PAM – Pluggable Authentication Modules for Linux and how to edit the defaults

Most of us have been using PAM when authenticating without really thinking about it, but for the few of us that have actually tried to make sense of it, PAM is the partner that always says “no”, unless otherwise stated. It’s the bane of any sysadmin’s existence when it comes to making system x secure, and it becomes a major pain point on and off when I forget about the normal rules of engagement.

Rules of Engagement

Session windows

To engage with PAM in any combative situation, please ensure belts and braces are on, and keep your arms and legs inside the vehicle at all times. Backup the /etc/pam.d/ directory, and make sure that you have one or two non-terminating sessions open on your system – ideally a console, and an ssh session.

The unexpected overwrite

In RHEL/Fedora like systems, PAM is configured to have two main files which are included by the rest of the PAM configuration: /etc/pam.d/system-auth-ac and /etc/pam.d/password-auth-ac.  Normally, system-auth and password-auth in the same /etc/pam.d directory are links to the above files. authconfig tools will overwrite the configuration in the files with a suffix of -ac. This means that if the changes need to be persistent and not overwritten, the symlinks can be set to the new location As follows:

ls -l /etc/pam.d/*-auth

lrwxrwxrwx. 1 root root 19 Feb 19 12:57 /etc/pam.d/fingerprint-auth -> fingerprint-auth-ac
lrwxrwxrwx. 1 root root 16 Feb 19 12:57 /etc/pam.d/password-auth -> password-auth-ac
lrwxrwxrwx. 1 root root 17 Feb 19 12:57 /etc/pam.d/smartcard-auth -> smartcard-auth-ac
lrwxrwxrwx. 1 root root 14 Feb 19 12:57 /etc/pam.d/system-auth -> system-auth-ac
[root@node1 pam.d]# rm password-auth
rm: remove symbolic link ‘password-auth’? y
[root@node1 pam.d]# ln -s password-auth-local password-auth

The Language

When the usual PAM translator (authconfig) is not enough to achieve the right system authentication, one has to start thinking about communicating directly with it. PAM has four different keywords for controlling authentication with the system:

awk '!/^[#\-]/{print $1}' /etc/pam.d/*  | sort | uniq


auth is used for providing some kind of challenge/response (depending on the module)  – usually username/password.

account is used to time or otherwise restrict the user account -i.e. user must use faillock, load all the sssd account requirements etc.

password is used to update the authtoken associated with the user account. This is mainly used to change passwords and it can be where the rules around local password strength can be formulated.

session is used to determine what the user needs before they are allowed a session: working home directory, to which system limits apply (open filehandles, number of terminals, etc.), and user keyring.

Each of these keywords has  these common modes:

required - if this fails return failure but continue executing anyway
[success=ok new_authtok_reqd=ok ignore=ignore default=bad]

requisite - if this fails return failure and die (don't even attempt to preauth)
[success=ok new_authtok_reqd=ok ignore=ignore default=die]

sufficient - this is enough for success and exit if nothing previously has failed
[success=done new_authtok_reqd=done default=ignore]

optional - we don't care unless this is the only module in the stack associated with this type
[success=ok new_authtok_reqd=ok default=ignore]

PAM execution stack

PAM executes everything sequentially unless told otherwise. The following snippet of password-auth-ac will:

# set environment variables
auth required
# delay (ms) by this amount if last time this user failed
# otherwise if absent check login.defs for specified delay
auth required delay=2000000
# check user authentication from the system 
# try_first_pass - use the password that has already been entered if any
# nullok allow - allow blank password
auth sufficient nullok try_first_pass
# succeed if uid is greater than 1000 and don't log success to the system log
auth requisite uid >= 1000 quiet_success
# deny everything else
auth required

If anything is sufficient and it succeeds then the execution stack exits for the component – i.e. auth successful when local user signs in with username/password.

PAM if statements

PAM doesn’t have very obvious if statements, but given the right parameters it allows jumps of execution. Below is a way of incorporating an SSSD back-end with PAM to allow users with IdM logins access to the system:

# check if the user is allowed to log in with preauthorisation (i.e. has faillock entries)
auth        required preauth silent audit deny=5 unlock_time=900
# skip two rules if successful 
# NOTE: ​default ignore  means sufficient
# and check if it's a unix user - use the password provided by the auth stack
auth        [success=2 new_authtok_reqd=done default=ignore] try_first_pass
# if it's not a unix user, then use sssd backend for logging in
auth        sufficient forward_pass
# otherwise fail 
auth        [default=die] authfail audit deny=5 unlock_time=900
# this is the skip step from pam_unix module
# it allows for resetting the faillock when necessary
auth        sufficient authsucc audit deny=5 unlock_time=900

The comments in line explain what each module is doing. The execution sequence reminds me a bit of jump statements in assembly language and it helps me to think about them in that manner.

Putting them all together gives us this auth section:

auth        required
auth        required delay=2000000
auth        required preauth silent audit deny=5 unlock_time=900
auth        [success=2 new_authtok_reqd=done default=ignore] try_first_pass 
auth        sufficient forward_pass
auth        [default=die] authfail audit deny=5 unlock_time=900 
auth        sufficient authsucc audit deny=5 unlock_time=900
auth        required

If we were to make slight adjustments to the above snippet, it may have the frightening effect of allowing users to log in without having the correct password:

auth        required
auth        required delay=2000000
auth        required preauth silent audit deny=5 unlock_time=900
# reducing this number from 2 to 1 (success=1)
auth        [success=1 new_authtok_reqd=done default=ignore] try_first_pass
auth        sufficient forward_pass
# swapping these two lines
auth        sufficient authsucc audit deny=5 unlock_time=900
auth        [default=die] authfail audit deny=5 unlock_time=900
auth        required


PAM is a very powerful, yet quite obscure tool. It can be configured to allow people in without even a valid password, or it can deny everyone access apart from every alternate Tuesday between 19:00 and 20:00 (in combination with other tools). Whenever I have configured it, I have found it useful to test for access allowed, access denied, and access locked in order to ensure predictable operation.

NOTE: To get more information around PAM, visit the man pages: (5) pam.conf , (8) pam ,  (5) password-auth

Identity Management (IPA) – The ‘on the side’ installation

Thoughts about IPA installation

IPA or IdM in its Red Hat productised form is a very neat product. It allows centralised authentication and policy management while providing that over secure channels (kerberos and TLS). IdM provides quite a few features and you may decide that you’re better off without some (saving the extra calories/effort for later) as your infrastructure may already provide those on the side.

This example installation is without DNS, without a CA, and without NTP (VM installations shouldn’t really be running NTP anyway).

Once you’re past the stage of convincing management that it’ll be good for you (and good for them) to allow this installation to happen, this is what you need to think about and discuss with the team managing Certificate Authorities, NTP servers, and DNS:

  • DNS – DNS zones need to be configured in such a way that IPA acts as a KDC to its own group of servers if there are existing KDC in a different realm in the environment, they will need to be in a different subdomain/domain. The SRV records will only return the IPA servers when queried about kerberos in this subdomain.
  • Certificates – IPA uses SSL for ldap and http. IPA could be acting as a Certificate Authority but not in this instance. Active Directory (or something else) may already be configured as a Certificate Authority which could allow you to present your windows team with a certificate request from IPA to sign in order to obtain a valid web certificate.
  • Time – a uniform time source across the estate IPA servers and clients. Think about business meetings, SSL, sex, humour, and trains – all require good timing.

NOTE: IPA/IdM used to have to provide certificates by default to its clients on installation. As this is no longer the case, IPA can be installed without a CA in an easier fashion than you’re used to. Give it a try.

Prerequisite Checking for IPA installation

NOTE: The following installation is for IPA version 4.1 and AD version 2012R2.

Check that you have:

  • Access to the right software packages via yum (normal RHEL/CentOS base repo should do)
  • Forward and reverse resolvable hostname
  • An entry in the /etc/hosts with the ip address and hostname
  • nscd off
  • An up-to-date OS installation
  • Still got your sanity (test to be performed by an external third party)

The certificate creation

Create your secret private key for your server. Here, we are using openssl to generate a private key:

mkdir /root/certs 
openssl genrsa -out /root/certs/http.$(hostname).key 2048

And then, the certificate request below:

And then the certificate request below:
[root@idm certs]# openssl req  \
-key /root/certs/http.idm.mgmt.linux.local.key \
-out /root/certs/$(hostname -f).csr -new

You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [XX]:GB
State or Province Name (full name) []:Norfolk
Locality Name (eg, city) [Default City]:Norwich
Organization Name (eg, company) [Default Company Ltd]:MGMT.LINUX.LOCAL
Organizational Unit Name (eg, section) []:She ITs And Giggles
Common Name (eg, your name or your server's hostname) []:idm.mgmt.linux.local  
Email Address []:root@localhost 

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:SheITsAndGiggles
An optional company name []:

Your AD certificate Authority should be able to sign this csr and then retrieve the CA chain with the cert for IPA installation. An example on how to sign certificates in a windows CA can be found here: Generate a Digital Certificate from CSR.

To examine the Certificates, open them as follows:

openssl x509 -in <certificate> -text

The Installation

The following step just downloads the software on the server. It doesn’t start any services:

yum -y install ipa-server ipa-server-trust-ad

Now for the fun part:

ipa-server-install --http-cert-file /root/certs/http.idm.mgmt.linux.local.crt \
--http-cert-file /root/certs/http.idm.mgmt.linux.local.key \
--http-pin SheITsAndGiggles \
--dirsrv-cert-file /root/certs/http.idm.mgmt.linux.local.crt \
--dirsrv-cert-file /root/certs/http.idm.mgmt.linux.local.key \
--dirsrv-pin SheITsAndGiggles --ca-cert-file ca-chain.p7b \
 -n mgmt.linux.local -r LINUX.LOCAL --mkhomedir

NOTE: I have used the same certificate and key for the http and directory servers. The p7b file that has been downloaded from the CA is the chain.

After the installation, you will need to open all the ports for the services that we are running and add some DNS entries to advertise those services:

systemctl enable firewalld
firewall-cmd --permanent --zone=public \
firewall-cmd --reload

To tell your IPA clients what you are serving you need to advertise the services via DNS. Find an example below:

_ldap._tcp IN SRV 0 100 389 idm
_ldap._udp IN SRV 0 100 389 idm
_kerberos._tcp IN SRV 0 100 88 idm
_kerberos._udp IN SRV 0 100 88 idm
_kerberos-master._tcp IN SRV 0 100 88 idm
_kerberos-master._udp IN SRV 0 100 88 idm
_kpasswd._tcp IN SRV 0 100 464 idm
_kpasswd._udp IN SRV 0 100 464 idm

Check that the installation is running as it should by getting kerberos credentials for your admin user and using admin to ssh on ipa:

[root@idm ~]# kinit admin
[root@idm ~]# ssh  admin@$(hostname -f)
Creating home directory for admin.