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

User · Answer

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     change the host ip to host docker internal  e g   LOGGER URL    http   host docker internal 8085 log  set the network mode to bridge  if you want to maintain the port forwarding  if not use host   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  As pointed out in previous answers  This should only be used in a local development environment  For more information read  https   docs docker com compose compose-file  network mode and https   docs docker com docker-for-windows networking  use-cases-and-workarounds

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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

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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   lt NO-CARRIER BROADCAST MULTICAST UP gt  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     If you are using Docker on MacOS or Windows 18 03   you can connect connect to the magic hostname host docker internal   Lastly  under Linux you can run your container in the host network namespace by setting --net host  in this case localhost on your host is the same as localhost inside the container  so containerized service will act like non-containerized services and will be accessible without any additional configuration

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Use --net  host  in your docker run command  then localhost in your docker container will point to your docker host

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For all platforms Docker v 20 10 and above  since December 14th 2020  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  -  quot host docker internal host-gateway quot    For macOS and Windows 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 MacOS with earlier 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

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I ve explored the various solution and I find this the least hacky solution   Define a static IP address for the bridge gateway IP  Add the gateway IP as an extra entry in the 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   quot redis quot      extra hosts        -  quot dockerhost 172 20 0 1 quot   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  quot dockerhost quot

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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

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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  gt  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

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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

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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 lt UP BROADCAST RUNNING MULTICAST gt   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   quot 3 7 quot   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   quot 192 168 32 0 20 quot   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

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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

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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

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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  https   github com moby moby pull 40007 issuecomment-578729356 https   github com docker for-linux issues 264 issuecomment-598864064

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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

[docker] How to access host port from docker container

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