January 9, 2015

Ubuntu/Debian 2 factor authentication using nfc

At work, we are currently testing 2 factor authentication for desktops/workstations. As we are already using some nfc based cards, we want to use them. Some quick research brought up pam_nfc which utilizes libnfc.

So we bought an ACR122U reader which is supported by libnfc

compile and install

Checkout source code and create a debian package for further automated distribution (e.g. Puppet,…)

svn checkout http://nfc-tools.googlecode.com/svn/trunk/ nfc-tools-read-only 

Now we need to adjust some requirements for libraries if we’re using Ubuntu as the debian control file requires libnfc2 (>= 1.5.0) which is not present in Ubuntu. Instead it’s called libnfc5 (1.7.0-3 atm in trusty)

cd nfc-tools-read-only/pam_nfc
sed -i 's,libnfc2 (>= 1.5.0),libnfc5 (1.7.0-3),' debian/control

Next, install some packages and headers for compiling the source

sudo apt-get install devscripts autoconf dh-autoreconf build-essential fakeroot libnfc-dev libpam0g-dev -y

Finally, build the package

debuild -us -uc

Answer this question with yes as the package will be built:

> This package has a Debian revision number but there does not seem to be
> an appropriate original tar file or .orig directory in the parent directory;
> (expected one of libpam-nfc_0.1.4.orig.tar.gz, libpam-nfc_0.1.4.orig.tar.bz2,
> libpam-nfc_0.1.4.orig.tar.lzma,  libpam-nfc_0.1.4.orig.tar.xz or pam_nfc.orig)
> continue anyway? (y/n)

Now we’ve got a deb package to install one folder up.


Grab the package built above and install it (or add it to your existing repo like freight or reprepro). This command might differ in version numbers and architecture and assumes you’re still in the current pam_nfc directory. Adjust to your needs.

sudo apt-get install ../libpam-nfc_0.1.4-0_amd64.deb

PAM configuration

Now we can add pam directives to the configuration files in /etc/pam.d/ to make it work. But in the past, we’ve encountered some problems with pam-auth-update in automated installations (via Puppet) so let’s create a description file for pam-auth-update to have it working the clean way.

Create /usr/share/pam-configs/nfc with content like this:

Name: NFC
Default: yes
Priority: 257
Auth-Type: Primary
	required	pam_nfc.so
	required	pam_nfc.so

This example is adding pam_nfc.so as required (so NFC tag and password will be asked) just before pam_unix.so. In our case, we are using Priority 251 so it skips local users (below pam_unix.so) but before pam_lsass.so (Active Directory using PowerBroker, you might not have this). If you want to use either username/password or nfc tag (ok, then it’s no more 2 factor auth ;) ), just change required to sufficient.

Enroll tags

Now we need to enroll tags for users, this is done via

pam-nfc-add <user>

Pairings will be put into /etc/pam_nfc.conf which can be enrolled company wide.


As always when testing PAM, keep an existing root shell to revert the settings in case it does not work.