Docker Container: Unable to connect to SQL Server by name - sql-server

Question

I created two container on the same network and one of them as a Sql Server instance running. In the other container (with SQL Tools) i'm able to connect to the SQL using the IP Address, but if i swith to machine name it fails.
I already tried do ping the machine and the dns is solving the right IP, i also tried dnslookup and it's also working. Does anyone as a clue on how to fix this?
Full test scenario:
Created new network
docker network create --driver=bridge specsnet
Run SQL Container
docker run -e 'ACCEPT_EULA=Y' -e 'SA_PASSWORD=Password!123456' -p 1401:1433 -d --name=TestDBServer --net=specsnet --rm microsoft/mssql-server-linux:latest
Run New Container with SQL Tools (to test connection)
docker run -it --net=specsnet --rm --entrypoint "bash" mcr.microsoft.com/mssql-tools:latest
Loaded some tools for troubleshooting (into SQL Tools container)
apt-get update | apt-get install telnet -y | apt-get install iputils-ping -y | apt-get install dnsutils -y | apt-get install nmap -y | apt-get install nano -y
Test Connection with IP (Success - IP was 172.18.0.2)
sqlcmd -S tcp:172.18.0.2,1433 -U sa -P 'Password!123456'
Test Connection with Name (Fails)
sqlcmd -S tcp:TestDBServer,1433 -U sa -P 'Password!123456'

Answer 1

So as Bjoern suggested i created a docker compose file and after doing some test i realized the issue was not fixed.
Then i started to manipulate the file, tweaking the properties, and discovered the problem was on the SQL container name (the container name had upper case letters). I set the SQL container name to "testdbserver" and everything worked fine.
Docker Compose File
version: '2'
services:
testdbserver:
image: microsoft/mssql-server-linux:latest
ports:
["1401:1433"]
environment:
SA_PASSWORD: Password!123456
ACCEPT_EULA: Y
networks:
- specsnet
sqltools:
image: mcr.microsoft.com/mssql-tools:latest
depends_on:
- testdbserver
networks:
- specsnet
networks:
specsnet:
driver: bridge
ipam:
config:
- subnet: 10.5.0.0/16
gateway: 10.5.0.1
Start SQL Tools Container on Bash Mode
docker-compose run sqltools bash
Execute SQL Test Connection (works now)
sqlcmd -S tcp:testdbserver,1433 -U sa -P 'Password!123456'

Answer 2

The TestDBServer part of your sqlcmd references a so-called Server Alias. These server aliases are not accessible due to the way you have set up the Docker networking currently. If you switch to Docker compose to set up the networking for you it should work.
An alternative approach would be to --link the containers in question together.

Answer 3

I had the same issue when using a gateway and a microservice:
Create network:
docker network create mynet
Add containers:
docker network connect mynet my-gateway
docker network connect mynet my_service_8080
Doing so, I wasn't able to communicate between both containers when using container name. E.g. when connecting to the gateway container and doing a curl to the service:
curl -i http://my_service_8080:8080/management/info
HTTP/1.1 400
Transfer-Encoding: chunked
Date: Tue, 23 Feb 2021 12:51:47 GMT
Connection: close
After doing a lot of research on the net which led to nothing, I finally had the idea to provide an alias name:
docker network connect --alias gateway mynet my-gateway
docker network connect --alias myservice mynet my_service_8080
Now, I can successfully communicate between the containers by using alias name as host name:
curl -i http://myservice:8080/management/info
HTTP/1.1 200
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0
X-Frame-Options: DENY
Content-Type: application/vnd.spring-boot.actuator.v2+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Tue, 23 Feb 2021 12:54:36 GMT
{"name":"myservice-app","description":"my service","version":"20210202"}
EDIT:
After some more digging, I found that communication using container names doesn't work if the name (or alias) contains an underline character. So if you experience problems, check your container names.