Restarting named container assigns different IP

Question

Im a trying to deploy my application using Docker and came across an issue that restarting named containers assigns a different IP to container. Maybe explaining what I am doing will better explain the issue:

• Postgres runs inside a separate container named "postgres"
$ PG_ID=$(docker run --name postgres postgres/image)
• My webapp container links to postgres container
  $ APP_ID=$(docker run --link postgres:postgres webapp/image)

Linking postgres container image to webapp container inserts in webapp container a hosts file entry with the IP of the postgres container. This allows me to point to postgres db within my webapp using postgres:5432 (I am using Django btw). This all works well except if for some reason postgres crashes.

Before I manually stop postgres process to simulate postgres process crashing I verify IP of postgres container:

$ docker inspect --format "{{.NetworkSettings.IPAddress}}" $PG_ID
172.17.0.73

Now to simulate crash I stop postgres container:

$ docker stop $PG_ID

If now I restart postgres by using

$ docker start $PG_ID

the ip of the container changes:

$ docker inspect --format "{{.NetworkSettings.IPAddress}}" $PG_ID
172.17.0.74

Therefore the IP which points to postgres container in webapp container is no longer correct. I though that by naming container docker assigns a name to it with specific configs so that you can reliably link between containers (both network and volumes). If the IP changes this seems to defeat the purpose.

If I have to restart my webapp process each time I postgres restarts, this does not seem any better than just using a single container to run both processes. Then I can use supervisor or something similar to keep both of them running and use localhost to link between processes.

I am still new to Docker so am I doing something wrong or is this a bug in docker?

Sloat

You may find this related question & answer useful. – Etherize

thanks. will take a look at ddns. hopefully wont be that difficult. Docker seems very cool however it seems needs some learning curve to use it properly... – Sloat

Answer 1

2nd UPDATE: maybe you already discovered this, but as workaround, I plan to map the service to share the database to the host interface (ej: with -p 5432:5432), and connect the webapps to the host IP (the IP of the docker0 interface: in my Ubuntu and CentOS, the IP is 172.17.42.1). If you restart the postgres container, the conteiner's IP will change, but I wil be accesible using 172.17.42.1:5432. The downside is that you are exposing that port to all the containers, and loose the fine-grained mapping that --link gives you.

--- OLD UPDATES:

CORRECTION: Docker will map 'postgres' to the container's IP in the /etc/hosts files, on the webapp container. So, in the webapp container, you can ping 'postgres', and it will be mapped to the IP.

1st UPDATE: I've seen that Docker generates and mounts /etc/hosts, /etc/resolv.conf, etc. to have always the correct information, but this does not apply when the linked container is restarted. So, I've assumed (wrongly) that Docker would update the hosts files.

-- ORIGINAL (wrong) response:

Add --hostname=postgres-db (you can use anythin, I'm using something different than 'postgres' to avoid confussion with the container name):

$ docker run --name postgres --hostname postgres-db postgres/image

Docker will map 'postgres-db' to the container's IP (check the contents of /etc/hosts on the webapp container).

This will allow you run 'ping postgres-db' from the webapp container. If the IP changes, Dockers will update /etc/hosts for you.

In the Django app, use 'postgres-db' instead of the IP (or whatever you use for --hostname of the container with PostgreSql).

Bye! Horacio

Stralsund

please check your facts. Docker when linking container only uses --name parameter. --hostname is only used to insert an entry in the postgres container, not in the webappcontainer. – Sloat

miki725: you are right, I get confused in my tests. I have used the same string for name and hostname, and assumed that the specified in 'hostname' were used in /etc/hosts... – Stralsund

Answer 2

According to https://docs.docker.com/engine/reference/commandline/run/, it should be possible to assign a static IP for your container -- at the time of container creation -- using the --ip option:

Example:

docker run -itd --ip 172.30.100.104 --name postgres postgres/image

....where 172.30.100.104 is a free IP address on a custom bridge/overlay network.

This should then retain the same IP address even if postgres container crashes/restarts.

Looks like this was released in Docker Engine v 1.10 or greater, therefore if you have a lower version, you have to upgrade first.

Unriddle

Answer 3

As of Docker 1.0 they implemented a stronger sense of linked containers. Now you can use the container instance name as if it were the host name.

Here is a link

I found a link that better describes your problem. And while that question was answered I wonder whether or not this ambassador pattern might not solve the problem... this assumes that the ambassador is more reliable than the services that link.

Sims

please read my question. once the parent container is restarted, its IP changes hence whatever is in the hosts file is no longer valid. – Sloat