.. _param-imtcp-socketbacklog:
.. _imtcp.parameter.input.socketbacklog:

SocketBacklog
=============

.. index::
   single: imtcp; SocketBacklog
   single: SocketBacklog

.. summary-start

Sets the backlog length for pending TCP connections.

.. summary-end

This parameter applies to :doc:`../../configuration/modules/imtcp`.

:Name: SocketBacklog
:Scope: input
:Type: integer
:Default: input=10% of configured connections
:Required?: no
:Introduced: 8.2502.0

Description
-----------
Specifies the backlog parameter passed to the ``listen()`` system call. This parameter defines the
maximum length of the queue for pending connections, which includes partially established connections
(those in the SYN-ACK handshake phase) and fully established connections waiting to be accepted by the
application.

**Available starting with the 8.2502.0 series.**

For more details, refer to the ``listen(2)`` man page.

By default, the value is set to 10% of the configured connections to accommodate modern workloads. It
can be adjusted to suit specific requirements, such as:

- **High rates of concurrent connection attempts**: Increasing this value helps handle bursts of
  incoming connections without dropping them.
- **Test environments with connection flooding**: Larger values are recommended to prevent SYN queue
  overflow.
- **Servers with low traffic**: Lower values may be used to reduce memory usage.

The effective backlog size is influenced by system-wide kernel settings, particularly
``net.core.somaxconn`` and ``net.ipv4.tcp_max_syn_backlog``. The smaller value between this parameter
and the kernel limits is used as the actual backlog.

Input usage
-----------
.. _param-imtcp-input-socketbacklog:
.. _imtcp.parameter.input.socketbacklog-usage:

.. code-block:: rsyslog

   input(type="imtcp" socketBacklog="128")

See also
--------
See also :doc:`../../configuration/modules/imtcp`.