From db50b83bc3cd634beb71f38978ad7d035c88ff11 Mon Sep 17 00:00:00 2001 From: Jan Engelhardt Date: Mon, 23 May 2011 18:38:09 +0200 Subject: 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 --- extensions/libxt_time.man | 39 +++++++++++++++++++++++++++++++-------- 1 file changed, 31 insertions(+), 8 deletions(-) (limited to 'extensions/libxt_time.man') 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 -- cgit v1.2.3