HOWTO: get Docker Containers under Centos 5 with Xen

Centos5 is getting long in the tooth, but then again, many of my servers are antiques that would find native Centos6 to be problematic.

A recent adventure in disaster recovery led me to upgrade several of my Xen DomU’s from CentOS 5 to CentOS 6, but I was distressed to discover that about the minimum you can get by with on RAM for CentOS6 is nearly 400MB. I wanted to host several CentOS6 VMs, but the thought of getting dinged to the tune of half-a-GByte of RAM plus several gigs of disk image didn’t sit well for lightweight systems.

The “in” thing for this kind of stuff is Containers, which neatly fit in the space between a full VM and something less capable such as a chroot jail. The question was, could I get CentOS 6 containers to work in a CentOS5 Dom0?

As a matter of fact, yes, and it was considerably less painful than expected!

I cheated and did the real dirty work using my desktop machine, which is running Fedora 20, hence is better supported for all the bleeding-edge tools. Actually, Ubuntu would probably be even better, but I’m at home with what I’ve got and besides, the idea is to make it as little work as possible given my particular working environment.

Step 1: Vagrant.

Vagrant is one of those products that everyone says is wonderful (including themselves), but it was hard to tell what it’s good for. As it turns out, what it’s good for is disposable VM’s.

Specifically, Vagrant allows the creation of VM “boxes” and the management of repositories of boxes. A “box” is a VM image plus the meta-data needed for Vagrant to deploy and run the VM.

So I yum-installed vagrant on my Fedora X86_64 system.

My selected victim was a basic CentOS 6 box, since for the VirtualBox VM environment.

vagrant box add centos65-x86_64-20131205

Step 2. Docker

It would have been more convenient to get a ready-made Centos6 Docker box, but most Docker-ready boxes in the public repo are for Ubuntu. So I did a “vagrant up” to cause the box image to download and launch, connected to the Centos6 guest, and Docker-ized it using this handy resource:

An alternative set of instructions:

The process is rather simple as long as you’re using the latest CentOS 6.5 release. Older kernels don’t have the necessary extensions, requiring a kernel upgrade first.

Step 3. Porting to Xen

Once docker was working, the challenge of getting the VM from VirtualBox to Xen presented itself. I was expecting a nightmare of fiddling with the disk image and generating a new initrd, but there was a pleasant surprise. All I had to do was convert the VM image from the “vmdk” format to a raw disk image, transfer the raw image to the Xen box, hack up a xen config and it booted right up!

The details:

On the Fedora desktop:

$ qemu-img convert -f vmdk virtualbox-centos6-containers.vmdk -O raw centos6-containers.img
$ rsync -avz --progress centos6-containers.img root@vmserver:/srv/xen/images/centos6-container/

File and directory names vary depending on your needs and preferences.

Now it’s time to connect to “vmserver”, which is my CentOS5 physical host.

I stole an existing XEN DomU pygrub-booting definition from another VM, changed the network and virtual disk definitions to provide a suitable environment. The disk definition itself looks like this:

disk = [ "tap:aio:/srv/xen/images/centos6-container/centos6-containers.img,xvda,w"]

xvda, incidentally is a standard Centos VM disk image, with a swap and LVM partition inside.

I launched the VM and behold! a Centos 6 Docker container DomU on a CentOS 5 Dom0 base.

Everything should be this easy.

[SOLVED] mail loops back to me (MX problem?) for virtual machine

Sometimes they just gang up on you.

I was migrating my sendmail server from a NAT address to a bridge address when it all started.

Xen has this really nasty habit of zapping your hardware MAC address if you don’t get the nat routing configure just right. There’s obviously some way to get it to revert, because occasionally for no obvious reason, the real MAC address will revert, but don’t try searching the web for an answer – all you’ll get is fruitless inquiries and flame responses (you shouldn’t be changing your MAC address, idiot!). Please. There are very good reasons why it’s useful to be able to set a custom MAC address. One place I worked coded their hardware asset IDs in the MAC to assist their DHCP server, for instance.

On the mousetech domain, I’d be happier if it didn’t happen. As it is, the MAC addresses of the primary and secondary NICs got swapped and I didn’t find out until I’d gotten most of the way through fixing things. So the former eth0 became eth1 and vice versa.

Shortly thereafter, outbound mail started bouncing with the infamous “mail loops back to me” message. Since I’d just done major relocation on the mail VM, I wasted a LOT of time messing around with sendmail options to no avail. Normally this message can be cured by putting in a valid MX record in DNS and/or adding all the mailserver alias names to the sendmail local-host-table (cw table). Not this time.

I was fairly sure that the problem had something to do with the fact that the physical host had been set up to forward all port 25 (smtp) requests to the mail VM and that somehow the wrong IP address was getting mixed in, but I could see the actual routing since it was all internal and specific to port 25 to boot.

Turns out I’d been sloppy when I fixed up the iptables forwarding. The correct version (after the NIC mixup) looks like this:

-A PREROUTING -i eth0 -p tcp --dport 25 -j DNAT --to-destination

Where I went wrong was in being lazy and omitting the NIC ID (eth0) when I repaired the damage that Xen did. As a result, BOTH NICs were being re-routed – the actual internet-facing NIC (which should be routed) and the internal DMZ bridge-facing NIC (which should not). As a result, traffic on port 25 for eth1 was being routed back on itself and sendmail complained.