Linux

Docker name resolution network problems – Tutorial

 

Hopefully, you will never really need to be reading this article, and you only happen to be here
because you are bored or did a wrong kind of search. Or you might actually be facing an issue where
your
Docker containers do not have Internet access anymore, even though they
used to work well, just recently, and you’ve made no changes to your environment.

This sounds like a very vague problem statement, but this is what I was facing all of a sudden. My
containers did not have network access, with the error like
Temporary failure resolving URL. It looked like an issue with
name resolution, and it bugged me extra, because it was not supposed to
happen. But we’re getting ahead of ourselves. Test host: Ubuntu with systemd – important for
latter. Let’s proceed slowly.

Problem in more detail

You have configured Docker on your system. It’s working well. You have multiple images, multiple
containers, and you even used
advanced network rules, and everything seems to be in perfect
order. Then, you notice that you can no longer do certain activities in your containers, like updates
or package installations. The best way to debug this is to attach a shell to a running container
instance, as I’ve explained in the intro guide. Indeed, inside a running container, you see something
like:

# apt-get update

Err:1 http://archive.ubuntu.com/ubuntu bionic
InRelease

Temporary failure resolving ‘archive.ubuntu.com’

0% [Connecting to security.ubuntu.com]

This looks like a DNS problem (name resolution). The container is unable to figure out how to
resolve domain names to IP addresses, and therefore it cannot connect to the servers to grab data, like
updates. There are two issues here. One, why did the problem come to bear all of a sudden? Two, we need
to actually fix the network. Now, I’d like to show you how to troubleshoot this in an elegant way.

Solution

First, let’s understand why the issue occurred. There’s no immediate answer here, but some things to
look at may include more than just your immediate environment. For example, your host may not have
changed, but the network could have – routers, network policies, DNS servers themselves. Since you
cannot normally control what’s happening outside your immediate setup, the best way to figure out where
the issue resides is by doing a step-by-step isolation of the problem.

  • If your host system has network and cannot resolve URLs, you will need to sort that out first.
    There’s most likely a problem between your host and the destination, and whatever network
    infrastructure exists in between.
  • If your host system has network and can correctly resolve URLs, the issue is in specifically
    how Docker containers resolve URLs. This is where we need to focus next.
  • Check if the Docker network interface is up and running (with a command like ip or ifconfig).
    If not, you will next to fix that first before moving on to the next step.

ifconfig docker0

  • Check if the container instance has an IP address. If not, you will need to fix that.

docker inspect <container name or ID> | grep -i “ipaddr”

  • Check if you get the same results inside the container if possible (ip or ifconfig). If the
    result does not match what you have seen in the previous command, you will need to fix that.
  • Check if you can ping the container from the outside (and vice versa, if the ping command is
    available). If ping works, this is probably a good indication that the network is configured
    correctly, and that there are no firewall rules blocking the traffic (most likely).

Name resolution

If all these checks return no strange problems or errors, the next step is to focus on DNS
resolution. The configuration for that will be available in the
/etc/resolv.conf configuration file. This is true both for physical
instances of Linux as well as virtual machines and containers. You will probably notice that the
container uses something like:

# See man:systemd-resolved.service(8) for details about the supported modes

# of operation for /etc/resolv.conf.

search xyz

nameserver X.Y.Z.W

The nameserver IP addresses will most likely be an external IP address (something like your ISP) or
the localhost (127.0.0.X). The question is: Do these match your host’s /etc/resolv.conf file?

The answer is, you will most probably have localhost defined in your host’s /etc/resolv.conf file.
Now, try that in your container. Edit the resolv.conf file and replace the IP address in the nameserver
line with the one that matches the host’s value = localhost. If that solves your issue, great. But most
likely, it won’t.

But then, at this point, you have no problem with network connectivity on your host. So we need to
figure out the REAL address of the DNS server in your environment. And this is further complicated by
the fact most modern Linux distributions use
systemd. The plot thickens.

Find out DNS with systemd

So yes. We need to figure out what the nameserver is, and we will need to use a systemd command for
that. Most likely, if you have systemd in your system, you are also using
systemd-resolve,
the network name resolution manager and service. The configuration is stored under
/etc/systemd/resolved.conf. But you can also obtain the results on the command line with the
systemd-resolve command:

systemd-resolve –status

Link 3 (wlp59s0)

Current Scopes: DNS

LLMNR setting: yes

MulticastDNS setting: no

DNSSEC setting: no

DNSSEC supported: no

DNS Servers: 10.50.34.1

2001:64c:1462:b023::1

DNS Domain: dedoimedo

What do we have here? A lot of interesting stuff. But what really matters is the line that reads DNS
Servers. This is what we want. Place this IP address into the container /etc/resolv.conf file and try
again. You should have the network working again.

Why this problem then?

Now, we can discuss the
why again. If you look at the systemd-resolve documentation, it is
possible that there was some change in your system (possibly even due to a regular update) whereby the
selected mode of operation causes some conflicts with the name resolution. In particular, if we look at
the way /etc/resolv.conf is handled, then the first mode states:

systemd-resolved maintains the /run/systemd/resolve/stub-resolv.conf file for
compatibility with traditional Linux programs. This file may be symlinked from /etc/resolv.conf. This
file lists the 127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of
search domains that are in use by systemd-resolved. The list of search domains is always kept
up-to-date. Note that /run/systemd/resolve/stub-resolv.conf should not be used directly by
applications, but only through a symlink from /etc/resolv.conf. This file may be symlinked from
/etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to systemd-resolved
with correct search domains settings. This mode of operation is recommended.

If one of the links in this equation is broken, or something changed, it’s possible that the Docker
service can’t really determine what gives, and you end up with no name resolution. So the resolution
[sic] is to provide the actual network DNS address, which the containers can understand and use. This
is a bit speculative on my side, but I think it’s quite correct.

Conclusion

Here we go, another mystery demisted, another windscreen demystified. Or something. I don’t like
half-magic solutions, but when you have a super-complicated, layered system infrastructure, sometimes
the solutions are just as bad as the problem. Not in that they don’t fix the issue – in that you have
less visibility and control than you respectfully should. But that’s the future of Linux – and
everything IT – endless abstraction.

On topic, hopefully this little guide should provide you with a relatively quick and painless fix
for your container adventures. If you’re using Docker, and the name resolution no longer works inside
the container instances, perhaps you should test the tips and tricks written above – and then build
your own images with the right fix in place, so you don’t need to fight this all over again. And there
you go, the end.

Cheers.

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