ksu - Kerberized super-user
=================================

SYNOPSIS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**ksu**  
    [ *target_user* ]
    [ **-n** *target_principal_name* ]
    [ **-c** *source_cache_name* ]
    [ **-k** ]
    [ **-D** ]
    [ **-r** time ]
    [ **-pf** ]
    [ **-l** *lifetime* ]
    [ **-z | Z** ]
    [ **-q** ]
    [ **-e** *command* [ args ...  ] ] [ **-a** [ args ...  ] ]


REQUIREMENTS
~~~~~~~~~~~~~~~~~

Must have Kerberos version 5 installed to compile *ksu*.  Must have a Kerberos version 5 server running to use *ksu*.

DESCRIPTION
~~~~~~~~~~~~~~~

*ksu* is a Kerberized version of the *su* program that has two missions: 
one is to securely change the real and effective user ID to that of the target user, 
and the other is to create a new security context.
For the sake of clarity, all references to and attributes of the user invoking the program will start with 'source'
(e.g. source user, source cache, etc.). 
Likewise, all references to and attributes of the target account will start with 'target'.

AUTHENTICATION
~~~~~~~~~~~~~~~~~~~~~

To fulfill the first mission, *ksu* operates in two phases: authentication and authorization.
Resolving the target principal name is the first step in authentication.  
The user can either specify his principal name with the *-n* option (e.g. *-n jqpublic\@USC.EDU* ) or
a default principal name will be assigned using a heuristic described in the *OPTIONS* section (see *-n* option).  
The target user name must be the first argument to *ksu*; if not specified root is the default.  
If '.' is specified then the target user will be the source user (e.g. *ksu* .).  
If the source user is root or the target user is the source user, no authentication or authorization takes place.  
Otherwise, *ksu* looks for an appropriate Kerberos ticket in the source cache.

The ticket can either be for the end-server or a ticket granting ticket (TGT) for the target principal's realm.  
If the ticket for the end-server is already in the cache, it's decrypted and verified.  
If it's not in the cache but the TGT is, the TGT is used to obtain the ticket for the end-server.  
The end-server ticket is then verified.  
If neither ticket is in the cache, but *ksu* is compiled with the *GET_TGT_VIA_PASSWD* define, the user will be prompted for a Kerberos password which will then be used to get a TGT.  
If the user is logged in remotely and does not have a secure channel, the password may be exposed.   
If neither ticket is in the cache and *GET_TGT_VIA_PASSWD* is not defined, authentication fails.

AUTHORIZATION
~~~~~~~~~~~~~~~~~~

This section describes authorization of the source user when *ksu* is invoked without the *-e* option.  
For a description of the -e option, see the OPTIONS section.

Upon successful authentication, *ksu* checks whether the target principal is authorized to access the target account. In the target user's home directory, *ksu* attempts to access two authorization files: *.k5login* and *.k5users*.  
In the .k5login file each line contains the name of a principal that is authorized to access the account.

For example::

       jqpublic@USC.EDU
       jqpublic/secure@USC.EDU
       jqpublic/admin@USC.EDU

The format of *.k5users* is the same, except the principal name may be followed by a list of commands that the principal is authorized to execute. (see the *-e* option in the *OPTIONS* section for details).

Thus if the target principal name is found in the *.k5login* file the source user is authorized to access the target account. 
Otherwise *ksu* looks in the *.k5users* file.  
If the target principal name is found without any trailing commands or followed only by '\*' then the source user is authorized.  
If either *.k5login* or *.k5users* exist but an appropriate entry for the target principal does not exist then access is denied. 
If neither file exists then the principal will be granted access to the account according to the aname->lname mapping rules (see krb5_anadd(8) for more details).  
Otherwise, authorization fails.

EXECUTION OF THE TARGET SHELL
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Upon successful authentication and authorization, *ksu* proceeds in a similar fashion to *su*.  
The environment is unmodified with the exception of USER, HOME and SHELL variables.  
If the target user is not root, USER gets set to the target user name.  
Otherwise USER remains unchanged. 
Both HOME and SHELL are set to the target login's default values.  
In addition, the environment variable *KRB5CCNAME* gets set to the name of the target cache.  
The real and effective user ID are changed to that of the target user.   
The target user's shell is then invoked (the shell name is specified in the password file).  
Upon termination of the shell, *ksu* deletes the target cache (unless *ksu* is invoked with the *-k* option).
This is implemented by first doing a fork and then an exec, instead of just exec, as done by *su*.

CREATING A NEW SECURITY CONTEXT
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*ksu* can be used to create a new security context for the target program (either the target shell, or command specified via the *-e* option). 
The target program inherits a set of credentials from the source user.  
By default, this set includes all of the credentials in the source cache plus any additional credentials obtained during authentication.  
The source user is able to limit the credentials in this set by using *-z* or *-Z* option.  
*-z* restricts the copy of tickets from the source cache to the target cache to only the tickets where client == the target principal name.  
The *-Z* option provides the target user with a fresh target cache (no creds in the cache).
Note that for security reasons, when the source user is root and target user is non-root, *-z* option is the default mode of operation.

While no authentication takes place if the source user is root or is the same as the target  user,  additional  tickets  can  still  be obtained  for the target cache.  
If *-n* is specified and no credentials can be copied to the target cache,  
the  source user is prompted for a Kerberos password (unless *-Z* specified or *GET_TGT_VIA_PASSWD* is undefined). 
If successful,  a  TGT is obtained from the  Kerberos server  and  stored  in the target cache.  
Otherwise, if a password is not provided (user hit return) *ksu* continues  in  a normal  mode of  operation (the target cache will not contain the desired TGT).  
If the wrong password is typed in, *ksu* fails.

*Side Note*: during authentication, only the tickets that could be obtained without providing a password are  cached  in  in  the  source cache.  


OPTIONS
~~~~~~~~~~~~~~~

       **-n** *target_principal_name*
                 Specify a Kerberos target principal name.  Used in authentication and authorization phases of *ksu*.

                 If *ksu* is invoked without *-n*, a default principal name is assigned via the following heuristic:

                 *Case 1: source user is non-root.*
                 If  the target user is the source user the default principal name is set to the default principal of the source cache. 
                 If the cache does not exist then the default principal name is set to target_user\@local_realm.  
                 If the source and target  users  are different  and  neither  ~target_user/*.k5users*  nor  ~target_user/*.k5login*  exist  
                 then  the  default  principal name is *target_user_login_name\@local_realm*. 
                 Otherwise, starting with the first principal listed below, *ksu* checks if  the  principal  is authorized  to  access the target account 
                 and whether there is a legitimate ticket for that principal in the source cache. 
                 If both conditions are met that principal becomes the default target principal, otherwise go to the next principal.

                 a) default principal of the source cache
                 b) target_user\@local_realm
                 c) source_user\@local_realm

                 If a-c fails try any principal for which there is a ticket in the source cache and that is authorized to  access  the  target account.   
                 If  that fails select the first principal that is authorized to access the target account from the above list.  
                 If none are authorized and *ksu* is configured with *PRINC_LOOK_AHEAD* turned on, select the default principal as follows:

                 For each candidate in the above list, select an authorized principal that has the same realm name and 
                 first part of the principal name equal to the prefix of the candidate.  
                 For example if candidate a) is *jqpublic\@ISI.EDU* and *jqpublic/secure\@ISI.EDU* is authorized to access the target account
                 then the default principal is set to *jqpublic/secure\@ISI.EDU*.

                 *Case 2: source user is root.*
                 If the target user is non-root then the default principal name is *target_user\@local_realm*.  
                 Else, if the source cache  exists the  default  principal name is set to the default principal of the source cache. 
                 If the source cache does not exist, default principal name is set to *root\@local_realm*.

       **-c** *source_cache_name*
                 Specify source cache name (e.g.  -c FILE:/tmp/my_cache).  
                 If *-c* option is not used then the name is obtained from  *KRB5CCNAME* environment  variable.   
                 If  *KRB5CCNAME* is not defined the source cache name is set to krb5cc_<source uid>.  
                 The target cache name is automatically set to krb5cc_<target uid>.(gen_sym()), 
                 where gen_sym generates a new number such  that  the  resulting cache does not already exist.
                 For example::

                        krb5cc_1984.2

       **-k**        
                 Do  not delete the target cache upon termination of the target shell or a command ( *-e* command).  
                 Without *-k*, *ksu* deletes the target cache.

       **-D**        
                 Turn on debug mode.

       **-z**    
                 Restrict the copy of tickets from the source cache to the target cache to only the tickets where client == the target principal name. 
                 Use the *-n* option if you want the tickets for other then the default principal. 
                 Note that the *-z* option is mutually exclusive with the *-Z* option.

       **-Z**        
                 Don't copy any tickets from the source cache to the target cache. 
                 Just create a fresh target cache, where the default principal name of the cache is initialized to the target principal name.  
                 Note that the *-Z* option is mutually exclusive with the *-z* option.

       **-q**        
                 Suppress the printing of status messages.

Ticket granting ticket options

       **-l lifetime -r time -pf**
                 The ticket granting ticket options only apply to the case where there are no appropriate tickets in the cache to authenticate
                 the  source  user. In this case if *ksu* is configured to prompt users for a Kerberos password (GET_TGT_VIA_PASSWD is defined),
                 the ticket granting ticket options that are specified will be used when getting a ticket granting ticket  from  the  Kerberos
                 server.

       **-l** *lifetime*
                 option  specifies  the lifetime to be requested for the ticket; if this option is not specified, the  default ticket lifetime
                 (configured by each site) is used instead.

       **-r** *time*   
                 option  specifies  that  the  *RENEWABLE*  option should be requested for the ticket, and specifies the desired total  lifetime of the ticket.

       **-p**        
                 option specifies that the PROXIABLE option should  be requested for the ticket.

       **-f**        
                 option specifies that the FORWARDABLE  option  should be requested for the ticket.

       **-e** *command* [args ...]
                 *ksu*  proceeds  exactly the same as if it was invoked without the *-e* option, 
                 except instead of executing the target shell, *ksu* executes the specified command 
                 Example of usage::
                      
                        ksu bob -e ls -lag

                 The authorization algorithm for *-e* is as follows:

                 If the source user is root or source user == target user, no authorization takes place  and  the  command  is  executed.   
                 If source  user  id  != 0, and ~target_user/*.k5users* file does not exist, authorization fails.  
                 Otherwise, ~target_user/*.k5users* file must have an appropriate entry for target principal to get authorized.

                 The *.k5users* file format:

                 A single principal entry on each line that may be followed by a list of commands that the principal is authorized to execute.
                 A principal name followed by a '\*' means that the user is authorized to execute any command. Thus, in the following example::

                     jqpublic@USC.EDU ls mail /local/kerberos/klist
                     jqpublic/secure@USC.EDU *
                     jqpublic/admin@USC.EDU

                 *jqpublic\@USC.EDU*  is only authorized to execute *ls*, *mail* and *klist* commands. 
                 *jqpublic/secure\@USC.EDU* is authorized to execute any command. 
                 *jqpublic/admin\@USC.EDU* is not authorized to execute any command.  
                 Note, that  *jqpublic/admin\@USC.EDU*  is  authorized to execute the target shell (regular *ksu*, without the *-e* option)
                 but *jqpublic\@USC.EDU* is not.

                 The  commands listed after the principal name must be either a full path names or just the program name.  
                 In the second case, CMD_PATH specifying the location of authorized programs must be defined at the compilation time of *ksu*.
                 Which command gets executed ?

                 If the source user is *root* or the target user is the source user or the user is authorized to execute any command ('\*' entry)
                 then  command can be either a full or a relative path leading to the target program.  
                 Otherwise, the user must specify either a full path or just the program name.

       **-a** *args*   
                 Specify arguments to be passed to the target shell.  
                 Note: that all flags and parameters following -a will be passed  to  the shell,  
                 thus  all  options  intended for *ksu* must precede *-a*.  

                 The *-a* option can be used to simulate the *-e* option if used as follows::
 
                     -a -c [command [arguments]].  
 
                 *-c* is interpreted by the c-shell to execute the command.


INSTALLATION INSTRUCTIONS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*ksu* can be compiled with the following 4 flags (see the Imakefile):

       **GET_TGT_VIA_PASSWD**
                 In case no appropriate tickets are found in the source cache, the user will be prompted for a Kerberos password. The password is then used to get a ticket granting ticket from the Kerberos server. The danger of configuring *ksu* with this macro is if the source user is loged in remotely and does not have a secure channel, the password may get exposed.

       **PRINC_LOOK_AHEAD**
                 During the resolution of the default principal name, *PRINC_LOOK_AHEAD* enables *ksu* to find principal names in the *.k5users* file as described in the *OPTIONS* section (see *-n* option).

       **CMD_PATH**
                 Specifies a list of directories containing programs that users are authorized to execute (via *.k5users* file).

       **HAS_GETUSERSHELL**
                 If the source user is non-root, *ksu* insists that the target user's shell to be invoked is a "legal shell". getusershell(3) is called to obtain the names of "legal shells". Note that the target user's shell is obtained from the passwd file.

       SAMPLE CONFIGURATION:
                 KSU_OPTS = -DGET_TGT_VIA_PASSWD -DPRINC_LOOK_AHEAD -DCMD_PATH='"/bin /usr/ucb /local/bin"

       PERMISSIONS FOR KSU
                 *ksu* should be owned by root and have the set user id  bit turned on.

       END-SERVER ENTRY
                 *ksu* attempts to get a ticket for the end server just as Kerberized telnet and rlogin.  Thus, there must be an entry for the server in the Kerberos database (e.g. host/nii.isi.edu\@ISI.EDU).  The keytab file must be in an appropriate location.

SIDE EFFECTS
~~~~~~~~~~~~~~~

*ksu* deletes all expired tickets from the source cache.

AUTHOR OF KSU:
~~~~~~~~~~~~~~~

GENNADY (ARI) MEDVINSKY

