Using a Yubikey for 2-factor authentication

For those of you who have not yet encountered a Yubikey, it is a physically small security key which empowers passwordless or 2-factor authentication in a number of different scenarios. They now support a huge number of protocols including One Time Passwords, NFC, FIDO2, U2F, Smart card, and OpenPGP.

Mine has been living on my keyring for several years now, and has proved invaluable for me accessing both my physical and electronic world.

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Configuring fail2ban on Debian – Part 1

If you are administering a Linux server on a publicly accessible IP address then you have no-doubt already noticed your log files filling up with repeated failed login attempts against all common protocols. While it may be possible to protected services to some extent with firewalls, nothing is going to protect you from weak password policies and software vulnerabilities.

There is, however, a great Open Source product that can do away with some of the noise and frustrate the spammers; fail2ban.

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Securing nginx with Letsencrypt

For several years now users have been taught to look for the green padlock in the address bar to ensure the site they are using is secure. But certificates are expensive due to the efforts that go in to proving your identity.

Letsencrypt are trying to help secure the web by issuing short term (90 day) certificates to users who can demonstrate through a simple challenge and response that they are in control of a host.

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Postfix and DKIM

openDKIM and Postfix on Ubuntu Server 12.04LTS

You probably found this post because you already know what DKIM is and want to implement it, but for those of you who need more information Wikipedia has a good entry on the subject;

This little walkthrough has been based upon a Ubuntu 12.04LTS server with Postfix; your mileage may very depending on your setup.  At a minimum you will need:

  1. Root access to your mail server
  2. Access to update the dns records for your domain

Firstly, install opendkim from the repositories.  There is some good information available there too.

sudo apt-get install opendkim

You must decide on what “selector” you wish to use.  The selector is essentially a word to describe the key you wish to use.  Here I am going to use the selector 201204 as the key became valid in April 2012 (cunning eh?).  Obviously, my domain will be


So onward.  Generate the keys in your home directory, using your chosen selector and domain.

opendkim-genkey -s 201204 -d

This will create two files, 201204.txt and 201204.private containing the public and private keys respectively.  You must publish the content of the public key to your authoritative DNS server.  Three things to note;

  1. openDKIM 2.5.2 on Ubuntu omits the k= after v=DKIM1;
  2. you should add the t=y flag to indicate to receiving servers that you are testing DKIM but not actively using it yet.
  3. You should use a short TTL (time to live) so that you can change the key without waiting ages for it to propagate though DNS.

DNS implementations vary, but you should end up with an entry something like this (I’ve shortened the key because of line breaks);

201204._domainkey IN TXT v=DKIM1; k=rsa; t=y; p=MIGfM..[snip]..QIDAQAB

Remember to reload your DNS configuration after addition.

Now, test the key…

opendkim-testkey -d -s 201204 -k 201204.private -vvv

And you should get output similar to below.

opendkim-testkey: key loaded from 201204.private
opendkim-testkey: checking key ''
opendkim-testkey: key not secure
opendkim-testkey: key OK

Note that opendkim is reporting that the key is not secure.  This relates to the fact that DNSSEC is not implemented on my DNS server and theoretically somebody could intercept the DNS lookup and replace it with their own key.

Once we are happy with the key test, move the private key to /etc/mail

cd /etc/mail
sudo mv /home/basil/201204.private .

Setup openDKIM by editing /etc/default/opendkim with your favourite editor and add the following line to the end.


Then edit /etc/opendkim.conf and add the following lines

KeyFile                 /etc/mail/201204.private
Selector                201204

And restart opendkim

sudo service opendkim restart

Lastly we configure postfix.  Edit /etc/postfix/ and add the lines to the end


And restart

sudo service postfix restart


So now you need to send some test messages through your newly configured postfix server.  At the receiving end you should see some new headers, one of which will show similar to below (note that Google recognize it as test mode as we still have the t=y flag applied).

DKIM pass
Screenshot of mail header showing a DKIM pass in test mode

Once you are happy that all is well, you can remove the testing flag and increase TTL.


Securing Apache for free with SSL and StartSSL

So you have a web-server and you want to serve content by SSL?  If you have found you way to here, you have probably discovered that browsers such as Firefox do not make it easy to use self-signed certificates; large scary error messages do not inspire confidence in most users.  But there is an easy answer – StartSSL.

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