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man: document the new PollLimitIntervalSec=/PollLimitBurst= settings

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Lennart Poettering 2023-09-18 17:51:49 +02:00
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man/systemd.socket.xml
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@ -868,21 +868,57 @@
<term><varname>TriggerLimitIntervalSec=</varname></term>
<term><varname>TriggerLimitBurst=</varname></term>
<listitem><para>Configures a limit on how often this socket unit may be activated within a specific time
interval. The <varname>TriggerLimitIntervalSec=</varname> may be used to configure the length of the time
interval in the usual time units <literal>us</literal>, <literal>ms</literal>, <literal>s</literal>,
<literal>min</literal>, <literal>h</literal>, … and defaults to 2s (See
<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details on
the various time units understood). The <varname>TriggerLimitBurst=</varname> setting takes a positive integer
value and specifies the number of permitted activations per time interval, and defaults to 200 for
<varname>Accept=yes</varname> sockets (thus by default permitting 200 activations per 2s), and 20 otherwise (20
activations per 2s). Set either to 0 to disable any form of trigger rate limiting. If the limit is hit, the
socket unit is placed into a failure mode, and will not be connectible anymore until restarted. Note that this
limit is enforced before the service activation is enqueued.</para>
<listitem><para>Configures a limit on how often this socket unit may be activated within a specific
time interval. The <varname>TriggerLimitIntervalSec=</varname> setting may be used to configure the
length of the time interval in the usual time units <literal>us</literal>, <literal>ms</literal>,
<literal>s</literal>, <literal>min</literal>, <literal>h</literal>, … and defaults to 2s (See
<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
details on the various time units understood). The <varname>TriggerLimitBurst=</varname> setting
takes a positive integer value and specifies the number of permitted activations per time interval,
and defaults to 200 for <varname>Accept=yes</varname> sockets (thus by default permitting 200
activations per 2s), and 20 otherwise (20 activations per 2s). Set either to 0 to disable any form of
trigger rate limiting.</para>
<para>If the limit is hit, the socket unit is placed into a failure mode, and will not be connectible
anymore until restarted. Note that this limit is enforced before the service activation is
enqueued.</para>
<para>Compare with <varname>PollLimitIntervalSec=</varname>/<varname>PollLimitBurst=</varname>
described below, which implements a temporary slowdown if a socket unit is flooded with incoming
traffic, as opposed to the permanent failure state
<varname>TriggerLimitIntervalSec=</varname>/<varname>TriggerLimitBurst=</varname> results in.</para>
<xi:include href="version-info.xml" xpointer="v230"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>PollLimitIntervalSec=</varname></term>
<term><varname>PollLimitBurst=</varname></term>
<listitem><para>Configures a limit on how often polling events on the file descriptors backing this
socket unit will be considered. This pair of settings is similar to
<varname>TriggerLimitIntervalSec=</varname>/<varname>TriggerLimitBurst=</varname> but instead of
putting a (fatal) limit on the activation frequency puts a (transient) limit on the polling
frequency. The expected parameter syntax and range are identical to that of the aforementioned
options, and can be disabled the same way.</para>
<para>If the polling limit is hit polling is temporarily disabled on it until the specified time
window passes. The polling limit hence slows down connection attempts if hit, but unlike the trigger
limit won't cause permanent failures. It's the recommended mechanism to deal with DoS attempts
through packet flooding.</para>
<para>The polling limit is enforced per file descriptor to listen on, as opposed to the trigger limit
which is enforced for the entire socket unit. This distinction matters for socket units that listen
on multiple file descriptors (i.e. have multiple <varname>ListenXYZ=</varname> stanzas).</para>
<para>These setting defaults to 150 (in case of <varname>Accept=yes</varname>) and 15 (otherwise)
polling events per 2s. This is considerably lower than the default values for the trigger limit (see
above) and means that the polling limit should typically ensure the trigger limit is never hit,
unless one of them is reconfigured or disabled.</para>
<xi:include href="version-info.xml" xpointer="v255"/></listitem>
</varlistentry>
</variablelist>
<xi:include href="systemd.service.xml" xpointer="shared-unit-options" />