Linux Server Hardening Utilizing Idempotency with Ansible: Half three


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Within the earlier articles, we launched idempotency as a option to strategy your server’s safety posture and checked out some particular Ansible examples, together with the kernel, system accounts, and IPtables. On this last article of the sequence, we’ll have a look at just a few extra server-hardening examples and discuss slightly extra about how the idempotency playbook is perhaps used.


Attributable to its decreased performance, and subsequently assault floor, the choice amongst a lot of OSs has been to introduce “chronyd” over “ntpd”. For those who’re new to “chrony” then fret not. It’s nonetheless utilizing the NTP (Community Time Protocol) that everyone knows and love however in a safer style.

The very first thing I do with Ansible throughout the “chrony.conf” file is alter the “bind tackle” and if my reminiscence serves there’s additionally a “command port” choice. These config choices enable Chrony to solely hear on the localhost. In different phrases you might be nonetheless syncing as regular with different upstream time servers (simply as NTP does) however no distant servers can question your time providers; solely your native machine has entry.

There’s extra info on the “bindcmdaddress” and “cmdport zero” on this Chrony web page ( below “2.5. How can I make chronyd safer?” which it’s best to learn for readability. This premise behind the touch upon that web page is a good suggestion: “you possibly can disable the web command sockets utterly by including cmdport zero to the configuration file”.

Moreover I might additionally deal with securing the file permissions for Chrony and demand that the service begins as anticipated identical to the syslog config above. In any other case guarantee that your time sources are sane, have a level of redundancy with a number of sources arrange after which copy the entire config file over utilizing Ansible.


You possibly can clearly have an effect on the extent of element included within the logs from a quantity items of software program on a server. Pondering again to what we’ve checked out in relation to syslog already you too can tweak that software’s config utilizing Ansible to your wants after which use the instance Ansible above as well as.


Apparently PAM (Pluggable Authentication Modules) has been part of Linux since 1997. It’s undeniably helpful (a standard use is that you may pressure SSH to make use of it for password logins, as per the SSH YAML file above). It’s extensible, subtle and might carry out helpful capabilities comparable to stopping brute pressure assaults on password logins utilizing a intelligent fee limiting system. The syntax varies slightly between OSes however in case you have the time then getting PAM working nicely (even if you happen to’re solely utilizing SSH keys and never passwords on your logins) is a worthwhile effort. Attackers like their very own customers on a system with plenty of usernames, one thing innocuous comparable to “webadmin” or related is perhaps simple to overlook on a server, and PAM may help you out on this respect.


We’ve checked out logging slightly already however what about capturing each “system name” that a kernel makes. The Linux kernel is a super-busy element of any system and logging nearly each single factor that a system does is a wonderful means of offering post-event forensics. This text will hopefully shed some mild on the place to start: Be aware the feedback in that article about efficiency, there’s little level in paying further for compute and disk IO useful resource since you’ve misconfigured your logging so spend a while getting it right could be my recommendation.

For considerations over disk house I’ll often change just a few strains within the file “/and so on/audit/auditd.conf” as a way to stop there firstly being too many log information created and secondly logs that develop very giant with out being rotated. That is additionally on the proviso that logs are being ingested upstream through one other mechanism too. Clearly the information permissions and the service beginning are additionally the fundamentals that you must cowl right here too. Usually file permissions for auditd are tight because it’s a “root” oriented service so there’s much less adjustments wanted right here typically.


With slightly studying you possibly can uncover which filesystems which might be made out there to your OS by default. It’s best to disable these (on the “modprode.d” file degree) with Ansible to stop bizarre issues being connected unwittingly to your servers. You’re lowering the assault floor with this strategy. The Ansible would possibly look one thing like this beneath for instance.

identify: Ensure that filesystems which aren’t wanted are pressured as off

lineinfile: dest=”/etcmodprobe.d/harden.conf” line=’set up squashfs /bin/true’ state=current


The outdated, however generally prevented on account of complexity, safety favorite, SELinux, needs to be set to “imposing” mode. Or, on the each least, set to log sensibly utilizing “permissive” mode. Permissive mode will at the least fill your auditd logs up with any right rule matches properly. When it comes to what Ansible appears to be like prefer it’s easy and is alongside these strains:

identify: Configure SElinux to be operating in permissive mode

exchange: path=”/and so on/selinux/config” regexp=’SELINUX=disabled’ exchange=’SELINUX=permissive’


For sure the compliance hardening playbook can be a great place to improve all of the packages (with some selective exclusions) on the system. Take note of the part regarding reboots and idempotency in a second nonetheless. With different mechanisms in place you won’t need to replace packages right here however as an alternative as per the Automation Paperwork article talked about in a second.


Now we’ve run by means of a few of the facets you’d need to have a look at when hardening on a server, let’s suppose slightly extra about how the playbook is perhaps used.

Relating to cloud platforms most of my skilled work has been on AWS and subsequently, most of the time, a contemporary AMI is launched after which a playbook is run excessive of it. There’s a mountain of element in a method of doing that on this article ( which you will be happy to find accommodates a mechanism to spawn a script or playbook.

It is very important notice, on the subject of idempotency, that it could take slightly extra effort initially to get your head across the logic concerned in with the ability to re-run Ansible repeatedly with out disturbing the required established order of your server property.

One factor to be completely sure of nonetheless (barring uncommon edge circumstances) is that after you apply your hardening for the very first time, on a brand new AMI or server construct, you’ll require a reboot. This is a vital aspect on account of a lot of system aspects not being altered accurately with out a reboot. These embody making use of kernel adjustments so alterations change into dwell, writing auditd guidelines as immutable config and likewise beginning or stopping providers to enhance the safety posture.

Be aware although that you simply’re most likely not going to need to execute all performs in a playbook each twenty or thirty minutes, comparable to updating all packages and stopping and restarting key customer-facing providers. In consequence it’s best to issue the logic into your Ansible in order that some duties solely run as soon as initially after which possibly write a “accomplished” placeholder file to the filesystem afterwards for referencing. There’s one million other ways of reaching a standing checker.

The great factor about Ansible is that the logic for rerunning playbooks is implicit and in contrast to shell scripts which for any such process will be arduous to code the logic into. Generally, comparable to updating the GRUB bootloader for instance, attempting to guess the numerous permutations of a system change will be painful.

Bedtime Studying

I nonetheless suppose that you may’t beat trial and error on the subject of computing. Expertise is valued for good motive.

Be warned that you simply’ll discover contradictory recommendation generally from the huge array of on-line sources on this space. Recommendation differs most likely due to the completely different use circumstances. The one option to harden the various flavours of OS to my thoughts is through a bespoke strategy. That is due to the environments that servers are used inside and the necessities of the safety framework or commonplace that an organisation wants to satisfy.

For OS hardening particulars you possibly can verify with sources such because the NSA (, the Cloud Safety Alliance (, proprietary coaching organisations comparable to GIAC ( who provide sources (, the varied CIS Benchmarks ( for trade consensus-based benchmarking, the SANS Institute (, NIST’s Pc Safety Analysis ( and naturally print media too.


Hopefully, you possibly can see how highly effective an idempotent server infrastructure is and are tempted to strive it for your self.

The ever-present risk of APT (Superior Persistent Risk) assaults on infrastructure, the place a profitable attacker will sit silently monitoring occasions after which when it’s opportune infiltrate deeper into an property, makes any such configuration extremely worthwhile.

The quantity of element that goes into the checks and configuration adjustments is vital to the worth that such an strategy will carry to an organisation. Just like the checks in a CI/CD pipeline they’re solely as ever pretty much as good as their protection.


Chris Binnie’s newest e-book, Linux Server Safety: Hack and Defend, reveals you make your servers invisible and carry out a wide range of assaults. You will discover out extra about DevSecOps, containers and Linux safety on his web site:


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