really - gain privilege or run commands a different user

really [options] [command args... ]

really checks whether the caller is allowed, and if it is it changes its uids and gids according to the command line options and executes the specified command.

If no options are specified, the uid will be set to 0 and the gids will be left unchanged.

If no command is specified, really will run $SHELL -i.

A caller is allowed if it has write access to /etc/inittab. This is most easily achieved by creating or using a suitable group, containing all the appropriate users, and making /etc/inittab group-owned by that group and group-writeable.

-u username | --user username

Sets the uid, gid, and supplementary group list, according to username's entry in the password and group databases.

-i username | --useronly username

Sets only the uid according to username's entry in the password database.

-I uid | --uidonly uid

Sets the uid to the numeric value uid (which need not correspond to any existing user in the password database).

-g groupname | --group groupname

groupname is looked up in the group database and its gid is appended to the process's supplementary groups list. If this is the first gid specified it will also be set as the primary gid.

-G gid | --gid gid

gid is appended to the process's supplementary groups list. (gid need not correspond to any existing group in the group database.) If this is the first gid specified it will also be set as the primary gid.

-z | --groupsclear

Clears the process's supplementary groups list. When using this option you must also specify -g or -G. The process's groups will then be exactly those specified. The relative position of -z in the argument list is not relevant.

--

Indicates the end of the options. The next argument (if present) will be interpreted as the command name, even if it starts with a hyphen.

really is designed so that installing it setuid root is extremely unlikely to compromise the security of any system. It will check using access(2) whether the real user is allowed to write to /etc/inittab and if this check fails really will exit without even attempting to parse its command line.

really is not designed to be resistant to malicious command line arguments. Do not allow untrusted processes to pass options to really, or to specify the command to be run. Whether it is safe to allow relatively untrusted processes to pass options to the command which is to be run depends on the behaviour of that command and its security status.

Attempting to use really to drop privilege is dangerous unless the calling environment is very well understood. There are many inherited process properties and resources which might be used by the callee to escalate its privilege to that of the (root-equivalent) caller. For this function, it is usually better to use userv if possible.

really does not manipulate the environment at all. The calling program is run in exactly the same environment as the caller passes to really. In particular, really will not add sbin directories to PATH so really-enabled accounts will usually need to have these directories on their configured PATH to start with.

SHELL is used to find the default shell to use in interactive mode (ie, when no command is specified).

This version of really was written by Ian Jackson <ian@chiark.greenend.org.uk>.

It and this manpage are Copyright (C) 1992-5,2003 Ian Jackson <ian@chiark.greenend.org.uk>.

really is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.

really is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this file; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

really is currently part of chiark-utils and is available for download from ftp.chiark.greenend.org.uk in /users/ian/chiark-utils/, in source and pre-compiled binary form, and also from Ian Jackson's cvsweb.

userv(1), access(2), setresuid(2), setresgid(2), setgroups(2)