lxc-attach

NAME

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] [-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.

AUTHOR

Daniel Lezcano <[email protected]>

2023-11-30