- update to 7.4.2 (bsc#1216123, CVE-2023-44487):
* The ``vcl_req_reset`` feature (controllable through the ``feature``
parameter, see `varnishd(1)`) has been added and enabled by default
to terminate client side VCL processing early when the client is
gone.
*req_reset* events trigger a VCL failure and are reported to
`vsl(7)` as ``Timestamp: Reset`` and accounted to ``main.req_reset``
in `vsc` as visible through ``varnishstat(1)``.
In particular, this feature is used to reduce resource consumption
of HTTP/2 "rapid reset" attacks (see below).
Note that *req_reset* events may lead to client tasks for which no
VCL is called ever. Presumably, this is thus the first time that
valid `vcl(7)` client transactions may not contain any ``VCL_call``
records.
* Added mitigation options and visibility for HTTP/2 "rapid reset"
attacks
Global rate limit controls have been added as parameters, which can
be overridden per HTTP/2 session from VCL using the new vmod ``h2``:
* The ``h2_rapid_reset`` parameter and ``h2.rapid_reset()`` function
define a threshold duration for an ``RST_STREAM`` to be classified
as "rapid": If an ``RST_STREAM`` frame is parsed sooner than this
duration after a ``HEADERS`` frame, it is accounted against the
rate limit described below.
* The ``h2_rapid_reset_limit`` parameter and
``h2.rapid_reset_limit()`` function define how many "rapid" resets
may be received during the time span defined by the
``h2_rapid_reset_period`` parameter / ``h2.rapid_reset_period()``
function before the HTTP/2 connection is forcibly closed with a
``GOAWAY`` and all ongoing VCL client tasks of the connection are
aborted.