Learn Bash Debugging Techniques the Hard Way

In this article I’m going to give you a hands-on introduction to standard bash debugging techniques.

In addition, you’ll learn some techniques to make your bash scripts more robust to failure.

This article uses the hard way method, which emphasises hands-on-keyboard work to embed the learning. You’re going to have to think and type to learn.

Syntax Checking Options

Start by creating this simple script:

$ mkdir -p lbthw_debugging
$ cd lbthw_debugging
$ cat > debug_script.sh << 'END'
A=some value
echo "${A}
echo "${B}"
$ chmod +x debug_script.sh

Now run it with the -n flag like this:

$ bash -n debug_script.sh -n

This flag only parses the script, rather than actually running it. It’s useful for detecting basic syntax errors.

You’ll see it’s broken. Fix it. Then run it again.

If you’re not sure how to fix, contact me.

Verbose and Trace Flags

Now run with -v to see the verbose output.

$ bash -v debug_script.sh

and then run with -x to trace the output:

$ bash -x debug_script.sh

What do you notice about the output of the commands? Read them carefully.

Do you see the problem?

Using these flags together can help debug scripts where there is an elementary error, or even just working out what’s going on when a script runs. I used -x only yesterday to figure out why a systemctl service wasn’t running or logging.


Material here based on the ‘advanced’ section of my book
Learn Bash the Hard Way.
Free preview available here.



Managing Variables

Variables are a core part of most serious bash scripts (and even one-liners!), so managing them is another important way to reduce the possibility of your script breaking.

Change your script to add the ‘set’ line immediately after the first line and see what happens:

set -o nounset
A="some value"
echo "${A}"
echo "${B}"

Now research what the nounset option does. Which set flag does this correspond to?

Now, without running it, try and figure out what this script will do. Will it run?

set -o nounset
A="some value"
echo "${A}"
echo "${B}"

I always set nounset on my scripts as a habit. It can catch many problems before they become serious.

Tracing Variables

If you are working with a particularly complex script, then you can get to the point where you are unsure what happened to a variable.

Try running this script and see what happens:

set -o nounset 
declare A="some value" 
function a { 
  echo "${BASH_SOURCE}>A A=${A} LINENO:${1}" 
trap "a $LINENO" DEBUG 
echo "${A}" 
A="another value" 
echo "${A}" 
echo "${B}"

There’s a problem with this code. The output is slightly wrong. Can you work out what is going on? If so, try and fix it.

You may need to refer to the bash man page, and make sure you understand quoting in bash properly.

It’s quite a tricky one to fix ‘properly’, so if you can’t fix it, or work out what’s wrong with it, then ask me directly and I will help.

Profiling Bash Scripts

Returning to the xtrace (or set -x flag), we can exploit its use of a PS variable to implement the profiling of a script:

set -o nounset
set -o xtrace
declare A="some value"
PS4='$(date "+%s%N => ")'
echo "${A}"
A="another value"
echo "${A}"
echo "${B}"
curl -q bbc.co.uk

From this you should be able to tell what PS4 does. Have a play with it, and read up and experiment with the other PS variables to get familiar with what they do.

NOTE: If you are on a Mac, then you might only get
second-level granularity on the date!


Linting with Shellcheck

Finally, here is a very useful tip for understanding bash more deeply and improving any bash scripts you come across.

Shellcheck is a website and a package available on most platforms that gives you advice to help fix and improve your shell scripts. Very often, its advice has prompted me to research more deeply and understand bash better.

Here is some example output from a script I found on my laptop:

$ shellcheck shrinkpdf.sh
In shrinkpdf.sh line 44:
          -dColorImageResolution=$3             \
                                 ^-- SC2086: Double quote to prevent globbing and word splitting.
In shrinkpdf.sh line 46:
          -dGrayImageResolution=$3              \
                                ^-- SC2086: Double quote to prevent globbing and word splitting.
In shrinkpdf.sh line 48:
          -dMonoImageResolution=$3              \
                                ^-- SC2086: Double quote to prevent globbing and word splitting.
In shrinkpdf.sh line 57:
        if [ ! -f "$1" -o ! -f "$2" ]; then
                      ^-- SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.
In shrinkpdf.sh line 60:
        ISIZE="$(echo $(wc -c "$1") | cut -f1 -d\ )"
                      ^-- SC2046: Quote this to prevent word splitting.
                      ^-- SC2005: Useless echo? Instead of 'echo $(cmd)', just use 'cmd'.
In shrinkpdf.sh line 61:
        OSIZE="$(echo $(wc -c "$2") | cut -f1 -d\ )"
                      ^-- SC2046: Quote this to prevent word splitting.
                      ^-- SC2005: Useless echo? Instead of 'echo $(cmd)', just use 'cmd'.

The most common reminders are regarding potential quoting issues, but you can see other useful tips in the above output, such as preferred arguments to the test construct, and advice on “useless” echos.



1) Find a large bash script on a social coding site such as GitHub, and run shellcheck over it. Contribute back any improvements you find.



Why Are Enterprises So Slow?



In this article I want to explain a few things about enterprises and their software, based on my experiences, and also describe what things need to be in place to make change  come about.

Have you ever found yourself saying things like:

  • Why are enterprises so slow?
  • How do they decide what to buy?
  • Why is it so hard to deliver things in an enterprise?

I worked for a large ‘enterprise’ organisation for a few years trying to deliver infrastructure software change, and found myself having to explain these things to developers who worked there, salespeople, external open source engineers, software engineers who worked for enterprise vendors, and even many, many people within that organisation.

A few of those people suggested I write these explanations up so that they could pass it on to their fellow salespeople/engineers etc..


The polygon of Enterprise despair


Before the enterprise, I worked for a startup that grew from a single room to 700+ people over 15 years.

‘Enterprise’ was a word often thrown at us when rejecting our software, usually in the sentence “your software isn’t enterprise enough”. I had no idea what that meant, but I have a much better idea now. It didn’t help that the people saying that were usually pretty clueless about software engineering.

Like many other software developers whose experience was in an unregulated startup environment, I had little respect for the concept of enterprise software. Seems I wasn’t alone.

When I finally got sick of the startup life I took a job at a huge organisation in financial services over 200 times as large. You don’t get much more ‘enterprise’ than that, but even within that context I was working in the ‘infrastructure team’, the part of the group that got beaten up for being (supposedly) slow to deliver, and then delivering less usable software than was desired. So it was like being in the enterprise, squared.

Over the time that I worked there, I got a great insight into the constraints on delivery that cause client frustration to happen, and – worse luck – I was responsible for helping to deliver change within it.

This is quite a long post, so I’ve broken this up into several parts to make it easier to digest:

  1. Thought Experiment
    • What would happen if an enterprise acted like a startup?
  2. Reducing Risk
    • Some ways enterprises reduce risk
    • The principle underlying these methods
  3. Cumulative Constraints
    • Consequences of the culture of risk reduction
  4. A New Hope?
    • What can be done?

1) Thought Experiment

Before we start, let’s imagine a counterfactual situation – imagine an enterprise acted like a startup. Showing how this doesn’t work (and therefore why it generally doesn’t happen) will help illustrate why some of the constraints that cause the slowdowns we see in large organisations exist.

First, let’s look at what a small team might do to change some software. We’ll make it a really simple example, and one you might well do routinely at home – upgrading a Linux distribution.

In both cases, the relationship is:

  • IT person
    • Manager

Here’s how the conversation might go at a really small startup:

‘Lean’ OS Upgrade – Small Company

  • Shall we upgrade the OS?
    • Yes, ok.
  • Oh, I’ve hit a problem. One of the falanges have stopped working.
    • OK, do some work to fix the transpondster.
  • Might take me a few hours
    • OK
  • … OK, done. Can you test?
    • Yup, looks good.
  • Great.

‘Lean’ OS Upgrade – Enterprise

  • Shall we upgrade the OS?
    • Yes.
  • OK, done.
    • Um, you brought down the payments system.
  • Whoops. I’ll roll back
    • OK.
  • Done. We’ll look into it.
    • Hi. The regulator called. They saw something on the news about payments being down. They want to know what happened.
  • Um, OK. I’ll write something up.
    • Thanks.
    • They read your write-up and have asked for evidence of who decided what when. They want a timeline.
  • I’ll check the emails.
    • By the way, you’re going to be audited in a couple of months. We’ll have to cancel all projects until then?
  • But we’ve got so much technical debt!
    • If we don’t get this right, they’ll shut us down and we’ll be fired.
    • OK, we have the results of the audit.
    • Audit has uncovered 59 other problems you need to solve.
  • OK…
    • We’ll have to drop other projects, and maybe lose some people.
  • Um, OK…
    • Oh, and my boss is being hauled in front of the regulator to justify what happened. If it doesn’t go well he’s out of a job and his boss might go to prison if they think something fishy is going on.

Now that’s a bad release…

That’s a worst-case scenario, but let’s unpick what these regulated enterprises do to mitigate both the risk and the consequences of the above scenario.


  • ‘Who owns this?’
  • ‘How is this maintained?’
  • ‘Who buys it?’
  • ‘Who’s signed off the deployment?’

2) Reducing Risk


‘Who Owns This?’ / ‘One Throat to Choke’

This is a big one. One of the most commonly-asked questions when architecting a solution within an enterprise is: ‘Who is responsible for that component/service/system?

In our enterprise ‘Lean OS Upgrade’ scenario above one of the first questions that will be asked is: ‘Who owns the operating system?’

That group will be identifiable through some internal system which tracks ownership of tools and technologies. Those identified as responsible will be responsible for some or all of the lifecycle management for that technology. This might include:

  • Upgrade management
  • Support (directly or via a vendor)
  • Security patching
  • Deciding who can and can’t use it
  • Overall policy on usage (expand/deprecate/continue usage)

This ownership results in ‘one throat to choke’ for audit functions. Much like the police will go after the drug dealer rather than the casual user, the audit functions of an enterprise will go after the formally responsible person or team than the (potentially thousands of) teams using an outdated version of a particular technology. There’s richer pickings there.

From ownership comes responsibility. A lot of the political footwork in an enterprise revolves around trying to not own technologies. Who wants to be responsible for Java usage across a technology function of dozens of thousands of staff, any of whom might be doing crazy stuff? You first, mate.

Enterprises and Vendors

This also explains enterprises’ love of vendor software over pure open source. If you’ve paid someone to maintain and support a technical stack, then they become responsible for that whole stack. That doesn’t solve all your problems (you still will need to integrate their software with your IT infrastructure, and things get fuzzier the closer you look at the resulting solution), but from a governance point of view you’ve successfully passed the buck.

What is governance?
IT Governance is a term that covers all the processes and structures that ensures the IT is appropriately managed in a way that satisfies those that govern the organisation. Being ‘out of governance’ (ie not conforming to standards) is considered a dangerous place to be, because you may be forced to spend money to get back ‘in’ to governance.

‘How is this maintained?’

Another aspect of managing software in an enterprise context is its maintenance. In our idealised startup above ‘Dev’ and ‘Ops’ were the same thing (ie, one person). Lo and behold you have DevOps!

Unfortunately, the DevOps slogan ‘you built it, you run it’ doesn’t usually work in an Enterprise context for a few reasons.

Partly it’s historical ie ‘it’s the way things have been done’ for decades, so there is a strong institutional bias towards not changing this. Jobs and heavily-invested-in processes depend on its persistence. But further bolstering this conservatism is the regulatory framework that governs how software is managed.


Regulations are rules created by regulators, who in turn are groups of people with power ultimately derived from government or other controlling authorities. So, effectively, they have the force of law as far as your business is concerned.

Regulators are not inclined to embrace fashionable new software deployment methods, and their paradigms are rooted in the experiences of software built in previous decades.

What does this mean? If your software is regulated, then it’s likely that your engineering (dev) and operations teams (ops) will be separate groups of people specialising in those roles, and one of the drivers of this is the regulations, which demand a separation to ensure that changes are under some kind of control and oversight.

Now, there is (arguably) a loophole here that some have exploited: regulations often talk about ‘separation of roles’ between engineering and operations, and don’t explicitly say that these roles need to be fulfilled by different people.

But if you’re a really big enterprise, that might be technically correct but effectively irrelevant. Why? Because, to ‘simplify’ things, these large enterprise often create a set of rules that cover all the regulations that may ever apply to their business across all jurisdictions. And those rules are generally the strictest you can imagine.

Added to that, those rules develop a life and culture of their own within the organisation independent of the regulator such that they can’t easily be brought into question.

Resistance is futile. Dev and Ops must be separate because that’s what we wrote down years ago.

So you can end up in a situation where you are forced to work in a way prescribed years ago by your internal regulations, which are in turn based on interpretations of regulations which were written years before that!

And if you want to change that, it will itself likely take years and agreement from multiple parties who are unlikely to want to risk losing their job so you can deliver your app slightly faster.

Obviously, this separation slows things down as engineering must make the code more tolerant to mistakes and failure so that another team can pick it up and carry it through to production. Or you just throw it over the wall and hope for the best. Either way, parties become more resistant to change.

Change Control

That’s not the only way in which the speed of change is reduced in an enterprise.

In order to ensure that changes to systems can be attributed to responsible individuals, there is usually some kind of system that tracks and audits changes. One person will raise a ‘change record’, which will usually involve filling out an enormous form, and then this change must be ‘signed off’ by one or more other person to ensure that changes don’t happen without due oversight.

In theory, the person signing off must carefully examine the change to ensure it is sensible and valid. In reality, most of the time trust relationships build up between change raiser and change validator which can speed things up. If the change is large and significant, then it is more likely to be closely scrutinised. There might also exist ‘standard changes’ or ‘templated changes’, which codify more routine and lower-risk updates and are pre-authorised. These must also be signed off before being deployed (usually at a higher level of responsibility, making it harder to achieve).

While in theory the change can be signed off in minutes, in reality change requests can take months as obscure fields in forms are filled out wrongly (‘you put the wrong code in field 44B! Start again.’), sign-off deadlines expire, change freezes come and go, and so on.

All this makes the effort of making changes far more onerous than it is elsewhere.

Security ‘Sign-Off’


If you’re working on something significant, such as a new product, or major release of a large-scale product, then it may become necessary to get what most people informally call ‘security sign-off’.

Processes around this vary from place to place, but essentially, one or more security experts descend at some point on your project and audit it.

I had imagined such reviews to be a very scientific process, but in reality it’s more like a medieval trial by ordeal. You get poked and prodded in various ways while questions are asked to determine weaknesses in your story.

This might involve a penetration test, a look at your code and documentation, or an interview with the engineers. There will likely be references to various ‘security standards’ you may or may not have read, which in turn are enforced with differing degrees of severity.

The outcome of this is usually some kind of report and a set of risks that have been identified. These risks (depending on their severity – I’ve never heard of there being none) may need to be ‘signed off’ by someone senior so that responsibility lies with them if there is a breach. That process in itself is arduous (especially when the senior doesn’t fully understand the risk) and can be repeated on a regular basis until it is sufficiently ‘mitigated’ through further engineering effort or process controls. After which it’s then re-reviewed. None of this is quick.

Summary: Corporate, not Individual Responsibility

If there’s a common thread to these factors in reducing risk, it is to shift responsibility and power from the individual to the corporate entity. If you’re a regulated, systemically-significant enterprise, then the last thing you or the public wants is for one person to wield too much power, either through knowledge of a system, or ability to alter that system in their own interests.

The corollary of this is that it is very hard for one person to make change by themselves. And, as we all know, if a task is given to multiple people to achieve together, then things get complicated and change slows up pretty fast as everyone must keep each other informed as to what everyone else is doing.

Once this principle of corporate responsibility is understood, then many other processes start to make sense. An example of one of these is sourcing (aka procurement: the process of buying software or other IT services).

Example – Sourcing

Working for such an enterprise, and before I stopped answering, I would get phoned up by salespeople all the time who seemed to imagine that I had a chequebook ready to sign for any technology I happened to like. The reality could not have been further from the truth.

What many people don’t expect is that to prevent a situation where one person could get too much power it can be the case that technical people have no direct control over the negotiation (or ‘sourcing process’) at all. What often happens is something close to this:

  • You go to senior person to get sign-off for a budget for purpose X
  • They agree
  • You document at least two options for products that fulfil that purpose
  • The ‘sourcing team’ take that document and negotiate with the suppliers
  • Some magic happens
  • You get told which supplier ‘won’

You can see why this process helps reduce the risk that someone takes a bribe to push a particular vendor solution (there’s also often strict rules around accepting so much as a coffee from a potential supplier), which is a good thing. On the other hand, this process can take months or even years. And might need to be repeated if the process takes so long that funding has disappeared or teams have been disbanded.

To complicate matters further, sourcing might have its own ‘preferred supplier lists‘ of companies that have been vetted and audited in the past. If your preferred supplier isn’t on that list (and hasn’t made a deal with one on those list), the process could take even longer.

3) Cumulative Constraints

What we have learned so far is that enterprises are fundamentally slowed down by attempts to reduce the power and individual responsibility in favour of corporate responsibility.

This usually results in:

  • More onerous change control
  • Higher bars for change planning
  • Higher bars for buying solutions
  • Higher bars for security requirements
  • Separation of engineering and ops functions

all of which slow down delivery. It’s like entropy. You can fight it, but in the end physics wins.

Now we’ll take a step outside these individual constraints to look at what happens when you structure a large scale enterprise organisation where its component groups are all fighting these same challenges.

Dependency Constraints

When you try and deliver in an enterprise you will find that your team has dependencies on other teams to provide you IT services.

The classic example of this is firewall changes. You, as a developer decide – in classic agile microservices/’all the shiny’ fashion – to create a new service running on a particular port on a set of hosts. You gulp Coke Zero all night and daub the code together to get a working prototype.

To allow connectivity, you need to open up some ports on the firewall. You raise a change, and discover the process involves updating a spreadsheet by hand and then raising a change request which requires at least a week’s notice. Your one night’s development is now going to elapse a week before you can try it out. And that’s hoping you filled everything out correctly and didn’t miss anything. If you did, then you have to go round again…

One of the joyous things about working in an unregulated startup is that if you see a problem in one of your dependencies you have the option of taking it over and running it yourself. Don’t like your cloud provider? Switch. Think your app might work better in erlang? Rewrite. Fed up with the firewall process? Write a script to do that, and move to gitops.

So why not do the same in the enterprise? Why not just ‘find your dependencies and eliminate them‘?

Some do indeed take this approach, and it costs them dearly. Either they have to spend great sums of money managing the processes required to maintain and stay ‘within governance’ for the technology they’ve decided to own, or they get hit with an audit sooner or later and get found out. At that point, they might go cap in hand to the infrastructure team, whose sympathy to their plight is in proportion to the amount of funding infrastructure is being offered to solve the problems for them…

The reality is that – as I said above – taking responsibility and owning a technology or layer of your stack brings with it real costs and risks that you may not be able to bear and stay in business.

So however great you are as a team, you’re delivery cadence is constrained to a local maxima based on your external dependencies, which are (effectively) non-negotiable.

This is a scaling up of the same constraints on individuals in favour of corporate power and responsibility. Just as it is significantly harder for you to make that much difference, it is harder for your team to make much difference, for the same structural reasons.

If you like this, you might like one of my books:

Learn Git the Hard WayLearn Terraform the Hard Way

Learn Bash the Hard Way


Cultural Constraints

Now that the ingredients for slow delivery are already there in a static, structural sense, let’s look at what happens when you ‘bake’ that structure over decades into the organisation and then try to make change within it.

Calcified Paradigms

Since reasoning about technology on a corporate scale is hard, creating change within it can only work at all if there are collective paradigms around which processes and functions can reason.

These paradigms become ingrained, and surfacing and reshaping these conceptual frameworks can be an effort that must repeated over and over across an organisation if you are to successfully make change.

The two big examples of this I’ve been aware of are the ‘machine paradigm’ and charging models, but one might add ‘secrets are used manually’ or many others that may also be bubbling under my conscious awareness.

The ‘machine paradigm’

Since von Neumann outlined the architecture of the computer, the view of the fundamental unit of computation as being a single discrete physical entity has held sway. Yes, you can share workloads on a single machine (mainframes still exist, for example, and two applications might use the same physical device), but for the broad mass of applications, the idea of needing a separate physical machine to run on (for performance or security reasons) has underpinned assumptions of applications’ design, build, test, and deploy phases.

Recently (mostly in the last 10 years), this paradigm has been modified by virtual machines, multiples of which sit on one larger machine that runs a hypervisor. Ironically, this has reinforced the ‘machine paradigm’, since for backward compatibility each VM has all the trappings of a physical machine, such as network interfaces, mac addresses, numbers of CPUs and so on. Whether you fill out a form and wait for a physical machine or a virtual machine to be provisioned makes little difference – you’re still in the machine paradigm.

Recently, aPaaSes, Kubernetes, and cloud computing have overthrown the idea that an application need sit on a ‘machine’, but the penetration of this novel (or old, if you used mainframes) idea, like the future, is unevenly distributed.

Charging models

Another paradigm that’s very hard to get traction on changing is charging models. How money moves around within an enterprise is a huge subject in itself, and has all sorts of secondary effects that are of no small interest to IT.

To grossly generalise, IT is moving from a ‘capex’ model to an ‘opex’ model. Instead of buying kit and software and then running it until it wears out (capex), the ‘new’ model is to rent software and services which can be easily scaled up and down as business demand requires.

Now, if you think IT in an enterprise is conservative, then prepare to deal with those that manage and handle the money! For good reason, they are as a rule very disinclined to change payment models within an organisation, since any change in process will result in bugs (old and new) being surfaced, institutional upheaval, and who knows what else.

The end result is that moving to these new models can be painful. Trying to cross-charge within an organisation of any size can result in surreal conversations about ‘wooden dollars’ (ie non-existent money exchanged in lieu of real money) or services being charged out to other parts of the business, but never paid for due to conversations that may or may not have been had outside your control.

Learned Helplessness

After decades of these habits of thoughts, you end up with several consequences:

  • Those who don’t like the way of working leave
  • Those that remain calcify into whole generations of employees
  • Those that remain tend to prize and prefer those that agree with their views

Suggestions of change to these groups of people result in entire generations, nay armies of employees that resist change.

The irony is that they are completely right. Most efforts to change do fail, and therefore most efforts to do so are wasted. The reasons are arguably circular, ie that change is resisted because it won’t work, and it won’t work because it’s resisted. But it’s also quite rational, since the reasons it won’t work are based on the external constraints that exist we have discussed above. But it’s simple game theory to follow the logic.

This has previously been described as the ‘square of despair‘:


Although I’d prefer to call it the ‘polygon of despair‘, since these four are fairly arbitrary. You could add to this list, for example:

  • Internal charging models
  • Change control
  • Institutional inertia
  • Audit
  • Regulation
  • Outdated paradigms

all of which have been discussed above.


The Decagon of Despair

4) A New Hope?

Is it all a lost cause? Is there really no hope for change? Does it always end up looking like this, at best a mass of compromises that feel like failure?


Well, no. But it is bloody hard. Here’s the things I think will stack the deck in your favour:

Senior Leadership Support

I think this is the big one. If you’re looking to swim against habits of thought, then stiff resolve is required. If senior management aren’t willing to sacrifice, aren’t united in favour of it, then all sorts of primary decisions and (equally important) second-guessed decisions made by underlings from different branches of the management tree that have conflicting aims.

People don’t like to talk about it, but it helps if people get fired for not constructively working with the changes. That tends to focus the mind. The classic precedent of this is point 6 of Jeff Bezos’s ‘API Mandate’:


You senior leadership will also need buckets of patience as the work to do this is very front-loaded, the pain being felt far earlier benefits being felt far later than the pain.

Reduce Complexity

Talking of pain, you will do yourself favours if you fight tooth and nail to reduce complexity. This may involve taking some risks as you call out that the entire effort may be ruined by compromises that defeat the purpose, or create bureaucratic or technical quicksand that your project will flounder in later.

Calling those dangers may get you a reputation, or even cost you your job. As the title of A Seat at the Table (a book I highly recommend on the subject) implies, it’s very close to a poker game.

Cross-functional Team

It might sound obvious to those that work in smaller companies, but it’s much easier to achieve change if you have a team of people that span the functions of your organisation working together. The collaboration not only benefits from seeing how things need be designed to fulfil requirements at an earlier stage, but more creative solutions are found by people who understand their function’s needs better, and the requirements of the project. If you want to go the skunkworks route, then the representatives of the other functions can tell you where your MVP shortcuts are going to bite you later on.

The alternative – and this is almost invariably much, much slower – is to ‘build, then check’. So you might spent several months building your solution before you find it’s fundamentally flawed based on some corporate rule or principle that can’t be questioned.

Use Your Cynical Old Hands

The flip side of those that constitute the ‘institutional inertia’ I described above is that many of those people know the organisation inside out. These people often lose heart regarding change not because they no longer care, but because they believe that when push comes to shove the changes won’t get support.

These people can be your biggest asset. The key is to persuade them that it’s possible, and that you need their help.

That can be hard for both sides, as your enthusiasm for change hits their brick wall, cemented by their hard-won (or lost) experience. They may give you messages that are hard to hear about how hard it will be. But don’t underestimate the loyalty and resilience you get if they are heard.


If you liked this post, you might also like:

Five Things I Did to Change a Team’s Culture

My 20-Year Experience of Software Development Methodologies

Things I Learned Managing Site Reliability for Some of the World’s Busiest Gambling Sites

A Checklist for Docker in the Enterprise (Updated)

Or one of my books:

Learn Git the Hard Way
Learn Terraform the Hard Way
Learn Bash the Hard Way



Anatomy of a Linux DNS Lookup – Part V – Two Debug Nightmares


In part V of this series of posts I take a slight detour to show you a couple of ‘debug nightmares’ that DNS threw at me.

Previous posts were:

Anatomy of a Linux DNS Lookup – Part I

Anatomy of a Linux DNS Lookup – Part II

Anatomy of a Linux DNS Lookup – Part III

Anatomy of a Linux DNS Lookup – Part IV

Both bugs threw up surprises not seen in previous posts…


Landrush and VMs

I’m a heavy Vagrant user, which I use mostly for testing (mostly) Kubernetes clusters of various kinds.

It’s key for these setups to work that DNS lookup between the various VMs works smoothly. By default, each VM is addressable only by IP address.

To solve this, I use a vagrant plugin called landrush. This works by creating a tiny ruby DNS server on the host that runs the VMs. This DNS server runs on port 10053, and keeps and returns records for the VMs that are running. So, for example, you might have two VMs running (vm1 and vm2), and landrush will ensure that your DNS lookups for these hosts (eg vm1.vagrant.test and vm2.vagrant.test) will point to the right local IP address for that VM.

It does this by creating IPTables rules on the VM host and the VMs themselves. These IPTables rules divert DNS requests to the DNS server running on port 10053, and if there’s no match, it will re-route the request to the original DNS server specified in that context.

Here’s a diagram that might help visualise this:


vagrant-landrush DNS server 

Above is a diagram that represents how Landrush DNS works with Vagrant. The box represents a host that’s running two Vagrant VMs (vm1 and vm2). These have the ipaddresses and respectively.

A DNS request on either vm is redirected from the host’s resolver (in this case systemd-resolved) to the host’s Landrush DNS server. This is achieved using an IPTables rule on the VM.

The Landrush DNS server keeps a small database of the host mappings to IPs given out by Vagrant and responds to any requests for vm1.vagrant.test or vm2.vagrant.test with the appropriate IP local address. If the request is for another address the request is forwarded on to the host’s configured DNS server (in this case DNSMasq).

Host lookups use the same IPTables mechanism to send DNS requests to the Landrush DNS server.

The Problem

Usually I use Ubuntu 16.04 machines for this, but when I tried 18.04 machines networking was failing on them:

$ curl google.com
curl: (6) Could not resolve host: google.com

At first I assumed the VM images themselves were faulty, but taking Landrush out of the equation restored networking fully.

Trying another tack, deleting the IPTables rule on the VM meant that networking worked also. So, mysteriously, the IPTables rule was not working. I tried stracing the two curl calls (working and not-working) to see what the difference was. There was a difference, but I had no idea why it might be happening.

As a next step I tried to take systemd-resolved and Landrush out of the equation (since that was new between 16.04 and 18.04). I did this by using different IPTables rules:

  • Direct requests to google’s DNS server rather than the Landrush failure (FAILED)
    • Showed that Landrush wasn’t the problem
  • Direct /etc/resolv.conf to a different address (changed to, and wire IPTables to Google’s DNS server (WORKED)
  • Direct /etc/resolv.conf to a different address (changed to, and wire IPTables to Google’s DNS server (FAILED)
    • Showed systemd-resolved not necessarily the problem

The fact that using instead of as a DNS server IP address led me to think that the fact that /etc/resolv.conf was pointed to a localhost address (ie one in the 127.0.0.* range) might be the problem.

A quick google led me here, which suggested that the problem was a sysctl setting:

sysctl -w net.ipv4.conf.all.route_localnet=1

And all was fixed.

The gory detail of the debugging is here.


sysctl settings are yet another thing that can affect and break DNS lookup!

These and more such settings are listed here.


DNSMasq, UDP=>TCP and Large DNS Responses

The second bug threw up another surprise.

We had an issue in production where DNS lookups were taking a very long time within an OpenShift Kubernetes cluster.

Strangely, it only affected some lookups and not others. Also, the time taken to do the lookup was consistent. This suggested that there was some kind of timeout on the first DNS server requested, after which it fell back to a ‘working’ one.

We did manual requests using dig to the local DNSMasq server on one of the hosts that was ‘failing’. The DNS request returned instantly, so we were scratching our heads. Then a colleague pointed out that the DNS response was rather longer than normal, which rang a bell.

Soon enough, he came back with this rfc, (RFC5966), which states:

   In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below),
   the normal behaviour of any DNS server needing to send a UDP response
   that would exceed the 512-byte limit is for the server to truncate
   the response so that it fits within that limit and then set the TC
   flag in the response header.  When the client receives such a
   response, it takes the TC flag as an indication that it should retry
   over TCP instead.

which, to summarise, means that if the DNS response is over 512 bytes, then the DNS server will send back a truncated response, and should make another request over TCP rather than UDP.

We never fixed the root cause here, but suspected that DNSMasq was not correctly returning the TCP to the client requesting. We found a setting that specified which interface DNSMasq would run against. By limiting this to one interface, requests worked again.

From this, we reasoned there was a bug in DNSMasq where if it was listening on more than one interface, and the upstream DNS request resulted in a response bigger than 512, then the response never reaches the original requester.


Another DNS surprise – DNS can stop working if the DNS response is over 512 bytes and the DNS client request program doesn’t handle this correctly.


DNS in Linux has even more surprises in store and things to check when things don’t go your way.

Here we saw how sysctl settings and plain old-fashioned bugs in seemingly battle-hardened code can affect your setup.

And we haven’t covered caching yet…


If you like this, you might like one of my books:

Learn Git the Hard WayLearn Terraform the Hard WayLearn Bash the Hard Way











Or you might like Docker in Practice

Anatomy of a Linux DNS Lookup – Part IV

In Anatomy of a Linux DNS Lookup – Part IPart II, and Part III I covered:

  • nsswitch
  • /etc/hosts
  • /etc/resolv.conf
  • ping vs host style lookups
  • systemd and its networking service
  • ifup and ifdown
  • dhclient
  • resolvconf
  • NetworkManager
  • dnsmasq

In Part IV I’ll cover how containers do DNS. Yes, that’s not simple either…

Other posts in the series:

Anatomy of a Linux DNS Lookup – Part I

Anatomy of a Linux DNS Lookup – Part II

Anatomy of a Linux DNS Lookup – Part III

Anatomy of a Linux DNS Lookup – Part V – Two Debug Nightmares

1) Docker and DNS

In part III we looked at DNSMasq, and learned that it works by directing DNS queries to the localhost address, and a process listening on port 53 there will accept the request.

So when you run up a Docker container, on a host set up like this, what do you expect to see in its /etc/resolv.conf?

Have a think, and try and guess what it will be.

Here’s the default output if you run a default Docker setup:

$  docker run  ubuntu cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.

search home


Where did the addresses and come from?

When I pondered this question, my first thought was that the container would inherit the /etc/resolv.conf settings from the host. But a little thought shows that that won’t always work.

If you have DNSmasq set up on the host, the /etc/resolv.conf file will be pointed at the loopback address. If this were passed through to the container, the container would look up DNS addresses from within its own networking context, and there’s no DNS server available within the container context, so the DNS lookups would fail.

‘A-ha!’ you might think: we can always use the host’s DNS server by using the host’s IP address, available from within the container as the default route:

root@79a95170e679:/# ip route                                                               
default via dev eth0                                       dev eth0 proto kernel scope link src     


Use the host?

From that we can work out that the ‘host’ is on the ip address:, so we could try manually pointing DNS at that using dig (you could also update the /etc/resolv.conf and then run ping, this just seems like a good time to introduce dig and its @ flag, which points the request at the ip address you specify):

root@79a95170e679:/# dig @ google.com | grep -A1 ANSWER.SECTION
google.com.             112     IN      A

However: that might work if you use DNSMasq, but if you don’t it won’t, as there’s no DNS server on the host to look up.

So Docker’s solution to this quandary is to bypass all that complexity and point your DNS lookups to Google’s DNS servers at and, ignoring whatever the host context is.

Anecdote: This was the source of my first problem with Docker back in 2013. Our corporate network blocked access to those IP addresses, so my containers couldn’t resolve URLs.

So that’s Docker containers, but container orchestrators such as Kubernetes can do different things again…


2) Kubernetes and DNS

The unit of container deployment in Kubernetes is a Pod. A pod is a set of co-located containers that (among other things) share the same IP address.

An extra challenge with Kubernetes is to forward requests for Kubernetes services to the right resolver (eg myservice.kubernetes.io) to the private network allocated to those service addresses. These addresses are said to be on the ‘cluster domain’. This cluster domain is configurable by the administrator, so it might be cluster.local or myorg.badger depending on the configuration you set up.

In Kubernetes you have four options for configuring how DNS lookup works within your pod.

  • Default

This (misleadingly-named) option takes the same DNS resolution path as the host the pod runs on, as in the ‘naive’ DNS lookup described earlier. It’s misleadingly named because it’s not the default! ClusterFirst is.

If you want to override the /etc/resolv.conf entries, you can in your config for the kubelet.

  • ClusterFirst

ClusterFirst does selective forwarding on the DNS request. This is achieved in one of two ways based on the configuration.

In the first, older and simpler setup, a rule was followed where if the cluster domain was not found in the request, then it was forwarded to the host.

In the second, newer approach, you can configure selective forwarding on an internal DNS

Here’s what the config looks like and a diagram lifted from the Kubernetes docs which shows the flow:

apiVersion: v1
kind: ConfigMap
  name: kube-dns
  namespace: kube-system
  stubDomains: |
    {"acme.local": [""]}
  upstreamNameservers: |
    ["", ""]

The stubDomains entry defines specific DNS servers to use for specific domains. The upstream servers are the servers we defer to when nothing else has picked up the DNS request.

This is achieved with our old friend DNSMasq running in a pod.


The other two options are more niche:

  • ClusterFirstWithHostNet

This applies if you use host network for your pods, ie you bypass the Docker networking setup to use the same network as you would directly on the host the pod is running on.

  • None

None does nothing to DNS but forces you to specify the DNS settings in the dnsConfig field in the pod specification.

CoreDNS Coming

And if that wasn’t enough, this is set to change again as CoreDNS comes to Kubernetes, replacing kube-dns. CoreDNS will offer a few benefits over kube-dns, being more configurabe and more efficient.

Find out more here.

If you’re interested in OpenShift networking, I wrote a post on that here. But that was for 3.6 so is likely out of date now.

End of Part IV

That’s part IV done. In it we covered.

  • Docker DNS lookups
  • Kubernetes DNS lookups
  • Selective forwarding (stub domains)
  • kube-dns


If you like this, you might like one of my books:

Learn Git the Hard WayLearn Terraform the Hard WayLearn Bash the Hard Way


Or you might like Docker in Practice

Anatomy of a Linux DNS Lookup – Part III

In Anatomy of a Linux DNS Lookup – Part I I covered:

  • nsswitch
  • /etc/hosts
  • /etc/resolv.conf
  • ping vs host style lookups

and in Anatomy of a Linux DNS Lookup – Part II I covered:

  • systemd and its networking service
  • ifup and ifdown
  • dhclient
  • resolvconf

and ended up here:

linux-dns-2 (2)

A (roughly) accurate map of what’s going on

Unfortunately, that’s not the end of the story. There’s still more things that can get involved. In Part III, I’m going to cover NetworkManager and dnsmasq and briefly show how they play a part.

Other posts in the series:

Anatomy of a Linux DNS Lookup – Part I

Anatomy of a Linux DNS Lookup – Part II

Anatomy of a Linux DNS Lookup – Part IV

Anatomy of a Linux DNS Lookup – Part V – Two Debug Nightmares

1) NetworkManager

As mentioned in Part II, we are now well away from POSIX standards and into Linux distribution-specific areas of DNS resolution management.

In my preferred distribution (Ubuntu), there is a service that’s available and often installed for me as a dependency of some other package I install called NetworkManager. It’s actually a service developed by RedHat in 2004 to help manage network interfaces for you.

What does this have to do with DNS? Install it to find out:

$ apt-get install -y network-manager

In my distribution, I get a config file.

$ cat /etc/NetworkManager/NetworkManager.conf


See that dns=dnsmasq there? That means that NetworkManager will use dnsmasq to manage DNS on the host.

2) dnsmasq

The dnsmasq program is that now-familiar thing: yet another level of indirection for /etc/resolv.conf.

Technically, dnsmasq can do a few things, but is primarily it acts as a DNS server that can cache requests to other DNS servers. It runs on port 53 (the standard DNS port), on all local network interfaces.

So where is dnsmasq running? NetworkManager is running:

$ ps -ef | grep NetworkManager
root     15048     1  0 16:39 ?        00:00:00 /usr/sbin/NetworkManager --no-daemon

But no dnsmasq process exists:

$ ps -ef | grep dnsmasq

Although it’s configured to be used, confusingly it’s not actually installed! So you’re going to install it.

Before you install it though, let’s check the state of /etc/resolv.conf.

$ cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search home

It’s not been changed by NetworkManager.

If dnsmasq is installed:

$ apt-get install -y dnsmasq

Then dnsmasq is up and running:

$ ps -ef | grep dnsmasq
dnsmasq  15286     1  0 16:54 ?        00:00:00 /usr/sbin/dnsmasq -x /var/run/dnsmasq/dnsmasq.pid -u dnsmasq -r /var/run/dnsmasq/resolv.conf -7 /etc/dnsmasq.d,.dpkg-dist,.dpkg-old,.dpkg-new --local-service --trust-anchor=.,19036,8,2,49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5

And /etc/resolv.conf has changed again!

root@linuxdns1:~# cat /etc/resolv.conf 
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search home

And netstat shows dnsmasq is serving on all interfaces at port 53:

$ netstat -nlp4
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address     Foreign Address State   PID/Program name
tcp        0      0*       LISTEN  15286/dnsmasq 
tcp        0      0*       LISTEN  15286/dnsmasq
tcp        0      0*       LISTEN  15286/dnsmasq
tcp        0      0*       LISTEN  1237/sshd
udp        0      0*               15286/dnsmasq
udp        0      0*               15286/dnsmasq  
udp        0      0*               15286/dnsmasq  
udp        0      0*               10758/dhclient
udp        0      0*               10530/dhclient
udp        0      0*               10185/dhclient

3) Unpicking dnsmasq

Now we are in a situation where all DNS queries are going to and from there what happens?

We can get a clue from looking again at the /var/run folder. The resolv.conf in resolvconf has been changed to point to where dnsmasq is being served:

$ cat /var/run/resolvconf/resolv.conf 
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search home

while there’s a new dnsmasq folder with its own resolv.conf.

$ cat /run/dnsmasq/resolv.conf 

which has the nameserver given to us by DHCP.

We can reason about this without looking too deeply, but what if we really want to know what’s going on?

4) Debugging Dnsmasq

Frequently I’ve found myself wondering what dnsmasq’s state is. Fortunately, you can get a good amount of information out of it if you set change this line in /etc/dnsmasq.conf:




and restart ​dnsmasq

Now, if you do a simple:

$ ping -c1 bbc.co.uk

you will see something like this in /var/log/syslog (the [...] indicates that the line’s start is the same as the previous one):

Jul  3 19:56:07 ubuntu-xenial dnsmasq[15372]: query[A] bbc.co.uk from
[...] forwarded bbc.co.uk to
[...] reply bbc.co.uk is
[...] reply bbc.co.uk is
[...] reply bbc.co.uk is
[...] reply bbc.co.uk is
[...] query[PTR] from
[...] forwarded to
[...] reply is NXDOMAIN

which shows what dnsmasq received, where the query was forwarded to, and what reply was received.

If the query is returned from the cache (or, more exactly, the local ‘time-to-live’ for the query has not expired), then it looks like this in the logs:

[...] query[A] bbc.co.uk from
[...] cached bbc.co.uk is
[...] cached bbc.co.uk is
[...] cached bbc.co.uk is
[...] cached bbc.co.uk is
[...] query[PTR] from

and if you ever want to know what’s in your cache, you can provoke dnsmasq into sending it to the same log file by sending the USR1 signal to the dnsmasq process id:

$ kill -SIGUSR1 <(cat /run/dnsmasq/dnsmasq.pid)

and the output of the dump looks like this:

Jul  3 15:08:08 ubuntu-xenial dnsmasq[15697]: time 1530630488                                                                                                                                 
[...] cache size 150, 0/5 cache insertions re-used unexpired cache entries.                                                                           
[...] queries forwarded 2, queries answered locally 0                                                                                                 
[...] queries for authoritative zones 0                                                                                                               
[...] server queries sent 2, retried or failed 0                                                                                         
[...] Host             Address         Flags      Expires                             
[...] linuxdns1    4FRI   H                                                                
[...] ip6-localhost    ::1             6FRI   H                                                                
[...] ip6-allhosts     ff02::3         6FRI   H                                                                
[...] ip6-localnet     fe00::          6FRI   H                                                                
[...] ip6-mcastprefix  ff00::          6FRI   H                                                                
[...] ip6-loopback     :               6F I   H                                                                
[...] ip6-allnodes     ff02:           6FRI   H                                                                
[...] bbc.co.uk   4F         Tue Jul  3 15:11:41 2018                                     
[...] bbc.co.uk  4F         Tue Jul  3 15:11:41 2018                                     
[...] bbc.co.uk    4F         Tue Jul  3 15:11:41 2018                                     
[...] bbc.co.uk  4F         Tue Jul  3 15:11:41 2018                                     
[...]           4 R  NX    Tue Jul  3 15:34:17 2018                                     
[...] localhost       4FRI   H                                                                
[...] <Root>           19036   8   2   SF I                                                                    
[...] ip6-allrouters   ff02::2         6FRI   H        

In the above output, I believe (but don’t know, and ‘?’ indicates a relatively wild guess on my part) that:

  • ‘4’ means IPv4
  • ‘6’ means IPv6
  • ‘H’ means address was read from an /etc/hosts file
  • ‘I’ ? ‘Immortal’ DNS value? (ie no time-to-live value?)
  • ‘F’ ?
  • ‘R’ ?
  • ‘S’?
  • ‘N’?
  • ‘X’


Alternatives to dnsmasq

dnsmasq is not the only option that can be passed to dns in NetworkManager. There’s none which does nothing to /etc/resolv,conf, default, which claims to ‘update resolv.conf  to reflect currently active connections’, and unbound, which communicates with the unbound service and dnssec-triggerd, which is concerned with DNS security and is not covered here.


End of Part III

That’s the end of Part III, where we covered the NetworkManager service, and its dns=dnsmasq setting.

Let’s briefly list some of the things we’ve come across so far:

  • nsswitch
  • /etc/hosts
  • /etc/resolv.conf
  • /run/resolvconf/resolv.conf
  • systemd and its networking service
  • ifup and ifdown
  • dhclient
  • resolvconf
  • NetworkManager
  • dnsmasq

















Or you might like Docker in Practice

Anatomy of a Linux DNS Lookup – Part II


In Anatomy of a Linux DNS Lookup – Part I I covered:

  • nsswitch
  • /etc/hosts
  • /etc/resolv.conf
  • ping vs host style lookups

and determined that most programs reference /etc/resolv.conf along the way to figuring out which DNS server to look up.

That stuff was more general linux behaviour (*) but here we move firmly into distribution-specific territory. I use ubuntu, but a lot of this will overlap with Debian and even CentOS-based distributions, and also differ from earlier or later Ubuntu versions.

(*) in fact, it’s subject to a POSIX standard, so
is not limited to Linux (I learned this from
a fantastic comment on the previous post)

In other words: your host is more likely to differ in its behaviour in specifics from here.

In Part II I’ll cover how resolv.conf can get updated, what happens when systemctl restart networking is run, and how dhclient gets involved.

Other posts in the series:

Anatomy of a Linux DNS Lookup – Part I

Anatomy of a Linux DNS Lookup – Part III

Anatomy of a Linux DNS Lookup – Part IV

Anatomy of a Linux DNS Lookup – Part V – Two Debug Nightmares

1) Updating /etc/resolv.conf by hand

We know that /etc/resolv.conf is (highly likely to be) referenced, so surely you can just add a nameserver to that file, and then your host will use that nameserver in addition to the others, right?

If you try that:

$ echo nameserver >> /etc/resolv.conf

it all looks good:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search home

until the network is restarted:

$ systemctl restart networking
$ cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search home

our nameserver has gone!

This is where those comments we ignored in Part I come in…

2) resolvconf

You see the phrase generated by resolvconf in the /etc/resolv.conf file above? This is our clue.

If you dig into what systemctl restart networking does, among many other things, it ends up calling a script: /etc/network/if-up.d/000resolvconf. Within this script is a call to resolvconf:

/sbin/resolvconf -a "${IFACE}.${ADDRFAM}"

A little digging through the man pages reveals that the -a flag allows us to:

Add or overwrite the record IFACE.PROG then run the update scripts
if updating is enabled.

So maybe we can call this directly to add a nameserver:

echo 'nameserver' | /sbin/resolvconf -a enp0s8.inet

Turns out we can!

$ cat /etc/resolv.conf  | grep nameserver

So we’re done now, right? This is how /etc/resolv.conf gets updated? Calling resolvconf adds it to a database somewhere, and then updates (if configured, whatever that means) the resolv.conf file


systemctl restart networking
root@linuxdns1:/etc# cat /etc/resolv.conf 
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search home

Argh! It’s gone again.

So systemctl restart networking does more than just run resolvconf. It must be getting the nameserver information from somewhere else. Where?

3) ifup/ifdown

Digging further into what systemctl restart networking does tells us a couple of things:

cat /lib/systemd/system/networking.service
ExecStartPre=-/bin/sh -c '[ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environment --list --exclude=lo)" ] && udevadm settle'
ExecStart=/sbin/ifup -a --read-environment
ExecStop=/sbin/ifdown -a --read-environment --exclude=lo

First, the networking ‘service’ restart is actually a ‘oneshot’ script that runs these commands:

/sbin/ifdown -a --read-environment --exclude=lo
/bin/sh -c '[ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environment --list --exclude=lo)" ] && udevadm settle'
/sbin/ifup -a --read-environment

The first line with ifdown brings down all the network interfaces (but excludes the local interface). (*)

(*) I’m unclear why this doesn’t boot me out of my
vagrant session in my example code (anyone know?).

The second line makes sure the system has done all it needs to do regarding the bringing of network interfaces down before going ahead and bringing them all back up with ifup in the third line. So the second thing we learn is that ifup and ifdown are what the networking service ‘actually’ runs.

The --read-environment flag is undocumented, and is there so that systemctl can play nice with it. A lot of people hate systemctl for this kind of thing.

Great. So what does ifup (and its twin, ifdown) do? To cut another long story short, it runs all the scripts in etc/network/if-pre-up.d/ and /etc/network/if-up.d/. These in turn might run other scripts, and so on.

One of the things it does (and I’m still not quite sure how – maybe udev is involved?) dhclient gets run.

4) dhclient

dhclient is a program that interacts with DHCP servers to negotiate the details of what IP address the network interface specified should use. It also can receive a DNS nameserver to use, which then gets placed in the /etc/resolv.conf.

Let’s cut to the chase and simulate what it does, but just on the enp0s3 interface on my example VM, having first removed the nameserver from the /etc/resolv.conf file:

$ sed -i '/nameserver.*/d' /run/resolvconf/resolv.conf
$ cat /etc/resolv.conf | grep nameserver
$ dhclient -r enp0s3 && dhclient -v enp0s3
Killed old client process
Internet Systems Consortium DHCP Client 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/enp0s8/08:00:27:1c:85:19
Sending on   LPF/enp0s8/08:00:27:1c:85:19
Sending on   Socket/fallback
DHCPDISCOVER on enp0s8 to port 67 interval 3 (xid=0xf2f2513e)
DHCPREQUEST of on enp0s8 to port 67 (xid=0x3e51f2f2)
DHCPACK of from
bound to -- renewal in 519 seconds.

$ cat /etc/resolv.conf | grep nameserver

So that’s where the nameserver comes from…

But hang on a sec – what’s that /run/resolvconf/resolv.conf doing there, when it should be /etc/resolv.conf?

Well, it turns out that /etc/resolv.conf isn’t always ‘just’ a file.

On my VM, it’s a symlink to the ‘real’ file stored in /run/resolvconf. This is a clue that the file is constructed at run time, and one of the reasons we’re told not to edit the file directly.

If the sed command above were to be run on the /etc/resolv.conf file directly then the behaviour above would be different and a warning thrown about /etc/resolv.conf not being a symlink (sed -i doesn’t handle symlinks cleverly – it just creates a fresh file).

dhclient offers the capability to override the DNS server given to you by DHCP if you dig a bit deeper into the supersede setting in /etc/dhcp/dhclient.conf

linux-dns-2 (2)

A (roughly) accurate map of what’s going on


End of Part II

That’s the end of Part II. Believe it or not that was a somewhat simplified version of what goes on, but I tried to keep it to the important and ‘useful to know’ stuff so you wouldn’t fall asleep. Most of that detail is around the twists and turns of the scripts that actually get run.

And we’re still not done yet. Part III will look at even more layers on top of these.

Let’s briefly list some of the things we’ve come across so far:

  • nsswitch
  • /etc/hosts
  • /etc/resolv.conf
  • /run/resolvconf/resolv.conf
  • systemd and its networking service
  • ifup and ifdown
  • dhclient
  • resolvconf

















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Anatomy of a Linux DNS Lookup – Part I


Since I work a lot with clustered VMs, I’ve ended up spending a lot of time trying to figure out how DNS lookups work. I applied ‘fixes’ to my problems from StackOverflow without really understanding why they work (or don’t work) for some time.

Eventually I got fed up with this and decided to figure out how it all hangs together. I couldn’t find a complete guide for this anywhere online, and talking to colleagues they didn’t know of any (or really what happens in detail)

So I’m writing the guide myself.

Turns out there’s quite a bit in the phrase ‘Linux does a DNS lookup’…

Other posts in the series:

Anatomy of a Linux DNS Lookup – Part II

Anatomy of a Linux DNS Lookup – Part III

Anatomy of a Linux DNS Lookup – Part IV

Anatomy of a Linux DNS Lookup – Part V – Two Debug Nightmares


“How hard can it be?”

These posts are intended to break down how a program decides how it gets an IP address on a Linux host, and the components that can get involved. Without understanding how these pieces fit together, debugging and fixing problems with (for example) dnsmasq, vagrant landrush, or resolvconf can be utterly bewildering.

It’s also a valuable illustration of how something so simple can get so very complex over time. I’ve looked at over a dozen different technologies and their archaeologies so far while trying to grok what’s going on.

I even wrote some automation code to allow me to experiment in a VM. Contributions/corrections are welcome.

Note that this is not a post on ‘how DNS works’. This is about everything up to the call to the actual DNS server that’s configured on a linux host (assuming it even calls a DNS server – as you’ll see, it need not), and how it might find out which one to go to, or how it gets the IP some other way.

1) There is no such thing as a ‘DNS Lookup’ call


This is NOT how it works

The first thing to grasp is that there is no single method of getting a DNS lookup done on Linux. It’s not a core system call with a clean interface.

There is, however, a standard C library call called which many programs use: getaddrinfo. But not all applications use this!

Let’s just take two simple standard programs: ping and host:

root@linuxdns1:~# ping -c1 bbc.co.uk | head -1
PING bbc.co.uk ( 56(84) bytes of data.
root@linuxdns1:~# host bbc.co.uk | head -1
bbc.co.uk has address

They both get the same result, so they must be doing the same thing, right?


Here’s the files that ping looks at on my host that are relevant to DNS:

root@linuxdns1:~# strace -e trace=open -f ping -c1 google.com
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libcap.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
open("/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 4
open("/etc/host.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 4
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
open("/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 4
open("/lib/x86_64-linux-gnu/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 4
PING google.com ( 56(84) bytes of data.
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 4
64 bytes from lhr25s12-in-f14.1e100.net ( icmp_seq=1 ttl=63 time=13.0 ms

and the same for host:

$ strace -e trace=open -f host google.com
[pid  9869] open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/libdst.cat", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid  9869] open("/usr/share/locale/en/libdst.cat", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid  9869] open("/usr/share/locale/en/LC_MESSAGES/libdst.cat", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid  9869] open("/usr/lib/ssl/openssl.cnf", O_RDONLY) = 6
[pid  9869] open("/usr/lib/x86_64-linux-gnu/openssl-1.0.0/engines/libgost.so", O_RDONLY|O_CLOEXEC) = 6[pid  9869] open("/etc/resolv.conf", O_RDONLY) = 6
google.com has address

You can see that while my ping looks at nsswitch.confhost does not. And they both look at /etc/resolv.conf.

We’re going to take these two .conf files in turn.

2) NSSwitch, and /etc/nsswitch.conf

We’ve established that applications can do what they like when they decide which DNS server to go to. Many apps (like ping) above can refer (depending on the implementation (*)) to NSSwitch via its config file /etc/nsswitch.conf.

(*) There’s a surprising degree of variation in
ping implementations. That’s a rabbit-hole I
didn’t want to get lost in.

NSSwitch is not just for DNS lookups. It’s also used for passwords and user lookup information (for example).

NSSwitch was originally created as part of the Solaris OS to allow applications to not have to hard-code which file or service they look these things up on, but defer them to this other configurable centralised place they didn’t have to worry about.

Here’s my nsswitch.conf:

passwd:         compat
group:          compat
shadow:         compat
gshadow:        files
hosts: files dns myhostname
networks:       files
protocols:      db files
services:       db files
ethers:         db files
rpc:            db files
netgroup:       nis

The ‘hosts’ line is the one we’re interested in. We’ve shown that ping cares about nsswitch.conf so let’s fiddle with it and see how we can mess with ping.

  • Set nsswitch.conf to only look at ‘files’

If you set the hosts line in nsswitch.conf to be ‘just’ files:

hosts: files

Then a ping to google.com will now fail:

$ ping -c1 google.com
ping: unknown host google.com

but localhost still works:

$ ping -c1 localhost
PING localhost ( 56(84) bytes of data.
64 bytes from localhost ( icmp_seq=1 ttl=64 time=0.039 ms

and using host still works fine:

host google.com
google.com has address

since, as we saw, it doesn’t care about nsswitch.conf

  • Set nsswitch.conf to only look at ‘dns’

If you set the hosts line in nsswitch.conf to be ‘just’ dns:

hosts: dns

Then a ping to google.com will now succeed again:

$ ping -c1 google.com
PING google.com ( 56(84) bytes of data.
64 bytes from lhr25s10-in-f174.1e100.net ( icmp_seq=1 ttl=63 time=8.01 ms

But localhost is not found this time:

$ ping -c1 localhost
ping: unknown host localhost

Here’s a diagram of what’s going on with NSSwitch by default wrt hosts lookup:

linux-dns-2 (1)

My default ‘hosts:‘ configuration in nsswitch.conf


3) /etc/resolv.conf

We’ve seen now that host and ping both look at this /etc/resolv.conf file.

Here’s what my /etc/resolv.conf looks like:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)

Ignore the first two lines – we’ll come back to those (they are significant, but you’re not ready for that ball of wool yet).

The nameserver lines specify the DNS servers to look up the host for.

If you hash out that line:


and run:

$ ping -c1 google.com
ping: unknown host google.com

it fails, because there’s no nameserver to go to (*).

* Another rabbit hole: host appears to fall back to if there’s no nameserver specified.

This file takes other options too. For example, if you add this line to the resolv.conf file:

search com

and then ping google (sic)

$ ping google
PING google.com ( 56(84) bytes of data.

it will try the .com domain automatically for you.

End of Part I

That’s the end of Part I. The next part will start by looking at how that resolv.conf gets created and updated.

Here’s what you covered above:

  • There’s no ‘DNS lookup’ call in the OS
  • Different programs figure out the IP of an address in different ways
    • For example, ping uses nsswitch, which in turn uses (or can use) /etc/hosts, /etc/resolv.conf and its own hostname to get the result
  • /etc/resolv.conf helps decide:
    • which addresses get called
    • which DNS server to look up

If you thought that was complicated, buckle up…
















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