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# SSL Application

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The ssl application provides secure communication over sockets.

## Description

The ssl application is an implementation of the TLS (previously known as SSL) and DTLS protocols in Erlang.

For current statement of standards compliance see the User's Guide.

## Dependencies

The SSL application uses the `Public_Key`, `Asn1` and `Crypto` application to handle public keys and encryption, hence these applications must be loaded for the SSL application to work. In an embedded environment this means they must be started with `application:start/1,2` before the SSL application is started.

## Configuration

The application environment configuration parameters in this section are defined for the SSL application. For more information about configuration parameters, see the `application` manual page in Kernel.

#### Note

All parameters including the wording 'session_ticket' are TLS-1.3 only configuration and other session parameters are prior to TLS-1.3 only configuration. DTLS versions are based on TLS versions see standard compliance for mapping.

The environment parameters can be set on the command line, for example:

`erl -ssl protocol_version "['tlsv1.2', 'tlsv1.1']"`

- `protocol_version =` `ssl:tls_version/0` | [`ssl:tls_version/0`] `<optional>` - Protocol supported by started clients and servers. If this option is not set, it defaults to all TLS protocols currently supported, more might be configurable, by the SSL application. This option can be overridden by the version option to `ssl:connect/2,3` and `ssl:listen/2`.

- `dtls_protocol_version =` `ssl:dtls_version/0` | [`ssl:dtls_version/0`] `<optional>` - Protocol supported by started clients and servers. If this option is not set, it defaults to all DTLS protocols currently supported, more might be configurable, by the SSL application. This option can be overridden by the version option to `ssl:connect/2,3` and `ssl:listen/2`.

- `session_lifetime = integer() <optional>` - Maximum lifetime of the session data in seconds. Defaults to 24 hours which is the maximum recommended lifetime by RFC 5246. However sessions may be invalidated earlier due to the maximum limitation of the session cache table.

- `session_cb = atom() <optional>` - Deprecated Since OTP-23.3 replaced by `client_session_cb` and `server_session_cb`

- `client_session_cb = atom() <optional>` - Since OTP-23.3 Name client of the session cache callback module that implements the `ssl_session_cache_api` behavior. Defaults to `ssl_client_session_cache_db`.

- `server_session_cb = atom() <optional>` - Since OTP-23.3 Name of the server session cache callback module that implements the `ssl_session_cache_api` behavior. Defaults to `ssl_server_session_cache_db`.

- `session_cb_init_args = proplist:proplist() <optional>` - Deprecated Since OTP-23.3 replaced by `client_session_cb_init_args` and `server_session_cb_init_args`

- `client_session_cb_init_args = proplist:proplist() <optional>` - List of extra user-defined arguments to the `init` function in the session cache callback module. Defaults to `[]`.

- `server_session_cb_init_args = proplist:proplist() <optional>` - List of extra user-defined arguments to the `init` function in the session cache callback module. Defaults to `[]`.

- `session_cache_client_max = integer() <optional>`
  Limits the growth of the clients session cache, that is how many sessions towards servers that are cached to be used by new client connections. If the maximum number of sessions is reached, the current cache entries will be invalidated regardless of their remaining lifetime. Defaults to 1000. Recommended ssl-8.2.1 or later for this option to work as intended.

- `session_cache_server_max = integer() <optional>` - Limits the growth of the servers session cache, that is how many client sessions are cached by the server. If the maximum number of sessions is reached, the current cache entries will be invalidated regardless of their remaining lifetime. Defaults to 1000. Recommended ssl-8.2.1 or later for this option to work as intended.

- `ssl_pem_cache_clean = integer() <optional>` - Number of milliseconds between PEM cache validations. Defaults to 2 minutes.

  Note: The cache can be reloaded by calling `ssl:clear_pem_cache/0`.

- `bypass_pem_cache = boolean() <optional>` - Introduced in ssl-8.0.2. Disables the PEM-cache. Can be used as a workaround for the PEM-cache bottleneck before ssl-8.1.1. Defaults to false.

- `alert_timeout = integer() <optional>` - Number of milliseconds between sending of a fatal alert and closing the connection. Waiting a little while improves the peers chances to properly receiving the alert so it may shutdown gracefully. Defaults to 5000 milliseconds.

- `internal_active_n = integer() <optional>` - For TLS connections this value is used to handle the internal socket. As the implementation was changed from an active once to an active N behavior (N = 100), for performance reasons, this option exist for possible tweaking or restoring of the old behavior (internal_active_n = 1) in unforeseen scenarios. The option will not affect erlang distribution over TLS that will always run in active N mode. Added in ssl-9.1 (OTP-21.2).

- `server_session_tickets_amount = integer() <optional>` - Number of session tickets sent by the server. It must be greater than 0. Defaults to 3.

- `server_session_ticket_lifetime = integer() <optional>` - Lifetime of session tickets sent by the server. Servers must not use any value greater than 604800 seconds (7 days). Expired tickets are automatically removed. Defaults to 7200 seconds (2 hours).

- `server_session_ticket_store_size = integer() <optional>` - Sets the maximum size of the server session ticket store (stateful tickets). Defaults to 1000. Size limit is enforced by dropping old tickets.

- `server_session_ticket_max_early_data = integer() <optional>` - Sets the maximum size of the early data that the server accepts and also configures its NewSessionTicket messages to include this same size limit in their early_data_indication extension. Defaults to 16384. Size limit is enforced by both client and server.

- `client_session_ticket_lifetime = integer() <optional>` - Lifetime of session tickets in the client ticket store. Expired tickets are automatically removed. Defaults to 7200 seconds (2 hours).

- `client_session_ticket_store_size = integer() <optional>` - Sets the maximum size of the client session ticket store. Defaults to 1000. Size limit is enforced by dropping old tickets.

## Error Logger and Event Handlers

The SSL application uses OTP logger. TLS/DTLS alerts are logged on notice level. Unexpected errors are logged on error level. These log entries will by default end up in the default Erlang log. The option `log_level` may be used to in run-time to set the log level of a specific TLS connection, which is handy when you want to use level debug to inspect the TLS handshake setup.

## See Also

`application`

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