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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | COMMON OPTIONS | EXAMPLES | COMPATIBILITY | NOTES | SECURITY | SEE ALSO | AUTHOR | COLOPHON |
LXC-ATTACH(1) LXC-ATTACH(1)
lxc-attach - start a process inside a running container.
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 ] [ -L, --pty-log file ] [ -v, --set-var variable ] [ --keep-
var variable ] [ -- command ]
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
master/slave pair on the host and attach any standard file
descriptors which refer to a terminal to the slave 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.
-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
archictecture 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.
-L, --pty-log file
Specify a file where the output of lxc-attach will be logged.
Important: When a standard file descriptor does not refer to a
pty output produced on it will not be logged.
-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.
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
/usr/local/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, CRIT, WARN, ERROR, NOTICE, INFO,
DEBUG.
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.
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
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.
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.
The -e and -s options should be used with care, as it may break the
isolation of the containers if used improperly.
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)
Daniel Lezcano <daniel.lezcano@free.fr>
This page is part of the lxc (Linux containers) project. Information
about the project can be found at ⟨http://linuxcontainers.org/⟩. If
you have a bug report for this manual page, send it to
lxc-devel@lists.linuxcontainers.org. This page was obtained from the
project's upstream Git repository ⟨git://github.com/lxc/lxc⟩ on
2017-07-05. If you discover any rendering problems in this HTML ver‐
sion of the page, or you believe there is a better or more up-to-date
source for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to man-pages@man7.org
2017-07-05 LXC-ATTACH(1)
Pages that refer to this page: lxc-attach(1), lxc-autostart(1), lxc-cgroup(1), lxc-checkconfig(1), lxc-checkpoint(1), lxc-config(1), lxc-console(1), lxc-copy(1), lxc-create(1), lxc-destroy(1), lxc-device(1), lxc-execute(1), lxc-freeze(1), lxc-info(1), lxc-ls(1), lxc-monitor(1), lxc-snapshot(1), lxc-start(1), lxc-stop(1), lxc-top(1), lxc-unfreeze(1), lxc-unshare(1), lxc-usernsexec(1), lxc-wait(1), lxc.container.conf(5), lxc.system.conf(5), lxc(7)