Authentication process design¶

See Design/Processes for an overview of how the Dovecot processes work.

There are four major classes in the code:

struct mech_module: Authentication (SASL) mechanism
struct password_scheme: Password scheme
struct passdb_module: Password database
struct userdb_module: User database

There are many implementations for each of these, and it’s simple to add more of them. They can also be added as plugins.

The code flow usually goes like:

The auth process listens for new authentication client connections.
A new authentication client (e.g. login process) connects to the login or auth-client UNIX socket.
Authentication client sends a request to begin a SASL authentication.
Authentication mechanism backend handles it (mech->auth_initial() and mech->auth_continue() in mech-*.c)
The mechanism either asks the passdbs to verify a username/password pair (auth_request_verify_plain()), or it looks up the credentials itself (auth_request_lookup_credentials()) and verifies that they are valid.
Success reply is sent to the authentication client.
If this is a login from login process, it creates a mail process by connects to the process type-specific socket (e.g. imap or pop3) and sending the authentication reply information to it.
The mail process connects to the auth-master UNIX socket and finishes the authentication request. This includes doing a userdb lookup, which is returned back to the mail process.

The authentication is fully asynchronous and it supports handling multiple requests in parallel.

It’s also possible to do passdb and userdb lookups directly without full authentication.

The login socket is mostly treated as untrusted. It’s not possible for it to authenticate users without actually providing the proper credentials. However, there are some fields that need to be trusted:

Client IP address and port

Local server IP address and port

Connecting proxy’s IP address and port

Client TLS certificate’s username and trust status. This means that if authentication is done via client TLS certificates, the auth process simply trusts the login process to verify the certificate. This breaks the trust model and should be fixed some day.

Authentication (SASL) mechanisms¶

These are SASL authentication mechanism implementations. See Authentication (SASL) Mechanisms for a list of mechanisms supported by Dovecot.

A new mechanism is created by filling a struct mech_module (in mech.h) and passing it to mech_register_module(). The struct fields are:

mech_name: The public name of the mechanism. This is shown to clients in the IMAP, POP3 and SMTP capability lists. If you create a new non-standard mechanism, please prefix it with “X-“.
flags: Describes how secure the mechanism is. Also MECH_SEC_PRIVATE flag specifies that the mechanism shouldn’t be advertised in the capability list. This is currently used only for APOP mechanism, which is defined by the POP3 protocol itself.
passdb_need_plain: This mechanism uses passdb’s verify_plain() function to verify the password’s validity. This means that the mechanism has access to the plaintext password. This is true only for plaintext mechanisms such as PLAIN and LOGIN. The main purpose of this flag is to make dovecot-auth complain at startup if there are no passdbs defined in the configuration file. Note that a configuration without any passdbs is valid with eg. GSSAPI mechanism which doesn’t need a passdb at all.
passdb_need_credentials: This mechanism uses passdb’s lookup_credentials() function. See below for description of the credentials.
auth_new(): Allocates a new struct auth_request. Typically with more complex mechanisms it really allocates a struct <mech>_auth_request which contains struct auth_request as the first field, followed by mechanism-specific fields.
auth_initial(request, data, data_size): This begins the authentication, data and data_size containing the initial response sent by the client (decoded, not in base64). Call request->callback() once you’re done (see below).
auth_continue(request, data, data_size): Continues the authentication. Works the same as auth_initial().
auth_free(): Free the request. Usually all the memory allocations for the request should be allocated from request->pool, so you can use mech_generic_auth_free() which simply frees the pool.

auth_initial() and auth_continue() can send SASL continuation requests. For success and failure replies, use one of these functions:

auth_request_success()
auth_request_fail()
auth_request_internal_failure(): Use this if you couldn’t figure out if the authentication succeeded or failed, for example because passdb lookup returned internal failure.

SASL authentication in general works like:

Client begins the authentication, optionally sending an “initial response”, meaning some data that the mechanism sees in auth_initial().
- Note that not all protocols support the initial response. For example IMAP supports it only if the server implements SASL-IR extension. Because of this mechanisms, such as PLAIN, support doing the authentication either in auth_initial() or in auth_continue().
- If the client initiates the authentication (ie. server’s initial reply is empty, such as with PLAIN mechanism) you can use mech_generic_auth_initial() instead of implementing your own.
Server processes the authentication request and replies back with auth_request_handler_reply().
- If the authentication failed, it’s placed into auth_failures array unless request->no_failure_delay=TRUE. The failures are flushed from the buffer once every 2 seconds to clients and mechanism->auth_free() is called.
- If the authentication succeeded and
  - there is a master connection associated with the request (IMAP/POP3 login), the authentication now waits for master connection to do a verification request. If this for some reason doesn’t happen in AUTH_REQUEST_TIMEOUT seconds (3,5 mins), it’s freed.
  - there isn’t a master connection (SMTP AUTH), the authentication is freed immediately.
Client processes the reply:
- If the authentication continues, it sends back more data which is processed in auth_continue(). Goto 2.
- If the authentication failed, it’s done.
If the authentication succeeded, the client connects to the post-login mail process, which in turn connects to auth and does the final auth verification.
- Besides verifying the authentication, auth process also does a userdb lookup to return the userdb information to the mail process.
- If the verification fails (normally because userdb lookup fails), the client gets “internal authentication failure”
- If the verification succeeds, the user is now logged in
- In either case, mechanism->auth_free() is called now.

Credentials¶

Most of the non-plaintext mechanisms need to verify the authentication by using a special hash of the user’s password. So either the passdb credentials lookup returns a plaintext password from which the hash can be created, or the hash directly. Both of these cases can be handled simply by calling auth_request_lookup_credentials() with the wanted password scheme.

Password schemes¶

struct password_scheme has fields:

name: Name of the scheme. This only shows up in configuration files and maybe in the passwords stored in passdb (“{scheme_name}password_hash”).
password_verify(plaintext, params, raw_password, raw_password_size, error): Returns 1 if raw_password hash matches the plaintext password given in plaintext parameter. The raw_password is in binary, i.e. not hex or base64-encoded.
password_generate(plaintext, params, raw_password_r, raw_password_size_r): Returns the password hash for given plaintext password.

The params can be used to specify some extra parameters:

user: Used if the password hash depends on the username (eg. with DIGEST-MD5).

rounds: Some schemes support a configurable number of hash rounds.

A new password scheme can be created simply by creating a struct password_scheme named <module_name>_scheme, compiling a shared object and placing it to $moduledir/auth/ directory.

Password databases¶

See Password databases (passdb) for a description of passdbs and a list of already implemented ones.

struct passdb_module contains fields:

cache_key: A string containing variables. When expanded, it uniquely identifies a passdb lookup. This is %u when the passdb lookup validity depends only on the username. With more complex databases such as SQL and LDAP this is created dynamically based on the password query in the configuration file. If there are multiple variables, they should be separated so that their contents don’t get mixed, for example %u<TAB>%r<TAB>%l. auth_cache_parse_key() can be used to easily create a cache key from a query string.
default_pass_scheme: The default scheme to use when it’s not explicitly specified with a “{scheme}” prefix.
blocking: If TRUE, the lookup is done in auth-worker process. This should be used if the lookup may block.
iface.preinit(pool, args): Allocate struct passdb_module from the pool and return it. This function is called before chrooting and before privileges are dropped from the auth process, so if should do things like read the configuration file. args contains the args parameter in the configuration file.
iface.init(module): The privileges have been dropped before calling this. module contains the structure returned by preinit(). Typically this function will do things like connect to the database.
iface.deinit(module): Close the connection to the password database and free all the used memory.
iface.verify_plain(auth_request, password, callback): Check if the given plaintext password matches. auth_request->wanted_credentials_scheme == NULL always. When the verification is done, call the given callback with the result in result parameter.
iface.lookup_credentials(auth_request, callback): Look up the password credentials. auth_request->wanted_credentials_scheme contains the credentials that the mechanism wants. Afterwards call passdb_handle_credentials() to finish the request.

Plaintext authentication mechanisms typically call verify_plain(), which is possible to implement with all the passdbs. Non-plaintext mechanisms typically call lookup_credentials(), which isn’t possible to implement always (eg. PAM). If it’s not possible to implement lookup_credentials(), the pointer can be left NULL.

If the passdb uses connections to external services, it’s preferred that they use non-blocking connections. Dovecot does this whenever possible (PostgreSQL and LDAP for example). If it’s not possible, set blocking = TRUE.

With both functions auth_request->passdb->passdb contains the passdb_module returned by your preinit() function. auth_request->user contains the username whose password is being verified. There’s no need to worry about master users here. It’s also possible to use any other fields in auth_request to do the lookup, such as service, local_ip or remote_ip if they exist. Often you want to let user to configure the lookup with variables (eg. SQL query). In that case you can use auth_request_get_var_expand_table() to retrieve the variable table for var_expand().

The passdb lookup can return one of the following results:

PASSDB_RESULT_INTERNAL_FAILURE: The lookup failed. For example SQL server is down.
PASSDB_RESULT_SCHEME_NOT_AVAILABLE: lookup_credentials() requested a scheme which isn’t in the passdb
PASSDB_RESULT_USER_UNKNOWN: The user doesn’t exist in the database.
PASSDB_RESULT_USER_DISABLED: The user is disabled either entirely, or for this specific login (eg. only POP3 logins allowed). This isn’t commonly implemented in passdbs.
PASSDB_RESULT_PASS_EXPIRED: The user’s password had expired. This isn’t commonly implemented in passdbs.
PASSDB_RESULT_NEXT: Internal use only: The “noauthenticate” field is set.
PASSDB_RESULT_PASSWORD_MISMATCH: The password given in verify_plain() wasn’t valid.
PASSDB_RESULT_OK: Success.

User databases¶

See User Databases (userdb) for a description of userdbs and a list of already implemented ones.

struct userdb_module is very similar to struct passdb_module. The lookup callback is a bit different though:

typedef void userdb_callback_t(enum userdb_result result,
                               struct auth_request *request);

result contains one of:

USERDB_RESULT_INTERNAL_FAILURE: The lookup failed. For example SQL server is down.
USERDB_RESULT_USER_UNKNOWN: The user doesn’t exist in the database.
USERDB_RESULT_OK: Success.

There is no equivalent for PASSDB_RESULT_USER_DISABLED currently. When logging in with IMAP or POP3, the user’s existence was already checked in passdb lookup, so only in rare conditions when a user is logging in at the same time as it’s being deleted, the userdb result is USER_UNKNOWN.

The results are added to the auth_request using auth_request_set_field() and auth_request_set_userdb_field().