How to access host port from docker container

Question

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.]+'

Answer 1

When running Docker natively on Linux, you can access host services using the IP address of the docker0 interface. From inside the container, this will be your default route.

For example, on my system:

$ ip addr show docker0
7: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default 
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever
    inet6 fe80::f4d2:49ff:fedd:28a0/64 scope link 
       valid_lft forever preferred_lft forever

And inside a container:

# ip route show
default via 172.17.0.1 dev eth0 
172.17.0.0/16 dev eth0  src 172.17.0.4 

It's fairly easy to extract this IP address using a simple shell script:

#!/bin/sh

hostip=$(ip route show | awk '/default/ {print $3}')
echo $hostip

You may need to modify the iptables rules on your host to permit connections from Docker containers. Something like this will do the trick:

# iptables -A INPUT -i docker0 -j ACCEPT

This would permit access to any ports on the host from Docker containers. Note that:

• iptables rules are ordered, and this rule may or may not do the right thing depending on what other rules come before it.

• you will only be able to access host services that are either (a) listening on INADDR_ANY (aka 0.0.0.0) or that are explicitly listening on the docker0 interface.

Answer 2

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.