People sign in with their Active Directory account

auth.ldap checks a username and password against the domain controllers
over LDAPS or StartTLS: found with a service account, checked by binding
as the person. Groups, nested ones included, give staff roles and places
in the tree. Accounts are tied to the objectGUID; the leader reads the
directory every 15 minutes, suspends whom it disabled or removed and
reinstates them after, and refuses a sync that would suspend most.

Entra ID sends no groups for someone in more than 200, only where to
fetch them; that was read as none and took an administrator's role away.
It is now left as it was, and logged. An account made by SCIM or OIDC
can be reinstated again.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/reference.md

@@ -37,6 +37,7 @@ rather than repeated.
 | `default_tpm` | `int` | [Auth](#auth) |
 | `signup_open` | `bool` | [Auth](#auth) |
 | `oidc` | `OIDC` | [Auth](#auth) |
+| `ldap` | `LDAP` | [Auth](#auth) |
 | `scim` | `SCIM` | [Auth](#auth) |
 | `max_key_ttl` | `time.Duration` | [Auth](#auth) |
 | `personal` | `Personal` | [Auth](#auth) |
@@ -147,6 +148,20 @@ rather than repeated.
 | `tokenize_timeout` | `time.Duration` | [KVIndex](#kvindex) |
 | `min_blocks` | `int` | [KVIndex](#kvindex) |
 | `max_blocks` | `int` | [KVIndex](#kvindex) |
+| `urls` | `[]string` | [LDAP](#ldap) |
+| `start_tls` | `bool` | [LDAP](#ldap) |
+| `ca_file` | `string` | [LDAP](#ldap) |
+| `bind_dn` | `string` | [LDAP](#ldap) |
+| `bind_password_file` | `string` | [LDAP](#ldap) |
+| `base_dn` | `string` | [LDAP](#ldap) |
+| `group_base_dn` | `string` | [LDAP](#ldap) |
+| `user_filter` | `string` | [LDAP](#ldap) |
+| `label` | `string` | [LDAP](#ldap) |
+| `create_accounts` | `bool` | [LDAP](#ldap) |
+| `roles` | `map[string][]string` | [LDAP](#ldap) |
+| `memberships` | `map[string]string` | [LDAP](#ldap) |
+| `sync_interval` | `time.Duration` | [LDAP](#ldap) |
+| `timeout` | `time.Duration` | [LDAP](#ldap) |
 | `contact` | `string` | [Landing](#landing) |
 | `notice` | `string` | [Landing](#landing) |
 | `language` | `string` | [Landing](#landing) |
@@ -619,6 +634,15 @@ OIDC is sign-in through an identity provider.
 
 Takes a [OIDC](#oidc) block.
 
+### `ldap`
+
+`LDAP`
+
+LDAP is sign-in against a directory, Active Directory above all, and
+the accounts kept in step with it.
+
+Takes a [LDAP](#ldap) block.
+
 ### `scim`
 
 `SCIM`
@@ -1427,6 +1451,106 @@ MaxBlocks bounds how many of a prompt's blocks are hashed and looked
 up. Beyond it a longer match still helps, but the difference no longer
 decides anything. Zero takes 4096.
 
+## LDAP
+
+### `urls`
+
+`[]string`
+
+URLs are the directory's servers, tried in turn: ldaps:// (636), or
+ldap:// (389) with StartTLS.
+
+### `start_tls`
+
+`bool`
+
+StartTLS upgrades an ldap:// connection before anything is sent. A
+password never crosses a connection without TLS: ldap:// without it
+is refused.
+
+### `ca_file`
+
+`string`
+
+CAFile is the authority the servers' certificates are checked against,
+when it is not the system's: an AD Certificate Services root.
+
+### `bind_dn`
+
+`string`
+
+BindDN and the file holding its password: a service account that may
+read users and groups. Nothing is written to the directory.
+
+### `bind_password_file`
+
+`string`
+
+### `base_dn`
+
+`string`
+
+BaseDN is where people are searched for, subtree included.
+
+### `group_base_dn`
+
+`string`
+
+GroupBaseDN is where their groups are; empty is BaseDN's domain root.
+
+### `user_filter`
+
+`string`
+
+UserFilter narrows who may sign in, ANDed with the login match: a
+group (memberOf=...) or an OU's people only. Empty is every enabled
+user object.
+
+### `label`
+
+`string`
+
+Label is what the form says to sign in with: "Sign in with your
+<label> account". Empty is "Active Directory".
+
+### `create_accounts`
+
+`bool`
+
+CreateAccounts makes an account, active, for a person the directory
+vouches for and the gateway does not know. Off, only existing accounts
+sign in this way.
+
+### `roles`
+
+`map[string][]string`
+
+Roles maps a staff role (viewer, operator, billing, admin) to the
+groups that give it, by name (CN) or whole DN. Set at every sign-in
+and sync: someone taken out of the group loses the role.
+
+### `memberships`
+
+`map[string]string`
+
+Memberships maps a group, by CN or DN, to a place in the tree —
+org:NAME, team:ORG/TEAM, project:ORG/TEAM/PROJECT — joined at sign-in
+and kept by the sync, which also takes away a place whose group the
+person has left.
+
+### `sync_interval`
+
+`time.Duration`
+
+SyncInterval is how often the leader reads the directory. Zero is
+15m; negative turns the sync off, leaving only sign-in.
+
+### `timeout`
+
+`time.Duration`
+
+Timeout bounds each round trip to a server. Zero is 10s.
+
 ## Landing
 
 ### `contact`