Selfhosted @lemmy.world grue @lemmy.world 2y ago

Why do some of my containers register their hostname with my DHCP server, but others don't?

...and more importantly, where is that setting stored so I can turn it on for all of them?

I realize this varies according to OS, so I'm specifically asking about my Turnkey Nextcloud and Turnkey Mediaserver containers, which are based on Debian Linux. It's perhaps worth noting that my Turnkey Syncthing container, which uses the same base OS, does register its hostname with my DHCP server. I've gone digging around in /etc/ etc. in each of the containers, but so far I haven't found any configuration differences that would explain the difference in behavior. (Also, if it matters, my DHCP server is the one included with OpenWRT and running on my router.)

By the way, I'm aware that the best answer might be "you have an X/Y problem and you really ought to use static IPs and/or setup a reverse proxy," but I haven't gotten to that point yet. Besides, I still want to satisfy my curiosity about DHCP and hostnames regardless.

EDIT: to be clear, this post is about LXC containers running directly in Proxmox, NOT Docker.

12 comments

You can pass hostnames in docker compose easily with

hostname: myhostname

Sometimes you can use container names too

container_name: myapp

It is that easy ☺️
- I'm asking about LXC containers, not Docker containers.
  
  Okay this is easy too to set a hostname on debian or ubuntu Container:
  
  hostnamectl set-hostname myname
The hostname resolution might be being done via mdns (https://en.m.wikipedia.org/wiki/Multicast_DNS). That is seperate to your actual DNS server, and I don't think you can centrally disable it. You would have to disable Avahi/ZeroConf/Bonjour on your computer to prevent resolution, or find a way to prevent the containers registering themselves?

https://linux.die.net/man/1/avahi-browse - might help you determine if it's being registered via mdns.
I’ll give you a pointer, the rest is up to you how to apply that in LXC

https://www.cyberciti.biz/faq/howto-get-linux-static-dhcp-address/
I don't have any knowledge about the specific containers, but the way this works on my Linux distro is via the:

send host-name "(hostname)";

option in dhclient.conf. Maybe you could try explicitly setting that option?
- That's part of what's confusing me: none of the containers even have an /etc/dhcp/dhclient.conf (and I don't think it's anywhere else either because $PATH_DHCLIENT_CONF is unset).
https://linuxcontainers.org/lxc/manpages/man5/lxc.container.conf.5.html

Maybe this can point you in the right direction.

I'm not familiar with LXC - do you put together your container from scratch kinda like a FreeBSD jail?

Where is that DHCP server? Are you sure they get actual DHCP leases?

Typically Docker's internal resolution is basically just injecting the hostnames in /etc/hosts, but Docker also supports macvlan and macvtap networks too which can expose a container network directly on the network.

But I think 99% of containers just use the default bridge network. I never heard of a Docker container running a DHCP client, most containers run the app process directly and don't do a full boot.

Where is that DHCP server? Are you sure they get actual DHCP leases?

The DHCP server is dnsmasq running on my OpenWRT router.

The "Active DHCP Leases" table has entries for the Syncthing and the Nextcloud containers (the former with the hostname listed as "syncthing" and the latter with it listed as "?"). Weirdly, it does not have an entry for the Mediaserver container, even though that container is getting a valid IP.

I also looked through the system logs and found DHCP-related messages for each (Nextcloud is .120, Mediaserver is .159, and Syncthing is .191):

Fri Jul 28 22:47:28 2023 daemon.info dnsmasq-dhcp[4023]: DHCPDISCOVER(br-lan) be:d2:c2:da:ba:fc
Fri Jul 28 22:47:28 2023 daemon.info dnsmasq-dhcp[4023]: DHCPOFFER(br-lan) 192.168.0.120 be:d2:c2:da:ba:fc
Fri Jul 28 22:47:28 2023 daemon.info dnsmasq-dhcp[4023]: DHCPDISCOVER(br-lan) be:d2:c2:da:ba:fc
Fri Jul 28 22:47:28 2023 daemon.info dnsmasq-dhcp[4023]: DHCPOFFER(br-lan) 192.168.0.120 be:d2:c2:da:ba:fc
Fri Jul 28 22:47:28 2023 daemon.info dnsmasq-dhcp[4023]: DHCPREQUEST(br-lan) 192.168.0.120 be:d2:c2:da:ba:fc
Fri Jul 28 22:47:28 2023 daemon.info dnsmasq-dhcp[4023]: DHCPACK(br-lan) 192.168.0.120 be:d2:c2:da:ba:fc


Wed Jul 26 21:45:18 2023 daemon.info dnsmasq-dhcp[4023]: DHCPDISCOVER(br-lan) a6:b6:02:9b:bd:e5
Wed Jul 26 21:45:18 2023 daemon.info dnsmasq-dhcp[4023]: DHCPOFFER(br-lan) 192.168.0.159 a6:b6:02:9b:bd:e5
Wed Jul 26 21:45:18 2023 daemon.info dnsmasq-dhcp[4023]: DHCPREQUEST(br-lan) 192.168.0.159 a6:b6:02:9b:bd:e5
Wed Jul 26 21:45:18 2023 daemon.info dnsmasq-dhcp[4023]: DHCPACK(br-lan) 192.168.0.159 a6:b6:02:9b:bd:e5


Wed Jul 26 21:45:14 2023 daemon.info dnsmasq-dhcp[4023]: DHCPDISCOVER(br-lan) 1e:44:37:54:4a:f8
Wed Jul 26 21:45:14 2023 daemon.info dnsmasq-dhcp[4023]: DHCPOFFER(br-lan) 192.168.0.191 1e:44:37:54:4a:f8
Wed Jul 26 21:45:14 2023 daemon.info dnsmasq-dhcp[4023]: DHCPDISCOVER(br-lan) 1e:44:37:54:4a:f8
Wed Jul 26 21:45:14 2023 daemon.info dnsmasq-dhcp[4023]: DHCPOFFER(br-lan) 192.168.0.191 1e:44:37:54:4a:f8
Wed Jul 26 21:45:14 2023 daemon.info dnsmasq-dhcp[4023]: DHCPREQUEST(br-lan) 192.168.0.191 1e:44:37:54:4a:f8
Wed Jul 26 21:45:14 2023 daemon.info dnsmasq-dhcp[4023]: DHCPACK(br-lan) 192.168.0.191 1e:44:37:54:4a:f8 syncthing

Typically Docker’s...

Again, this post is about LXC containers running directly in Proxmox, NOT DOCKER.

12 comments