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docs: add information about terminals

Users can get very confused by how terminals work with runc, and the
quite confusing "terminal: ..." option. Add a document which goes
through all of the important parts of terminal handling in runc, in the
hopes that we can just point people to this as an explanation.

Signed-off-by: Avi Deitcher <>
[cyphar: quite a large rewrite to fix factual errors and structure]
Co-authored-by: Avi Deitcher <>
Signed-off-by: Aleksa Sarai <>
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cyphar and deitch committed Feb 22, 2018
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# Terminals and Standard IO #
*Note that the default configuration of `runc` (foreground, new terminal) is
generally the best option for most users. This document exists to help explain
what the purpose of the different modes is, and to try to steer users away from
common mistakes and misunderstandings.*
In general, most processes on Unix (and Unix-like) operating systems have 3
standard file descriptors provided at the start, collectively referred to as
"standard IO" (`stdio`):
* `0`: standard-in (`stdin`), the input stream into the process
* `1`: standard-out (`stdout`), the output stream from the process
* `2`: standard-error (`stderr`), the error stream from the process
When creating and running a container via `runc`, it is important to take care
to structure the `stdio` the new container's process receives. In some ways
containers are just regular processes, while in other ways they're an isolated
sub-partition of your machine (in a similar sense to a VM). This means that the
structure of IO is not as simple as with ordinary programs (which generally
just use the file descriptors you give them).
## Other File Descriptors ##
Before we continue, it is important to note that processes can have more file
descriptors than just `stdio`. By default in `runc` no other file descriptors
will be passed to the spawned container process. If you wish to explicitly pass
file descriptors to the container you have to use the `--preserve-fds` option.
These ancillary file descriptors don't have any of the strange semantics
discussed further in this document (those only apply to `stdio`) -- they are
passed untouched by `runc`.
It should be noted that `--preserve-fds` does not take individual file
descriptors to preserve. Instead, it takes how many file descriptors (not
including `stdio` or `LISTEN_FDS`) should be passed to the container. In the
following example:
% runc run --preserve-fds 5 <container>
`runc` will pass the first `5` file descriptors (`3`, `4`, `5`, `6`, and `7` --
assuming that `LISTEN_FDS` has not been configured) to the container.
In addition to `--preserve-fds`, `LISTEN_FDS` file descriptors are passed
automatically to allow for `systemd`-style socket activation. To extend the
above example:
% LISTEN_PID=$pid_of_runc LISTEN_FDS=3 runc run --preserve-fds 5 <container>
`runc` will now pass the first `8` file descriptors (and it will also pass
`LISTEN_FDS=3` and `LISTEN_PID=1` to the container). The first `3` (`3`, `4`,
and `5`) were passed due to `LISTEN_FDS` and the other `5` (`6`, `7`, `8`, `9`,
and `10`) were passed due to `--preserve-fds`. You should keep this in mind if
you use `runc` directly in something like a `systemd` unit file. To disable
this `LISTEN_FDS`-style passing just unset `LISTEN_FDS`.
**Be very careful when passing file descriptors to a container process.** Due
to some Linux kernel (mis)features, a container with access to certain types of
file descriptors (such as `O_PATH` descriptors) outside of the container's root
file system can use these to break out of the container's pivoted mount
namespace. [This has resulted in CVEs in the past.][CVE-2016-9962]
## <a name="terminal-modes" /> Terminal Modes ##
`runc` supports two distinct methods for passing `stdio` to the container's
primary process:
* [new terminal](#new-terminal) (`terminal: true`)
* [pass-through](#pass-through) (`terminal: false`)
When first using `runc` these two modes will look incredibly similar, but this
can be quite deceptive as these different modes have quite different
By default, `runc spec` will create a configuration that will create a new
terminal (`terminal: true`). However, if the `terminal: ...` line is not
present in `config.json` then pass-through is the default.
*In general we recommend using new terminal, because it means that tools like
`sudo` will work inside your container. But pass-through can be useful if you
know what you're doing, or if you're using `runc` as part of a non-interactive
### <a name="new-terminal"> New Terminal ###
In new terminal mode, `runc` will create a brand-new "console" (or more
precisely, a new pseudo-terminal using the container's namespaced
`/dev/pts/ptmx`) for your contained process to use as its `stdio`.
When you start a process in new terminal mode, `runc` will do the following:
1. Create a new pseudo-terminal.
2. Pass the slave end to the container's primary process as its `stdio`.
3. Send the master end to a process to interact with the `stdio` for the
container's primary process ([details below](#runc-modes)).
It should be noted that since a new pseudo-terminal is being used for
communication with the container, some strange properties of pseudo-terminals
might surprise you. For instance, by default, all new pseudo-terminals
translate the byte `'\n'` to the sequence `'\r\n'` on both `stdout` and
`stderr`. In addition there are [a whole range of `ioctls(2)` that can only
interact with pseudo-terminal `stdio`][tty_ioctl(4)].
> **NOTE**: In new terminal mode, all three `stdio` file descriptors are the
> same underlying file. The reason for this is to match how a shell's `stdio`
> looks to a process (as well as remove race condition issues with having to
> deal with multiple master pseudo-terminal file descriptors). However this
> means that it is not really possible to uniquely distinguish between `stdout`
> and `stderr` from the caller's perspective.
### <a name="pass-through"> Pass-Through ###
If you have already set up some file handles that you wish your contained
process to use as its `stdio`, then you can ask `runc` to pass them through to
the contained process (this is not necessarily the same as `--preserve-fds`'s
passing of file descriptors -- [details below](#runc-modes)). As an example
(assuming that `terminal: false` is set in `config.json`):
% echo input | runc run some_container > /tmp/log.out 2>& /tmp/log.err
Here the container's various `stdio` file descriptors will be substituted with
the following:
* `stdin` will be sourced from the `echo input` pipeline.
* `stdout` will be output into `/tmp/log.out` on the host.
* `stderr` will be output into `/tmp/log.err` on the host.
It should be noted that the actual file handles seen inside the container may
be different [based on the mode `runc` is being used in](#runc-modes) (for
instance, the file referenced by `1` could be `/tmp/log.out` directly or a pipe
which `runc` is using to buffer output, based on the mode). However the net
result will be the same in either case. In principle you could use the [new
terminal mode](#new-terminal) in a pipeline, but the difference will become
more clear when you are introduced to [`runc`'s detached mode](#runc-modes).
## <a name="runc-modes" /> `runc` Modes ##
`runc` itself runs in two modes:
* [foreground](#foreground)
* [detached](#detached)
You can use either [terminal mode](#terminal-modes) with either `runc` mode.
However, there are considerations that may indicate preference for one mode
over another. It should be noted that while two types of modes (terminal and
`runc`) are conceptually independent from each other, you should be aware of
the intricacies of which combination you are using.
*In general we recommend using foreground because it's the most
straight-forward to use, with the only downside being that you will have a
long-running `runc` process. Detached mode is difficult to get right and
generally requires having your own `stdio` management.*
### Foreground ###
The default (and most straight-forward) mode of `runc`. In this mode, your
`runc` command remains in the foreground with the container process as a child.
All `stdio` is buffered through the foreground `runc` process (irrespective of
which terminal mode you are using). This is conceptually quite similar to
running a normal process interactively in a shell (and if you are using `runc`
in a shell interactively, this is what you should use).
Because the `stdio` will be buffered in this mode, some very important
peculiarities of this mode should be kept in mind:
* With [new terminal mode](#new-terminal), the container will see a
pseudo-terminal as its `stdio` (as you might expect). However, the `stdio` of
the foreground `runc` process will remain the `stdio` that the process was
started with -- and `runc` will copy all `stdio` between its `stdio` and the
container's `stdio`. This means that while a new pseudo-terminal has been
created, the foreground `runc` process manages it over the lifetime of the
* With [pass-through mode](#pass-through), the foreground `runc`'s `stdio` is
**not** passed to the container. Instead, the container's `stdio` is a set of
pipes which are used to copy data between `runc`'s `stdio` and the
container's `stdio`. This means that the container never has direct access to
host file descriptors (aside from the pipes created by the container runtime,
but that shouldn't be an issue).
The main drawback of the foreground mode of operation is that it requires a
long-running foreground `runc` process. If you kill the foreground `runc`
process then you will no longer have access to the `stdio` of the container
(and in most cases this will result in the container dying abnormally due to
`SIGPIPE` or some other error). By extension this means that any bug in the
long-running foreground `runc` process (such as a memory leak) or a stray
OOM-kill sweep could result in your container being killed **through no fault
of the user**. In addition, there is no way in foreground mode of passing a
file descriptor directly to the container process as its `stdio` (like
`--preserve-fds` does).
These shortcomings are obviously sub-optimal and are the reason that `runc` has
an additional mode called "detached mode".
### Detached ###
In contrast to foreground mode, in detached mode there is no long-running
foreground `runc` process once the container has started. In fact, there is no
long-running `runc` process at all. However, this means that it is up to the
caller to handle the `stdio` after `runc` has set it up for you. In a shell
this means that the `runc` command will exit and control will return to the
shell, after the container has been set up.
You can run `runc` in detached mode in one of the following ways:
* `runc run -d ...` which operates similar to `runc run` but is detached.
* `runc create` followed by `runc start` which is the standard container
lifecycle defined by the OCI runtime specification (`runc create` sets up the
container completely, waiting for `runc start` to begin execution of user
The main use-case of detached mode is for higher-level tools that want to be
wrappers around `runc`. By running `runc` in detached mode, those tools have
far more control over the container's `stdio` without `runc` getting in the
way (most wrappers around `runc` like `cri-o` or `containerd` use detached mode
for this reason).
Unfortunately using detached mode is a bit more complicated and requires more
care than the foreground mode -- mainly because it is now up to the caller to
handle the `stdio` of the container.
#### Detached Pass-Through ####
In detached mode, pass-through actually does what it says on the tin -- the
`stdio` file descriptors of the `runc` process are passed through (untouched)
to the container's `stdio`. The purpose of this option is to allow a user to
set up `stdio` for a container themselves and then force `runc` to just use
their pre-prepared `stdio` (without any pseudo-terminal funny business). *If
you don't see why this would be useful, don't use this option.*
**You must be incredibly careful when using detached pass-through (especially
in a shell).** The reason for this is that by using detached pass-through you
are passing host file descriptors to the container. In the case of a shell,
usually your `stdio` is going to be a pseudo-terminal (on your host). A
malicious container could take advantage of TTY-specific `ioctls` like
`TIOCSTI` to fake input into the **host** shell (remember that in detached
mode, control is returned to your shell and so the terminal you've given the
container is being read by a shell prompt).
There are also several other issues with running non-malicious containers in a
shell with detached pass-through (where you pass your shell's `stdio` to the
* Output from the container will be interleaved with output from your shell (in
a non-deterministic way), without any real way of distinguishing from where a
particular piece of output came from.
* Any input to `stdin` will be non-deterministically split and given to either
the container or the shell (because both are blocked on a `read(2)` of the
same FIFO-style file descriptor).
They are all related to the fact that there is going to be a race when either
your host or the container tries to read from (or write to) `stdio`. This
problem is especially obvious when in a shell, where usually the terminal has
been put into raw mode (where each individual key-press should cause `read(2)`
to return).
> **NOTE**: There is also currently a [known problem][issue-1721] where using
> detached pass-through will result in the container hanging if the `stdout` or
> `stderr` is a pipe (though this should be a temporary issue).
#### Detached New Terminal ####
When creating a new pseudo-terminal in detached mode, and fairly obvious
problem appears -- how do we use the new terminal that `runc` created? Unlike
in pass-through, `runc` has created a new set of file descriptors that need to
be used by *something* in order for container communication to work.
The way this problem is resolved is through the use of Unix domain sockets.
There is a feature of Unix sockets called `SCM_RIGHTS` which allows a file
descriptor to be sent through a Unix socket to a completely separate process
(which can then use that file descriptor as though they opened it). When using
`runc` in detached new terminal mode, this is how a user gets access to the
pseudo-terminal's master file descriptor.
To this end, there is a new option (which is required if you want to use `runc`
in detached new terminal mode): `--console-socket`. This option takes the path
to a Unix domain socket which `runc` will connect to and send the
pseudo-terminal master file descriptor down. The general process for getting
the pseudo-terminal master is as follows:
1. Create a Unix domain socket at some path, `$socket_path`.
2. Call `runc run` or `runc create` with the argument `--console-socket
3. Using `recvmsg(2)` retrieve the file descriptor sent using `SCM_RIGHTS` by
4. Now the manager can interact with the `stdio` of the container, using the
retrieved pseudo-terminal master.
After `runc` exits, the only process with a copy of the pseudo-terminal master
file descriptor is whoever read the file descriptor from the socket.
> **NOTE**: Currently `runc` doesn't support abstract socket addresses (due to
> it not being possible to pass an `argv` with a null-byte as the first
> character). In the future this may change, but currently you must use a valid
> path name.
In order to help users make use of detached new terminal mode, we have provided
a [Go implementation in the `go-runc` bindings][containerd/go-runc.Socket], as
well as [a simple client][recvtty].
[recvtty]: /contrib/cmd/recvtty

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