Provided by: lxc-utils_4.0.12-0ubuntu1~20.04.1_amd64 
NAME
lxc-attach - start a process inside a running container.
SYNOPSIS
lxc-attach {-n, --name name} [-f, --rcfile config_file] [-a, --arch arch] [-e, --elevated-privileges
privileges] [-s, --namespaces namespaces] [-R, --remount-sys-proc] [--keep-env] [--clear-env]
[-v, --set-var variable] [--keep-var variable] [-u, --uid uid] [-g, --gid gid] [-- command]
DESCRIPTION
lxc-attach runs the specified command inside the container specified by name. The container has to be
running already.
If no command is specified, the current default shell of the user running lxc-attach will be looked up
inside the container and executed. This will fail if no such user exists inside the container or the
container does not have a working nsswitch mechanism.
Previous versions of lxc-attach simply attached to the specified namespaces of a container and ran a
shell or the specified command without first allocating a pseudo terminal. This made them vulnerable to
input faking via a TIOCSTI ioctl call after switching between userspace execution contexts with different
privilege levels. Newer versions of lxc-attach will try to allocate a pseudo terminal file descriptor
pair on the host and attach any standard file descriptors which refer to a terminal to the container side
of the pseudo terminal before executing a shell or command. Note, that if none of the standard file
descriptors refer to a terminal lxc-attach will not try to allocate a pseudo terminal. Instead it will
simply attach to the containers namespaces and run a shell or the specified command.
OPTIONS
-f, --rcfile config_file
Specify the configuration file to configure the virtualization and isolation functionalities for
the container.
This configuration file if present will be used even if there is already a configuration file
present in the previously created container (via lxc-create).
-a, --arch arch
Specify the architecture which the kernel should appear to be running as to the command executed.
This option will accept the same settings as the lxc.arch option in container configuration files,
see lxc.conf(5). By default, the current architecture of the running container will be used.
-e, --elevated-privileges privileges
Do not drop privileges when running command inside the container. If this option is specified, the
new process will not be added to the container's cgroup(s) and it will not drop its capabilities
before executing.
You may specify privileges, in case you do not want to elevate all of them, as a pipe-separated
list, e.g. CGROUP|LSM. Allowed values are CGROUP, CAP and LSM representing cgroup, capabilities
and restriction privileges respectively. (The pipe symbol needs to be escaped, e.g. CGROUP\|LSM or
quoted, e.g. "CGROUP|LSM".)
Warning: This may leak privileges into the container if the command starts subprocesses that
remain active after the main process that was attached is terminated. The (re-)starting of daemons
inside the container is problematic, especially if the daemon starts a lot of subprocesses such as
cron or sshd. Use with great care.
-s, --namespaces namespaces
Specify the namespaces to attach to, as a pipe-separated list, e.g. NETWORK|IPC. Allowed values
are MOUNT, PID, UTSNAME, IPC, USER and NETWORK. This allows one to change the context of the
process to e.g. the network namespace of the container while retaining the other namespaces as
those of the host. (The pipe symbol needs to be escaped, e.g. MOUNT\|PID or quoted, e.g.
"MOUNT|PID".)
Important: This option implies -e.
-R, --remount-sys-proc
When using -s and the mount namespace is not included, this flag will cause lxc-attach to remount
/proc and /sys to reflect the current other namespace contexts.
Please see the Notes section for more details.
This option will be ignored if one tries to attach to the mount namespace anyway.
--keep-env
Keep the current environment for attached programs. This is the current default behaviour (as of
version 0.9), but is is likely to change in the future, since this may leak undesirable
information into the container. If you rely on the environment being available for the attached
program, please use this option to be future-proof. In addition to current environment variables,
container=lxc will be set.
--clear-env
Clear the environment before attaching, so no undesired environment variables leak into the
container. The variable container=lxc will be the only environment with which the attached program
starts.
-v, --set-var variable
Set an additional environment variable that is seen by the attached program in the container. It
is specified in the form of "VAR=VALUE", and can be specified multiple times.
--keep-var variable
Keep a specified environment variable. It can only be specified in conjunction with --clear-env,
and can be specified multiple times.
-u, --uid uid
Executes the command with user ID uid inside the container.
--g, --gid gid
Executes the command with group ID gid inside the container.
COMMON OPTIONS
These options are common to most of lxc commands.
-?, -h, --help
Print a longer usage message than normal.
--usage
Give the usage message
-q, --quiet
mute on
-P, --lxcpath=PATH
Use an alternate container path. The default is /var/lib/lxc.
-o, --logfile=FILE
Output to an alternate log FILE. The default is no log.
-l, --logpriority=LEVEL
Set log priority to LEVEL. The default log priority is ERROR. Possible values are : FATAL, ALERT,
CRIT, WARN, ERROR, NOTICE, INFO, DEBUG, TRACE.
Note that this option is setting the priority of the events log in the alternate log file. It do
not have effect on the ERROR events log on stderr.
-n, --name=NAME
Use container identifier NAME. The container identifier format is an alphanumeric string.
--rcfile=FILE
Specify the configuration file to configure the virtualization and isolation functionalities for
the container.
This configuration file if present will be used even if there is already a configuration file
present in the previously created container (via lxc-create).
--version
Show the version number.
EXAMPLES
To spawn a new shell running inside an existing container, use
lxc-attach -n container
To restart the cron service of a running Debian container, use
lxc-attach -n container -- /etc/init.d/cron restart
To deactivate the network link eth1 of a running container that does not have the NET_ADMIN capability,
use either the -e option to use increased capabilities, assuming the ip tool is installed:
lxc-attach -n container -e -- /sbin/ip link delete eth1
Or, alternatively, use the -s to use the tools installed on the host outside the container:
lxc-attach -n container -s NETWORK -- /sbin/ip link delete eth1
COMPATIBILITY
Attaching completely (including the pid and mount namespaces) to a container requires a kernel of version
3.8 or higher, or a patched kernel, please see the lxc website for details. lxc-attach will fail in that
case if used with an unpatched kernel of version 3.7 and prior.
Nevertheless, it will succeed on an unpatched kernel of version 3.0 or higher if the -s option is used to
restrict the namespaces that the process is to be attached to to one or more of NETWORK, IPC and UTSNAME.
Attaching to user namespaces is supported by kernel 3.8 or higher with enabling user namespace.
NOTES
The Linux /proc and /sys filesystems contain information about some quantities that are affected by
namespaces, such as the directories named after process ids in /proc or the network interface information
in /sys/class/net. The namespace of the process mounting the pseudo-filesystems determines what
information is shown, not the namespace of the process accessing /proc or /sys.
If one uses the -s option to only attach to the pid namespace of a container, but not its mount namespace
(which will contain the /proc of the container and not the host), the contents of /proc will reflect that
of the host and not the container. Analogously, the same issue occurs when reading the contents of
/sys/class/net and attaching to just the network namespace.
To work around this problem, the -R flag provides the option to remount /proc and /sys in order for them
to reflect the network/pid namespace context of the attached process. In order not to interfere with the
host's actual filesystem, the mount namespace will be unshared (like lxc-unshare does) before this is
done, essentially giving the process a new mount namespace, which is identical to the hosts's mount
namespace except for the /proc and /sys filesystems.
Previous versions of lxc-attach suffered a bug whereby a user could attach to a containers namespace
without being placed in a writeable cgroup for some critical subsystems. Newer versions of lxc-attach
will check whether a user is in a writeable cgroup for those critical subsystems. lxc-attach might thus
fail unexpectedly for some users (E.g. on systems where an unprivileged user is not placed in a writeable
cgroup in critical subsystems on login.). However, this behavior is correct and more secure.
SECURITY
The -e and -s options should be used with care, as it may break the isolation of the containers if used
improperly.
SEE ALSO
lxc(7), lxc-create(1), lxc-copy(1), lxc-destroy(1), lxc-start(1), lxc-stop(1), lxc-execute(1), lxc-
console(1), lxc-monitor(1), lxc-wait(1), lxc-cgroup(1), lxc-ls(1), lxc-info(1), lxc-freeze(1), lxc-
unfreeze(1), lxc-attach(1), lxc.conf(5)
AUTHOR
Daniel Lezcano <daniel.lezcano@free.fr>
2022-02-04 lxc-attach(1)