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gh-45048: Document the availability and uses of the SO_ constants #132752

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gh-45048: Document the availability and uses of the SO_ constants.
ukarroum Apr 20, 2025
196aa97
gh-45048: Removed >= vista constrain on SO_BROADCAST support on windows
ukarroum Apr 20, 2025
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282 changes: 282 additions & 0 deletions Doc/library/socket.rst
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Expand Up @@ -411,6 +411,288 @@ Constants

.. _socket-unix-constants:

Socket level options
''''''''''''''''''''

These constants are passed to :meth:`socket.setsockopt` and :meth:`socket.getsockopt`
with a level of :const:`SOL_SOCKET` to set and get socket settings. Their availability
and behavior is dependent upon the underlying OS. Basic descriptions of each option are
documented below but your OS's documentation will be far more reliable. The following
table serves a basic compatibility guide.

Unless otherwise noted, these options are available on Linux versions >= 2.3,
FreeBSD >= 2 and Windows >= 2000.

======================= =========== =========== ================
Socket Option Linux FreeBSD Windows
======================= =========== =========== ================
**SO_DEBUG** ✓ ✓ ✓ [1]_
**SO_ACCEPTCONN** ✓ ✓ ✓
**SO_REUSEADDR** ✓ ✓ ✓
**SO_EXCLUSIVEADDRUSE** ✓
**SO_KEEPALIVE** ✓ ✓ ✓
**SO_DONTROUTE** ✓ ✓ ✓ [1]_
**SO_BROADCAST** ✓ ✓ ✓
**SO_USELOOPBACK** ✓ ✓ >= Vista [1]_
**SO_LINGER** ✓ ✓ ✓
**SO_OOBINLINE** ✓ ✓ ✓
**SO_REUSEPORT** ✓ >= 3.9 ✓
**SO_SNDBUF** ✓ ✓ ✓
**SO_RCVBUF** ✓ ✓ ✓
**SO_SNDLOWAT** ✓ ✓ ✓ [1]_
**SO_RCVLOWAT** ✓ ✓ ✓ [1]_
**SO_SNDTIMEO** ✓ ✓ ✓
**SO_RCVTIMEO** ✓ ✓ ✓
**SO_ERROR** ✓ ✓ ✓
**SO_TYPE** ✓ ✓ ✓
**SO_SETFIB** ✓ >= 7.1
**SO_PASSCRED** ✓
**SO_PEERCRED** ✓
**SO_PASSSEC** ✓ >= 2.6.2
**SO_PEERSEC** ✓ >= 2.6.2
**SO_BINDTODEVICE** ✓
**SO_PRIORITY** ✓
**SO_MARK** ✓ >= 2.6.25
**SO_DOMAIN** ✓ >= 2.6.32
**SO_PROTOCOL** ✓ >= 2.6.32 ✓ >= 8.3
======================= =========== =========== ================

.. [1] On Windows these options exist purely for compatibility
and are `not functional <https://msdn.microsoft.com/en-us/library/windows/desktop/ms740532(v=vs.85).aspx>`_.

.. data:: SOL_SOCKET

This is the level parameter passed to :meth:`socket.setsockopt`
and :meth:`socket.getsockopt` to change socket level parameters.

For example, this is how to change a socket's sending buffer size. ::

>>> s.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)
16384
>>> s.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, 2048)
>>> s.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)
2048

.. data:: SO_DEBUG

Setting this option to 1 on a socket causes it to print out debugging
information into the kernel log. (Viewable with dmesg)

.. Availability: Linux, Windows `(non functional) <https://msdn.microsoft.com/en-us/library/windows/desktop/ms740532(v=vs.85).aspx>`_

.. data:: SO_ACCEPTCONN

Returns 1 if the socket is marked to accept connections with
:meth:`socket.listen`. This option is read only.

.. Availability: Linux, Windows

.. data:: SO_REUSEADDR

Allows a socket to be able to bind to an address that was previously
bound to. When a socket shuts down, whether by virtue of the program
exiting or an explicit call to shut it down, it takes the OS some time
to terminate existing connections and perform the proper shut down
procedure. ``SO_REUSEADDR`` allows bypassing of this behavior and permits
another socket to be able to bind to the address.

.. Availability: Linux, Windows

.. data:: SO_EXCLUSIVEADDRUSE

Allows overriding ``SO_REUSEADDR`` behavior for exclusive access to an
address and port for high availability services.

.. seealso::

`Microsoft's documentation <https://msdn.microsoft.com/en-us/library/windows/desktop/cc150667(v=vs.85).aspx>`_
on this option.

.. Availability: Windows

.. data:: SO_KEEPALIVE

Set to 1 to enable sending of keep-alive packets on certain types of
sockets.

.. Availability: Linux, Windows

.. data:: SO_DONTROUTE

Indicates that packets sent through this socket should be routed
through the interface it is bound to.

.. Availability: Linux, Windows `(non functional) <https://msdn.microsoft.com/en-us/library/windows/desktop/ms740532(v=vs.85).aspx>`_

.. data:: SO_BROADCAST

This option may be set to 1 to enable broadcasting messages, if
supported by the protocol. Note that IPV6 (:const:`AF_INET6`) does
not support broadcasting.

.. Availability: Linux, Windows >= Vista.

.. data:: SO_USELOOPBACK

.. Availability: FreeBSD, Windows >= Vista `(non functional) <https://msdn.microsoft.com/en-us/library/windows/desktop/ms740532(v=vs.85).aspx>`_

.. data:: SO_LINGER

When a socket is set to linger, a call to :meth:`~socket.close` or
:meth:`~socket.shutdown` will not return until all queued messages
for the socket have been successfully sent or the linger timeout
is reached. If the socket is closed due to the program exiting, it
will linger in the background.

The value for this option is a struct on most operating systems.
Consult your OS documentation for the struct's details.

.. Availability: Linux, Windows

.. data:: SO_OOBINLINE

Allows receiving of out of band data in the normal stream.

.. seealso::

`Out-of-band data <https://www.ibm.com/support/knowledgecenter/en/ssw_i5_54/rzab6/coobd.htm>`_
for an in depth explanation.

.. Availability: Linux, Windows

.. data:: SO_REUSEPORT

.. Availability: Linux >= 3.9 (for TCP/UDP)

.. data:: SO_SNDBUF
SO_RCVBUF

Sets the size of the sending and receiving buffers for this socket in
bytes. Most operating systems impose an upper limit on the size of
these buffers.

.. Availability: Linux, Windows

.. data::
SO_RCVLOWAT
SO_SNDLOWAT

``SO_RCVLOWAT`` sets the minimum number of bytes that must be present in
the socket's internal receive buffer before they are returned by a
read call. ``SO_SNDLOWAT`` similiary sets the minimum bytes before data is
sent from the send buffer to the socket protocol.

SO_SNDLOWAT is read-only on Linux and SO_RCVLOWAT is read-only on
Linux versions below 2.4.

.. Availability: Linux, Windows `(non functional) <https://msdn.microsoft.com/en-us/library/windows/desktop/ms740532(v=vs.85).aspx>`_

.. data:: SO_RCVTIMEO
SO_SNDTIMEO

Specifies the amount of time send and receive calls for this socket will
block before timing out. The default timeout of zero means that operations
will never time out.

This is independent of :meth:`~socket.settimeout`.

On Linux this is a `struct timeval`, on Windows this is an integer.

.. Availability: Linux, Windows

.. data:: SO_ERROR

Gets and resets the pending socket error.

.. Availability: Linux, Windows

.. data:: SO_TYPE

Gets the type of the socket. This type corresponds to the type as
defined in the :func:`socket.socket` function. Examples include
:const:`SOCK_STREAM` and :const:`SOCK_DGRAM`.

.. Availability: Linux, Windows

.. data:: SO_SETFIB

Sets the FIB (routing table) for the socket.

Availability: FreeBSD >= 7.1

.. versionadded:: 3.1

.. data:: SO_PASSCRED
SO_PEERCRED

Allows for the passing of SCM credentials over unix sockets. These
are passed as ancillary messages which can be received using :func:`~socket.recvmsg`

.. versionadded:: 3.3

.. Availability: Linux

.. data:: SO_PASSSEC
SO_PEERSEC

Allows for the passing of security contexts over unix sockets.

.. versionadded:: 3.6

.. Availability: Linux >= 2.6.2

.. data:: SO_BINDTODEVICE

Binds a socket to a particular network interface device like "eth0",
When bound, only packets received from that particular device are
processsed by the socket.

.. versionadded:: 3.3

.. Availability: Linux

.. data:: SO_PRIORITY

Sets the protocol-defined priority for each packet sent on this socket.
Packets with higher priority may be processed first depending on the
queueing mechanism of the network interface.

.. versionadded:: 3.4

.. Availability: Linux

.. data:: SO_MARK

Sets the mark on each packet sent through this socket. This may be used
for routing or packet filtering.

.. versionadded:: 3.5

.. Availability: Linux >= 2.6.25

.. data:: SO_DOMAIN
SO_PROTOCOL

Passing ``SO_DOMAIN`` to :meth:`~socket.getsockopt` allows for the retrival
of the ``family`` value as defined in the :func:`socket.socket` function.
``SO_PROTOCOL`` returns the ``proto`` value. The protocol value can be the
exact protocol used such as ``IPPROTO_TCP`` even if 0 was passed in to specify
the default protocol.

Both these options are read only.

The value returned for the ``family`` is an integer which is the value of
the constants above like :const:`AF_INET`. In order to get the const name
value back you can use the AddressFamily enum. ::

>>> family = s.getsockopt(socket.SOL_SOCKET, socket.SO_DOMAIN)
>>> socket.AddressFamily(family)
<AddressFamily.AF_INET: 2>

Availability: Linux >= 2.6.32.

.. versionadded:: 3.6

.. data:: SO_*
SOMAXCONN
MSG_*
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