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# The Kerberos 5 GSS-API Mechanism

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This topic describes and lists security features regarding Java Generic Security Services (Java GSS) for Kerberos 5.

This GSS-API mechanism is defined by RFC 1964, supplemented with RFC 4121 under the Official Internet Protocol Standards process. See RFC 1964 and RFC 4121.

This topic also describes the Object Identifier (OID) for the Kerberos V5 mechanism, in the Official Internet Protocol Standards process, the encryption types, and the `krb5.conf` settings supported by Java GSS.

Note: Java GSS features have been in each version of Java SE since 1.4.2.

## The OID for the Kerberos V5 Mechanism

According to RFC 1964 section 1, the OID for Java Generic Security Services (Java GSS) for Kerberos 5 is defined as 1.2.840.113554.1.2.

## Java GSS/Kerberos Supported Encryption Types

Each major release of Java SE has added features or additional supported encryption types for GSS/Kerberos. However, support for new encryption types have been included in update editions of Java SE 5 and Java SE 6.

The following table lists the preferred order of encryption types supported in editions of Java up to Java SE 8.

Java SE 7 Supported Encryption Types
Name | etype Number
--- | ---
aes256-cts | 18
aes128-cts | 17
aes128-cts | 23
des3-cbc-sha1 | 16
des-cbc-md5 | 3
des-cbc-crc | 1

Note that `des-cbc-crc` and `dec-cbc-md5` are disabled by default in JDK 8.

A user can restrict the usage of encryption for various purposes in `krb5.conf`, in the `[libdefaults]` section.

For more detailed information about the AES encryption types (AES128 and AES256 and the RC4-HMAC encryption type, see Java GSS for Java SE 6.

For more detailed information about the DES and Triple DES encryption types, see Java GSS for Java SE 5 and Support for Triple DES Encryption.

## Supported krb5.conf Settings

In Java SE 7, the following are the supported parameters.

    [libdefaults]
    default_realm
    allow_weak_crypto

    dns_lookup_kdc
    dns_lookup_realm
    dns_fallback

    default_checksum
    safe_checksum_type
    ap_req_checksum_type
    default_keytab_name

    default_tkt_enctypes
    permitted_enctypes
    default_tgs_enctypes

    no_addresses
    noaddresses

    renewable
    proxiable
    forwardable

    kdc_default_options
    clockskew

    kdc_timeout
    udp_preference_limit

    [realms]
      REALM.NAME = {
        kdc =
      }

    [capaths]
      A = {
        I = .
        B = I
      }

    [domain_realm]
      domain=REALM

The following are the defaults for the `krb5.conf` file parameters.

    no_addresses = true
    noaddresses = true
    dns_fallback = true (false in JDK 6)
    dns_lookup_kdc = true (false in JDK 6)
    dns_lookup_realm = true (false in JDK 6)
    allow_weak_crypto = false (true in JDK 7)
    kdc_timeout = 30000
    max_retries = 3
    udp_preference_limit = -1
    clockskew = 300
    renewable = false
    proxiable = false
    forwardable = false

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