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.c | 24 ++++++++++++++---------- extensions/libxt_time.man | 39 +++++++++++++++++++++++++++++++-------- 2 files changed, 45 insertions(+), 18 deletions(-) diff --git a/extensions/libxt_time.c b/extensions/libxt_time.c index dfb0d5b7..44c05b8f 100644 --- a/extensions/libxt_time.c +++ b/extensions/libxt_time.c @@ -26,8 +26,10 @@ enum { O_WEEKDAYS, O_LOCAL_TZ, O_UTC, + O_KERNEL_TZ, F_LOCAL_TZ = 1 << O_LOCAL_TZ, F_UTC = 1 << O_UTC, + F_KERNEL_TZ = 1 << O_KERNEL_TZ, }; static const char *const week_days[] = { @@ -46,7 +48,9 @@ static const struct xt_option_entry time_opts[] = { {.name = "localtz", .id = O_LOCAL_TZ, .type = XTTYPE_NONE, .excl = F_UTC}, {.name = "utc", .id = O_UTC, .type = XTTYPE_NONE, - .excl = F_LOCAL_TZ}, + .excl = F_LOCAL_TZ | F_KERNEL_TZ}, + {.name = "kerneltz", .id = O_KERNEL_TZ, .type = XTTYPE_NONE, + .excl = F_UTC}, XTOPT_TABLEEND, }; @@ -63,7 +67,7 @@ static void time_help(void) "[!] --weekdays value List of weekdays on which to match, sep. by comma\n" " (Possible days: Mon,Tue,Wed,Thu,Fri,Sat,Sun or 1 to 7\n" " Defaults to all weekdays.)\n" -" --localtz/--utc Time is interpreted as UTC/local time\n"); +" --kerneltz Work with the kernel timezone instead of UTC\n"); } static void time_init(struct xt_entry_match *m) @@ -79,9 +83,6 @@ static void time_init(struct xt_entry_match *m) /* ...and have no date-begin or date-end boundary */ info->date_start = 0; info->date_stop = INT_MAX; - - /* local time is default */ - info->flags |= XT_TIME_LOCAL_TZ; } static time_t time_parse_date(const char *s, bool end) @@ -273,6 +274,12 @@ static void time_parse(struct xt_option_call *cb) info->daytime_stop = time_parse_minutes(cb->arg); break; case O_LOCAL_TZ: + fprintf(stderr, "WARNING: --localtz is being replaced by " + "--kerneltz, since \"local\" is ambiguous. Note the " + "kernel timezone has caveats - " + "see manpage for details.\n"); + /* fallthrough */ + case O_KERNEL_TZ: info->flags |= XT_TIME_LOCAL_TZ; break; case O_MONTHDAYS: @@ -285,9 +292,6 @@ static void time_parse(struct xt_option_call *cb) if (cb->invert) info->weekdays_match ^= XT_TIME_ALL_WEEKDAYS; break; - case O_UTC: - info->flags &= ~XT_TIME_LOCAL_TZ; - break; } } @@ -423,8 +427,8 @@ static void time_save(const void *ip, const struct xt_entry_match *match) } time_print_date(info->date_start, "--datestart"); time_print_date(info->date_stop, "--datestop"); - if (!(info->flags & XT_TIME_LOCAL_TZ)) - printf(" --utc"); + if (info->flags & XT_TIME_LOCAL_TZ) + printf(" --kerneltz"); } static struct xtables_match time_match = { 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