| [1166] | 1 | 6.10 The Restricted Shell
|
|---|
| 2 | =========================
|
|---|
| 3 |
|
|---|
| 4 | If Bash is started with the name 'rbash', or the '--restricted' or '-r'
|
|---|
| 5 | option is supplied at invocation, the shell becomes restricted. A
|
|---|
| 6 | restricted shell is used to set up an environment more controlled than
|
|---|
| 7 | the standard shell. A restricted shell behaves identically to 'bash'
|
|---|
| 8 | with the exception that the following are disallowed or not performed:
|
|---|
| 9 |
|
|---|
| 10 | * Changing directories with the 'cd' builtin.
|
|---|
| 11 | * Setting or unsetting the values of the 'SHELL', 'PATH', 'HISTFILE',
|
|---|
| 12 | 'ENV', or 'BASH_ENV' variables.
|
|---|
| 13 | * Specifying command names containing slashes.
|
|---|
| 14 | * Specifying a filename containing a slash as an argument to the '.'
|
|---|
| 15 | builtin command.
|
|---|
| 16 | * Specifying a filename containing a slash as an argument to the
|
|---|
| 17 | 'history' builtin command.
|
|---|
| 18 | * Specifying a filename containing a slash as an argument to the '-p'
|
|---|
| 19 | option to the 'hash' builtin command.
|
|---|
| 20 | * Importing function definitions from the shell environment at
|
|---|
| 21 | startup.
|
|---|
| 22 | * Parsing the value of 'SHELLOPTS' from the shell environment at
|
|---|
| 23 | startup.
|
|---|
| 24 | * Redirecting output using the '>', '>|', '<>', '>&', '&>', and '>>'
|
|---|
| 25 | redirection operators.
|
|---|
| 26 | * Using the 'exec' builtin to replace the shell with another command.
|
|---|
| 27 | * Adding or deleting builtin commands with the '-f' and '-d' options
|
|---|
| 28 | to the 'enable' builtin.
|
|---|
| 29 | * Using the 'enable' builtin command to enable disabled shell
|
|---|
| 30 | builtins.
|
|---|
| 31 | * Specifying the '-p' option to the 'command' builtin.
|
|---|
| 32 | * Turning off restricted mode with 'set +r' or 'set +o restricted'.
|
|---|
| 33 |
|
|---|
| 34 | These restrictions are enforced after any startup files are read.
|
|---|
| 35 |
|
|---|
| 36 | When a command that is found to be a shell script is executed (*note
|
|---|
| 37 | Shell Scripts::), 'rbash' turns off any restrictions in the shell
|
|---|
| 38 | spawned to execute the script.
|
|---|
| 39 |
|
|---|
| 40 | The restricted shell mode is only one component of a useful restricted
|
|---|
| 41 | environment. It should be accompanied by setting 'PATH' to a value that
|
|---|
| 42 | allows execution of only a few verified commands (commands that allow
|
|---|
| 43 | shell escapes are particularly vulnerable), leaving the user in a
|
|---|
| 44 | non-writable directory other than his home directory after login, not
|
|---|
| 45 | allowing the restricted shell to execute shell scripts, and cleaning the
|
|---|
| 46 | environment of variables that cause some commands to modify their
|
|---|
| 47 | behavior (e.g., 'VISUAL' or 'PAGER').
|
|---|
| 48 |
|
|---|
| 49 | Modern systems provide more secure ways to implement a restricted
|
|---|
| 50 | environment, such as 'jails', 'zones', or 'containers'.
|
|---|
| 51 |
|
|---|
