.. _bsd_sockets_interface:

BSD Sockets
###########

.. contents::
    :local:
    :depth: 2

Overview
********

Zephyr offers an implementation of a subset of the BSD Sockets API (a part
of the POSIX standard). This API allows to reuse existing programming experience
and port existing simple networking applications to Zephyr.

Here are the key requirements and concepts which governed BSD Sockets
compatible API implementation for Zephyr:

* Has minimal overhead, similar to the requirement for other
  Zephyr subsystems.
* Is namespaced by default, to avoid name conflicts with well-known
  names like ``close()``, which may be part of libc or other POSIX
  compatibility libraries.
  If enabled by :kconfig:`CONFIG_NET_SOCKETS_POSIX_NAMES`, it will also
  expose native POSIX names.

BSD Sockets compatible API is enabled using :kconfig:`CONFIG_NET_SOCKETS`
config option and implements the following operations: ``socket()``, ``close()``,
``recv()``, ``recvfrom()``, ``send()``, ``sendto()``, ``connect()``, ``bind()``,
``listen()``, ``accept()``, ``fcntl()`` (to set non-blocking mode),
``getsockopt()``, ``setsockopt()``, ``poll()``, ``select()``,
``getaddrinfo()``, ``getnameinfo()``.

Based on the namespacing requirements above, these operations are by
default exposed as functions with ``zsock_`` prefix, e.g.
:c:func:`zsock_socket` and :c:func:`zsock_close`. If the config option
:kconfig:`CONFIG_NET_SOCKETS_POSIX_NAMES` is defined, all the functions
will be also exposed as aliases without the prefix. This includes the
functions like ``close()`` and ``fcntl()`` (which may conflict with
functions in libc or other libraries, for example, with the filesystem
libraries).

Another entailment of the design requirements above is that the Zephyr
API aggressively employs the short-read/short-write property of the POSIX API
whenever possible (to minimize complexity and overheads). POSIX allows
for calls like ``recv()`` and ``send()`` to actually process (receive
or send) less data than requested by the user (on ``SOCK_STREAM`` type
sockets). For example, a call ``recv(sock, 1000, 0)`` may return 100,
meaning that only 100 bytes were read (short read), and the application
needs to retry call(s) to receive the remaining 900 bytes.

The BSD Sockets API uses file descriptors to represent sockets. File
descriptors are small integers, consecutively assigned from zero, shared
among sockets, files, special devices (like stdin/stdout), etc. Internally,
there is a table mapping file descriptors to internal object pointers.
The file descriptor table is used by the BSD Sockets API even if the rest
of the POSIX subsystem (filesystem, stdin/stdout) is not enabled.

.. _secure_sockets_interface:

Secure Sockets
**************

Zephyr provides an extension of standard POSIX socket API, allowing to create
and configure sockets with TLS protocol types, facilitating secure
communication. Secure functions for the implementation are provided by
mbedTLS library. Secure sockets implementation allows use of both TLS and DTLS
protocols with standard socket calls. See :c:enum:`net_ip_protocol_secure` type
for supported secure protocol versions.

To enable secure sockets, set the :kconfig:`CONFIG_NET_SOCKETS_SOCKOPT_TLS`
option. To enable DTLS support, use :kconfig:`CONFIG_NET_SOCKETS_ENABLE_DTLS`
option.

TLS credentials subsystem
=========================

TLS credentials must be registered in the system before they can be used with
secure sockets. See :c:func:`tls_credential_add` for more information.

When a specific TLS credential is registered in the system, it is assigned with
numeric value of type :c:type:`sec_tag_t`, called a tag. This value can be used
later on to reference the credential during secure socket configuration with
socket options.

The following TLS credential types can be registered in the system:

- ``TLS_CREDENTIAL_CA_CERTIFICATE``
- ``TLS_CREDENTIAL_SERVER_CERTIFICATE``
- ``TLS_CREDENTIAL_PRIVATE_KEY``
- ``TLS_CREDENTIAL_PSK``
- ``TLS_CREDENTIAL_PSK_ID``

An example registration of CA certificate (provided in ``ca_certificate``
array) looks like this:

.. code-block:: c

   ret = tls_credential_add(CA_CERTIFICATE_TAG, TLS_CREDENTIAL_CA_CERTIFICATE,
                            ca_certificate, sizeof(ca_certificate));

By default certificates in DER format are supported. PEM support can be enabled
in mbedTLS settings.

Secure Socket Creation
======================

A secure socket can be created by specifying secure protocol type, for instance:

.. code-block:: c

   sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TLS_1_2);

Once created, it can be configured with socket options. For instance, the
CA certificate and hostname can be set:

.. code-block:: c

   sec_tag_t sec_tag_opt[] = {
           CA_CERTIFICATE_TAG,
   };

   ret = setsockopt(sock, SOL_TLS, TLS_SEC_TAG_LIST,
                    sec_tag_opt, sizeof(sec_tag_opt));

.. code-block:: c

   char host[] = "google.com";

   ret = setsockopt(sock, SOL_TLS, TLS_HOSTNAME, host, sizeof(host) - 1);

Once configured, socket can be used just like a regular TCP socket.

Several samples in Zephyr use secure sockets for communication. For a sample use
see e.g. :ref:`echo-server sample application <sockets-echo-server-sample>` or
:ref:`HTTP GET sample application <sockets-http-get>`.

Secure Sockets options
======================

Secure sockets offer the following options for socket management:

.. doxygengroup:: secure_sockets_options

API Reference
*************

BSD Sockets
===========

.. doxygengroup:: bsd_sockets

TLS Credentials
===============

.. doxygengroup:: tls_credentials