Docker containers cannot resolve DNS

I just installed docker on CentOS 8. Everything went fine, I’ve run the hello_world test container and the result was successful. The issue is that inside the containers I cannot resolve any DNS queries. For example when I type ping 8.8.8.8 (on any kind of base container such as ubuntu os centOS) i get the correct answer but when I try running ping google.com I don’t get any answer, meaning that the container is unable to resolve the DNS for any URL.

I’ve also tried running a base container like this:

docker run busybox ping -c 1 192.203.230.10

ang I’m getting this (correct) output:

[server@localhost ~]$ docker run busybox ping -c 1 192.203.230.10
PING 192.203.230.10 (192.203.230.10): 56 data bytes
64 bytes from 192.203.230.10: seq=0 ttl=51 time=32.413 ms
--- 192.203.230.10 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 32.413/32.413/32.413 ms

But now trying resolving the domain google.com with the command:

docker run busybox nslookup google.com

I get this output:

[server@localhost ~]$ docker run busybox nslookup google.com
nslookup: write to '8.8.8.8': No route to host
nslookup: write to '8.8.4.4': No route to host
;; connection timed out; no servers could be reached

I’m finding this problem really frustrating since I’ve not found a way to solve it. I'm attaching for completeness a cat /etc/resolv.conf of both the machine running docker and the container itself

On the machine with CentOS 8:

[server@localhost ~]$ cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4

Inside any docker container:

bash-4.4# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4

Which looks like a normal and working configuration, but when i try to ping google.com (inside the containers) I get:

bash-4.4# ping google.com
ping: google.com: Try again

Tell me which command to run to get any info you’d find useful to help me solve this issue, I’ll post it here asap.

1 Answer

The solution to the problem seems to be adding docker as a trusted interface on firewalld using the command:

firewall-cmd --permanent --zone=trusted --add-interface=docker0

and then reloading firewalld with:

firewall-cmd --reload

After it should work correctly (remember to reboot)