Electing Default Gateway within container is really opaque

[fbuecklers]

I have created two bridges with the docker network API one bridged named local the other one is named isolated.

docker network create --driver bridge --subnet=172.17.0.0/16 -o com.docker.network.bridge.name=local local 
docker network create --driver bridge --subnet=172.28.0.0/16 -o com.docker.network.bridge.enable_ip_masquerade=false -o com.docker.network.bridge.name=isolated isolated

When I run a container with: docker run --rm -it --net=local busybox
and attach my isolated network afterward, i have the following routing table within the container:

/ # route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.28.0.1      0.0.0.0         UG    0      0        0 eth1
172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 eth0
172.28.0.0      0.0.0.0         255.255.0.0     U     0      0        0 eth1

so eth1 becomes the default gateway (the isolated bridge) which was not my attempt.
If i change the network attach order: starting in the isolated network and connecting to the local network my routing table results in:

/ # route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.28.0.1      0.0.0.0         UG    0      0        0 eth0
172.28.0.0      0.0.0.0         255.255.0.0     U     0      0        0 eth0
172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 eth1

So my gateway always ends in the isolated bridge. After a lot of try and error my founding is, that the default gateway is elected by the name order of the network. Naming my default network to allow_default solves the problem, but the solution seams really ugly.

[bboreham]

Seems relevant: #1141 (comment)

[aboch]

Thanks @fbuecklers @bboreham
Given the logic is now documented, I will go ahead and close this issue.
Feel free to ocntinue the discussion, though.