Nginx reverse proxy not finding other internal Docker container using hostname - docker

Question

I have two docker containers. One runs Kestrel (172.17.0.3), The other runs Nginx (172.17.0.4) using a reverse proxy to connect to Kestrel. Nginx connects fine when I use internal Docker ip of Kestrel container but when I try to connect to Kestrel using container's hostname in nginx.conf (kestral) I get following error:
2020/06/30 00:23:03 [emerg] 58#58: host not found in upstream "kestrel" in /etc/nginx/nginx.conf:7
nginx: [emerg] host not found in upstream "kestrel" in /etc/nginx/nginx.conf:7
I launched containers with these two lines
docker run -d --name kestrel --restart always -h kestrel mykestrelimage
docker run -d --name nginx --restart always -p 80:80 -h nginx mynginximage
My nginx.conf file below.
http {
# I've tried with and without line below that I found on Stackoverflow
resolver 127.0.0.11 ipv6=off;
server {
listen 80;
location / {
# lines below don't work
# proxy_pass http//kestrel:80;
# proxy_pass http//kestrel
# proxy_pass http//kestrel:80/;
# proxy_pass http//kestrel/;
# when I put internal docker ip of Kestrel server works fine
proxy_pass http://172.17.0.3:80/;
}
}
}
events {
}

Answer 1

I figured out solution to my problem. There were two issues.
First problem: By default Docker uses default bridge network when creating containers. The default Docker bridge network does not resolve DNS though. You have to create a custom bridge network and then specify network when creating docker containers. The below allowed me to ping between containers using hostname
docker network create --driver=bridge mycustomnetwork
docker run -d --name=kestrel --restart=always -h kestrel.local --network=mycustomnetwork mykestrelimage
docker run -d --name=nginx --restart always -p 80:80 -h nginx.local --network=mycustomnetwork mynginximage
Second problem: Even though it was only one kestrel server for some reason Nginx required that I setup an upstream section in /etc/nginx/nginx.conf
http {
upstream backendservers {
server kestrel;
}
server {
listen 80;
location / {
proxy_pass http://backendservers/;
}
}
}
events {
}