Provided by: lxc-utils_3.0.2-0ubuntu4_amd64 bug

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] [--
                  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  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.

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
              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.

       -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.

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, 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.

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>

                                            2018-08-25                              lxc-attach(1)