Provided by: krb5-doc_1.21.3-3_all 
NAME
krb5.conf - Kerberos configuration file
The krb5.conf file contains Kerberos configuration information, including the locations of
KDCs and admin servers for the Kerberos realms of interest, defaults for the current realm
and for Kerberos applications, and mappings of hostnames onto Kerberos realms. Normally,
you should install your krb5.conf file in the directory /etc. You can override the
default location by setting the environment variable KRB5_CONFIG. Multiple
colon-separated filenames may be specified in KRB5_CONFIG; all files which are present
will be read. Starting in release 1.14, directory names can also be specified in
KRB5_CONFIG; all files within the directory whose names consist solely of alphanumeric
characters, dashes, or underscores will be read.
STRUCTURE
The krb5.conf file is set up in the style of a Windows INI file. Lines beginning with '#'
or ';' (possibly after initial whitespace) are ignored as comments. Sections are headed
by the section name, in square brackets. Each section may contain zero or more relations,
of the form:
foo = bar
or:
fubar = {
foo = bar
baz = quux
}
Placing a '*' after the closing bracket of a section name indicates that the section is
final, meaning that if the same section appears within a later file specified in
KRB5_CONFIG, it will be ignored. A subsection can be marked as final by placing a '*'
after either the tag name or the closing brace.
The krb5.conf file can include other files using either of the following directives at the
beginning of a line:
include FILENAME
includedir DIRNAME
FILENAME or DIRNAME should be an absolute path. The named file or directory must exist and
be readable. Including a directory includes all files within the directory whose names
consist solely of alphanumeric characters, dashes, or underscores. Starting in release
1.15, files with names ending in ".conf" are also included, unless the name begins with
".". Included profile files are syntactically independent of their parents, so each
included file must begin with a section header. Starting in release 1.17, files are read
in alphanumeric order; in previous releases, they may be read in any order.
The krb5.conf file can specify that configuration should be obtained from a loadable
module, rather than the file itself, using the following directive at the beginning of a
line before any section headers:
module MODULEPATH:RESIDUAL
MODULEPATH may be relative to the library path of the krb5 installation, or it may be an
absolute path. RESIDUAL is provided to the module at initialization time. If krb5.conf
uses a module directive, kdc.conf(5) should also use one if it exists.
SECTIONS
The krb5.conf file may contain the following sections:
┌───────────────┬──────────────────────────────────┐
│[libdefaults] │ Settings used by the Kerberos V5 │
│ │ library │
├───────────────┼──────────────────────────────────┤
│[realms] │ Realm-specific contact │
│ │ information and settings │
├───────────────┼──────────────────────────────────┤
│[domain_realm] │ Maps server hostnames to │
│ │ Kerberos realms │
├───────────────┼──────────────────────────────────┤
│[capaths] │ Authentication paths for │
│ │ non-hierarchical cross-realm │
├───────────────┼──────────────────────────────────┤
│[appdefaults] │ Settings used by some Kerberos │
│ │ V5 applications │
├───────────────┼──────────────────────────────────┤
│[plugins] │ Controls plugin module │
│ │ registration │
└───────────────┴──────────────────────────────────┘
Additionally, krb5.conf may include any of the relations described in kdc.conf(5), but it
is not a recommended practice.
[libdefaults]
The libdefaults section may contain any of the following relations:
allow_des3
Permit the KDC to issue tickets with des3-cbc-sha1 session keys. In future
releases, this flag will allow des3-cbc-sha1 to be used at all. The default value
for this tag is false. (Added in release 1.21.)
allow_rc4
Permit the KDC to issue tickets with arcfour-hmac session keys. In future
releases, this flag will allow arcfour-hmac to be used at all. The default value
for this tag is false. (Added in release 1.21.)
allow_weak_crypto
If this flag is set to false, then weak encryption types (as noted in
Encryption_types in kdc.conf(5)) will be filtered out of the lists
default_tgs_enctypes, default_tkt_enctypes, and permitted_enctypes. The default
value for this tag is false.
canonicalize
If this flag is set to true, initial ticket requests to the KDC will request
canonicalization of the client principal name, and answers with different client
principals than the requested principal will be accepted. The default value is
false.
ccache_type
This parameter determines the format of credential cache types created by kinit(1)
or other programs. The default value is 4, which represents the most current
format. Smaller values can be used for compatibility with very old implementations
of Kerberos which interact with credential caches on the same host.
clockskew
Sets the maximum allowable amount of clockskew in seconds that the library will
tolerate before assuming that a Kerberos message is invalid. The default value is
300 seconds, or five minutes.
The clockskew setting is also used when evaluating ticket start and expiration
times. For example, tickets that have reached their expiration time can still be
used (and renewed if they are renewable tickets) if they have been expired for a
shorter duration than the clockskew setting.
default_ccache_name
This relation specifies the name of the default credential cache. The default is
FILE:/tmp/krb5cc_%{uid}. This relation is subject to parameter expansion (see
below). New in release 1.11.
default_client_keytab_name
This relation specifies the name of the default keytab for obtaining client
credentials. The default is FILE:/etc/krb5/user/%{euid}/client.keytab. This
relation is subject to parameter expansion (see below). New in release 1.11.
default_keytab_name
This relation specifies the default keytab name to be used by application servers
such as sshd. The default is FILE:/etc/krb5.keytab. This relation is subject to
parameter expansion (see below).
default_rcache_name
This relation specifies the name of the default replay cache. The default is dfl:.
This relation is subject to parameter expansion (see below). New in release 1.18.
default_realm
Identifies the default Kerberos realm for the client. Set its value to your
Kerberos realm. If this value is not set, then a realm must be specified with
every Kerberos principal when invoking programs such as kinit(1).
default_tgs_enctypes
Identifies the supported list of session key encryption types that the client
should request when making a TGS-REQ, in order of preference from highest to
lowest. The list may be delimited with commas or whitespace. See Encryption_types
in kdc.conf(5) for a list of the accepted values for this tag. Starting in release
1.18, the default value is the value of permitted_enctypes. For previous releases
or if permitted_enctypes is not set, the default value is aes256-cts-hmac-sha1-96
aes128-cts-hmac-sha1-96 aes256-cts-hmac-sha384-192 aes128-cts-hmac-sha256-128
des3-cbc-sha1 arcfour-hmac-md5 camellia256-cts-cmac camellia128-cts-cmac.
Do not set this unless required for specific backward compatibility purposes; stale
values of this setting can prevent clients from taking advantage of new stronger
enctypes when the libraries are upgraded.
default_tkt_enctypes
Identifies the supported list of session key encryption types that the client
should request when making an AS-REQ, in order of preference from highest to
lowest. The format is the same as for default_tgs_enctypes. Starting in release
1.18, the default value is the value of permitted_enctypes. For previous releases
or if permitted_enctypes is not set, the default value is aes256-cts-hmac-sha1-96
aes128-cts-hmac-sha1-96 aes256-cts-hmac-sha384-192 aes128-cts-hmac-sha256-128
des3-cbc-sha1 arcfour-hmac-md5 camellia256-cts-cmac camellia128-cts-cmac.
Do not set this unless required for specific backward compatibility purposes; stale
values of this setting can prevent clients from taking advantage of new stronger
enctypes when the libraries are upgraded.
dns_canonicalize_hostname
Indicate whether name lookups will be used to canonicalize hostnames for use in
service principal names. Setting this flag to false can improve security by
reducing reliance on DNS, but means that short hostnames will not be canonicalized
to fully-qualified hostnames. If this option is set to fallback (new in release
1.18), DNS canonicalization will only be performed the server hostname is not found
with the original name when requesting credentials. The default value is true.
dns_lookup_kdc
Indicate whether DNS SRV records should be used to locate the KDCs and other
servers for a realm, if they are not listed in the krb5.conf information for the
realm. (Note that the admin_server entry must be in the krb5.conf realm
information in order to contact kadmind, because the DNS implementation for kadmin
is incomplete.)
Enabling this option does open up a type of denial-of-service attack, if someone
spoofs the DNS records and redirects you to another server. However, it's no worse
than a denial of service, because that fake KDC will be unable to decode anything
you send it (besides the initial ticket request, which has no encrypted data), and
anything the fake KDC sends will not be trusted without verification using some
secret that it won't know.
dns_uri_lookup
Indicate whether DNS URI records should be used to locate the KDCs and other
servers for a realm, if they are not listed in the krb5.conf information for the
realm. SRV records are used as a fallback if no URI records were found. The
default value is true. New in release 1.15.
enforce_ok_as_delegate
If this flag to true, GSSAPI credential delegation will be disabled when the
ok-as-delegate flag is not set in the service ticket. If this flag is false, the
ok-as-delegate ticket flag is only enforced when an application specifically
requests enforcement. The default value is false.
err_fmt
This relation allows for custom error message formatting. If a value is set, error
messages will be formatted by substituting a normal error message for %M and an
error code for %C in the value.
extra_addresses
This allows a computer to use multiple local addresses, in order to allow Kerberos
to work in a network that uses NATs while still using address-restricted tickets.
The addresses should be in a comma-separated list. This option has no effect if
noaddresses is true.
forwardable
If this flag is true, initial tickets will be forwardable by default, if allowed by
the KDC. The default value is false.
ignore_acceptor_hostname
When accepting GSSAPI or krb5 security contexts for host-based service principals,
ignore any hostname passed by the calling application, and allow clients to
authenticate to any service principal in the keytab matching the service name and
realm name (if given). This option can improve the administrative flexibility of
server applications on multihomed hosts, but could compromise the security of
virtual hosting environments. The default value is false. New in release 1.10.
k5login_authoritative
If this flag is true, principals must be listed in a local user's k5login file to
be granted login access, if a .k5login(5) file exists. If this flag is false, a
principal may still be granted login access through other mechanisms even if a
k5login file exists but does not list the principal. The default value is true.
k5login_directory
If set, the library will look for a local user's k5login file within the named
directory, with a filename corresponding to the local username. If not set, the
library will look for k5login files in the user's home directory, with the filename
.k5login. For security reasons, .k5login files must be owned by the local user or
by root.
kcm_mach_service
On macOS only, determines the name of the bootstrap service used to contact the KCM
daemon for the KCM credential cache type. If the value is -, Mach RPC will not be
used to contact the KCM daemon. The default value is org.h5l.kcm.
kcm_socket
Determines the path to the Unix domain socket used to access the KCM daemon for the
KCM credential cache type. If the value is -, Unix domain sockets will not be used
to contact the KCM daemon. The default value is /var/run/.heim_org.h5l.kcm-socket.
kdc_default_options
Default KDC options (Xored for multiple values) when requesting initial tickets.
By default it is set to 0x00000010 (KDC_OPT_RENEWABLE_OK).
kdc_timesync
Accepted values for this relation are 1 or 0. If it is nonzero, client machines
will compute the difference between their time and the time returned by the KDC in
the timestamps in the tickets and use this value to correct for an inaccurate
system clock when requesting service tickets or authenticating to services. This
corrective factor is only used by the Kerberos library; it is not used to change
the system clock. The default value is 1.
noaddresses
If this flag is true, requests for initial tickets will not be made with address
restrictions set, allowing the tickets to be used across NATs. The default value
is true.
permitted_enctypes
Identifies the encryption types that servers will permit for session keys and for
ticket and authenticator encryption, ordered by preference from highest to lowest.
Starting in release 1.18, this tag also acts as the default value for
default_tgs_enctypes and default_tkt_enctypes. The default value for this tag is
aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96 aes256-cts-hmac-sha384-192
aes128-cts-hmac-sha256-128 des3-cbc-sha1 arcfour-hmac-md5 camellia256-cts-cmac
camellia128-cts-cmac.
plugin_base_dir
If set, determines the base directory where krb5 plugins are located. The default
value is the krb5/plugins subdirectory of the krb5 library directory. This
relation is subject to parameter expansion (see below) in release 1.17 and later.
preferred_preauth_types
This allows you to set the preferred preauthentication types which the client will
attempt before others which may be advertised by a KDC. The default value for this
setting is "17, 16, 15, 14", which forces libkrb5 to attempt to use PKINIT if it is
supported.
proxiable
If this flag is true, initial tickets will be proxiable by default, if allowed by
the KDC. The default value is false.
qualify_shortname
If this string is set, it determines the domain suffix for single-component
hostnames when DNS canonicalization is not used (either because
dns_canonicalize_hostname is false or because forward canonicalization failed).
The default value is the first search domain of the system's DNS configuration. To
disable qualification of shortnames, set this relation to the empty string with
qualify_shortname = "". (New in release 1.18.)
rdns If this flag is true, reverse name lookup will be used in addition to forward name
lookup to canonicalizing hostnames for use in service principal names. If
dns_canonicalize_hostname is set to false, this flag has no effect. The default
value is true.
realm_try_domains
Indicate whether a host's domain components should be used to determine the
Kerberos realm of the host. The value of this variable is an integer: -1 means not
to search, 0 means to try the host's domain itself, 1 means to also try the
domain's immediate parent, and so forth. The library's usual mechanism for
locating Kerberos realms is used to determine whether a domain is a valid realm,
which may involve consulting DNS if dns_lookup_kdc is set. The default is not to
search domain components.
renew_lifetime
(duration string.) Sets the default renewable lifetime for initial ticket
requests. The default value is 0.
spake_preauth_groups
A whitespace or comma-separated list of words which specifies the groups allowed
for SPAKE preauthentication. The possible values are:
┌─────────────┬───────────────────────────────┐
│edwards25519 │ Edwards25519 curve (RFC 7748) │
└─────────────┴───────────────────────────────┘
│P-256 │ NIST P-256 curve (RFC 5480) │
├─────────────┼───────────────────────────────┤
│P-384 │ NIST P-384 curve (RFC 5480) │
├─────────────┼───────────────────────────────┤
│P-521 │ NIST P-521 curve (RFC 5480) │
└─────────────┴───────────────────────────────┘
The default value for the client is edwards25519. The default value for the KDC is
empty. New in release 1.17.
ticket_lifetime
(duration string.) Sets the default lifetime for initial ticket requests. The
default value is 1 day.
udp_preference_limit
When sending a message to the KDC, the library will try using TCP before UDP if the
size of the message is above udp_preference_limit. If the message is smaller than
udp_preference_limit, then UDP will be tried before TCP. Regardless of the size,
both protocols will be tried if the first attempt fails.
verify_ap_req_nofail
If this flag is true, then an attempt to verify initial credentials will fail if
the client machine does not have a keytab. The default value is false.
client_aware_channel_bindings
If this flag is true, then all application protocol authentication requests will be
flagged to indicate that the application supports channel bindings when operating
over a secure channel. The default value is false.
[realms]
Each tag in the [realms] section of the file is the name of a Kerberos realm. The value
of the tag is a subsection with relations that define the properties of that particular
realm. For each realm, the following tags may be specified in the realm's subsection:
admin_server
Identifies the host where the administration server is running. Typically, this is
the primary Kerberos server. This tag must be given a value in order to
communicate with the kadmind(8) server for the realm.
auth_to_local
This tag allows you to set a general rule for mapping principal names to local user
names. It will be used if there is not an explicit mapping for the principal name
that is being translated. The possible values are:
RULE:exp
The local name will be formulated from exp.
The format for exp is [n:string](regexp)s/pattern/replacement/g. The
integer n indicates how many components the target principal should have.
If this matches, then a string will be formed from string, substituting the
realm of the principal for $0 and the n'th component of the principal for $n
(e.g., if the principal was johndoe/admin then [2:$2$1foo] would result in
the string adminjohndoefoo). If this string matches regexp, then the s//[g]
substitution command will be run over the string. The optional g will cause
the substitution to be global over the string, instead of replacing only the
first match in the string.
DEFAULT
The principal name will be used as the local user name. If the principal
has more than one component or is not in the default realm, this rule is not
applicable and the conversion will fail.
For example:
[realms]
ATHENA.MIT.EDU = {
auth_to_local = RULE:[2:$1](johndoe)s/^.*$/guest/
auth_to_local = RULE:[2:$1;$2](^.*;admin$)s/;admin$//
auth_to_local = RULE:[2:$2](^.*;root)s/^.*$/root/
auth_to_local = DEFAULT
}
would result in any principal without root or admin as the second component to be
translated with the default rule. A principal with a second component of admin
will become its first component. root will be used as the local name for any
principal with a second component of root. The exception to these two rules are
any principals johndoe/*, which will always get the local name guest.
auth_to_local_names
This subsection allows you to set explicit mappings from principal names to local
user names. The tag is the mapping name, and the value is the corresponding local
user name.
default_domain
This tag specifies the domain used to expand hostnames when translating Kerberos 4
service principals to Kerberos 5 principals (for example, when converting
rcmd.hostname to host/hostname.domain).
disable_encrypted_timestamp
If this flag is true, the client will not perform encrypted timestamp
preauthentication if requested by the KDC. Setting this flag can help to prevent
dictionary attacks by active attackers, if the realm's KDCs support SPAKE
preauthentication or if initial authentication always uses another mechanism or
always uses FAST. This flag persists across client referrals during initial
authentication. This flag does not prevent the KDC from offering encrypted
timestamp. New in release 1.17.
http_anchors
When KDCs and kpasswd servers are accessed through HTTPS proxies, this tag can be
used to specify the location of the CA certificate which should be trusted to issue
the certificate for a proxy server. If left unspecified, the system-wide default
set of CA certificates is used.
The syntax for values is similar to that of values for the pkinit_anchors tag:
FILE: filename
filename is assumed to be the name of an OpenSSL-style ca-bundle file.
DIR: dirname
dirname is assumed to be an directory which contains CA certificates. All files in
the directory will be examined; if they contain certificates (in PEM format), they
will be used.
ENV: envvar
envvar specifies the name of an environment variable which has been set to a value
conforming to one of the previous values. For example, ENV:X509_PROXY_CA, where
environment variable X509_PROXY_CA has been set to FILE:/tmp/my_proxy.pem.
kdc The name or address of a host running a KDC for that realm. An optional port
number, separated from the hostname by a colon, may be included. If the name or
address contains colons (for example, if it is an IPv6 address), enclose it in
square brackets to distinguish the colon from a port separator. For your computer
to be able to communicate with the KDC for each realm, this tag must be given a
value in each realm subsection in the configuration file, or there must be DNS SRV
records specifying the KDCs.
kpasswd_server
Points to the server where all the password changes are performed. If there is no
such entry, DNS will be queried (unless forbidden by dns_lookup_kdc). Finally,
port 464 on the admin_server host will be tried.
master_kdc
The name for primary_kdc prior to release 1.19. Its value is used as a fallback if
primary_kdc is not specified.
primary_kdc
Identifies the primary KDC(s). Currently, this tag is used in only one case: If an
attempt to get credentials fails because of an invalid password, the client
software will attempt to contact the primary KDC, in case the user's password has
just been changed, and the updated database has not been propagated to the replica
servers yet. New in release 1.19.
v4_instance_convert
This subsection allows the administrator to configure exceptions to the
default_domain mapping rule. It contains V4 instances (the tag name) which should
be translated to some specific hostname (the tag value) as the second component in
a Kerberos V5 principal name.
v4_realm
This relation is used by the krb524 library routines when converting a V5 principal
name to a V4 principal name. It is used when the V4 realm name and the V5 realm
name are not the same, but still share the same principal names and passwords. The
tag value is the Kerberos V4 realm name.
[domain_realm]
The [domain_realm] section provides a translation from hostnames to Kerberos realms. Each
tag is a domain name, providing the mapping for that domain and all subdomains. If the
tag begins with a period (.) then it applies only to subdomains. The Kerberos realm may
be identified either in the realms section or using DNS SRV records. Tag names should be
in lower case. For example:
[domain_realm]
crash.mit.edu = TEST.ATHENA.MIT.EDU
.dev.mit.edu = TEST.ATHENA.MIT.EDU
mit.edu = ATHENA.MIT.EDU
maps the host with the name crash.mit.edu into the TEST.ATHENA.MIT.EDU realm. The second
entry maps all hosts under the domain dev.mit.edu into the TEST.ATHENA.MIT.EDU realm, but
not the host with the name dev.mit.edu. That host is matched by the third entry, which
maps the host mit.edu and all hosts under the domain mit.edu that do not match a preceding
rule into the realm ATHENA.MIT.EDU.
If no translation entry applies to a hostname used for a service principal for a service
ticket request, the library will try to get a referral to the appropriate realm from the
client realm's KDC. If that does not succeed, the host's realm is considered to be the
hostname's domain portion converted to uppercase, unless the realm_try_domains setting in
[libdefaults] causes a different parent domain to be used.
[capaths]
In order to perform direct (non-hierarchical) cross-realm authentication, configuration is
needed to determine the authentication paths between realms.
A client will use this section to find the authentication path between its realm and the
realm of the server. The server will use this section to verify the authentication path
used by the client, by checking the transited field of the received ticket.
There is a tag for each participating client realm, and each tag has subtags for each of
the server realms. The value of the subtags is an intermediate realm which may
participate in the cross-realm authentication. The subtags may be repeated if there is
more then one intermediate realm. A value of "." means that the two realms share keys
directly, and no intermediate realms should be allowed to participate.
Only those entries which will be needed on the client or the server need to be present. A
client needs a tag for its local realm with subtags for all the realms of servers it will
need to authenticate to. A server needs a tag for each realm of the clients it will
serve, with a subtag of the server realm.
For example, ANL.GOV, PNL.GOV, and NERSC.GOV all wish to use the ES.NET realm as an
intermediate realm. ANL has a sub realm of TEST.ANL.GOV which will authenticate with
NERSC.GOV but not PNL.GOV. The [capaths] section for ANL.GOV systems would look like
this:
[capaths]
ANL.GOV = {
TEST.ANL.GOV = .
PNL.GOV = ES.NET
NERSC.GOV = ES.NET
ES.NET = .
}
TEST.ANL.GOV = {
ANL.GOV = .
}
PNL.GOV = {
ANL.GOV = ES.NET
}
NERSC.GOV = {
ANL.GOV = ES.NET
}
ES.NET = {
ANL.GOV = .
}
The [capaths] section of the configuration file used on NERSC.GOV systems would look like
this:
[capaths]
NERSC.GOV = {
ANL.GOV = ES.NET
TEST.ANL.GOV = ES.NET
TEST.ANL.GOV = ANL.GOV
PNL.GOV = ES.NET
ES.NET = .
}
ANL.GOV = {
NERSC.GOV = ES.NET
}
PNL.GOV = {
NERSC.GOV = ES.NET
}
ES.NET = {
NERSC.GOV = .
}
TEST.ANL.GOV = {
NERSC.GOV = ANL.GOV
NERSC.GOV = ES.NET
}
When a subtag is used more than once within a tag, clients will use the order of values to
determine the path. The order of values is not important to servers.
[appdefaults]
Each tag in the [appdefaults] section names a Kerberos V5 application or an option that is
used by some Kerberos V5 application[s]. The value of the tag defines the default
behaviors for that application.
For example:
[appdefaults]
telnet = {
ATHENA.MIT.EDU = {
option1 = false
}
}
telnet = {
option1 = true
option2 = true
}
ATHENA.MIT.EDU = {
option2 = false
}
option2 = true
The above four ways of specifying the value of an option are shown in order of decreasing
precedence. In this example, if telnet is running in the realm EXAMPLE.COM, it should, by
default, have option1 and option2 set to true. However, a telnet program in the realm
ATHENA.MIT.EDU should have option1 set to false and option2 set to true. Any other
programs in ATHENA.MIT.EDU should have option2 set to false by default. Any programs
running in other realms should have option2 set to true.
The list of specifiable options for each application may be found in that application's
man pages. The application defaults specified here are overridden by those specified in
the realms section.
[plugins]
• pwqual interface
• kadm5_hook interface
• clpreauth and kdcpreauth interfaces
Tags in the [plugins] section can be used to register dynamic plugin modules and to turn
modules on and off. Not every krb5 pluggable interface uses the [plugins] section; the
ones that do are documented here.
New in release 1.9.
Each pluggable interface corresponds to a subsection of [plugins]. All subsections
support the same tags:
disable
This tag may have multiple values. If there are values for this tag, then the named
modules will be disabled for the pluggable interface.
enable_only
This tag may have multiple values. If there are values for this tag, then only the
named modules will be enabled for the pluggable interface.
module This tag may have multiple values. Each value is a string of the form
modulename:pathname, which causes the shared object located at pathname to be
registered as a dynamic module named modulename for the pluggable interface. If
pathname is not an absolute path, it will be treated as relative to the
plugin_base_dir value from [libdefaults].
For pluggable interfaces where module order matters, modules registered with a module tag
normally come first, in the order they are registered, followed by built-in modules in the
order they are documented below. If enable_only tags are used, then the order of those
tags overrides the normal module order.
The following subsections are currently supported within the [plugins] section:
ccselect interface
The ccselect subsection controls modules for credential cache selection within a cache
collection. In addition to any registered dynamic modules, the following built-in modules
exist (and may be disabled with the disable tag):
k5identity
Uses a .k5identity file in the user's home directory to select a client principal
realm Uses the service realm to guess an appropriate cache from the collection
hostname
If the service principal is host-based, uses the service hostname to guess an
appropriate cache from the collection
pwqual interface
The pwqual subsection controls modules for the password quality interface, which is used
to reject weak passwords when passwords are changed. The following built-in modules exist
for this interface:
dict Checks against the realm dictionary file
empty Rejects empty passwords
hesiod Checks against user information stored in Hesiod (only if Kerberos was built with
Hesiod support)
princ Checks against components of the principal name
kadm5_hook interface
The kadm5_hook interface provides plugins with information on principal creation,
modification, password changes and deletion. This interface can be used to write a plugin
to synchronize MIT Kerberos with another database such as Active Directory. No plugins
are built in for this interface.
kadm5_auth interface
The kadm5_auth section (introduced in release 1.16) controls modules for the kadmin
authorization interface, which determines whether a client principal is allowed to perform
a kadmin operation. The following built-in modules exist for this interface:
acl This module reads the kadm5.acl(5) file, and authorizes operations which are
allowed according to the rules in the file.
self This module authorizes self-service operations including password changes, creation
of new random keys, fetching the client's principal record or string attributes,
and fetching the policy record associated with the client principal.
clpreauth and kdcpreauth interfaces
The clpreauth and kdcpreauth interfaces allow plugin modules to provide client and KDC
preauthentication mechanisms. The following built-in modules exist for these interfaces:
pkinit This module implements the PKINIT preauthentication mechanism.
encrypted_challenge
This module implements the encrypted challenge FAST factor.
encrypted_timestamp
This module implements the encrypted timestamp mechanism.
hostrealm interface
The hostrealm section (introduced in release 1.12) controls modules for the host-to-realm
interface, which affects the local mapping of hostnames to realm names and the choice of
default realm. The following built-in modules exist for this interface:
profile
This module consults the [domain_realm] section of the profile for authoritative
host-to-realm mappings, and the default_realm variable for the default realm.
dns This module looks for DNS records for fallback host-to-realm mappings and the
default realm. It only operates if the dns_lookup_realm variable is set to true.
domain This module applies heuristics for fallback host-to-realm mappings. It implements
the realm_try_domains variable, and uses the uppercased parent domain of the
hostname if that does not produce a result.
localauth interface
The localauth section (introduced in release 1.12) controls modules for the local
authorization interface, which affects the relationship between Kerberos principals and
local system accounts. The following built-in modules exist for this interface:
default
This module implements the DEFAULT type for auth_to_local values.
rule This module implements the RULE type for auth_to_local values.
names This module looks for an auth_to_local_names mapping for the principal name.
auth_to_local
This module processes auth_to_local values in the default realm's section, and
applies the default method if no auth_to_local values exist.
k5login
This module authorizes a principal to a local account according to the account's
.k5login(5) file.
an2ln This module authorizes a principal to a local account if the principal name maps to
the local account name.
certauth interface
The certauth section (introduced in release 1.16) controls modules for the certificate
authorization interface, which determines whether a certificate is allowed to
preauthenticate a user via PKINIT. The following built-in modules exist for this
interface:
pkinit_san
This module authorizes the certificate if it contains a PKINIT Subject Alternative
Name for the requested client principal, or a Microsoft UPN SAN matching the
principal if pkinit_allow_upn is set to true for the realm.
pkinit_eku
This module rejects the certificate if it does not contain an Extended Key Usage
attribute consistent with the pkinit_eku_checking value for the realm.
dbmatch
This module authorizes or rejects the certificate according to whether it matches
the pkinit_cert_match string attribute on the client principal, if that attribute
is present.
PKINIT OPTIONS
NOTE:
The following are PKINIT-specific options. These values may be specified in
[libdefaults] as global defaults, or within a realm-specific subsection of
[libdefaults], or may be specified as realm-specific values in the [realms] section. A
realm-specific value overrides, not adds to, a generic [libdefaults] specification.
The search order is:
1. realm-specific subsection of [libdefaults]:
[libdefaults]
EXAMPLE.COM = {
pkinit_anchors = FILE:/usr/local/example.com.crt
}
2. realm-specific value in the [realms] section:
[realms]
OTHERREALM.ORG = {
pkinit_anchors = FILE:/usr/local/otherrealm.org.crt
}
3. generic value in the [libdefaults] section:
[libdefaults]
pkinit_anchors = DIR:/usr/local/generic_trusted_cas/
Specifying PKINIT identity information
The syntax for specifying Public Key identity, trust, and revocation information for
PKINIT is as follows:
FILE:filename[,keyfilename]
This option has context-specific behavior.
In pkinit_identity or pkinit_identities, filename specifies the name of a
PEM-format file containing the user's certificate. If keyfilename is not
specified, the user's private key is expected to be in filename as well.
Otherwise, keyfilename is the name of the file containing the private key.
In pkinit_anchors or pkinit_pool, filename is assumed to be the name of an
OpenSSL-style ca-bundle file.
DIR:dirname
This option has context-specific behavior.
In pkinit_identity or pkinit_identities, dirname specifies a directory with files
named *.crt and *.key where the first part of the file name is the same for
matching pairs of certificate and private key files. When a file with a name
ending with .crt is found, a matching file ending with .key is assumed to contain
the private key. If no such file is found, then the certificate in the .crt is not
used.
In pkinit_anchors or pkinit_pool, dirname is assumed to be an OpenSSL-style hashed
CA directory where each CA cert is stored in a file named hash-of-ca-cert.#. This
infrastructure is encouraged, but all files in the directory will be examined and
if they contain certificates (in PEM format), they will be used.
In pkinit_revoke, dirname is assumed to be an OpenSSL-style hashed CA directory
where each revocation list is stored in a file named hash-of-ca-cert.r#. This
infrastructure is encouraged, but all files in the directory will be examined and
if they contain a revocation list (in PEM format), they will be used.
PKCS12:filename
filename is the name of a PKCS #12 format file, containing the user's certificate
and private key.
PKCS11:[module_name=]modname[:slotid=slot-id][:token=token-label][:certid=cert-id][:certlabel=cert-label]
All keyword/values are optional. modname specifies the location of a library
implementing PKCS #11. If a value is encountered with no keyword, it is assumed to
be the modname. If no module-name is specified, the default is opensc-pkcs11.so.
slotid= and/or token= may be specified to force the use of a particular smard card
reader or token if there is more than one available. certid= and/or certlabel= may
be specified to force the selection of a particular certificate on the device. See
the pkinit_cert_match configuration option for more ways to select a particular
certificate to use for PKINIT.
ENV:envvar
envvar specifies the name of an environment variable which has been set to a value
conforming to one of the previous values. For example, ENV:X509_PROXY, where
environment variable X509_PROXY has been set to FILE:/tmp/my_proxy.pem.
PKINIT krb5.conf options
pkinit_anchors
Specifies the location of trusted anchor (root) certificates which the client
trusts to sign KDC certificates. This option may be specified multiple times.
These values from the config file are not used if the user specifies X509_anchors
on the command line.
pkinit_cert_match
Specifies matching rules that the client certificate must match before it is used
to attempt PKINIT authentication. If a user has multiple certificates available
(on a smart card, or via other media), there must be exactly one certificate chosen
before attempting PKINIT authentication. This option may be specified multiple
times. All the available certificates are checked against each rule in order until
there is a match of exactly one certificate.
The Subject and Issuer comparison strings are the RFC 2253 string representations
from the certificate Subject DN and Issuer DN values.
The syntax of the matching rules is:
[relation-operator]component-rule ...
where:
relation-operator
can be either &&, meaning all component rules must match, or ||, meaning
only one component rule must match. The default is &&.
component-rule
can be one of the following. Note that there is no punctuation or
whitespace between component rules.
<SUBJECT>regular-expression
<ISSUER>regular-expression
<SAN>regular-expression
<EKU>extended-key-usage-list
<KU>key-usage-list
extended-key-usage-list is a comma-separated list of required Extended Key
Usage values. All values in the list must be present in the certificate.
Extended Key Usage values can be:
• pkinit
• msScLogin
• clientAuth
• emailProtection
key-usage-list is a comma-separated list of required Key Usage values. All
values in the list must be present in the certificate. Key Usage values can
be:
• digitalSignature
• keyEncipherment
Examples:
pkinit_cert_match = ||<SUBJECT>.*DoE.*<SAN>.*@EXAMPLE.COM
pkinit_cert_match = &&<EKU>msScLogin,clientAuth<ISSUER>.*DoE.*
pkinit_cert_match = <EKU>msScLogin,clientAuth<KU>digitalSignature
pkinit_eku_checking
This option specifies what Extended Key Usage value the KDC certificate presented
to the client must contain. (Note that if the KDC certificate has the pkinit
SubjectAlternativeName encoded as the Kerberos TGS name, EKU checking is not
necessary since the issuing CA has certified this as a KDC certificate.) The
values recognized in the krb5.conf file are:
kpKDC This is the default value and specifies that the KDC must have the
id-pkinit-KPKdc EKU as defined in RFC 4556.
kpServerAuth
If kpServerAuth is specified, a KDC certificate with the id-kp-serverAuth
EKU will be accepted. This key usage value is used in most commercially
issued server certificates.
none If none is specified, then the KDC certificate will not be checked to verify
it has an acceptable EKU. The use of this option is not recommended.
pkinit_dh_min_bits
Specifies the size of the Diffie-Hellman key the client will attempt to use. The
acceptable values are 1024, 2048, and 4096. The default is 2048.
pkinit_identities
Specifies the location(s) to be used to find the user's X.509 identity information.
If this option is specified multiple times, each value is attempted in order until
certificates are found. Note that these values are not used if the user specifies
X509_user_identity on the command line.
pkinit_kdc_hostname
The presence of this option indicates that the client is willing to accept a KDC
certificate with a dNSName SAN (Subject Alternative Name) rather than requiring the
id-pkinit-san as defined in RFC 4556. This option may be specified multiple times.
Its value should contain the acceptable hostname for the KDC (as contained in its
certificate).
pkinit_pool
Specifies the location of intermediate certificates which may be used by the client
to complete the trust chain between a KDC certificate and a trusted anchor. 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.
pkinit_revoke
Specifies the location of Certificate Revocation List (CRL) information to be used
by the client when verifying the validity of the KDC certificate presented. This
option may be specified multiple times.
PARAMETER EXPANSION
Starting with release 1.11, several variables, such as default_keytab_name, allow
parameters to be expanded. Valid parameters are:
┌──────────────────┬──────────────────────────────────┐
│%{TEMP} │ Temporary directory │
├──────────────────┼──────────────────────────────────┤
│%{uid} │ Unix real UID or Windows SID │
├──────────────────┼──────────────────────────────────┤
│%{euid} │ Unix effective user ID or │
│ │ Windows SID │
├──────────────────┼──────────────────────────────────┤
│%{USERID} │ Same as %{uid} │
├──────────────────┼──────────────────────────────────┤
│%{null} │ Empty string │
├──────────────────┼──────────────────────────────────┤
│%{LIBDIR} │ Installation library directory │
├──────────────────┼──────────────────────────────────┤
│%{BINDIR} │ Installation binary directory │
├──────────────────┼──────────────────────────────────┤
│%{SBINDIR} │ Installation admin binary │
│ │ directory │
├──────────────────┼──────────────────────────────────┤
│%{username} │ (Unix) Username of effective │
│ │ user ID │
├──────────────────┼──────────────────────────────────┤
│%{APPDATA} │ (Windows) Roaming application │
│ │ data for current user │
├──────────────────┼──────────────────────────────────┤
│%{COMMON_APPDATA} │ (Windows) Application data for │
│ │ all users │
├──────────────────┼──────────────────────────────────┤
│%{LOCAL_APPDATA} │ (Windows) Local application data │
│ │ for current user │
├──────────────────┼──────────────────────────────────┤
│%{SYSTEM} │ (Windows) Windows system folder │
├──────────────────┼──────────────────────────────────┤
│%{WINDOWS} │ (Windows) Windows folder │
├──────────────────┼──────────────────────────────────┤
│%{USERCONFIG} │ (Windows) Per-user MIT krb5 │
│ │ config file directory │
├──────────────────┼──────────────────────────────────┤
│%{COMMONCONFIG} │ (Windows) Common MIT krb5 config │
│ │ file directory │
└──────────────────┴──────────────────────────────────┘
SAMPLE KRB5.CONF FILE
Here is an example of a generic krb5.conf file:
[libdefaults]
default_realm = ATHENA.MIT.EDU
dns_lookup_kdc = true
dns_lookup_realm = false
[realms]
ATHENA.MIT.EDU = {
kdc = kerberos.mit.edu
kdc = kerberos-1.mit.edu
kdc = kerberos-2.mit.edu
admin_server = kerberos.mit.edu
primary_kdc = kerberos.mit.edu
}
EXAMPLE.COM = {
kdc = kerberos.example.com
kdc = kerberos-1.example.com
admin_server = kerberos.example.com
}
[domain_realm]
mit.edu = ATHENA.MIT.EDU
[capaths]
ATHENA.MIT.EDU = {
EXAMPLE.COM = .
}
EXAMPLE.COM = {
ATHENA.MIT.EDU = .
}
FILES
/etc/krb5.conf
SEE ALSO
syslog(3)
AUTHOR
MIT
COPYRIGHT
1985-2024, MIT