slow or timeout of dns resolving inside docker - dns

Question

On host machine, it's very fast to lookup a domain. But inside docker container, it's much
slower and sometimes timeout.
The host machine is a virtual host, and it's dns server address is 127.0.0.1 (weird but true). So I've tried to modify /etc/resolv.conf inside container and set the dns server to be 172.x (host's address). As a result, I didn't see any good effect.
I've also tried to set the container's dns server to be a self-built one (101.x), but still, it's slow to look up a domain. Another weird thing is that ping 101.x is very fast.
I'm confused about this phenomenon, anyone can explain and help?

Answer 1

I am not sure of why resolving DNS is slow in the containers, but I have procedure which I follow to resolve the DNS in the docker containers.
To verify DNS resolution issue:
# docker run busybox nslookup google.com
Server: 8.8.8.8
Address 1: 8.8.8.8
nslookup: can't resolve 'google.com'
Find out the DNS server used in your machine :
# nm-tool |grep DNS
DNS: 172.24.100.50
DNS: 10.1.100.50
Run it again using DNS IP found in the above step which resolves the DNS issue:
# docker run --dns 172.24.100.50 busybox nslookup google.com
Server: 172.24.100.50
Address 1: 172.24.100.50 indc01.radisys.com
Name: google.com
Address 1: 2607:f8b0:4009:80c::200e ord36s01-in-x0e.1e100.net
Address 2: 172.217.4.110 ord36s04-in-f14.1e100.net
To resolve it permanently add the following content as below to a new file:
root#labadmin-VirtualBox:/home/labadmin# cat /etc/docker/daemon.json
{
"dns" : ["172.24.100.50", "8.8.8.8"]
}
More info on Docker DNS configuration.
Restart the docker service and verify it again:
# docker run busybox nslookup google.com
Server: 172.24.100.50
Address 1: 172.24.100.50 indc01.radisys.com
Name: google.com
Address 1: 2607:f8b0:4009:801::200e ord30s31-in-x0e.1e100.net
Address 2: 172.217.4.238 ord30s31-in-f14.1e100.net
Check it by running the container:
# docker run -it e02e811dd08f
/ # ping google.com
PING google.com (172.217.4.238): 56 data bytes
64 bytes from 172.217.4.238: seq=0 ttl=47 time=251.506 ms
64 bytes from 172.217.4.238: seq=1 ttl=47 time=245.621 ms
Hope this helps.