Access host database from a docker container

Question

From the 18.03 docs:

I want to connect from a container to a service on the host

The host has a changing IP address (or none if you have no network access). From 18.03 onwards our recommendation is to connect to the special DNS name host.docker.internal, which resolves to the internal IP address used by the host.

The gateway is also reachable as gateway.docker.internal.

EXAMPLE: Here's what I use for my MySQL connection string inside my container to access the MySQL instance on my host:

mysql://host.docker.internal:3306/my_awesome_database

There are several long standing discussions about how to do this in a consistent, well understood and portable way. No complete resolution but I'll link you to the discussions below.

In any event you many want to try using the --add-host option to docker run to add the ip address of the host into the container's /etc/host file. From there it's trivial to connect to the host on any required port:

Adding entries to a container hosts file

You can add other hosts into a container's /etc/hosts file by using one or more --add-host flags. This example adds a static address for a host named docker:

 $ docker run --add-host=docker:10.180.0.1 --rm -it debian
    $$ ping docker
    PING docker (10.180.0.1): 48 data bytes
    56 bytes from 10.180.0.1: icmp_seq=0 ttl=254 time=7.600 ms
    56 bytes from 10.180.0.1: icmp_seq=1 ttl=254 time=30.705 ms
    ^C--- docker ping statistics ---
    2 packets transmitted, 2 packets received, 0% packet loss
    round-trip min/avg/max/stddev = 7.600/19.152/30.705/11.553 ms

Note: Sometimes you need to connect to the Docker host, which means getting the IP address of the host. You can use the following shell commands to simplify this process:

 $ alias hostip="ip route show 0.0.0.0/0 | grep -Eo 'via \S+' | awk '{ print $2 }'"
 $ docker run  --add-host=docker:$(hostip) --rm -it debian

Documentation:

https://docs.docker.com/engine/reference/commandline/run/

Discussions on accessing host from container:

https://github.com/docker/docker/issues/1143

https://github.com/docker/docker/issues/10023

From Docker 17.06 onwards, a special Mac-only DNS name is available in docker containers that resolves to the IP address of the host. It is:

docker.for.mac.localhost

The documentation is here: https://docs.docker.com/docker-for-mac/networking/#httphttps-proxy-support

Use host.docker.internal from Docker 18.03 onwards.

Other answers did not work well for me. My container could not resolve host ip using host.docker.internal. There are two ways

1. Sharing host network --net=host:

   docker run -it --net=host  myimage
   

2. Using docker's ip address, which is usually 172.17.0.1. You can check it by calling ifconfig command and grabbing inet addr of docker interface

   user@ubuntu:~$ ifconfig
   docker0   Link encap:Ethernet  HWaddr 02:42:a4:a2:b2:f1  
     inet addr:172.17.0.1  Bcast:0.0.0.0  Mask:255.255.0.0
     inet6 addr: fe80::42:a4ff:fea2:b2f1/64 Scope:Link
   

Once you have this ip address, you can pass it as an argument to docker run and then to application or as I do it, map the location of jdbc.properties via volume to the directory on host machine, so you can manage the file externally.

  docker run -it -v /host_dir/docker_jdbc_config:${jetty_base}/var/config myimage

NOTE: Your database might not allow external connections. In case of postgresql, you need to edit 2 files, as described here and here:

1. Edit postgresql.conf to listen on all addresses. By default it will point to localhost.

   listen_addresses = '*'
   

2. Edit pg_hba.conf to allow connections from all addresses. Add on the last line:

   host     all             all             0.0.0.0/0               md5
   

IMPORTANT: Last step updating database access is not recommended for production use unless you are really sure what you are doing.