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KSU(1) MIT Kerberos KSU(1)

NAME
ksu - Kerberized super-user

SYNOPSIS
ksu [ target_user ] [ -n target_principal_name ] [ -c source_cache_name
] [ -k ] [ -r time ] [ -p | -P] [ -f | -F] [ -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.

NOTE:
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: authentica-
tion and authorization. Resolving the target principal name is the
first step in authentication. The user can either specify his princi-
pal 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 OP-
TIONS section (see -n option). The target user name must be the first
argument to ksu; if not specified root is the default. If . is speci-
fied 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 au-
thentication 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 ex-
posed. 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 in-
voked 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(5)
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 exe-
cute (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 appro-
priate entry for the target principal does not exist then access is de-
nied. If neither file exists then the principal will be granted access
to the account according to the aname->lname mapping rules. 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 excep-
tion 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 ter-
mination of the shell, ksu deletes the target cache (unless ksu is in-
voked 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 princi-
pal 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.

NOTE:
During authentication, only the tickets that could be obtained with-
out providing a password are cached in the source cache.

OPTIONS
-n target_principal_name
Specify a Kerberos target principal name. Used in authentica-
tion and authorization phases of ksu.

If ksu is invoked without -n, a default principal name is as-
signed 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 au-
thorized 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 ex-
ample if candidate a) is jqpublic@ISI.EDU and jqpublic/se-
cure@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 op-
tion is not used then the name is obtained from KRB5CCNAME environ-
ment variable. If KRB5CCNAME is not defined the source cache name
is set to krb5cc_<source uid>. The target cache name is automati-
cally set to krb5cc_<target uid>.(gen_sym()), where gen_sym gener-
ates a new number such that the resulting cache does not already ex-
ist. 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.

-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 ex-
clusive 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 princi-
pal 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 -p -P -f -F
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
(duration string.) Specifies the lifetime to be requested for
the ticket; if this option is not specified, the default ticket
lifetime (12 hours) is used instead.

-r time
(duration string.) Specifies that the renewable option should
be requested for the ticket, and specifies the desired total
lifetime of the ticket.

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

-P specifies that the proxiable option should not be requested for
the ticket, even if the default configuration is to ask for
proxiable tickets.

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

-F option specifies that the forwardable option should not be re-
quested for the ticket, even if the default configuration is to
ask for forwardable tickets.

-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 ex-
ecutes 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 au-
thorization takes place and the command is executed. If source
user id != 0, and ~target_user/.k5users file does not exist, au-
thorization fails. Otherwise, ~target_user/.k5users file must
have an appropriate entry for target principal to get autho-
rized.

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 exe-
cute any command. jqpublic/admin@USC.EDU is not authorized to
execute any command. Note, that jqpublic/admin@USC.EDU is au-
thorized 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 exe-
cuted?

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 four flags:

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 logged 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 op-
tion).

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

HAVE_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"

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

ksu attempts to get a ticket for the end server just as Kerberized tel-
net 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

ENVIRONMENT
See kerberos(7) for a description of Kerberos environment variables.

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