libxt_time: deprecate --localtz option, document kernel TZ caveats

Comparing against the kernel time zone has significant caveats. This
patch adds documentation about the issue, and makes --utc the default
setting for libxt_time.

Furthremore, throw a warning on using the "--localtz" option, to avoid
confusion with one's shell TZ environment variable, and rename it to
"--kerneltz" to be explicit about whose timezone will be used.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

1 files changed, 31 insertions, 8 deletions

diff --git a/extensions/libxt_time.man b/extensions/libxt_time.man
index 4aff7ff5..1d677b94 100644
--- a/extensions/libxt_time.man
+++ b/extensions/libxt_time.man
@@ -1,5 +1,6 @@
 This matches if the packet arrival time/date is within a given range. All
-options are optional, but are ANDed when specified.
+options are optional, but are ANDed when specified. All times are interpreted
+as UTC by default.
 .TP
 \fB\-\-datestart\fP \fIYYYY\fP[\fB\-\fP\fIMM\fP[\fB\-\fP\fIDD\fP[\fBT\fP\fIhh\fP[\fB:\fP\fImm\fP[\fB:\fP\fIss\fP]]]]]
 .TP
@@ -29,13 +30,35 @@ Only match on the given weekdays. Possible values are \fBMon\fP, \fBTue\fP,
 to \fB7\fP, respectively. You may also use two-character variants (\fBMo\fP,
 \fBTu\fP, etc.).
 .TP
-\fB\-\-utc\fP
-Interpret the times given for \fB\-\-datestart\fP, \fB\-\-datestop\fP,
-\fB\-\-timestart\fP and \fB\-\-timestop\fP to be UTC.
-.TP
-\fB\-\-localtz\fP
-Interpret the times given for \fB\-\-datestart\fP, \fB\-\-datestop\fP,
-\fB\-\-timestart\fP and \fB\-\-timestop\fP to be local kernel time. (Default)
+\fB\-\-kerneltz\fP
+Use the kernel timezone instead of UTC to determine whether a packet meets the
+time regulations.
+.PP
+About kernel timezones: Linux keeps the system time in UTC, and always does so.
+On boot, system time is initialized from a referential time source. Where this
+time source has no timezone information, such as the x86 CMOS RTC, UTC will be
+assumed. If the time source is however not in UTC, userspace should provide the
+correct system time and timezone to the kernel once it has the information.
+.PP
+Local time is a feature on top of the (timezone independent) system time. Each
+process has its own idea of local time, specified via the TZ environment
+variable. The kernel also has its own timezone offset variable. The TZ
+userspace environment variable specifies how the UTC-based system time is
+displayed, e.g. when you run date(1), or what you see on your desktop clock.
+The TZ string may resolve to different offsets at different dates, which is
+what enables the automatic time-jumping in userspace. when DST changes. The
+kernel's timezone offset variable is used when it has to convert between
+non-UTC sources, such as FAT filesystems, to UTC (since the latter is what the
+rest of the system uses).
+.PP
+The caveat with the kernel timezone is that Linux distributions may ignore to
+set the kernel timezone, and instead only set the system time. Even if a
+particular distribution does set the timezone at boot, it is usually does not
+keep the kernel timezone offset - which is what changes on DST - up to date.
+ntpd will not touch the kernel timezone, so running it will not resolve the
+issue. As such, one may encounter a timezone that is always +0000, or one that
+is wrong half of the time of the year. As such, \fBusing \-\-kerneltz is highly
+discouraged.\fP
 .PP
 EXAMPLES. To match on weekends, use:
 .IP