The term "container" (on Linux) refers to the high-level combination of two concrete low-level interfaces: namespaces and control groups (cgroups).
I like to think of namespaces as converting certain global variables in the kernel to local variables for each process. These local variables are then inherited to child processes. chroots are the simplest example, although they predate namespaces. Ordinarily you'd think of a system as having a single root directory; somewhere in the kernel is a global variable DIR root. But in fact, each process has its own root directory pointer in the process structure. Most of those pointers have the same value in every process, but if you run chroot, you change that pointer for the current process and all its children.
The list of possible namespaces is in the clone(2) and unshare(2) manpages (`man 2 clone` and `man 2 unshare`); look for the options starting with CLONE_NEW. They all change some pointer in the process structure, either to a substructure of the original pointer (like chroot does), a deep copy of the structure, or to a new, empty structure.
CLONE_NEWIPC changes the pointer for routing System V IPCs. CLONE_NEWNET changes the pointer for the list of network device to a new structure with just a new loopback interface. CLONE_NEWNS copies the mount table instead of keeping a pointer to it, so your process can unmount filesystems without affecting the rest of the system, or vice versa. CLONE_NEWPID changes the pointer to pid 1 / the process ID table to point to yourself (effectively a chroot for process IDs). CLONE_NEWUSER changes the interpretation of user IDs, so you can have UID 0 in your process be a non-zero UID in the outside system. CLONE_NEWUTS creates a copy of the structure containing the machine's hostname, instead of keeping a pointer to the global structure.
cgroups are resource control. They let you say that a certain process tree has some maximum amount of RAM, or CPU shares, or so forth. This is useful for making containers perform the way you want, but doesn't really affect their semantics.
I'd guess that it's just a matter of how you view it - putting a minimum number of CPU shares on the root cgroup is the same as putting a maximum on the rest of the cgroups, right? But maybe one or the other documentation is wrong.
Both manpage and documentation are probably right, though I'd expect CPU throttling to be significant enough at least to be mentioned.
I'm not so sure about your guess, because that apparently works only when CPU is fully utilized. I'm not so sure about my reading comprehension either.
How do the copies of the variables that become local relate to the originals? What happens at
read/write, what happens if the host OS changes them? In other words, what is the semantics of
sharing/copying these variables?
For a chroot, you get a pointer to a subdirectory of the host root directory. Changes within that directory are visible in both directions. CLONE_NEWPID and CLONE_NEWUSER work similarly; every process has a PID and a UID outside of the container (that is, in the root namespace), but a subset of PIDs and UIDs are visible in the container, with their own values. Creating a process in a PID namespace causes it to get a PID counting from 1 in that namespace, as well as a PID counting from 1 in the parent namespace. A user account in a user namespace has a value (which could be 0) in the namespace, as well as a mapped value in the parent namespace.
CLONE_NEWIPC and CLONE_NEWNET create new, empty structures for the IPC namespace and network stack. Changes in one namespace aren't visible to another. You can move network devices between namespaces by using `ip link set dev eth1 netns 1234`, which will move eth1 out of the current process's network namespace and into process 1234's namespace. (This is occasionally useful with physical devices, but more useful with virtual devices like veth and macvlan.)
CLONE_NEWNS and CLONE_NEWUTS create a deep copy of the current namespace's mount table and hostname/domainname strings, respectively. Further changes in one namespace do not affect the other.
Has that been written up somewhere in a suitably abstract form?
Is there a list of variables that are affected, and how they are affected.
In particular, I wonder about network interfaces. Say your hardware
has a network interface that's got MAC address 0b:21:b5:e2:11:22 and
IPv4 address 123.234.34.45, how are these addresses affeced by
cloning?
You get a completely separate network stack. None of the network devices are copied/cloned. You can move a network device into the container, but it's no longer accessible in the host.
Since most people don't have spare physical devices, there are a couple of approaches using virtual devices. You can create a "veth" device pair, which is basically a two-ended loopback connection. Move one end of the veth into the container, configure them as 192.168.1.1 and 2 (or whatever), and set up NAT. Or you can create a "macvlan" device, which hangs off an existing device and generates a new MAC address. Any traffic destined for the macvlan's MAC address goes to the macvlan device; any other traffic goes to the parent device. So I can move the macvlan into the container and assign it the address of 123.234.34.46, and it will ARP for that address using its own MAC address.
The container also has its own routing table, iptables (firewall) rule set, etc. And anyone listening on a broadcast interface in the host won't get packets destined to the container, or vice versa. It's basically like a virtual machine.
Thanks for the description. I guess container networking like Weave works by 'hijacking' veth and executing NAT-like address translation.
like a virtual machine.
That's surprising. Containers were advertised to me as being much more lightweight than conventional virtualisation (e.g. VMware, Xen), because the former, unlike the latter, share the ambient operating system.
completely separate network stack
What does that mean exactly, does the container copy the actual code, or is the network stack's code shared, just run in a separate address space?
I like to think of namespaces as converting certain global variables in the kernel to local variables for each process. These local variables are then inherited to child processes. chroots are the simplest example, although they predate namespaces. Ordinarily you'd think of a system as having a single root directory; somewhere in the kernel is a global variable DIR root. But in fact, each process has its own root directory pointer in the process structure. Most of those pointers have the same value in every process, but if you run chroot, you change that pointer for the current process and all its children.
The list of possible namespaces is in the clone(2) and unshare(2) manpages (`man 2 clone` and `man 2 unshare`); look for the options starting with CLONE_NEW. They all change some pointer in the process structure, either to a substructure of the original pointer (like chroot does), a deep copy of the structure, or to a new, empty structure.
CLONE_NEWIPC changes the pointer for routing System V IPCs. CLONE_NEWNET changes the pointer for the list of network device to a new structure with just a new loopback interface. CLONE_NEWNS copies the mount table instead of keeping a pointer to it, so your process can unmount filesystems without affecting the rest of the system, or vice versa. CLONE_NEWPID changes the pointer to pid 1 / the process ID table to point to yourself (effectively a chroot for process IDs). CLONE_NEWUSER changes the interpretation of user IDs, so you can have UID 0 in your process be a non-zero UID in the outside system. CLONE_NEWUTS creates a copy of the structure containing the machine's hostname, instead of keeping a pointer to the global structure.
cgroups are resource control. They let you say that a certain process tree has some maximum amount of RAM, or CPU shares, or so forth. This is useful for making containers perform the way you want, but doesn't really affect their semantics.