Connecting across containers in Docker network - docker

Question

I have this:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
e3252abd7587 cdt-tests "/bin/bash /home/new…" 5 seconds ago Exited (1) 22 seconds ago cdt-tests
f492760705e3 cdt-server "/bin/bash /usr/loca…" 52 seconds ago Up About a minute 0.0.0.0:3040->3040/tcp cdt-server
89c5a28855df mongo "docker-entrypoint.s…" 55 seconds ago Up About a minute 27017/tcp cdt-mongo
1eaa4aa684a9 selenium/standalone-firefox:3.4.0-chromium "/opt/bin/entry_poin…" 59 seconds ago Up About a minute 4444/tcp cdt-selenium
the cdt-tests container, is attempting to make connections to the other containers in the same network. the network looks like this:
$ docker network inspect cdt-net # this yields the below json
[
{
"Name": "cdt-net",
"Id": "8c2b486e950076130860e0c6c09f06eaf8cccf02127786b80bf7cc169f8eae0f",
"Created": "2018-01-23T21:52:34.5021152Z",
"Scope": "local",
"Driver": "bridge",
"EnableIPv6": false,
"IPAM": {
"Driver": "default",
"Options": {},
"Config": [
{
"Subnet": "172.18.0.0/16",
"Gateway": "172.18.0.1"
}
]
},
"Internal": false,
"Attachable": false,
"Ingress": false,
"ConfigFrom": {
"Network": ""
},
"ConfigOnly": false,
"Containers": {
"1eaa4aa684a9d7c1ad7a1b7ac28418b100e6b8d7a22ceb2a97cf51e4487c5fb2": {
"Name": "cdt-selenium",
"EndpointID": "674ce85e14339e67e767ab9844cd2fd1356fc3e60ab050e1cd1457e4168ac9fc",
"MacAddress": "02:42:ac:12:00:02",
"IPv4Address": "172.18.0.2/16",
"IPv6Address": ""
},
"89c5a28855dfde05fe9db9a35bbe7bce232eb56d9024022785d2a65570c423b5": {
"Name": "cdt-mongo",
"EndpointID": "ed497939965363cd194b4fea8e6a26ee47ef7f24bef56c9726003a897be83dd1",
"MacAddress": "02:42:ac:12:00:03",
"IPv4Address": "172.18.0.3/16",
"IPv6Address": ""
},
"f492760705e30be4fe8469ae422e96548ee2192f41314e3815762a9e39a4cc82": {
"Name": "cdt-server",
"EndpointID": "17e8bd6f7735a52669f0fe92b2d5717c7a3ae6954c108af3f29c13233ef20ee6",
"MacAddress": "02:42:ac:12:00:04",
"IPv4Address": "172.18.0.4/16",
"IPv6Address": ""
}
},
"Options": {},
"Labels": {}
}
]
in my cdt-tests container, I run these commands:
export CDT_SELENIUM_HOST="cdt-selenium.cdt-net";
export OPENSHIFT_NODEJS_IP="127.0.0.1";
export OPENSHIFT_NODEJS_PORT="3040";
export CDT_SERVER_HOST="127.0.0.1";
export CDT_SERVER_PORT=3040;
export OPENSHIFT_MONGODB_DB_HOST="127.0.0.1"
export OPENSHIFT_MONGODB_DB_PORT=27017
nc -zv "$CDT_SELENIUM_HOST" 4444 > /dev/null 2>&1
nc_exit=$?
if [ ${nc_exit} -eq 0 ]; then
echo "selenium server is live"
else
echo "selenium server is NOT live"
exit 1;
fi
nc -zv "$OPENSHIFT_MONGODB_DB_HOST" 27017 > /dev/null 2>&1
nc_exit=$?
if [ ${nc_exit} -eq 0 ]; then
echo "mongo server is live"
else
echo "mongo server is NOT live"
exit 1;
fi
nc -zv "$CDT_SERVER_HOST" 3040 > /dev/null 2>&1
nc_exit=$?
if [ ${nc_exit} -eq 0 ]; then
echo "cdt server is live"
else
echo "cdt server is NOT live"
exit 1;
fi
and all of those connection tests fail. Does anyone know how to connect between containers in the same Docker network? is there some surefire pattern to use?

Answer 1

It looks like you're trying to use 127.0.0.1 as the address for your other containers. Docker containers all have a unique ip address in an isolated network space. Much like your own physical host, 127.0.0.1 is a special address that means "this container". So since none of those services are running in the container in which you're running your tests, you can't connect to anything.
You need to use the ip address of the container running the service you want to test. Because ip addresses change with every deployment, it's not convenient to use the literal address. You need some way to get the information dynamically. For this reason, Docker maintains a DNS service on each network so that you can simply use the name of a container like any other hostname.
For example, in your environment, you could set:
export OPENSHIFT_MONGODB_DB_HOST="cdt-mongo"
And then your mongo test should succeed. And so forth for the other _HOST and _IP variables you're using.

Answer 2

I could assure you that the way #larsks mentioned would work well.
But there is still an easy way. I could see container cdt-tests is in the same network with other containers based on your description above. So you may attach all containers to a user-defined bridge network created by docker network create -d bridge mynet.
When starting containers, just add --net=mynet flag.
E.g.docker run -tid --name=cdt-server --net=mynet cdt-server .
Thus, there is no need to add any ENVs into your container(s). Since user-defined bridge networks provide automatic DNS resolution between containers. You may see introduction here.