get the ip of the host machine from the docker container

Question

I need to get the ip address in the network (eth0) of the server on which this container is located from inside the docker container.

inside the container (alpine:3.18) with ifconfig:

/ # ifconfig
eth0      Link encap:Ethernet  HWaddr 02:42:AC:11:00:04  
          inet addr:172.17.0.4  Bcast:172.17.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:65535  Metric:1
          RX packets:49 errors:0 dropped:0 overruns:0 frame:0
          TX packets:54 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:6433 (6.2 KiB)  TX bytes:5904 (5.7 KiB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

and outside the container, the host has:

eth0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=6460<TSO4,TSO6,CHANNEL_IO,PARTIAL_CSUM,ZEROINVERT_CSUM>
    ether 9c:3e:53:92:e9:86
    inet6 fe80::fb:c60b:fa61:92bd%en0 prefixlen 64 secured scopeid 0xc 
    inet 192.168.1.63 netmask 0xffffff00 broadcast 192.168.1.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect
    status: active

how can I get it inside the container? is it possible to throw something into a container, such as a docker socket:

 -v /var/run/docker.sock:/var/run/docker.sock

This is a continuation of the question "launching new containers from a container"

I understand that there is an option to do it outside the container by reading the file, but I would like to know how to do it through the command

Answer 1

You can grab the host IP address and pass it to the container as an environment variable.

After experimenting a bit, I came up with the command hostname -I | awk '{print $1;}' to get the IP address. I don't know if that always works because the host might have multiple IP addresses and I'm not certain that this command selects the correct one. But it seems to work on my machine.

To illustrate, if you run

docker run --rm -e HOST_IP=$(hostname -I | awk '{print $1;}') debian env

you should see that the HOST_IP variable is set in the container.