I have a docker container running jenkins. As part of the build process, I need to access a web server that is run locally on the host machine. Is there a way the host web server (which can be configured to run on a port) can be exposed to the jenkins container?
EDIT: I'm running docker natively on a Linux machine.
UPDATE:
In addition to @larsks answer below, to get the IP address of the Host IP from the host machine, I do the following:
ip addr show docker0 | grep -Po 'inet \K[\d.]+'
This question is related to
docker
docker-container
For linux systems, you can – starting from major version 20.04 of the docker engine – now also communicate with the host via host.docker.internal. This won't work automatically, but you need to provide the following run flag:
--add-host=host.docker.internal:host-gateway
See
This is an old question and had many answers, but none of those fit well enough to my context. In my case, the containers are very lean and do not contain any of the networking tools necessary to extract the host's ip address from within the container.
Also, usin the --net="host" approach is a very rough approach that is not applicable when one wants to have well isolated network configuration with several containers.
So, my approach is to extract the hosts' address at the host's side, and then pass it to the container with --add-host parameter:
$ docker run --add-host=docker-host:`ip addr show docker0 | grep -Po 'inet \K[\d.]+'` image_name
or, save the host's IP address in an environment variable and use the variable later:
$ DOCKERIP=`ip addr show docker0 | grep -Po 'inet \K[\d.]+'`
$ docker run --add-host=docker-host:$DOCKERIP image_name
And then the docker-host is added to the container's hosts file, and you can use it in your database connection strings or API URLs.
I've explored the various solution and I find this the least hacky solution:
extra_hosts directive.The only downside is if you have multiple networks or projects doing this, you have to ensure that their IP address range do not conflict.
Here is a Docker Compose example:
version: '2.3'
services:
redis:
image: "redis"
extra_hosts:
- "dockerhost:172.20.0.1"
networks:
default:
ipam:
driver: default
config:
- subnet: 172.20.0.0/16
gateway: 172.20.0.1
You can then access ports on the host from inside the container using the hostname "dockerhost".
For me (Windows 10, Docker Engine v19.03.8) it was a mix of https://stackoverflow.com/a/43541732/7924573 and https://stackoverflow.com/a/50866007/7924573 .
version: '3.7'
services:
server:
build: .
ports:
- "5000:5000"
network_mode: bridge
or alternatively: Use --net="bridge" if you are not using docker-compose (similar to https://stackoverflow.com/a/48806927/7924573)For docker-compose using bridge networking to create a private network between containers, the accepted solution using docker0 doesn't work because the egress interface from the containers is not docker0, but instead, it's a randomly generated interface id, such as:
$ ifconfig
br-02d7f5ba5a51: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.32.1 netmask 255.255.240.0 broadcast 192.168.47.255
Unfortunately that random id is not predictable and will change each time compose has to recreate the network (e.g. on a host reboot). My solution to this is to create the private network in a known subnet and configure iptables to accept that range:
Compose file snippet:
version: "3.7"
services:
mongodb:
image: mongo:4.2.2
networks:
- mynet
# rest of service config and other services removed for clarity
networks:
mynet:
name: mynet
ipam:
driver: default
config:
- subnet: "192.168.32.0/20"
You can change the subnet if your environment requires it. I arbitrarily selected 192.168.32.0/20 by using docker network inspect to see what was being created by default.
Configure iptables on the host to permit the private subnet as a source:
$ iptables -I INPUT 1 -s 192.168.32.0/20 -j ACCEPT
This is the simplest possible iptables rule. You may wish to add other restrictions, for example by destination port. Don't forget to persist your iptables rules when you're happy they're working.
This approach has the advantage of being repeatable and therefore automatable. I use ansible's template module to deploy my compose file with variable substitution and then use the iptables and shell modules to configure and persist the firewall rules, respectively.
You can access the local webserver which is running in your host machine in two ways.
Approach 1 with public IP
Use host machine public IP address to access webserver in Jenkins docker container.
Approach 2 with the host network
Use "--net host" to add the Jenkins docker container on the host's network stack. Containers which are deployed on host's stack have entire access to the host interface. You can access local webserver in docker container with a private IP address of the host machine.
NETWORK ID NAME DRIVER SCOPE
b3554ea51ca3 bridge bridge local
2f0d6d6fdd88 host host local
b9c2a4bc23b2 none null local
Start a container with the host network
Eg: docker run --net host -it ubuntu and run ifconfig to list all available network IP addresses which are reachable from docker container.
Eg: I started a nginx server in my local host machine and I am able to access the nginx website URLs from Ubuntu docker container.
docker run --net host -it ubuntu
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
a604f7af5e36 ubuntu "/bin/bash" 22 seconds ago Up 20 seconds ubuntu_server
Accessing the Nginx web server (running in local host machine) from Ubuntu docker container with private network IP address.
root@linuxkit-025000000001:/# curl 192.168.x.x -I
HTTP/1.1 200 OK
Server: nginx/1.15.10
Date: Tue, 09 Apr 2019 05:12:12 GMT
Content-Type: text/html
Content-Length: 612
Last-Modified: Tue, 26 Mar 2019 14:04:38 GMT
Connection: keep-alive
ETag: "5c9a3176-264"
Accept-Ranges: bytes
Solution with docker-compose:
For accessing to host-based service, you can use network_mode parameter
https://docs.docker.com/compose/compose-file/#network_mode
version: '3'
services:
jenkins:
network_mode: host
EDIT 2020-04-27: recommended for use only in local development environment.
Currently the easiest way to do this on Mac and Windows is using host host.docker.internal, that resolves to host machine's IP address. Unfortunately it does not work on linux yet (as of April 2018).
When you have two docker images "already" created and you want to put two containers to communicate with one-another.
For that, you can conveniently run each container with its own --name and use the --link flag to enable communication between them. You do not get this during docker build though.
When you are in a scenario like myself, and it is your
docker build -t "centos7/someApp" someApp/
That breaks when you try to
curl http://172.17.0.1:localPort/fileIWouldLikeToDownload.tar.gz > dump.tar.gz
and you get stuck on "curl/wget" returning no "route to host".
The reason is security that is set in place by docker that by default is banning communication from a container towards the host or other containers running on your host. This was quite surprising to me, I must say, you would expect the echosystem of docker machines running on a local machine just flawlessly can access each other without too much hurdle.
The explanation for this is described in detail in the following documentation.
http://www.dedoimedo.com/computers/docker-networking.html
Two quick workarounds are given that help you get moving by lowering down the network security.
The simplest alternative is just to turn the firewall off - or allow all. This means running the necessary command, which could be systemctl stop firewalld, iptables -F or equivalent.
Hope this information helps you.
On Linux, add --add-host=host.docker.internal:host-gateway to your Docker command to enable this feature. (See below for Docker Compose configuration.)
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.
To enable this in Docker Compose on Linux, add the following lines to the container definition:
extra_hosts: - "host.docker.internal:host-gateway"
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
Same as above but use docker.for.mac.host.internal instead.
Same as above but use docker.for.mac.localhost instead.
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
I created a docker container for doing exactly that https://github.com/qoomon/docker-host
You can then simply use container name dns to access host system e.g.
curl http://dockerhost:9200
Use --net="host" in your docker run command, then localhost in your docker container will point to your docker host.
We found that a simpler solution to all this networking junk is to just use the domain socket for the service. If you're trying to connect to the host anyway, just mount the socket as a volume, and you're on your way. For postgresql, this was as simple as:
docker run -v /var/run/postgresql:/var/run/postgresql
Then we just set up our database connection to use the socket instead of network. Literally that easy.
Source: Stackoverflow.com