1KSU(1) MIT Kerberos KSU(1)
2
6 ksu - Kerberized super-user
7
9 ksu [ target_user ] [ -n target_principal_name ] [ -c source_cache_name
10 ] [ -k ] [ -r time ] [ -p | -P] [ -f | -F] [ -l lifetime ] [ -z | Z ] [
11 -q ] [ -e command [ args ... ] ] [ -a [ args ... ] ]
12
14 Must have Kerberos version 5 installed to compile ksu. Must have a
15 Kerberos version 5 server running to use ksu.
16
18 ksu is a Kerberized version of the su program that has two missions:
19 one is to securely change the real and effective user ID to that of the
20 target user, and the other is to create a new security context.
21 NOTE:
23 For the sake of clarity, all references to and attributes of the
24 user invoking the program will start with "source" (e.g., "source
25 user", "source cache", etc.).
26 Likewise, all references to and attributes of the target account
28 will start with "target".
29
31 To fulfill the first mission, ksu operates in two phases: authentica‐
32 tion and authorization. Resolving the target principal name is the
33 first step in authentication. The user can either specify his princi‐
34 pal name with the -n option (e.g., -n jqpublic@USC.EDU) or a default
35 principal name will be assigned using a heuristic described in the OP‐
36 TIONS section (see -n option). The target user name must be the first
37 argument to ksu; if not specified root is the default. If . is speci‐
38 fied then the target user will be the source user (e.g., ksu .). If
39 the source user is root or the target user is the source user, no au‐
40 thentication or authorization takes place. Otherwise, ksu looks for an
41 appropriate Kerberos ticket in the source cache.
42 The ticket can either be for the end-server or a ticket granting ticket
44 (TGT) for the target principal's realm. If the ticket for the
45 end-server is already in the cache, it's decrypted and verified. If
46 it's not in the cache but the TGT is, the TGT is used to obtain the
47 ticket for the end-server. The end-server ticket is then verified. If
48 neither ticket is in the cache, but ksu is compiled with the
49 GET_TGT_VIA_PASSWD define, the user will be prompted for a Kerberos
50 password which will then be used to get a TGT. If the user is logged
51 in remotely and does not have a secure channel, the password may be ex‐
52 posed. If neither ticket is in the cache and GET_TGT_VIA_PASSWD is not
53 defined, authentication fails.
54
56 This section describes authorization of the source user when ksu is in‐
57 voked without the -e option. For a description of the -e option, see
58 the OPTIONS section.
59 Upon successful authentication, ksu checks whether the target principal
61 is authorized to access the target account. In the target user's home
62 directory, ksu attempts to access two authorization files: .k5login and
63 .k5users. In the .k5login file each line contains the name of a prin‐
64 cipal that is authorized to access the account.
65 For example:
67 jqpublic@USC.EDU
69 jqpublic/secure@USC.EDU
70 jqpublic/admin@USC.EDU
71 The format of .k5users is the same, except the principal name may be
73 followed by a list of commands that the principal is authorized to exe‐
74 cute (see the -e option in the OPTIONS section for details).
75 Thus if the target principal name is found in the .k5login file the
77 source user is authorized to access the target account. Otherwise ksu
78 looks in the .k5users file. If the target principal name is found
79 without any trailing commands or followed only by * then the source
80 user is authorized. If either .k5login or .k5users exist but an appro‐
81 priate entry for the target principal does not exist then access is de‐
82 nied. If neither file exists then the principal will be granted access
83 to the account according to the aname->lname mapping rules. Otherwise,
84 authorization fails.
85
87 Upon successful authentication and authorization, ksu proceeds in a
88 similar fashion to su. The environment is unmodified with the excep‐
89 tion of USER, HOME and SHELL variables. If the target user is not
90 root, USER gets set to the target user name. Otherwise USER remains
91 unchanged. Both HOME and SHELL are set to the target login's default
92 values. In addition, the environment variable KRB5CCNAME gets set to
93 the name of the target cache. The real and effective user ID are
94 changed to that of the target user. The target user's shell is then
95 invoked (the shell name is specified in the password file). Upon ter‐
96 mination of the shell, ksu deletes the target cache (unless ksu is in‐
97 voked with the -k option). This is implemented by first doing a fork
98 and then an exec, instead of just exec, as done by su.
99
101 ksu can be used to create a new security context for the target program
102 (either the target shell, or command specified via the -e option). The
103 target program inherits a set of credentials from the source user. By
104 default, this set includes all of the credentials in the source cache
105 plus any additional credentials obtained during authentication. The
106 source user is able to limit the credentials in this set by using -z or
107 -Z option. -z restricts the copy of tickets from the source cache to
108 the target cache to only the tickets where client == the target princi‐
109 pal name. The -Z option provides the target user with a fresh target
110 cache (no creds in the cache). Note that for security reasons, when
111 the source user is root and target user is non-root, -z option is the
112 default mode of operation.
113 While no authentication takes place if the source user is root or is
115 the same as the target user, additional tickets can still be obtained
116 for the target cache. If -n is specified and no credentials can be
117 copied to the target cache, the source user is prompted for a Kerberos
118 password (unless -Z specified or GET_TGT_VIA_PASSWD is undefined). If
119 successful, a TGT is obtained from the Kerberos server and stored in
120 the target cache. Otherwise, if a password is not provided (user hit
121 return) ksu continues in a normal mode of operation (the target cache
122 will not contain the desired TGT). If the wrong password is typed in,
123 ksu fails.
124 NOTE:
126 During authentication, only the tickets that could be obtained with‐
127 out providing a password are cached in the source cache.
128
130 -n target_principal_name
131 Specify a Kerberos target principal name. Used in authentica‐
132 tion and authorization phases of ksu.
133 If ksu is invoked without -n, a default principal name is as‐
135 signed via the following heuristic:
136 • Case 1: source user is non-root.
138 If the target user is the source user the default principal
140 name is set to the default principal of the source cache. If
141 the cache does not exist then the default principal name is
142 set to target_user@local_realm. If the source and target
143 users are different and neither ~target_user/.k5users nor
144 ~target_user/.k5login exist then the default principal name is
145 target_user_login_name@local_realm. Otherwise, starting with
146 the first principal listed below, ksu checks if the principal
147 is authorized to access the target account and whether there
148 is a legitimate ticket for that principal in the source cache.
149 If both conditions are met that principal becomes the default
150 target principal, otherwise go to the next principal.
151 a. default principal of the source cache
153 b. target_user@local_realm
155 c. source_user@local_realm
157 If a-c fails try any principal for which there is a ticket in
159 the source cache and that is authorized to access the target
160 account. If that fails select the first principal that is au‐
161 thorized to access the target account from the above list. If
162 none are authorized and ksu is configured with
163 PRINC_LOOK_AHEAD turned on, select the default principal as
164 follows:
165 For each candidate in the above list, select an authorized
167 principal that has the same realm name and first part of the
168 principal name equal to the prefix of the candidate. For ex‐
169 ample if candidate a) is jqpublic@ISI.EDU and jqpublic/se‐
170 cure@ISI.EDU is authorized to access the target account then
171 the default principal is set to jqpublic/secure@ISI.EDU.
172 • Case 2: source user is root.
174 If the target user is non-root then the default principal name
176 is target_user@local_realm. Else, if the source cache exists
177 the default principal name is set to the default principal of
178 the source cache. If the source cache does not exist, default
179 principal name is set to root\@local_realm.
180 -c source_cache_name
182 Specify source cache name (e.g., -c FILE:/tmp/my_cache). If -c op‐
183 tion is not used then the name is obtained from KRB5CCNAME environ‐
184 ment variable. If KRB5CCNAME is not defined the source cache name
185 is set to krb5cc_<source uid>. The target cache name is automati‐
186 cally set to krb5cc_<target uid>.(gen_sym()), where gen_sym gener‐
187 ates a new number such that the resulting cache does not already ex‐
188 ist. For example:
189 krb5cc_1984.2
191 -k Do not delete the target cache upon termination of the target
193 shell or a command (-e command). Without -k, ksu deletes the
194 target cache.
195 -z Restrict the copy of tickets from the source cache to the target
197 cache to only the tickets where client == the target principal
198 name. Use the -n option if you want the tickets for other then
199 the default principal. Note that the -z option is mutually ex‐
200 clusive with the -Z option.
201 -Z Don't copy any tickets from the source cache to the target
203 cache. Just create a fresh target cache, where the default
204 principal name of the cache is initialized to the target princi‐
205 pal name. Note that the -Z option is mutually exclusive with
206 the -z option.
207 -q Suppress the printing of status messages.
209 Ticket granting ticket options:
211 -l lifetime -r time -p -P -f -F
213 The ticket granting ticket options only apply to the case where
214 there are no appropriate tickets in the cache to authenticate
215 the source user. In this case if ksu is configured to prompt
216 users for a Kerberos password (GET_TGT_VIA_PASSWD is defined),
217 the ticket granting ticket options that are specified will be
218 used when getting a ticket granting ticket from the Kerberos
219 server.
220 -l lifetime
222 (Time duration string.) Specifies the lifetime to be requested
223 for the ticket; if this option is not specified, the default
224 ticket lifetime (12 hours) is used instead.
225 -r time
227 (Time duration string.) Specifies that the renewable option
228 should be requested for the ticket, and specifies the desired
229 total lifetime of the ticket.
230 -p specifies that the proxiable option should be requested for the
232 ticket.
233 -P specifies that the proxiable option should not be requested for
235 the ticket, even if the default configuration is to ask for
236 proxiable tickets.
237 -f option specifies that the forwardable option should be requested
239 for the ticket.
240 -F option specifies that the forwardable option should not be re‐
242 quested for the ticket, even if the default configuration is to
243 ask for forwardable tickets.
244 -e command [args ...]
246 ksu proceeds exactly the same as if it was invoked without the
247 -e option, except instead of executing the target shell, ksu ex‐
248 ecutes the specified command. Example of usage:
249 ksu bob -e ls -lag
251 The authorization algorithm for -e is as follows:
253 If the source user is root or source user == target user, no au‐
255 thorization takes place and the command is executed. If source
256 user id != 0, and ~target_user/.k5users file does not exist, au‐
257 thorization fails. Otherwise, ~target_user/.k5users file must
258 have an appropriate entry for target principal to get autho‐
259 rized.
260 The .k5users file format:
262 A single principal entry on each line that may be followed by a
264 list of commands that the principal is authorized to execute. A
265 principal name followed by a * means that the user is authorized
266 to execute any command. Thus, in the following example:
267 jqpublic@USC.EDU ls mail /local/kerberos/klist
269 jqpublic/secure@USC.EDU *
270 jqpublic/admin@USC.EDU
271 jqpublic@USC.EDU is only authorized to execute ls, mail and
273 klist commands. jqpublic/secure@USC.EDU is authorized to exe‐
274 cute any command. jqpublic/admin@USC.EDU is not authorized to
275 execute any command. Note, that jqpublic/admin@USC.EDU is au‐
276 thorized to execute the target shell (regular ksu, without the
277 -e option) but jqpublic@USC.EDU is not.
278 The commands listed after the principal name must be either a
280 full path names or just the program name. In the second case,
281 CMD_PATH specifying the location of authorized programs must be
282 defined at the compilation time of ksu. Which command gets exe‐
283 cuted?
284 If the source user is root or the target user is the source user
286 or the user is authorized to execute any command (* entry) then
287 command can be either a full or a relative path leading to the
288 target program. Otherwise, the user must specify either a full
289 path or just the program name.
290 -a args
292 Specify arguments to be passed to the target shell. Note that
293 all flags and parameters following -a will be passed to the
294 shell, thus all options intended for ksu must precede -a.
295 The -a option can be used to simulate the -e option if used as
297 follows:
298 -a -c [command [arguments]].
300 -c is interpreted by the c-shell to execute the command.
302
304 ksu can be compiled with the following four flags:
305 GET_TGT_VIA_PASSWD
307 In case no appropriate tickets are found in the source cache,
308 the user will be prompted for a Kerberos password. The password
309 is then used to get a ticket granting ticket from the Kerberos
310 server. The danger of configuring ksu with this macro is if the
311 source user is logged in remotely and does not have a secure
312 channel, the password may get exposed.
313 PRINC_LOOK_AHEAD
315 During the resolution of the default principal name,
316 PRINC_LOOK_AHEAD enables ksu to find principal names in the
317 .k5users file as described in the OPTIONS section (see -n op‐
318 tion).
319 CMD_PATH
321 Specifies a list of directories containing programs that users
322 are authorized to execute (via .k5users file).
323 HAVE_GETUSERSHELL
325 If the source user is non-root, ksu insists that the target
326 user's shell to be invoked is a "legal shell". getusershell(3)
327 is called to obtain the names of "legal shells". Note that the
328 target user's shell is obtained from the passwd file.
329 Sample configuration:
331 KSU_OPTS = -DGET_TGT_VIA_PASSWD -DPRINC_LOOK_AHEAD -DCMD_PATH='"/bin /usr/ucb /local/bin"
333 ksu should be owned by root and have the set user id bit turned on.
335 ksu attempts to get a ticket for the end server just as Kerberized tel‐
337 net and rlogin. Thus, there must be an entry for the server in the
338 Kerberos database (e.g., host/nii.isi.edu@ISI.EDU). The keytab file
339 must be in an appropriate location.
340
342 ksu deletes all expired tickets from the source cache.
343
345 GENNADY (ARI) MEDVINSKY
346
348 See kerberos for a description of Kerberos environment variables.
349
351 kerberos, kinit
352
354 MIT
355
357 1985-2023, MIT
3581.21.2 KSU(1)