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We have a RHEL 5 box running some POE components that require the XS version of Scalar::Util.  However, the yum repos appear to have the non-XS version included, so every upgrade of perl breaks the Perl code that requires Scalar::Util.
The fix is here:

http://www.cybersprocket.com/2011/programming-languages/weaken-is-only-available-with-the-xs-version-of-scalarutil/

cpan
> force install Scalar::Util


But why is this? If you check out the “Diagnostics” section for Scalar::Util at CPAN, you’ll see:

NAME is only available with the XS version of Scalar::Util

Scalar::Util contains both perl and C implementations of many of its functions so that those without access to a C compiler may still use it. However some of the functions are only available when a C compiler was available to compile the XS version of the extension

Meaning – the update from yum must install the perl-only impelementation for cross-compatibility. What’s of most interest here is that “force install Scalar::Util” isn’t actually the full answer – you have to have gcc installed as well, so that there’s a C-compiler available during the CPAN forced reinstall. In case your production systems don’t have gcc installed already.

(Originally drafted November 2nd, 2007, finally finished and posted much later)
As I posted last night, we built a new Fedora Core 7 box last night for PHP testing. Whenever at all possible, I leave SELinux enabled on new systems in Enforcing mode. Oracle 10g hasn’t had any issues with it, Oracle 11i EBusiness Suite hasn’t had any issues with it, and my NFS and FTP servers run without at hitch. The Oracle systems are RHEL4 (Red Hat Enterprise Linux 4), and the NFS and FTP servers are RHEL5.

However, this new PHP webserver caused a few glitches. I feel a little silly for not catching this as being an SELinux problem earlier, but since it’s caused 0 issues in 9 months of use in production, I didn’t even consider it initially.

What we initially saw was 0 errors from PHP – all the pages would run without error. PHP.ini has the following lines:

sendmail_from = from@domain.com
sendmail_path = /usr/sbin/sendmail -t -i

and testing cat mail.txt | /usr/sbin/sendmail -t -i as a non-root user delivered mail properly as well. Combine that with /var/log/maillog being completely empty for every test page loaded, and it was sure that the mail wasn’t getting TO postfix (our preferred localhost MTA).

So, I looked at the /var/log/httpd/error_log for apache and found:

sh: /usr/sbin/sendmail: Permission denied
sh: /usr/sbin/sendmail: Permission denied
sh: /usr/sbin/sendmail: Permission denied
sh: /usr/sbin/sendmail: Permission denied
sh: /usr/sbin/sendmail: Permission denied

But I knew that non-root users could access sendmail as defined in php.ini, so I finally decided to tail /var/log/messages and saw:

Nov 2 11:05:41 $(servername) setroubleshoot: SELinux is preventing the sh from using potentially mislabeled files sendmail.postfix (sendmail_exec_t). For complete SELinux messages. run sealert -l c9001c48-5d48-4b7c-9fd7-8400544daa8f

So now to fix it…
This is surprisingly simple, actually. The sad part is, we had this problem, fixed it, forgot about it, had it again, and I blogged it… and lost the post. so this has been sitting in my “drafts” folder for about 10 months now:
setsebool httpd_can_sendmail=true
service httpd restart
service postfix restart
And retry sending mail. There’s a few posts about sendmail and having to change permissions on home directories or on “main.cf”, but I use postfix, and not sendmail, so I don’t know how effective or necessary those changes are.

(Edit: repost on 2/23/2012 because of a DB problem losing the original)

I ran into a problem 2 years ago where I couldn’t remember the native packet capture tool for Solaris and couldn’t install tcpdump, so i thought I’d put down as many as many native packet capture commands as I knew, by OS, in a single place.  I’ll update this as I find more, since there’s hundreds of Operating systems out there.

• AIX: iptrace: /usr/sbin/iptrace [ -a ] [ -b ][ -e ] [ -u ] [ -PProtocol_list ] [ -iInterface ] [ -pPort_list ] [ -sHost [ -b ] ] [ -dHost ] [ -L Log_size ] [ -B ] [ -T ] [ -S snap_length] LogFile
• FreeBSD: tcpdump (I think): tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ] [ -C file_size ] [ -F file ] [ -i interface ] [ -m module ] [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ] [ -E algo:secret ] [ expression ]
• HP-UX: nettl: nettl requires a daemon start, and other setup: /usr/sbin/nettl -traceon kind… -entity subsystem… [-card dev_name…] [-file tracename] [-m bytes] [-size portsize] [-tracemax maxsize] [-n num_files] [-mem init_mem [max_mem]] [-bind cpu_id] [-timer timer_value]
• Linux 2.4 and higher:
  • tcpdump (some distros): tcpdump [ -AdDefKlLnNOpqRStuUvxX ] [ -c count ] [ -C file_size ] [ -G rotate_seconds ] [ -F file ] [ -i interface ] [ -m module ] [ -M secret ] [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ] [ -W filecount ] [ -E spi@ipaddr algo:secret,… ] [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ] [ expression ]
  • wireshark (some distros, used to be called “ethereal”): GUI-config, no command-line, use tethereal (now tshark) for that
  • tshark: tshark [ -a <capture autostop condition> ] … [ -b <capture ring buffer option>] … [ -B <capture buffer size (Win32 only)> ]  [ -c <capture packet count> ] [ -C <configuration profile> ] [ -d <layer type>==<selector>,<decode-as protocol> ] [ -D ] [ -e <field> ] [ -E <field print option> ] [ -f <capture filter> ] [ -F <file format> ] [ -h ] [ -i <capture interface>|- ] [ -l ] [ -L ] [ -n ] [ -N <name resolving flags> ] [ -o <preference setting> ] … [ -p ] [ -q ] [ -r <infile> ] [ -R <read (display) filter> ] [ -s <capture snaplen> ] [ -S ] [ -t ad|a|r|d|e ] [ -T pdml|psml|ps|text|fields ] [ -v ] [ -V ] [ -w <outfile>|- ] [ -x ] [ -X <eXtension option>] [ -y <capture link type> ] [ -z <statistics> ] [ <capture filter> ]
• Mac OSX: tcpdump (among others): tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ] [ -C file_size ] [ -F file ] [ -i interface ] [ -m module ] [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ] [ -E algo:secret ] [ expression ]
• Solaris: snoop: snoop [ -aPDSvVNC ] [ -d device ] [ -s snaplen ] [ -c maxcount ] [ -i filename ] [ -o filename ] [ -n filename ] [ -t [ r | a | d ] ] [ -p first [ , last ] ] [ -x offset [ , length ] ] [ expression ]
• Windows 2000, XP, 2003, Vista, 2008 and beyond:
  • netmon (not installed by default): GUI config, filter creation info here
  • wireshark (available by download): GUI-config, no command-line

Any others anyone wants added (or corrected), just comment or email and I’ll update this.
(Edit 7/29/08 – change tcpdump link)
(Edit 10/13/08 – add tshark info, thanks Jefferson!, and wireshark on Windows)
(Edit 2/23/2012 – repost since a DB problem lost this post.  Thanks wayback machine!)