Projects/KDC Discovery
This project will implement Kerberos service discovery by DNS as specified by draft-mccallum-kitten-krb-service-discovery-02. The draft currently specifies a new URI DNS record type.
The current method of KDC discovery using DNS SRV records has the following drawbacks:
- Only UDP and TCP protocols can be specified
- Multiple queries are needed to discover both protocol records
- The DNS administrator has no influence on client protocol use
Contents
Design
The client performs a DNS lookup for one or more of the following URI records:
- _kerberos-adm.REALM (Admin service)
-
_krb5kdc.REALM (KDC)Changed to: _kerberos.REALM (KDC) in code and draft - _kpasswd.REALM (Password service)
The URI record includes a priority and weight, and contains a URI string of the krb5srv scheme name, flags, transport, and target (containing optional port and path) fields, separated by colons.
- krb5srv:[flags]:transport:host[:port][/path]
The priority and weight are as defined in RFC 2782. Weight may or may not be implemented, and priority must be implemented. On the initial KDC contact, all KDCs will be tried according to priority regardless of master status. On the fallback contact, master KDCs will be tried according to priority, excluding non-masters.
The flags field contains zero or more flag characters. Currently the only valid character is M, indicating that the record is for a master server. On the initial contact, if a non-master KDC has answered and returns an error such as PREAUTH_FAILED, entries that are marked as master will be contacted.
The transport field indicates the transport type for the given host address. It can be either "tcp", "udp", or "kkdcp" for MS-KKDCP.
The host field in the udp and tcp transport cases can be a hostname, IPv4 address, or bracket-enclosed IPv6 address, and can be followed by a port extension. A host for the MS-KKDCP transport type uses a https:// URL, and can include a port and/or path extension.
Discovery using this new method should be attempted before searching SRV records.
Examples
- _kerberos IN URI 10 1 "krb5srv:M:kkdcp:https://kdc.example.com/path"
- _kerberos IN URI 20 1 "krb5srv::kkdcp:https://kdc2.example.com"
- _kerberos-adm IN URI 10 1 "krb5srv::tcp:192.168.1.20:1333"
Implementation
src/lib/krb5/os/dnsglue.c: k5_try_realm_txt_rr() has existing TXT lookup code, but only retrieves a realm name from the record. Make a generalized TXT lookup function to pass the result to a new parsing function for the URI format.
Resources
- https://tools.ietf.org/html/draft-mccallum-kitten-krb-service-discovery-02
- http://mailman.mit.edu/pipermail/krbdev/2016-May/012588.html
- KDC Discovery may fail with URI entries served by RHEL7 bind: https://bugzilla.redhat.com/show_bug.cgi?id=1388534
