Access docker container from host using containers name

Question

I am developing a service and using there docker compose to spin services like postgres, redis, elasticsearch. I have a web application that is based on RubyOnRails and writes and reads from all those services.

Here is my docker-compose.yml

version: '2'  services:   redis:     image: redis:2.8     networks:       - frontapp    elasticsearch:     image: elasticsearch:2.2     networks:       - frontapp    postgres:       image: postgres:9.5     environment:       POSTGRES_USER: elephant       POSTGRES_PASSWORD: smarty_pants       POSTGRES_DB: elephant     volumes:       - /var/lib/postgresql/data     networks:       - frontapp  networks:   frontapp:     driver: bridge 

And i can ping containers within this network

$ docker-compose run redis /bin/bash root@777501e06c03:/data# ping postgres PING postgres (172.20.0.2): 56 data bytes 64 bytes from 172.20.0.2: icmp_seq=0 ttl=64 time=0.346 ms 64 bytes from 172.20.0.2: icmp_seq=1 ttl=64 time=0.047 ms ... 

So far so good. Now I want to run ruby on rails application on my host machine but be able to access postgres instance with url like postgresql://username:password@postgres/database currently that is not possible

$ ping postgres ping: unknown host postgres 

I can see my network in docker

$ docker network ls NETWORK ID          NAME                DRIVER ac394b85ce09        bridge              bridge               0189d7e86b33        elephant_default    bridge               7e00c70bde3b        elephant_frontapp   bridge               a648554a72fa        host                host                 4ad9f0f41b36        none                null  

And I can see an interface to it

$ ifconfig  br-0189d7e86b33 Link encap:Ethernet  HWaddr 02:42:76:72:bb:c2             inet addr:172.18.0.1  Bcast:0.0.0.0  Mask:255.255.0.0           inet6 addr: fe80::42:76ff:fe72:bbc2/64 Scope:Link           UP BROADCAST MULTICAST  MTU:1500  Metric:1           RX packets:36 errors:0 dropped:0 overruns:0 frame:0           TX packets:60 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:0            RX bytes:2000 (2.0 KB)  TX bytes:8792 (8.7 KB)  br-7e00c70bde3b Link encap:Ethernet  HWaddr 02:42:e7:d1:fe:29             inet addr:172.20.0.1  Bcast:0.0.0.0  Mask:255.255.0.0           inet6 addr: fe80::42:e7ff:fed1:fe29/64 Scope:Link           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1           RX packets:1584 errors:0 dropped:0 overruns:0 frame:0           TX packets:1597 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:0            RX bytes:407137 (407.1 KB)  TX bytes:292299 (292.2 KB) ... 

But i am not sure what should I do next. I tried to play a bit with /etc/resolv.conf, mainly with nameserver directive, but that had no effect.

I would appreciate any help of suggestions how to configure this setup correctly.

UPDATE

After browsing through Internet resources I managed to assign static IP addresses to boxes. For now it is enough for me to continue development. Here is my current docker-compose.yml

version: '2'  services:   redis:     image: redis:2.8     networks:       frontapp:         ipv4_address: 172.25.0.11    elasticsearch:     image: elasticsearch:2.2     networks:       frontapp:         ipv4_address: 172.25.0.12    postgres:       image: postgres:9.5     environment:       POSTGRES_USER: elephant       POSTGRES_PASSWORD: smarty_pants       POSTGRES_DB: elephant     volumes:       - /var/lib/postgresql/data     networks:       frontapp:         ipv4_address: 172.25.0.10  networks:   frontapp:     driver: bridge     ipam:       driver: default       config:         - subnet: 172.25.0.0/16           gateway: 172.25.0.1 

Answer 1

There is a opensource application that solves this issue, it's called DNS Proxy Server, here some examples from official repository

It's a DNS server that solves containers hostnames, if could not found a hostname that matches then solve it from internet as well

Start DNS Server

$ docker run --hostname dns.mageddo --restart=unless-stopped -p 5380:5380 \ -v /var/run/docker.sock:/var/run/docker.sock \ -v /etc/resolv.conf:/etc/resolv.conf \ defreitas/dns-proxy-server 

It will be set automatically as your default DNS (and recover to the original when it stops)

Creating some containers for test

checking docker-compose file

$ cat docker-compose.yml version: '3' services:   nginx-1:     image: nginx     hostname: nginx-1.docker     network_mode: bridge    linux-1:     image: alpine     hostname: linux-1.docker     command: sh -c 'apk add --update bind-tools && tail -f /dev/null'     network_mode: bridge # that way he can solve others containers names even inside, solve nginx-2, for example 

starting containers

$ docker-compose up 

Solving containers

from host

nslookup nginx-1.docker Server:     13.0.0.5 Address:    13.0.0.5#53 Non-authoritative answer: Name:   nginx-1.docker Address: 13.0.0.6 

from another container

$ docker-compose exec linux-1 ping nginx-1.docker PING nginx-1.docker (13.0.0.6): 56 data bytes 64 bytes from 13.0.0.6: seq=0 ttl=64 time=0.034 ms 

As well it solves internet hostnames

$ nslookup google.com Server:     13.0.0.5 Address:    13.0.0.5#53  Non-authoritative answer: Name:   google.com Address: 216.58.202.78 

Answer 2

I'm using a bash script to update /etc/hosts. Why this solution?

• Short script, easy to review (didn't want to give some un-reviewed application with lots of dependencies access to the Docker socket (which means root access))
• It uses docker events to run every time a container is started or stopped (other solutions posted here run every second in a loop, which is way less efficient)
• Updates /etc/hosts, no separate DNS server needed.
• Only dependencies are bash, mktemp, grep, xargs, sed, jq and docker, all of which I had already installed.

Just put the script somewhere, e.g. /usr/local/bin/docker-update-hosts:

#!/usr/bin/env bash set -e -u -o pipefail  hosts_file=/etc/hosts begin_block="# BEGIN DOCKER CONTAINERS" end_block="# END DOCKER CONTAINERS"  if ! grep -Fxq "$begin_block" "$hosts_file"; then     echo -e "\n${begin_block}\n${end_block}\n" >> "$hosts_file" fi  (echo "| container start |" && docker events) | \ while read event; do     if [[ "$event" == *" container start "* ]] || [[ "$event" == *" network disconnect "* ]]; then         hosts_file_tmp="$(mktemp)"         docker container ls -q | xargs -r docker container inspect | \         jq -r '.[]|"\(.NetworkSettings.Networks[].IPAddress|select(length > 0) // "# no ip address:") \(.Name|sub("^/"; "")|sub("_1$"; ""))"' | \         sed -ne "/^${begin_block}$/ {p; r /dev/stdin" -e ":a; n; /^${end_block}$/ {p; b}; ba}; p" "$hosts_file" \         > "$hosts_file_tmp"         chmod 644 "$hosts_file_tmp"         mv "$hosts_file_tmp" "$hosts_file"     fi done 

Note: The script removes the _1 suffix added by docker-compose from container names. If you don't want that just remove |sub("_1$"; "") from the script.

You can use a systemd service to run this synchronously with Docker: /etc/systemd/system/docker-update-hosts.service:

[Unit] Description=Update Docker containers in /etc/hosts Requires=docker.service After=docker.service PartOf=docker.service  [Service] ExecStart=/usr/local/bin/docker-update-hosts  [Install] WantedBy=docker.service 

To activate, run:

systemctl daemon-reload systemctl enable docker-update-hosts.service systemctl start docker-update-hosts.service