Posts Tagged ‘howto


Updating CentOS 4.9

I recently booted up a long-powered-off test system for a customer, and realized it was still CentOS 4.7. up2date gives one of these error messages:

Error: Bad/Outdated Mirrorlist and Baseurl

[Errno -1] Header is not complete.
Trying other mirror.

To fix, first run this to update to a newer version of yum:

for i in \
libxml2-2.6.16-12.6.i386.rpm \
libxml2-python-2.6.16-12.6.i386.rpm \
readline-4.3-13.i386.rpm \
python-2.3.4-14.7.el4.i386.rpm \
python-elementtree-1.2.6-5.el4.centos.i386.rpm \
sqlite-3.3.6-2.i386.rpm \
python-sqlite-1.1.7-1.2.1.i386.rpm \
elfutils-0.97.1-5.i386.rpm \
popt-1.9.1-32_nonptl.i386.rpm \
rpm-libs-4.3.3-32_nonptl.i386.rpm \
rpm-4.3.3-32_nonptl.i386.rpm \
rpm-python-4.3.3-32_nonptl.i386.rpm \
python-urlgrabber-2.9.8-2.noarch.rpm \
yum-metadata-parser-1.0-8.el4.centos.i386.rpm \
rpm -Uvh${i};

(note: the order is important for the dependencies, and the trailing “\” prevents bash from spitting out interpretation errors, but makes it readable.)

Then edit /etc/yum.repos.d/CentOS-Base.repo: comment out mirrorlist directives, and in each enabled section add baseurl=$basearch, baseurl=$basearch, etc.

Note the change from to – the CentOS Vault is handling RPM headers differently than yum in CentOS 4.9 expects. The archive does not.


DNS Configuration for AD

Many are the times we’ve run into DNS configuration problems with Microsoft AD.  After being asked for advice a few more times than normal this year, I’ve pulled together several emails for this list of “Troubleshooting Microsoft AD-integrated DNS” highlights below.  We’ll first cover the generic topics of checking the configuration of your server configuration,  then the configuration of the zones themselves. For each topic, we’ll do a checklist followed by an explanation.

Server configuration:


  1. Is the server (Windows 2003 or higher) pointing to itself for primary DNS in the network configuration?
  2. If a standalone DC: Does the server have *no* secondary DNS in the network configuration?
  3. If there are multiple DCs: Does the server list only other DCs in the secondary DNS server list in the advanced network configuration?
  4. Does the server have proper forwarders in the DNS server configuration (to the parent domain or to the ISP, but not both)?
  5. In a command prompt, run the following:
    ipconfig /registerdns
    net stop netlogon
    net start netlogon
  6. Read DNS and System logs to make sure there are no issues being reported.
  7. wait 20 minutes


One of the major problems we run into is that customers will put the ISP DNS servers in the network configuration on the DC, not in the DNS Forwarders list in the DNS Server configuration.  The DC *is* a DNS server.  It needs to talk to itself, so that it can register crucial DNS settings in its own database.  If its own database can’t find the information requested (such as, then the DNS Server service is responsible for looking that data up, and then caching it so that it’s readily available for other clients, too.  This misconfiguration also has the problem of generating DDNS update requests back to the ISP DNS servers, which are ignored at best, and a security leak at worst (like for military/government installations).

I like to tell my Unix customers “the first rule of administering Active Directory is to go get another cup of coffee.” This forces them to take their hands off the keyboard and wait for cross-site replication (hopefully) before making another change.  It’s a good reminder for the seasoned Windows admins, as well.

Zone Configuration

Reverse Lookup Zones

We’ll cover reverse lookup zones before forward lookup zones, for two reasons: 1) customers screw up reverse lookup configuration much more often than forward lookup configuration ; 2) no SRV records in Reverse zones (normally).


If you have non-Microsoft DNS servers or multiple AD domains in your environment

  1. Does the server have reverse DNS zones defined?
  2. Does any *other* server (in the DNS Forwarders configuration list) have the same reverse DNS zone defined?
  3. Do the defined reverse zones allow “unsecured dynamic updates”?
  4. Are all IP subnets in your network defined as reverse DNS zones on the primary DNS servers (the last forwarders in the network before the ISP)?
  5. Do you have aging and scavenging turned on in the server settings?  If so (you should), do you have all clients automatically renewing their records (Windows clients will by default)?

If you only have a single AD domain, or no non-Microsoft DNS servers

  1. Does the server have reverse DNS zones defined for all IP subnets (including IPv6) in your network?
  2. Do those reverse DNS zones allow dynamic updates?
  3. Is aging of old records enabled with sane no-refresh and refresh values  in the reverse zones?


Each DNS Zone is a database.  There can only be one authoritative owner of the database, defined by the SOA record on the Zone.  Any other DNS servers get their information from this SOA, either by normal queries, or by zone transfer (AD replication does a kind of zone transfer).  If two servers are set up with the same zone (create reverse DNS zone in and, for example), then there is no mechanism to transfer the information between those two servers.

For example: any individual client will only talk to the DNS server it’s configured to talk to ( gets its DNS info from and gets its information from Each client will also send updates only to its own DNS server.  This means that will register its IP with, and will register its IP with  These two records will never be synched between and  Therefore, when asks “who has”, will answer “nobody!”.

The DNS admin must fix this problem by manually registering all of the records from in the zone stored in, deleting the zone from, and then setting up a forwarder or conditional forwarder to  Now, that same query results in looking in its own database, finding no answer, and reaching out to its forwarders to ask, “who has”.  Similarly, when goes to register, it is directed, via the SOA record, to send that registration to  This is why reverse zones often need to allow unsecured dynamic updates.

Forward Lookup Zones

I have a customer who needs this much data now – I’ll follow up with the Forward Lookup zones in a separate post later this week.


PHP mail(), Apache, and SELinux (FC7)

(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 =
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 “”, 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)


Network packet capture on different OSes

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:

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!)

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