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Adjusting limits to mitigate denial of service

'dbus-daemon --system' has several arbitrary limits which are a trade-off
between working correctly when not under attack, and preventing local
denial of service attacks. System administrators with particularly hostile
local users should review these limits and tune them if necessary.

In particular, the fix for CVE-2014-3639 in dbus-1.8.8 makes it difficult
for local users to prevent connections completely, but they can still
introduce a delay which increases with larger authentication timeout
(auth_timeout) values, by opening many parallel connections from
different processes and never completing the authentication handshake.
As a result, dbus 1.8.8 also reduced the auth_timeout from 30 seconds
to 5 seconds to mitigate this delay. However, this change resulted in
boot failures on some systems because systemd could not authenticate
sufficiently quickly while the system was busy, and was reverted in 1.8.12.

On fast systems with hostile local users, administrators can reduce this
delay by returning to the 5 second timeout (or any other value in
milliseconds), by saving this as /etc/dbus-1/system-local.conf or a file
matching /etc/dbus-1/system.d/*.conf:

    <limit name="auth_timeout">5000</limit>

If applying this change, please reboot several times and check the
syslog or Journal for messages containing "Connection has not authenticated
soon enough, closing it". Seeing that message while not subject to a
denial-of-service attack indicates that the auth_timeout has been set
too short.