[Bug 273807] tcpdump is not able to set honor the timezone of the PC as set in /etc/localtime

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273807

            Bug ID: 273807
           Summary: tcpdump is not able to set honor the timezone of the
                    PC as set in /etc/localtime
           Product: Base System
           Version: 14.0-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: mike@sentex.net

Summary discussion here 
https://lists.freebsd.org/archives/freebsd-stable/2023-September/001448.html

AMD64 VM of FreeBSD 14 from Sept 1st.  TCPDUMP (in the base and in the ports)
will not work properly when the timezone is set to something other than UTC. 

e.g. tcpdump -nei pflog0 will display log entries always in UTC, despite what
/etc/localtime is set to. 

des@freebsd.org wrote in the email thread

In stable/13, tcpdump reads /etc/localtime very early, and long before
entering capability mode:

 72111 tcpdump  0.007527 NAMI  "/etc/localtime"
 72111 tcpdump  0.007541 RET   open 3
 72111 tcpdump  0.007549 CALL  read(0x3,0x1a9058bb78c0,0xd6b8)
 72111 tcpdump  0.007627 RET   read 2298/0x8fa
 72111 tcpdump  0.007634 CALL  close(0x3)
 72111 tcpdump  0.007642 RET   close 0
 [...]
 72111 tcpdump  0.024369 CALL  cap_enter
 72111 tcpdump  0.024381 RET   cap_enter 0

In main and stable/14, it enters capability mode immediately before the
first attempt to read /etc/localtime, which fails:

   745 tcpdump  0.069967829 CALL  cap_enter
   745 tcpdump  0.070015646 RET   cap_enter 0
   745 tcpdump  0.070139522 CALL 
fstatat(AT_FDCWD,0x1c377723d38e,0x1c3773430d00
,0)
   745 tcpdump  0.070196299 NAMI  "/etc/localtime"
   745 tcpdump  0.070240578 RET   fstatat -1 errno 94 Not permitted in
capability mode
   745 tcpdump  0.070487574 CALL 
fstatat(AT_FDCWD,0x1c377723d38e,0x1c3773430cd0,0)
   745 tcpdump  0.070550458 NAMI  "/etc/localtime"
   745 tcpdump  0.070593003 RET   fstatat -1 errno 94 Not permitted in
capability mode

The simplest workaround is to call tzset(3) before entering capability
mode.

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