Cannot apt-get update from inside my docker container if connected to bridge network

Solution 1:

After hours and hours I was able to solve the problem

The MTU of the docker's bridge network has to match to the MTU of host's the network adapter

In my case the MTU of eth0 (host) was set to 1450 while MTU of docker0 was set to 1500

You can change the MTU by either

• editing your docker.service as described in https://stackoverflow.com/questions/35300497/docker-container-not-connecting-to-https-endpoints
• add "mtu": to /etc/docker/daemon.json and systemctl restart docker

In case you don't have a /etc/docker/daemon.json just create one:

# /etc/docker/daemon.json
# adjust the MTU accordingly to the hosts network adapter

{
    "mtu":1450
}

Don't forget to restart docker.service: systemctl restart docker.service

Further details:

If you wan't to check the settings use ip and compare the mtu value

$ ip a

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> **mtu 1450** qdisc fq_codel state UP group default qlen 1000
    ...
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> **mtu 1500** qdisc noqueue state DOWN group default 
    ...

Note, that docker0 states always 1500 and changes it's value only if a container ist connected to that network

$ ip a
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc fq_codel state UP group default qlen 1000
    ...
3: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP group default 
    ...
17: vethe4b452f@if16: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue master docker0 state UP group default 
    ...

Custom Network

I also tried to just create a custom network with a defined MTU instead of setting the MTU via /etc/docker/daemon.json. This did not work and I don't know why

docker network create --opt com.docker.network.mtu=1450 CustomMTU