Apache Reference: http_core, ListenBacklog
Maximum Length of the Queue of Pending Connections
Syntax: ListenBacklog A
Example: ListenBacklog 100
Since: Apache 1.3
This directive specifies the maximum length of the queue of
pending connections, as used by listen(2). Generally, no tuning
is needed or desired, although on some systems it is desirable
to increase tuning when under a TCP SYN flood attack. In many
cases, the operating system will limit the backlog parameter to
the listen(2) system call to a smaller number. This limitation
varies from one operating system to the next. Also, note that
many operating systems do not use the backlog exactly as
specified, but instead use a number based on (but normally
larger than) that set.