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Wiki source code of KerberosAndLDAP

Version 36.1 by Sirius Rayner-Karlsson on 2024/05/09 10:00
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1 {{box cssClass="floatinginfobox" title="**Contents**"}}
2 {{toc/}}
3 {{/box}}
4
5 = Debian =
6
7 The guide I followed was https://wiki.debian.org/LDAP/OpenLDAPSetup#Kerberos which while it worked required some minor tweaks. I obtained edit privileges for the Debian Wiki and updated the guide with the fixes that I found. I however have a Synology NAS and that can run an LDAP Server. So this guide differs from the upstream Debian Guide.
8
9 Assumption is that you have installed the LDAP Server package on your NAS and gone through initial configuration steps, so it has a domain, there is a DN you can bind as and so forth. It is also assumed you have a Debian system (12.5 or later, though this guide should work with 11.x and likely 10.x as well) that will become your KDC and KAdmin server.
10
11 Recommendation is that you create actual ##.ldif## files rather than use here-documents as used in this guide. It is far easier to make adjustments to things if you have a file to edit rather than having to type it all out again or paste it and then have to try and make edits to it without making mistakes.
12
13 **The guide is for illustration. Expectation is that you do not follow it verbatim but adapt it to your needs.**
14
15
16 === Install packages ===
17
18 (% class="wikigeneratedid" %)
19 The packages you need are [[krb5-kdc-ldap>>url:https://packages.debian.org/krb5-kdc-ldap]], [[krb5-admin-server>>url:https://packages.debian.org/krb5-admin-server]] for the actual KDC and [[schema2ldif>>url:https://packages.debian.org/schema2ldif]]  plus [[slapd>>https://packages.debian.org/search?keywords=slapd]] for adding the schema and ##slappasswd##. They are to be installed on your designated Debian host.
20
21 (% style="color:#400" %)
22 ##{{{$ sudo apt install krb5-kdc-ldap krb5-admin-server schema2ldif}}}##
23
24
25 === Load kerberos LDAP schema ===
26
27 Now you need to load the kerberos schema into the LDAP server on the Synology. Use the ##cn=config## DN.
28
29 (% style="color:#400" %)
30 ##{{{$ zcat /usr/share/doc/krb5-kdc-ldap/kerberos.openldap.ldif.gz | ldapadd -H ldaps://nas.example.com/ -D cn=config -W Enter LDAP Password: adding new entry "cn=kerberos,cn=schema,cn=config" $}}}##
31
32
33 === Create Index on krbPrincipalName ===
34
35 Having an index on the ##krbPrincipalName## improves performance. Synology OpenLDAP does not use ##mdb## format, it uses ##bdb##. Debian ##slapd## uses ##mdb## format. It is different database format, but the principle is the same. Again, as you are modifying config, the DN is ##cn=config##. Use the main password you set for the Synology LDAP server. Create a file ##step1.ldif## with the following content:
36
37 (% style="color:#400" %)
38 ##{{{dn: olcDatabase={1}bdb,cn=config add: olcDbIndex olcDbIndex: krbPrincipalName eq,pres,sub}}}##
39
40
41 and apply it with
42
43 (% style="color:#400" %)
44 ##{{{$ ldapmodify -H ldaps://nas.example.com/ -D cn=config -W -f step1.ldif Enter LDAP Password: modifying entry "olcDatabase={1}bdb,cn=config" $}}}##
45
46
47 === Create principals kadmin and kdc ===
48
49 Next, you create and configure two entries which will be used by the Kerberos servers to connect to OpenLDAP. Not running the Kerberos KDC and Admin Server on the same host as OpenLDAP, these steps are required. Keeping things confined, everything will be created under a separate ##organizationalUnit##. My guide differs from the official Debian guide here. Due to Synology OpenLDAP having a strict password policy, it was necessary to adjust the DNs of ##kdc## and ##kadmin##. The official guide use placeholder passwords which does not work with the Synology LDAP server.
50 Generate the passwords upfront with ##slappasswd -h {SSHA}##. Then create a file ##step2.ldif## with the following content:
51
52 (% style="color:#400" %)
53 ##{{{dn: ou=kerberos,dc=example,dc=com objectClass: organizationalUnit objectClass: top ou: kerberos dn: cn=kdc,ou=kerberos,dc=example,dc=com cn: kdc sn: kdc objectClass: person objectClass: pwdPolicy pwdAttribute: userPassword pwdMinLength: 8 pwdCheckQuality: 2 pwdPolicySubentry: cn=kdc,ou=kerberos,dc=example,dc=com userPassword: {SSHA}<password-hash> description: Kerberos KDC Account dn: cn=kadmin,ou=kerberos,dc=example,dc=com cn: kadmin sn: kadmin objectClass: person objectClass: pwdPolicy pwdAttribute: userPassword pwdMinLength: 8 pwdCheckQuality: 2 pwdPolicySubentry: cn=kadmin,ou=kerberos,dc=example,dc=com userPassword: {SSHA}<password-hash> description: Kerberos KDC Account}}}##
54
55 Apply it with
56
57 (% style="color:#400" %)
58 ##{{{$ ldapadd -H ldaps://nas.example.com/ -D uid=root,cn=users,dc=example,dc=com -W -f step2.ldif Enter LDAP Password: adding new entry "ou=kerberos,dc=example,dc=com" adding new entry "cn=kdc,ou=kerberos,dc=example,dc=com" adding new entry "cn=kadmin,ou=kerberos,dc=example,dc=com"}}}##
59
60
61
62 === Grant kdc and kadmin permissions ===
63
64 This switches back to the ##cn=config## DN as you are changing the permissions.
65
66 $ ldapmodify -H ldaps:~/~/ds723.trudheim.com -W -D cn=config <<EOF
67
68 dn: olcDatabase={1}bdb,cn=config
69 add: olcAccess
70 olcAccess: {0}to attrs=krbPrincipalKey
71 by anonymous auth
72 by dn.exact="cn=kdc,ou=kerberos,dc=trudheim,dc=com" write
73 by dn.exact="cn=kadmin,ou=kerberos,dc=trudheim,dc=com" write
74 by self write
75 by * none
76 -
77 add: olcAccess
78 olcAccess: {1}to dn.subtree="cn=krbContainer,ou=kerberos,dc=example,dc=com"
79 by dn.exact="cn=kdc,ou=kerberos,dc=trudheim,dc=com" write
80 by dn.exact="cn=kadmin,ou=kerberos,dc=trudheim,dc=com" write
81 by * none
82
83 EOF
84
85 Enter LDAP Password:
86 modifying entry "olcDatabase={1}bdb,cn=config"
87
88 $
89
90
91 Note that we now reference our kdc and kadmin accounts and we grant them permission to the krbContainer which will house all our kerberos principals. Give both of them write access, because we do want to have the ability to track last login and lock accounts if there are login failures. We like security.
92
93
94 === Create krb5.conf ===
95
96 Over to adjusting /etc/krb5.conf so that it will point to the right thing later. It should look something like this:
97
98
99 {{{[libdefaults]
100 default_realm = EXAMPLE.COM
101 dns_lookup_realm = false
102 dns_lookup_kdc = false
103 ticket_lifetime = 24h
104 forwardable = true
105 proxiable = true
106 rdns = false
107
108 [realms]
109 EXAMPLE.COM = {
110 kdc = debian.example.com
111 admin_server = debian.example.com
112 default_domain = example.com
113 }
114 [domain_realm]
115 .example.com = EXAMPLE.COM
116 example.com = EXAMPLE.COM}}}
117
118 Make sure your designated debian server have ports 88, 464 and 749 open, both for TCP and UDP, in its firewall. 88 is for the kdc, 464 and 749 is for kadmin.
119
120
121 === Create kdc.conf ===
122
123 Next, we need to write up /etc/krb5kdc/kdc.conf. Something like this should work
124
125
126 [libdefaults]
127
128 [realms]
129 TRUDHEIM.COM = {
130 database_module = openldap_ldapconf
131 max_life = 7d
132 max_renewable_life = 6d
133 }
134
135 [dbdefaults]
136 ldap_kerberos_container_dn = cn=krbContainer,ou=kerberos,dc=trudheim,dc=com
137
138 [dbmodules]
139 openldap_ldapconf = {
140 db_library = kldap
141 disable_last_success = false
142 disable_lockout = false
143 ldap_conns_per_server = 5
144 ldap_servers = ldaps:~/~/ds723.trudheim.com
145 ldap_kdc_dn = "cn=kdc,ou=kerberos,dc=trudheim,dc=com"
146 ldap_kadmind_dn = "cn=kadmin,ou=kerberos,dc=trudheim,dc=com"
147 ldap_service_password_file = /etc/krb5kdc/service.keyfile
148 }
149
150
151 === Create kadm5.acl ===
152
153 Then you need to create ##/etc/krb5kdc/kadm5.acl and put in it##
154
155 ##*/admin@EXAMPLE.COM *##
156
157
158 so that administrator principals can run kadmin. Now we are ready to create the domain. And that we do with
159
160
161 === Create the kerberos domain ===
162
163 #
164
165 kdb5_ldap_util -D uid=root,cn=users,dc=trudheim,dc=com -H ldaps:~/~/ds723.trudheim.com -r TRUDHEIM.COM create -subtrees dc=trudheim,dc=com -maxtktlife '7 Days' -maxrenewlife '6 Days' -s
166 Password for "uid=root,cn=users,dc=trudheim,dc=com":
167 Initializing database for realm 'TRUDHEIM.COM'
168 You will be prompted for the database Master Password.
169 It is important that you NOT FORGET this password.
170 Enter KDC database master key:
171 Re-enter KDC database master key to verify:
172
173 kdb5_ldap_util -D uid=root,cn=users,dc=trudheim,dc=com -H ldaps:~/~/ds723.trudheim.com stashsrvpw -f /etc/krb5kdc/service.keyfile cn=kdc,ou=kerberos,dc=trudheim,dc=com
174 Password for "uid=root,cn=users,dc=trudheim,dc=com":
175 Password for "cn=kdc,ou=kerberos,dc=trudheim,dc=com":
176 Re-enter password for "cn=kdc,ou=kerberos,dc=trudheim,dc=com":
177
178 kdb5_ldap_util -D uid=root,cn=users,dc=trudheim,dc=com -H ldaps:~/~/ds723.trudheim.com stashsrvpw -f /etc/krb5kdc/service.keyfile cn=kadmin,ou=kerberos,dc=trudheim,dc=com
179 Password for "uid=root,cn=users,dc=trudheim,dc=com":
180 Password for "cn=kadmin,ou=kerberos,dc=trudheim,dc=com":
181 Re-enter password for "cn=kadmin,ou=kerberos,dc=trudheim,dc=com":
182
183
184 === Create the first regular principals ===
185
186 Here, you will run kadmin.local to create first a regular user, and then an admin version of that user.
187
188 # kadmin.local
189
190 Authenticating as principal root/admin@TRUDHEIM.COM with password.
191 kadmin.local:  addprinc user
192
193 No policy specified for user@TRUDHEIM.COM; defaulting to no policy
194 Enter password for principal "user@TRUDHEIM.COM":
195 Re-enter password for principal "user@TRUDHEIM.COM":
196 Principal "user@TRUDHEIM.COM" created.
197
198 kadmin.local:  addprinc user/admin
199 No policy specified for user/admin@TRUDHEIM.COM; defaulting to no policy
200 Enter password for principal "user/admin@TRUDHEIM.COM":
201 Re-enter password for principal "user/admin@TRUDHEIM.COM":
202 Principal "user/admin@TRUDHEIM.COM" created.
203 kadmin.local:  q
204
205 #
206
207 Worth to note here is that [[user@EXAMPLE.COM>>mailto:user@EXAMPLE.COM]] and [[user/admin@EXAMPLE.COM>>mailto:user/admin@EXAMPLE.COM]] can have (and should have) different passwords as the admin variant is allowed to do things to the kerberos database. And this is why you want to have the registering of failures to login enabled. Should you have the system exposed to the internet, you can and should expect intrusion attempts. Having Kerberos deployed makes it harder for perpetrators to gain access, but not impossible.
208
209 If you later kerberise your storage and leverage it for NFS4 mounts from your NAS, you can have NFS exposed to the internet as well. Unless someone has a valid kerberos ticket, even if they somehow could mount the share, they see nothing on it without the krbtgt.
210
211
212 === Test your new principal ===
213
214 $ kinit [[user@EXAMPLE.COM>>mailto:user@EXAMPLE.COM]]
215
216 Password for user@EXAMPLE.COM:
217
218 $ klist
219 Ticket cache: FILE:/tmp/krb5cc_1000
220 Default principal: user@EXAMPLE.COM
221
222 Valid starting     Expires            Service principal
223 09/05/24 08:07:35  10/05/24 08:07:35  krbtgt/EXAMPLE.COM@EXAMPLE.COM
224
225 $
226
227
228 === Set up pam and sssd ===
229
230