lxc-attach(1) — Linux manual page


lxc-attach(1)                                              lxc-attach(1)

NAME         top

       lxc-attach - start a process inside a running container.

SYNOPSIS         top

       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         top

       lxc-attach runs the specified command inside the container speci‐
       fied 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 ex‐
       ecuted. This will fail if no such user exists inside the contain‐
       er 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  com‐
       mand  without  first allocating a pseudo terminal. This made them
       vulnerable to input faking via a TIOCSTI ioctl call after switch‐
       ing between userspace execution contexts with different privilege
       levels. Newer versions of lxc-attach will try to allocate a pseu‐
       do terminal file descriptor pair on the host and attach any stan‐
       dard 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         top

       -f, --rcfile config_file
              Specify the configuration file to configure the virtual‐
              ization 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 pre‐
              viously 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 con‐
              tainer configuration files, see lxc.conf(5). By default,
              the current architecture of the running container will be

       -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 el‐
              evate all of them, as a pipe-separated list, e.g.
              CGROUP|LSM. Allowed values are CGROUP, CAP and LSM repre‐
              senting 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 problem‐
              atic, especially if the daemon starts a lot of subprocess‐
              es 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, UT‐
              SNAME, 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 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 the environment before attaching, so no undesired
              environment variables leak into the container. The vari‐
              able 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 (use numerical value)
              uid inside the container.

       --g, --gid gid
              Executes the command with group ID (use numerical value)
              gid inside the container.

COMMON OPTIONS         top

       These options are common to most of lxc commands.

       -?, -h, --help
              Print a longer usage message than normal.

              Give the usage message

       -q, --quiet
              mute on

       -P, --lxcpath=PATH
              Use an alternate container path. The default is

       -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 ER‐
              ROR. Possible values are : FATAL, ALERT, CRIT, WARN, ER‐
              ROR, 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 ef‐
              fect on the ERROR events log on stderr.

       -n, --name=NAME
              Use container identifier NAME.  The container identifier
              format is an alphanumeric string.

              Specify the configuration file to configure the virtual‐
              ization 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 pre‐
              viously created container (via lxc-create).

              Show the version number.

EXAMPLES         top

       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-at‐
       tach 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         top

       The Linux /proc and /sys filesystems contain information about
       some quantities that are affected by namespaces, such as the di‐
       rectories named after process ids in /proc or the network inter‐
       face 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 net‐
       work/pid namespace context of the attached process. In order not
       to interfere with the host's actual filesystem, the mount name‐
       space 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 unexpected‐
       ly for some users (E.g. on systems where an unprivileged user is
       not placed in a writeable cgroup in critical subsystems on lo‐
       gin.). However, this behavior is correct and more secure.

SECURITY         top

       The -e and -s options should be used with care, as it may break
       the isolation of the containers if used improperly.

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of the lxc (Linux containers) project.  Infor‐
       mation 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
       ⟨https://github.com/lxc/lxc.git⟩ on 2024-06-14.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2024-06-05.)  If you discover any rendering
       problems in this HTML version 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 manual page), send a mail to

                               2024-04-03                  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-update-config(1)lxc-usernsexec(1)lxc-wait(1)lxc.container.conf(5)lxc.system.conf(5)lxc(7)