corresponds to the empty result set. 🧚
  • onerr {CONTINUE|stop} — this directive lets you pick the behavior when the remote server returns an error during a search. The default, continue, means it returns success. If the value is set to stop, the error is returned to the client. 🔥
  • protocol-version {0,2,3} — says what protocol version must be used to contact the remote server. If set to 0 (the default), the proxy uses the same protocol version the client used; otherwise the requested protocol is used. The proxy returns unwillingToPerform if an operation that's incompatible with the requested protocol is attempted. 💅
  • proxy-whoami {NO|yes} — turns on proxying of the WhoAmI extended operation. If this option is given, back-ldap will replace slapd's original WhoAmI routine with its own. On slapd sessions that were authenticated by back-ldap, the WhoAmI request will be forwarded to the remote LDAP server. Other sessions will be handled by the local slapd, as before. This option is mainly useful in conjunction with Proxy Authorization. 👑
  • quarantine <interval>,<num>[;<interval>,<num>[...]] — turns on quarantine of URIs that returned LDAP_UNAVAILABLE, so that a reconnect attempt only happens at given intervals instead of any time a client requests an operation. The pattern goes: retry only after at least interval seconds have elapsed since the last attempt, for exactly num times; then use the next pattern. If num for the last pattern is "+", it retries forever; otherwise no more retries occur. The process can be restarted by resetting the olcDbQuarantine attribute of the database entry in the configuration backend. 🔁
  • rebind-as-user {NO|yes} — if this option is given, the client's bind credentials are remembered for rebinds — like when trying to re-establish a broken connection, or when chasing a referral, if chase-referrals is set to yes. Note tho: the connection is not re-established automatically after it was dropped due to idle-timeout or conn-ttl. 😭
  • session-tracking-request {NO|yes} — adds session tracking control for all requests. The client's IP and hostname, and the identity associated to each request (if known), are sent to the remote server for informational purposes. This directive is incompatible with setting protocol-version to 2. 🚫
  • single-conn {NO|yes} — discards the current cached connection when the client rebinds. 💫
  • t-f-support {NO|yes|discover} — enable if the remote server supports absolute filters (see RFC 4526 for the deets). If set to discover, support is detected by reading the remote server's root DSE. 🔍
  • timeout [<op>=]<val> [...] — this directive lets you set per-operation timeouts. Operations can be:
<op> ::= bind, add, delete, modrdn, modify, compare, search

The overall duration of the search operation is controlled either by the timelimit parameter or by server-side enforced time limits (see timelimit and limits in slapd.conf(5) for details). This timeout parameter controls how long the target can be irresponsive before the operation is aborted. Timeout is meaningless for the remaining operations, unbind and abandon, which don't imply any response — and it's not yet implemented in currently supported extended operations. If no operation is specified, the timeout val affects all supported operations.

Note: if the timelimit is exceeded, the operation is cancelled (according to the cancel directive) — and the protocol does not provide any means to rollback operations, so the client will not be notified about the result of the operation, which may eventually succeed or not. 😬 In case the timeout is exceeded during a bind operation, the connection is destroyed, according to RFC4511.
Note: in some cases, this backend may issue binds prior to other operations (e.g. to bind anonymously or with some prescribed identity according to the idassert-bind directive). In this case, the timeout of the operation that resulted in the bind is used.
  • tls — specify TLS settings for regular connections. The full vibe is:
tls {none|[try-]start|[try-]propagate|ldaps} [starttls=no]
[tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>]
[tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand]
[tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>]
[tls_ecname=<names>] [tls_crlcheck=none|peer|all]

If the first parameter is not "none", then this configures the TLS settings to be used for regular connections. The StartTLS extended operation will be used when establishing the connection unless the URI directive protocol scheme is ldaps://. In that case this keyword may only be set to "ldaps" and the StartTLS operation will not be used. 🔐

With propagate, the proxy issues the StartTLS operation only if the original connection has a TLS layer set up. The try- prefix instructs the proxy to continue operations if the StartTLS operation failed — its use is not recommended. 🙈

Hiii bestie, so the TLS situation in here is 💅 vibes: all the TLS settings default to the exact same thing as the main slapd TLS settings — except for three lil divas:

  • tls_reqcert defaults to "demand" (she's strict, respect it)
  • tls_reqsan defaults to "allow"
  • starttls gets absolutely overshadowed by the first keyword, so it's just… ignored. 🔇 not her moment.
  • use-temporary-conn {NO|yes} — set it to yes and a temporary connection gets created whenever a thread is competing with other threads for a shared one. Set it to anything else and your girl just waits (patiently, iconically) until the shared connection is available again.

🌈 ACCESS CONTROL 🌈

Okay so real talk: the ldap backend does not honor all the ACL semantics the way slapd.access(5) describes them. 😭 Not her fault, it's just how it is.

Generally speaking, access checking is delegated to the remote server(s). Live your life, let the remote handle it. ✨

The only thing honored locally is read (=r) access to the entry pseudo-attribute and to the other attribute values of the entries that come back from the search operation — and that check is performed by the frontend. Just that one. No more. Periodt. 💅

🦋 OVERLAYS 🦋

The LDAP backend gives you basic proxying functionalities to many overlays. Two of them are getting their flowers specifically called out:

They deserve a special mention, and honestly? They ate. 👑

Conversely, there's also a whole squad of overlays that are best used in conjunction with the LDAP backend:

  • proxycache — allows caching of LDAP search requests (queries) in a local database. See slapo-pcache(5) for the details, bestie.
  • rwm — provides DN rewrite and attribute/objectClass mapping capabilities to the underlying database. See slapo-rwm(5).

So many options, so many vibes. 🌸

🎀 FILES 🎀

  • /etc/openldap/slapd.conf — the default slapd configuration file

👑 SEE ALSO 👑

slapd.conf(5), slapd-config(5), slapd-meta(5), slapo-chain(5), slapo-pcache(5), slapo-rwm(5), slapo-translucent(5), slapd(8), ldap(3).

Go read 'em, iykyk. 📚✨

💖 AUTHOR 💖

Howard Chu, with enhancements by Pierangelo Masarati

🌟 And that's the whole page, bestie! This is OpenLDAP 2.7.2, dated 2026/10/21, and the page itself is SLAPD-LDAP(5). Stan a classic. 🫶💅✨
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