Preparing for World IPv6 Day: I changed my MTU to 1280

In preparation for World IPv6 Day on 8 June 2011, I thought I’d check my connectivity..

I visited a test-ipv6 page .. but the page took too long to open, had broken images, timeouts and readiness score 1/10 !

test.ipv6 with mtu 1500 - Problems!
test.ipv6 with mtu 1500 - Problems!

So I changed my MTU to 1280 and tested again

(In linux you might type: /sbin/ifconfig eth0 mtu 1280)

test.ipv6 with mtu 1280 - No Problems!
test.ipv6 with mtu 1280 - No Problems!

I then found this relevant discussion: IPv6 and path MTU discovery black holes

UPDATE 4/6/2011: An MTU of 1492 also seems to work correctly in my setup (as @chatasos prefers


Red Hat Certificate Exam Preparation

Marios asked me to elaborate a little on the Red Hat Certification Exams and my preparation.

I found the lab based exams much better than other multiple choice exams. It doesn’t matter if you do something using the command line, a gui tool, or if you write a program yourself to do the job. It’s real world simulation!

The preparation was self-study using the following:

No book covering RHEL6 existed while I was preparing, but I believe one is available now (ISBN 0321767950) and others are on their way (e.g. ISBN 0071765654).

I also recently discovered OpenRHCE by @texastwister, an effort to collaboratively prepare study materials to help candidates achieve Red Hat’s RHCE certification, under a CC license.

I found the journey of answering all the above quite a rewording experience. I have learned new things and I can definately troubleshoot SELinux issues faster than before ;-)

IPv6 reverse DNS in 60″

Your allocation: 2001:db8/32
You LAN: 2001:db8:2:2001/64
Your device: 2001:db8:2:2001::11 (mydevice.local)

Let «host» do the dirty work for you!

sotiris@jumbo:~$ host 2001:db8:2:2001::11
Host not found: 3(NXDOMAIN)

The zone for your /32 is

$TTL 1d
@    IN    SOA ( 42 1h 15m 30d 10m )
     IN    NS    localhost.
;    IN    NS    localhost. ; delegate 2001:db8:2:2001/64

The zone for your /64 is

$TTL 1d
@    IN    SOA ( 42 1h 15m 30d 10m )
     IN    NS    localhost.
; IN PTR mydevice.local.

Reconfig and run host again

sotiris@jumbo:~$ host 2001:db8:2:2001::11 domain name pointer mydevice.local.

Resolvers Vs. Authoritative DNS, part 2

Με τον καινούριο Κανονισμό Διαχείρισης και Εκχώρησης Ονομάτων Χώρου (Domain Names) με κατάληξη .GR (pdf link), που δημοσιεύτηκε από την ΕΕΤΤ στις 14/04/2011, γίνεται υποχρεωτικό πλέον αυτό που είχα γράψει πριν δύο χρόνια στο Resolvers Vs. Authoritative DNS

Αρθρο 17, παράγραφος 28:
Στην περίπτωση που ένας Καταχωρητής Ονομάτων Χώρου λειτουργεί και ως Πάροχος υπηρεσιών πρόσβασης στο Διαδίκτυο (ISP), οι Λύτες (Resolvers) στο δίκτυο πρόσβασης δεν πρέπει να είναι Αρμόδιοι Εξυπηρετητές Ονομάτων (Authoritative Domain Name Server) για άλλα αρχεία ζώνης Ονομάτων Χώρου, με εξαίρεση τα Ονόματα Χώρου στα οποία ο ίδιος ο Πάροχος υπηρεσιών πρόσβασης στο Διαδίκτυο (ISP) είναι Φορέας/δικαιούχος.

Nice work EETT!

Block spam or report it?

Personally, I don’t block spam. I keep receiving it because I find it interesting to study.

Where does it come from? Single IP? Single domain? Botnet?
Who does it target? list of harvested emails or specifically targeted?
Is it sent aggressively? Hundreds per minute or a few per hour?
What is it about? Meds? Gambling? Porn? Business deals? Visit a site?
Fake sender? valid or just random username with legit domain?
Fake subject? Trying to avoid spam detection?
Small body? Long body? With random chars to avoid checksum checking?
Imagespam? Distorted image?
Sent by professional spammers or amateur (Greek) marketers? :>

After all this, I always report is upstream using Spamcop (HabuL add-on).