kdc.conf

The kdc.conf file supplements krb5.conf for programs which are typically only used on a KDC, such as the krb5kdc and kadmind daemons and the kdb5_util program. Relations documented here may also be specified in krb5.conf; for the KDC programs mentioned, krb5.conf and kdc.conf will be merged into a single configuration profile.

Normally, the kdc.conf file is found in the KDC state directory, LOCALSTATEDIR/krb5kdc. You can override the default location by setting the environment variable KRB5_KDC_PROFILE.

Please note that you need to restart the KDC daemon for any configuration changes to take effect.

Structure

The kdc.conf file is set up in the same format as the krb5.conf file.

Sections

The kdc.conf file may contain the following sections:

[kdcdefaults]	Default values for KDC behavior
[realms]	Realm-specific database configuration and settings
[dbdefaults]	Default database settings
[dbmodules]	Per-database settings
[logging]	Controls how Kerberos daemons perform logging

[kdcdefaults]

With one exception, relations in the [kdcdefaults] section specify default values for realm variables, to be used if the [realms] subsection does not contain a relation for the tag. See the [realms] section for the definitions of these relations.

• host_based_services
• kdc_ports
• kdc_tcp_ports
• no_host_referral
• restrict_anonymous_to_tgt

kdc_max_dgram_reply_size

Specifies the maximum packet size that can be sent over UDP. The default value is 4096 bytes.

[realms]

Each tag in the [realms] section is the name of a Kerberos realm. The value of the tag is a subsection where the relations define KDC parameters for that particular realm. The following example shows how to define one parameter for the ATHENA.MIT.EDU realm:

[realms]
    ATHENA.MIT.EDU = {
        max_renewable_life = 7d 0h 0m 0s
    }

The following tags may be specified in a [realms] subsection:

acl_file

(String.) Location of the access control list file that kadmind uses to determine which principals are allowed which permissions on the Kerberos database. The default value is LOCALSTATEDIR/krb5kdc/kadm5.acl. For more information on Kerberos ACL file see kadm5.acl.

database_module

(String.) This relation indicates the name of the configuration section under [dbmodules] for database-specific parameters used by the loadable database library. The default value is the realm name. If this configuration section does not exist, default values will be used for all database parameters.

database_name

(String, deprecated.) This relation specifies the location of the Kerberos database for this realm, if the DB2 module is being used and the [dbmodules] configuration section does not specify a database name. The default value is LOCALSTATEDIR/krb5kdc/principal.

default_principal_expiration

(Absolute time string.) Specifies the default expiration date of principals created in this realm. The default value is 0, which means no expiration date.

default_principal_flags

(Flag string.) Specifies the default attributes of principals created in this realm. The format for this string is a comma-separated list of flags, with ‘+’ before each flag that should be enabled and ‘-‘ before each flag that should be disabled. The postdateable, forwardable, tgt-based, renewable, proxiable, dup-skey, allow-tickets, and service flags default to enabled.

There are a number of possible flags:

allow-tickets

Enabling this flag means that the KDC will issue tickets for this principal. Disabling this flag essentially deactivates the principal within this realm.

dup-skey

Enabling this flag allows the principal to obtain a session key for another user, permitting user-to-user authentication for this principal.

forwardable

Enabling this flag allows the principal to obtain forwardable tickets.

hwauth

If this flag is enabled, then the principal is required to preauthenticate using a hardware device before receiving any tickets.

no-auth-data-required

Enabling this flag prevents PAC data from being added to service tickets for the principal.

ok-as-delegate

If this flag is enabled, it hints the client that credentials can and should be delegated when authenticating to the service.

ok-to-auth-as-delegate

Enabling this flag allows the principal to use S4USelf tickets.

postdateable

Enabling this flag allows the principal to obtain postdateable tickets.

preauth

If this flag is enabled on a client principal, then that principal is required to preauthenticate to the KDC before receiving any tickets. On a service principal, enabling this flag means that service tickets for this principal will only be issued to clients with a TGT that has the preauthenticated bit set.

proxiable

Enabling this flag allows the principal to obtain proxy tickets.

pwchange

Enabling this flag forces a password change for this principal.

pwservice

If this flag is enabled, it marks this principal as a password change service. This should only be used in special cases, for example, if a user’s password has expired, then the user has to get tickets for that principal without going through the normal password authentication in order to be able to change the password.

renewable

Enabling this flag allows the principal to obtain renewable tickets.

service

Enabling this flag allows the the KDC to issue service tickets for this principal.

tgt-based

Enabling this flag allows a principal to obtain tickets based on a ticket-granting-ticket, rather than repeating the authentication process that was used to obtain the TGT.

dict_file

(String.) Location of the dictionary file containing strings that are not allowed as passwords. If none is specified or if there is no policy assigned to the principal, no dictionary checks of passwords will be performed.

host_based_services

(Whitespace- or comma-separated list.) Lists services which will get host-based referral processing even if the server principal is not marked as host-based by the client.

iprop_enable

(Boolean value.) Specifies whether incremental database propagation is enabled. The default value is false.

iprop_master_ulogsize

(Integer.) Specifies the maximum number of log entries to be retained for incremental propagation. The default value is 1000. Prior to release 1.11, the maximum value was 2500.

iprop_slave_poll

(Delta time string.) Specifies how often the slave KDC polls for new updates from the master. The default value is 2m (that is, two minutes).

iprop_port

(Port number.) Specifies the port number to be used for incremental propagation. This is required in both master and slave configuration files.

iprop_resync_timeout

(Delta time string.) Specifies the amount of time to wait for a full propagation to complete. This is optional in configuration files, and is used by slave KDCs only. The default value is 5 minutes (5m).

iprop_logfile

(File name.) Specifies where the update log file for the realm database is to be stored. The default is to use the database_name entry from the realms section of the krb5 config file, with .ulog appended. (NOTE: If database_name isn’t specified in the realms section, perhaps because the LDAP database back end is being used, or the file name is specified in the [dbmodules] section, then the hard-coded default for database_name is used. Determination of the iprop_logfile default value will not use values from the [dbmodules] section.)

kadmind_port

(Port number.) Specifies the port on which the kadmind daemon is to listen for this realm. The assigned port for kadmind is 749, which is used by default.

key_stash_file

(String.) Specifies the location where the master key has been stored (via kdb5_util stash). The default is LOCALSTATEDIR/krb5kdc/.k5.REALM, where REALM is the Kerberos realm.

kdc_ports

(Whitespace- or comma-separated list.) Lists the ports on which the Kerberos server should listen for UDP requests, as a comma-separated list of integers. The default value is 88,750, which are the assigned Kerberos port and the port historically used by Kerberos V4.

kdc_tcp_ports

(Whitespace- or comma-separated list.) Lists the ports on which the Kerberos server should listen for TCP connections, as a comma-separated list of integers. If this relation is not specified, the compiled-in default is not to listen for TCP connections at all.

If you wish to change this (note that the current implementation has little protection against denial-of-service attacks), the standard port number assigned for Kerberos TCP traffic is port 88.

master_key_name

(String.) Specifies the name of the principal associated with the master key. The default is K/M.

master_key_type

(Key type string.) Specifies the master key’s key type. The default value for this is aes256-cts-hmac-sha1-96. For a list of all possible values, see Encryption types.

max_life

(Time duration string.) Specifies the maximum time period for which a ticket may be valid in this realm. The default value is 24 hours.

max_renewable_life

(Time duration string.) Specifies the maximum time period during which a valid ticket may be renewed in this realm. The default value is 0.

no_host_referral

(Whitespace- or comma-separated list.) Lists services to block from getting host-based referral processing, even if the client marks the server principal as host-based or the service is also listed in host_based_services. no_host_referral = * will disable referral processing altogether.

des_crc_session_supported

(Boolean value). If set to true, the KDC will assume that service principals support des-cbc-crc for session key enctype negotiation purposes. If allow_weak_crypto in [libdefaults] is false, or if des-cbc-crc is not a permitted enctype, then this variable has no effect. Defaults to true.

reject_bad_transit

(Boolean value.) If set to true, the KDC will check the list of transited realms for cross-realm tickets against the transit path computed from the realm names and the capaths section of its krb5.conf file; if the path in the ticket to be issued contains any realms not in the computed path, the ticket will not be issued, and an error will be returned to the client instead. If this value is set to false, such tickets will be issued anyways, and it will be left up to the application server to validate the realm transit path.

If the disable-transited-check flag is set in the incoming request, this check is not performed at all. Having the reject_bad_transit option will cause such ticket requests to be rejected always.

This transit path checking and config file option currently apply only to TGS requests.

The default value is true.

restrict_anonymous_to_tgt

(Boolean value.) If set to true, the KDC will reject ticket requests from anonymous principals to service principals other than the realm’s ticket-granting service. This option allows anonymous PKINIT to be enabled for use as FAST armor tickets without allowing anonymous authentication to services. The default value is false.

supported_enctypes

(List of key:salt strings.) Specifies the default key/salt combinations of principals for this realm. Any principals created through kadmin will have keys of these types. The default value for this tag is aes256-cts-hmac-sha1-96:normal aes128-cts-hmac-sha1-96:normal des3-cbc-sha1:normal arcfour-hmac-md5:normal. For lists of possible values, see Keysalt lists.

[dbdefaults]

The [dbdefaults] section specifies default values for some database parameters, to be used if the [dbmodules] subsection does not contain a relation for the tag. See the [dbmodules] section for the definitions of these relations.

• ldap_kerberos_container_dn
• ldap_kdc_dn
• ldap_kadmind_dn
• ldap_service_password_file
• ldap_servers
• ldap_conns_per_server

[dbmodules]

The [dbmodules] section contains parameters used by the KDC database library and database modules. Each tag in the [dbmodules] section is the name of a Kerberos realm or a section name specified by a realm’s database_module parameter. The following example shows how to define one database parameter for the ATHENA.MIT.EDU realm:

[dbmodules]
    ATHENA.MIT.EDU = {
        disable_last_success = true
    }

The following tags may be specified in a [dbmodules] subsection:

database_name

This DB2-specific tag indicates the location of the database in the filesystem. The default is LOCALSTATEDIR/krb5kdc/principal.

db_library

This tag indicates the name of the loadable database module. The value should be db2 for the DB2 module and kldap for the LDAP module.

disable_last_success

If set to true, suppresses KDC updates to the “Last successful authentication” field of principal entries requiring preauthentication. Setting this flag may improve performance. (Principal entries which do not require preauthentication never update the “Last successful authentication” field.). First introduced in version 1.9.

disable_lockout

If set to true, suppresses KDC updates to the “Last failed authentication” and “Failed password attempts” fields of principal entries requiring preauthentication. Setting this flag may improve performance, but also disables account lockout. First introduced in version 1.9.

ldap_conns_per_server

This LDAP-specific tag indicates the number of connections to be maintained per LDAP server.

ldap_kadmind_dn

This LDAP-specific tag indicates the default bind DN for the kadmind daemon. kadmind does a login to the directory as this object. This object should have the rights to read and write the Kerberos data in the LDAP database.

ldap_kdc_dn

This LDAP-specific tag indicates the default bind DN for the krb5kdc daemon. The KDC does a login to the directory as this object. This object should have the rights to read the Kerberos data in the LDAP database, and to write data unless disable_lockout and disable_last_success are true.

ldap_kerberos_container_dn

This LDAP-specific tag indicates the DN of the container object where the realm objects will be located.

ldap_servers

This LDAP-specific tag indicates the list of LDAP servers that the Kerberos servers can connect to. The list of LDAP servers is whitespace-separated. The LDAP server is specified by a LDAP URI. It is recommended to use ldapi: or ldaps: URLs to connect to the LDAP server.

ldap_service_password_file

This LDAP-specific tag indicates the file containing the stashed passwords (created by kdb5_ldap_util stashsrvpw) for the ldap_kadmind_dn and ldap_kdc_dn objects. This file must be kept secure.

The following tag may be specified directly in the [dbmodules] section to control where database modules are loaded from:

db_module_dir

This tag controls where the plugin system looks for database modules. The value should be an absolute path.

[logging]

The [logging] section indicates how krb5kdc and kadmind perform logging. The keys in this section are daemon names, which may be one of:

admin_server

Specifies how kadmind performs logging.

kdc

Specifies how krb5kdc performs logging.

default

Specifies how either daemon performs logging in the absence of relations specific to the daemon.

Values are of the following forms:

FILE=filename or FILE:filename

This value causes the daemon’s logging messages to go to the filename. If the = form is used, the file is overwritten. If the : form is used, the file is appended to.

STDERR

This value causes the daemon’s logging messages to go to its standard error stream.

CONSOLE

This value causes the daemon’s logging messages to go to the console, if the system supports it.

DEVICE=<devicename>

This causes the daemon’s logging messages to go to the specified device.

SYSLOG[:severity[:facility]]

This causes the daemon’s logging messages to go to the system log.

The severity argument specifies the default severity of system log messages. This may be any of the following severities supported by the syslog(3) call, minus the LOG_ prefix: EMERG, ALERT, CRIT, ERR, WARNING, NOTICE, INFO, and DEBUG.

The facility argument specifies the facility under which the messages are logged. This may be any of the following facilities supported by the syslog(3) call minus the LOG_ prefix: KERN, USER, MAIL, DAEMON, AUTH, LPR, NEWS, UUCP, CRON, and LOCAL0 through LOCAL7.

If no severity is specified, the default is ERR. If no facility is specified, the default is AUTH.

In the following example, the logging messages from the KDC will go to the console and to the system log under the facility LOG_DAEMON with default severity of LOG_INFO; and the logging messages from the administrative server will be appended to the file /var/adm/kadmin.log and sent to the device /dev/tty04.

[logging]
    kdc = CONSOLE
    kdc = SYSLOG:INFO:DAEMON
    admin_server = FILE:/var/adm/kadmin.log
    admin_server = DEVICE=/dev/tty04

PKINIT options

Note

The following are pkinit-specific options. These values may be specified in [kdcdefaults] as global defaults, or within a realm-specific subsection of [realms]. Also note that a realm-specific value over-rides, does not add to, a generic [kdcdefaults] specification. The search order is:

1. realm-specific subsection of [realms]:

   [realms]
       EXAMPLE.COM = {
           pkinit_anchors = FILE:/usr/local/example.com.crt
       }
   

2. generic value in the [kdcdefaults] section:

   [kdcdefaults]
       pkinit_anchors = DIR:/usr/local/generic_trusted_cas/
   

For information about the syntax of some of these options, see Specifying PKINIT identity information in krb5.conf.

pkinit_anchors

Specifies the location of trusted anchor (root) certificates which the KDC trusts to sign client certificates. This option is required if pkinit is to be supported by the KDC. This option may be specified multiple times.

pkinit_dh_min_bits

Specifies the minimum number of bits the KDC is willing to accept for a client’s Diffie-Hellman key. The default is 2048.

pkinit_allow_upn

Specifies that the KDC is willing to accept client certificates with the Microsoft UserPrincipalName (UPN) Subject Alternative Name (SAN). This means the KDC accepts the binding of the UPN in the certificate to the Kerberos principal name. The default value is false.

Without this option, the KDC will only accept certificates with the id-pkinit-san as defined in RFC 4556. There is currently no option to disable SAN checking in the KDC.

pkinit_eku_checking

This option specifies what Extended Key Usage (EKU) values the KDC is willing to accept in client certificates. The values recognized in the kdc.conf file are:

kpClientAuth

This is the default value and specifies that client certificates must have the id-pkinit-KPClientAuth EKU as defined in RFC 4556.

scLogin

If scLogin is specified, client certificates with the Microsoft Smart Card Login EKU (id-ms-kp-sc-logon) will be accepted.

none

If none is specified, then client certificates will not be checked to verify they have an acceptable EKU. The use of this option is not recommended.

pkinit_identity

Specifies the location of the KDC’s X.509 identity information. This option is required if pkinit is to be supported by the KDC.

pkinit_kdc_ocsp

Specifies the location of the KDC’s OCSP.

pkinit_pool

Specifies the location of intermediate certificates which may be used by the KDC to complete the trust chain between a client’s certificate and a trusted anchor. This option may be specified multiple times.

pkinit_revoke

Specifies the location of Certificate Revocation List (CRL) information to be used by the KDC when verifying the validity of client certificates. This option may be specified multiple times.

pkinit_require_crl_checking

The default certificate verification process will always check the available revocation information to see if a certificate has been revoked. If a match is found for the certificate in a CRL, verification fails. If the certificate being verified is not listed in a CRL, or there is no CRL present for its issuing CA, and pkinit_require_crl_checking is false, then verification succeeds.

However, if pkinit_require_crl_checking is true and there is no CRL information available for the issuing CA, then verification fails.

pkinit_require_crl_checking should be set to true if the policy is such that up-to-date CRLs must be present for every CA.

Encryption types

Any tag in the configuration files which requires a list of encryption types can be set to some combination of the following strings. Encryption types marked as “weak” are available for compatibility but not recommended for use.

des-cbc-crc	DES cbc mode with CRC-32 (weak)
des-cbc-md4	DES cbc mode with RSA-MD4 (weak)
des-cbc-md5	DES cbc mode with RSA-MD5 (weak)
des-cbc-raw	DES cbc mode raw (weak)
des3-cbc-raw	Triple DES cbc mode raw (weak)
des3-cbc-sha1 des3-hmac-sha1 des3-cbc-sha1-kd	Triple DES cbc mode with HMAC/sha1
des-hmac-sha1	DES with HMAC/sha1 (weak)
aes256-cts-hmac-sha1-96 aes256-cts AES-256	CTS mode with 96-bit SHA-1 HMAC
aes128-cts-hmac-sha1-96 aes128-cts AES-128	CTS mode with 96-bit SHA-1 HMAC
arcfour-hmac rc4-hmac arcfour-hmac-md5	RC4 with HMAC/MD5
arcfour-hmac-exp rc4-hmac-exp arcfour-hmac-md5-exp	Exportable RC4 with HMAC/MD5 (weak)
camellia256-cts-cmac camellia256-cts	Camellia-256 CTS mode with CMAC
camellia128-cts-cmac camellia128-cts	Camellia-128 CTS mode with CMAC
des	The DES family: des-cbc-crc, des-cbc-md5, and des-cbc-md4 (weak)
des3	The triple DES family: des3-cbc-sha1
aes	The AES family: aes256-cts-hmac-sha1-96 and aes128-cts-hmac-sha1-96
rc4	The RC4 family: arcfour-hmac
camellia	The Camellia family: camellia256-cts-cmac and camellia128-cts-cmac

The string DEFAULT can be used to refer to the default set of types for the variable in question. Types or families can be removed from the current list by prefixing them with a minus sign (“-”). Types or families can be prefixed with a plus sign (“+”) for symmetry; it has the same meaning as just listing the type or family. For example, “DEFAULT -des” would be the default set of encryption types with DES types removed, and “des3 DEFAULT” would be the default set of encryption types with triple DES types moved to the front.

While aes128-cts and aes256-cts are supported for all Kerberos operations, they are not supported by very old versions of our GSSAPI implementation (krb5-1.3.1 and earlier). Services running versions of krb5 without AES support must not be given AES keys in the KDC database.

Keysalt lists

Kerberos keys for users are usually derived from passwords. Kerberos commands and configuration parameters that affect generation of keys take lists of enctype-salttype (“keysalt”) pairs, known as keysalt lists. Each keysalt pair is an enctype name followed by a salttype name, in the format enc:salt. Individual keysalt list members are separated by comma (”,”) characters or space characters. For example:

kadmin -e aes256-cts:normal,aes128-cts:normal

would start up kadmin so that by default it would generate password-derived keys for the aes256-cts and aes128-cts encryption types, using a normal salt.

To ensure that people who happen to pick the same password do not have the same key, Kerberos 5 incorporates more information into the key using something called a salt. The supported salt types are as follows:

normal	default for Kerberos Version 5
v4	the only type used by Kerberos Version 4 (no salt)
norealm	same as the default, without using realm information
onlyrealm	uses only realm information as the salt
afs3	AFS version 3, only used for compatibility with Kerberos 4 in AFS
special	generate a random salt

Sample kdc.conf File

Here’s an example of a kdc.conf file:

[kdcdefaults]
    kdc_ports = 88

[realms]
    ATHENA.MIT.EDU = {
        kadmind_port = 749
        max_life = 12h 0m 0s
        max_renewable_life = 7d 0h 0m 0s
        master_key_type = aes256-cts-hmac-sha1-96
        supported_enctypes = aes256-cts-hmac-sha1-96:normal aes128-cts-hmac-sha1-96:normal
        database_module = openldap_ldapconf
    }

[logging]
    kdc = FILE:/usr/local/var/krb5kdc/kdc.log
    admin_server = FILE:/usr/local/var/krb5kdc/kadmin.log

[dbdefaults]
    ldap_kerberos_container_dn = cn=krbcontainer,dc=mit,dc=edu

[dbmodules]
    openldap_ldapconf = {
        db_library = kldap
        disable_last_success = true
        ldap_kdc_dn = "cn=krbadmin,dc=mit,dc=edu"
            # this object needs to have read rights on
            # the realm container and principal subtrees
        ldap_kadmind_dn = "cn=krbadmin,dc=mit,dc=edu"
            # this object needs to have read and write rights on
            # the realm container and principal subtrees
        ldap_service_password_file = /etc/kerberos/service.keyfile
        ldap_servers = ldaps://kerberos.mit.edu
        ldap_conns_per_server = 5
    }

FILES

LOCALSTATEDIR/krb5kdc/kdc.conf

SEE ALSO

krb5.conf, krb5kdc, kadm5.acl