From inside of a Docker container, how do I connect to the localhost of the machine?

Question

So I have a Nginx running inside a docker container, I have a mysql running on localhost, I want to connect to the MySql from within my Nginx. The MySql is running on localhost and not exposing a port to the outside world, so its bound on localhost, not bound on the ip address of the machine.

Is there any way to connect to this MySql or any other program on localhost from within this docker container?

This question is different from "How to get the IP address of the docker host from inside a docker container" due to the fact that the IP address of the docker host could be the public IP or the private IP in the network which may or may not be reachable from within the docker container (I mean public IP if hosted at AWS or something). Even if you have the IP address of the docker host it does not mean you can connect to docker host from within the container given that IP address as your Docker network may be overlay, host, bridge, macvlan, none etc which restricts the reachability of that IP address.

Answer 1

Edit:

If you are using Docker-for-mac or Docker-for-Windows 18.03+, just connect to your mysql service using the host host.docker.internal (instead of the 127.0.0.1 in your connection string).

If you are using Docker-for-Linux 20.10.0+, you can also use the host host.docker.internal if you started your Docker container with the --add-host host.docker.internal:host-gateway option.

Otherwise, read below

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TLDR

Use --network="host" in your docker run command, then 127.0.0.1 in your docker container will point to your docker host.

Note: This mode only works on Docker for Linux, per the documentation.

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Note on docker container networking modes

Docker offers different networking modes when running containers. Depending on the mode you choose you would connect to your MySQL database running on the docker host differently.

docker run --network="bridge" (default)

Docker creates a bridge named docker0 by default. Both the docker host and the docker containers have an IP address on that bridge.

on the Docker host, type sudo ip addr show docker0 you will have an output looking like:

[vagrant@docker:~] $ sudo ip addr show docker0 4: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default     link/ether 56:84:7a:fe:97:99 brd ff:ff:ff:ff:ff:ff     inet 172.17.42.1/16 scope global docker0        valid_lft forever preferred_lft forever     inet6 fe80::5484:7aff:fefe:9799/64 scope link        valid_lft forever preferred_lft forever 

So here my docker host has the IP address 172.17.42.1 on the docker0 network interface.

Now start a new container and get a shell on it: docker run --rm -it ubuntu:trusty bash and within the container type ip addr show eth0 to discover how its main network interface is set up:

root@e77f6a1b3740:/# ip addr show eth0 863: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000     link/ether 66:32:13:f0:f1:e3 brd ff:ff:ff:ff:ff:ff     inet 172.17.1.192/16 scope global eth0        valid_lft forever preferred_lft forever     inet6 fe80::6432:13ff:fef0:f1e3/64 scope link        valid_lft forever preferred_lft forever 

Here my container has the IP address 172.17.1.192. Now look at the routing table:

root@e77f6a1b3740:/# route Kernel IP routing table Destination     Gateway         Genmask         Flags Metric Ref    Use Iface default         172.17.42.1     0.0.0.0         UG    0      0        0 eth0 172.17.0.0      *               255.255.0.0     U     0      0        0 eth0 

So the IP Address of the docker host 172.17.42.1 is set as the default route and is accessible from your container.

root@e77f6a1b3740:/# ping 172.17.42.1 PING 172.17.42.1 (172.17.42.1) 56(84) bytes of data. 64 bytes from 172.17.42.1: icmp_seq=1 ttl=64 time=0.070 ms 64 bytes from 172.17.42.1: icmp_seq=2 ttl=64 time=0.201 ms 64 bytes from 172.17.42.1: icmp_seq=3 ttl=64 time=0.116 ms 

docker run --network="host"

Alternatively you can run a docker container with network settings set to host. Such a container will share the network stack with the docker host and from the container point of view, localhost (or 127.0.0.1) will refer to the docker host.

Be aware that any port opened in your docker container would be opened on the docker host. And this without requiring the -p or -P docker run option.

IP config on my docker host:

[vagrant@docker:~] $ ip addr show eth0 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000     link/ether 08:00:27:98:dc:aa brd ff:ff:ff:ff:ff:ff     inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0        valid_lft forever preferred_lft forever     inet6 fe80::a00:27ff:fe98:dcaa/64 scope link        valid_lft forever preferred_lft forever 

and from a docker container in host mode:

[vagrant@docker:~] $ docker run --rm -it --network=host ubuntu:trusty ip addr show eth0 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000     link/ether 08:00:27:98:dc:aa brd ff:ff:ff:ff:ff:ff     inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0        valid_lft forever preferred_lft forever     inet6 fe80::a00:27ff:fe98:dcaa/64 scope link        valid_lft forever preferred_lft forever 

As you can see both the docker host and docker container share the exact same network interface and as such have the same IP address.

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Connecting to MySQL from containers

bridge mode

To access MySQL running on the docker host from containers in bridge mode, you need to make sure the MySQL service is listening for connections on the 172.17.42.1 IP address.

To do so, make sure you have either bind-address = 172.17.42.1 or bind-address = 0.0.0.0 in your MySQL config file (my.cnf).

If you need to set an environment variable with the IP address of the gateway, you can run the following code in a container :

export DOCKER_HOST_IP=$(route -n | awk '/UG[ \t]/{print $2}') 

then in your application, use the DOCKER_HOST_IP environment variable to open the connection to MySQL.

Note: if you use bind-address = 0.0.0.0 your MySQL server will listen for connections on all network interfaces. That means your MySQL server could be reached from the Internet ; make sure to setup firewall rules accordingly.

Note 2: if you use bind-address = 172.17.42.1 your MySQL server won't listen for connections made to 127.0.0.1. Processes running on the docker host that would want to connect to MySQL would have to use the 172.17.42.1 IP address.

host mode

To access MySQL running on the docker host from containers in host mode, you can keep bind-address = 127.0.0.1 in your MySQL configuration and all you need to do is to connect to 127.0.0.1 from your containers:

[vagrant@docker:~] $ docker run --rm -it --network=host mysql mysql -h 127.0.0.1 -uroot -p Enter password: Welcome to the MySQL monitor.  Commands end with ; or \g. Your MySQL connection id is 36 Server version: 5.5.41-0ubuntu0.14.04.1 (Ubuntu)  Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.  Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners.  Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.  mysql> 

note: Do use mysql -h 127.0.0.1 and not mysql -h localhost; otherwise the MySQL client would try to connect using a unix socket.

Answer 2

For all platforms

Docker v 20.10 and above (since December 14th 2020)

Use your internal IP address or connect to the special DNS name host.docker.internal which will resolve to the internal IP address used by the host.

On Linux, add --add-host=host.docker.internal:host-gateway to your Docker command to enable this feature. To enable this in Docker Compose on Linux, add the following lines to the container definition:

extra_hosts:     - "host.docker.internal:host-gateway" 

For older macOS and Windows versions of Docker

Docker v 18.03 and above (since March 21st 2018)

Use your internal IP address or connect to the special DNS name host.docker.internal which will resolve to the internal IP address used by the host.

Linux support pending https://github.com/docker/for-linux/issues/264

For older macOS versions of Docker

Docker for Mac v 17.12 to v 18.02

Same as above but use docker.for.mac.host.internal instead.

Docker for Mac v 17.06 to v 17.11

Same as above but use docker.for.mac.localhost instead.

Docker for Mac 17.05 and below

To access host machine from the docker container you must attach an IP alias to your network interface. You can bind whichever IP you want, just make sure you're not using it to anything else.

sudo ifconfig lo0 alias 123.123.123.123/24

Then make sure that you server is listening to the IP mentioned above or 0.0.0.0. If it's listening on localhost 127.0.0.1 it will not accept the connection.

Then just point your docker container to this IP and you can access the host machine!

To test you can run something like curl -X GET 123.123.123.123:3000 inside the container.

The alias will reset on every reboot so create a start-up script if necessary.

Solution and more documentation here: https://docs.docker.com/docker-for-mac/networking/#use-cases-and-workarounds