cntr

Say no to $ apt install vim in containers! cntr is a replacement for docker exec that brings all your developers tools with you. This is done by mounting the file system from one container or the host into the target container by creating a nested container with the help of a FUSE filesystem. This allows to ship minimal runtime image in production and limit the surface for exploits.

Cntr was also published in Usenix ATC 2018. See bibtex for citation.

Demo

In this two minute recording you learn all the basics of cntr:

Features

For convenience cntr supports container names/identifier for the following container engines natively:
- docker
- LXC
- LXD
- rkt
- systemd-nspawn
For other container engines cntr also takes process ids (PIDs) instead of container names.

Installation

Pre-build static-linked binary

For linux x86_64 we build static binaries for every release. More platforms can added on request. See the release tab for pre-build tarballs. At runtime only commandline utils of the container engine in questions are required.

Build from source

All you need for compilation is rust + cargo. Checkout rustup.rs on how to get a working rust toolchain. Then run:

Either:

console $ cargo install cntr

Or the latest master:

console $ cargo install --git https://github.com/Mic92/cntr

For offline builds we also provided a tarball with all dependencies bundled here for compilation with cargo-vendor.

Usage

At a high-level cntr provides two subcommands: attach and exec:

attach: Allows you to attach to a container with your own native shell/commands. Cntr will mount the container at /var/lib/cntr. The container itself will run unaffected as the mount changes are not visible to container processes.
- Example: cntr attach <container_id> where container_id can be a container identifier or process id (see examples below).
exec: Once you are in the container, you can also run commands from the container filesystem itself. Since those might need their native mount layout at / instead of /var/lib/cntr, cntr provides exec subcommand to chroot to container again and also resets the environment variables that might have been changed by the shell.
- Example: cntr exec <command> where command is an executable in the container

Note: Cntr needs to run on the same host as the container. It does not work if the container is running in a virtual machine while cntr is running on the hypervisor.

console $ cntr --help Usage: cntr COMMAND [ARGUMENTS ...] Enter or executed in container positional arguments: command Command to run (either "attach" or "exec") arguments Arguments for command optional arguments: -h,--help show this help message and exit

console $ cntr attach --help Usage: subcommand attach [OPTIONS] ID [COMMAND] [ARGUMENTS ...] Enter container positional arguments: id container id, container name or process id command command to execute after attach (default: $SHELL) arguments arguments passed to command optional arguments: -h,--help show this help message and exit --effective-user EFFECTIVE_USER effective username that should be owner of new created files on the host --type TYPE Container type (docker|lxc|rkt|process_id|nspawn, default: all)

console $ cntr exec --help Usage: subcommand exec [COMMAND] [ARGUMENTS ...] Execute command in container filesystem positional arguments: command command to execute (default: $SHELL) arguments Arguments to pass to command optional arguments: -h,--help show this help message and exit

Docker

1: Find out the container name/container id: console $ docker run --name boxbusy -ti busybox $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 55a93d71b53b busybox "sh" 22 seconds ago Up 20 seconds boxbusy

Either provide a container id...

console $ cntr attach 55a93d71b53b [root@55a93d71b53b:/var/lib/cntr]# echo "I am in a container!" [root@55a93d71b53b:/var/lib/cntr]# ip addr 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever 40: eth0@if41: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0 inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0 valid_lft forever preferred_lft forever [root@55a93d71b53b:/var/lib/cntr]# vim etc/resolv.conf

...or the container name. Use cntr exec to execute container native commands (while running in the cntr shell).

console $ cntr attach boxbusy [root@55a93d71b53b:/var/lib/cntr]# cntr exec sh -c 'busybox | head -1'

Also see Dockerfile.example on how to build a docker container with cntr.

LXD

1: Create a container and start it

2: Attach to the container with cntr

console $ cntr attach amazed-sailfish $ cat etc/hostname amazed-sailfish

LXC

1: Create a container and start it

console $ lxc-create --name ubuntu -t download -- -d ubuntu -r xenial -a amd64 $ lxc-start --name ubuntu -F ... Ubuntu 16.04.4 LTS ubuntu console ubuntu login: $ lxc-ls ubuntu

2: Attach to container with cntr:

console $ cntr attach ubuntu [root@ubuntu2:/var/lib/cntr]# cat etc/os-release NAME="Ubuntu" VERSION="16.04.4 LTS (Xenial Xerus)" ID=ubuntu ID_LIKE=debian PRETTY_NAME="Ubuntu 16.04.4 LTS" VERSION_ID="16.04" HOME_URL="http://www.ubuntu.com/" SUPPORT_URL="http://help.ubuntu.com/" BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/" VERSION_CODENAME=xenial UBUNTU_CODENAME=xenial

rkt

1: Find out the container uuid:

console $ rkt run --interactive=true docker://busybox $ rkt list UUID APP IMAGE NAME STATE CREATED STARTED NETWORKS c2d2e87e busybox registry-1.docker.io/library/busybox:latest running 6 minutes ago 6 minutes ago default:ip4=172.16.28.3

2: Attach with cntr

```console

make sure your container is still running!

$ cntr attach c2d2e87e

Finally not the old ugly top!

[gen0@rkt-c2d2e87e-e798-4341-ae93-26f6cbb7c017:/var/lib/cntr]# htop ... ```

With cntr you can also debug stage1 of rkt - even there is no support from rkt itself.

console $ ps aux | grep stage1 joerg 13546 0.0 0.0 120808 1608 pts/12 S+ 11:10 0:00 grep --binary-files=without-match --directories=skip --color=auto stage1 root 22232 0.0 0.0 54208 2656 pts/7 S+ 10:54 0:00 stage1/rootfs/usr/lib/ld-linux-x86-64.so.2 stage1/rootfs/usr/bin/systemd-nspawn --boot --notify-ready=yes --register=true --link-journal=try-guest --quiet --uuid=c2d2e87e-e798-4341-ae93-26f6cbb7c017 --machine=rkt-c2d2e87e-e798-4341-ae93-26f6cbb7c017 --directory=stage1/rootfs --capability=CAP_AUDIT_WRITE,CAP_CHOWN,CAP_DAC_OVERRIDE,CAP_FSETID,CAP_FOWNER,CAP_KILL,CAP_MKNOD,CAP_NET_RAW,CAP_NET_BIND_SERVICE,CAP_SETUID,CAP_SETGID,CAP_SETPCAP,CAP_SETFCAP,CAP_SYS_CHROOT -- --default-standard-output=tty --log-target=null --show-status=0

Therefore we use the process id instead of the container uuid:

```console $ cntr attach 22232

new and exiting territory!

[root@turingmachine:/var/lib/cntr]# mount | grep pods sysfs on /var/lib/cntr/var/lib/rkt/pods/run/c2d2e87e-e798-4341-ae93-26f6cbb7c017/stage1/rootfs/sys type sysfs (ro,nosuid,nodev,noexec,relatime) tmpfs on /var/lib/cntr/var/lib/rkt/pods/run/c2d2e87e-e798-4341-ae93-26f6cbb7c017/stage1/rootfs/sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755) cgroup on /var/lib/cntr/var/lib/rkt/pods/run/c2d2e87e-e798-4341-ae93-26f6cbb7c017/stage1/rootfs/sys/fs/cgroup/memory type cgroup (ro,nosuid,nodev,noexec,relatime,memory) ```

systemd-nspawn

1: Start container

console $ wget https://cloud-images.ubuntu.com/releases/16.04/release/ubuntu-16.04-server-cloudimg-amd64-root.tar.xz $ mkdir /var/lib/machines/ubuntu $ tar -xf ubuntu-16.04-server-cloudimg-amd64-root.tar.xz -C /var/lib/machines/ubuntu $ systemd-nspawn -b -M ubuntu $ machinectl list MACHINE CLASS SERVICE OS VERSION ADDRESSES ubuntu container systemd-nspawn ubuntu 16.04 -

2: Attach console $ cntr attach ubuntu

Generic process id

The minimal information needed by cntr is the process id of a container process you want to attach to.

```console

Did you now chromium uses namespaces too?

$ ps aux | grep 'chromium --type=renderer' joerg 17498 11.7 1.0 1394504 174256 ? Sl 15:16 0:08 /usr/bin/chromium ```

In this case 17498 is the pid we are looking for.

```console $ cntr attach 17498

looks quite similar to our system, but with less users

[joerg@turingmachine cntr]$ ls -la / total 240 drwxr-xr-x 23 nobody nogroup 23 Mar 13 15:05 . drwxr-xr-x 23 nobody nogroup 23 Mar 13 15:05 .. drwxr-xr-x 2 nobody nogroup 3 Mar 13 15:14 bin drwxr-xr-x 4 nobody nogroup 16384 Jan 1 1970 boot drwxr-xr-x 24 nobody nogroup 4120 Mar 13 14:56 dev drwxr-xr-x 52 nobody nogroup 125 Mar 13 15:14 etc drwxr-xr-x 3 nobody nogroup 3 Jan 8 16:17 home drwxr-xr-x 8 nobody nogroup 8 Feb 9 22:10 mnt dr-xr-xr-x 306 nobody nogroup 0 Mar 13 09:38 proc drwx------ 22 nobody nogroup 43 Mar 13 15:09 root ... ```

How it works

Cntr is container-agnostic: Instead of interfacing with container engines, it implements the underlying operating system API. It treats every container as a group of processes, that it can inherit properties from.

Cntr inherits the following container properties: * Namespaces (mount, uts, pid, net, cgroup, ipc) * Cgroups * Apparamor/selinux * Capabilities * User/group ids * Environment variables * The following files: /etc/passwd, /etc/hostname, /etc/hosts, /etc/resolv.conf

Under the hood it spawns a shell or user defined program that inherits the full context of the container and mount itself as a fuse filesystem.

We extensively evaluated the correctness and performance of cntr's filesystem using xfstests and a wide range of filesystem performance benchmarks (iozone, pgbench, dbench, fio, fs-mark, postmark, ...)

Related projects

nsenter
- Only covers linux namespaces and the user is limited to tools installed in the containers
toolbox
- Does attach from a container to the host, this is the opposite of what Cntr is doing

Bibtex

We published a paper with all technical details about Cntr in Usenix ATC 2018.

bibtex @inproceedings{cntr-atc18, author = {J{\"o}rg Thalheim and Pramod Bhatotia and Pedro Fonseca and Baris Kasikci}, title = {Cntr: Lightweight {OS} Containers}, booktitle = {2018 {USENIX} Annual Technical Conference ({USENIX} {ATC} 18)}, year = {2018}, }