ubuntu - Allow docker container to connect to a local/host postgres database

Question

I've recently been playing around with Docker and QGIS and have installed a container following the instructions in this tutorial.

Everything works great, although I am unable to connect to a localhost postgres database that contains all my GIS data. I figure this is because my postgres database is not configured to accept remote connections and have been editing the postgres conf files to allow remote connections using the instructions in this article.

I'm still getting an error message when I try and connect to my database running QGIS in Docker: could not connect to server: Connection refused Is the server running on host "localhost" (::1) and accepting TCP/IP connections to port 5433? The postgres server is running, and I've edited my pg_hba.conf file to allow connections from a range of IP addresses (172.17.0.0/32). I had previously queried the IP address of the docker container using docker ps and although the IP address changes, it has so far always been in the range 172.17.0.x

Any ideas why I can't connect to this database? Probably something very simple I imagine!

I'm running Ubuntu 14.04; Postgres 9.3

Answer 1

TL;DR

1. Use 172.17.0.0/16 as IP address range, not 172.17.0.0/32.
2. Don't use localhost to connect to the PostgreSQL database on your host, but the host's IP instead. To keep the container portable, start the container with the --add-host=database:<host-ip> flag and use database as hostname for connecting to PostgreSQL.
3. Make sure PostgreSQL is configured to listen for connections on all IP addresses, not just on localhost. Look for the setting listen_addresses in PostgreSQL's configuration file, typically found in /etc/postgresql/9.3/main/postgresql.conf (credits to @DazmoNorton).

Long version

172.17.0.0/32 is not a range of IP addresses, but a single address (namly 172.17.0.0). No Docker container will ever get that address assigned, because it's the network address of the Docker bridge (docker0) interface.

When Docker starts, it will create a new bridge network interface, that you can easily see when calling ip a:

$ ip a
...
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN 
    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

As you can see, in my case, the docker0 interface has the IP address 172.17.42.1 with a netmask of /16 (or 255.255.0.0). This means that the network address is 172.17.0.0/16.

The IP address is randomly assigned, but without any additional configuration, it will always be in the 172.17.0.0/16 network. For each Docker container, a random address from that range will be assigned.

This means, if you want to grant access from all possible containers to your database, use 172.17.0.0/16.