How can I reach servers on the network from my swift docker (5.6.1) container?


I'm trying to retrieve some json from a server in our network.
The problem I'm having is that I get an error: 'Could not resolve host:'
I'm using the 'swift:5.6.1-slim' image as the source for my container.

So my question is, Is there any special I need to do to my container to get access to servers on the network?

(If I spin up a simple busybox container, I can ping devices on the network and even Internet just fine. 'docker run -dit --name alpine1 alpine ash')

	func fetch(url: URL, headers: [String: String] = [:]) {
		var request = URLRequest(url: url)
		request.httpMethod = "GET"
		request.allHTTPHeaderFields = headers

		URLSession.shared.dataTask(with: request) { (data, response, error) in
			print(error?.localizedDescription ?? "")
			guard let data = data else {
				print("No Data")
			print(String(data: data, encoding: .utf8)!)
			print("Hello, World, It's \(Date())")

I started my container as follows:
docker run -dit --rm --name NowBoarding now-boarding:0.1.0.RC1

P.s. These are my first steps at using swift in a docker container...

I'd avoid the Swift question for now and just establish a shell within that container and see if you can use ping to hit the server. If it fails the DNS lookup, try taking a look at the contents of /etc/hosts and see how you're doing your DNS.

This seems highly likely to be relying on a corporate DNS server that is not getting propagated into the docker container.

I tried that, using "docker run -it --rm swift /bin/bash" and then do a ping, but the image doesn't have the ping command.

root@cadff2ae8c3e:/# ping
bash: ping: command not found

Can it be that this image wasn't meant to do outgoing traffic?

P.s. the hosts file:

root@cadff2ae8c3e:/# cat /etc/hosts	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::0	ip6-localnet
ff00::0	ip6-mcastprefix
ff02::1	ip6-allnodes
ff02::2	ip6-allrouters	cadff2ae8c3e

Sorry, I meant /etc/resolv.conf. Lazy morning.

As for ping, no, it just means it's not installed. You can do an apt-get update and an apt-get install iputils-ping to get it installed.

Installed ping to test and you were correct. The container has networking capabilities.

I modified my code to download a json from a server on the internet and that succeeds. Downloading it from the server in our company's network still fails....

So I guess I have to put our company's DNS server address in /etc/resolv.conf I guess?

docker run -it --rm now-boarding:0.1.0.RC1 cat /etc/resolv.conf                                                                                          ─╯
# DNS requests are forwarded to the host. DHCP DNS options are ignored.

Or do I have to do something in docker itself (because the address is something from docker)?

I suspect docker is attempting to populate that with something from the host but it's not sufficient. is going to be a local network connection of some kind. Are you running Docker for Mac, Docker for Windows, or Docker on Linux?

Working on a mac here.
You've been a great help. I got it working now.

Container networking | Docker Documentation says I should pass the DNS server address using the --dns argument and that works. I can now access the resource I wanted.

This was the trick:

docker run -it --dns x.x.x.x --name NowBoarding now-boarding:0.1.0.RC1

Thanks for your swift help here ;-)