Stuck with a Docker image DNS lookup error? Fear not, our Support Team is here with a solution.
At Bobcares, coming up with solutions for queries, big and small is a part of our Server Management Service s.
Let’s take a look at how our Support Team resolved the DNS lookup error today.
About DNS lookup errors
Have you been experiencing a number of obscure errors with your applications?? This might be due to Docker’s internet connectivity issues. According to our Support Tech, this may be a result of failing DNS lookups. Resolving this issue will also help you bid adieu to the pesky docker image DNS lookup error.
Narrowing down docker image DNS lookup error
First, we will check the basic internet connectivity by pinging a public IP address. If it succeeds, you will get a similar output as seen below:
$ docker run busybox ping -c 1 192.203.230.10 # Ping a London-based NASA root nameserver PING 192.203.230.10 (192.203.230.10): 56 data bytes 64 bytes from 192.203.230.10: seq=0 ttl=53 time=113.866 ms --- 192.203.230.10 ping statistics --- 1 packets transmitted, 1 packets received, 0% packet loss round-trip min/avg/max = 113.866/113.866/113.866 ms
After that, let’s try resolving google.com domain as shown below:
$ docker run busybox nslookup google.com Server: 8.8.8.8 Address 1: 8.8.8.8 nslookup: can't resolve 'google.com'
If you get a “can’t resolve” message as seen above, it indicates a problem with resolving DNS.
Why does docker image DNS lookup error occur?
Before we dive into resolving the issue, let’s take a look at why the DNS lookup errors keep occurring. In fact, a DNS server that is locally defined in the /etc/resolv.conf file, containers by default, use 8.8.8.8, Google’s public DNS server to resolve DNS.
Hence, the error occurs, then this process is interrupted. Interestingly, in some places, network administrators intentionally block public DNS servers to ensure that the network’s own DNS server is used instead.
In such cases, Docker containers that rely on the default configuration are unable to resolve DNS. Thereby making the internet unusable from within those containers.
Quick fix for docker image DNS lookup error
Our Support Engineers have come up with a quick fix to ensure that the docker container runs with a custom DNS server.
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First, we will track down the address of the DNS server from within Ubuntu with the following command:
$ nmcli dev show | grep 'IP4.DNS' IP4.DNS[1]: 10.0.0.2
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Then, we will run a docker container with the DNS server obtained in the previous step. We will include the –dns flag as well with the command as seen below:
$ docker run --dns 10.0.0.2 busybox nslookup google.com Server: 10.0.0.2 Address 1: 10.0.0.2 Name: google.com Address 1: 2a00:1450:4008:811::200e lhr26s02-in-x200e.1e100.net Address 2: 216.58.198.173 lhr25s10-in-f14.1e100.net
Permanent fix for docker image DNS lookup error
If you prefer a more permanent fix, our Support Engineers have that covered as well.
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First, we will change the DNS settings of the Docker daemon by creating the daemon configuration file at /etc/docker/daemon.json.
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Then, configure the daemon configuration file with a set of two DNS, namely, the network DNS server and the Google DNS server. The latter works as a backup in case the network DNS server is not available.
/etc/docker/daemon.json: "dns": ["10.0.0.2", "8.8.8.8"]