Principal names and DNS¶
Kerberos clients can do DNS lookups to canonicalize service principal names. This can cause difficulties when setting up Kerberos application servers, especially when the client’s name for the service is different from what the service thinks its name is.
Service principal names¶
A frequently used kind of principal name is the host-based service principal name. This kind of principal name has two components: a service name and a hostname. For example, imap/imap.example.com is the principal name of the “imap” service on the host “imap.example.com”. Other possible service names for the first component include “host” (remote login services such as ssh), “HTTP”, and “nfs” (Network File System).
Service administrators often publish well-known hostname aliases that they would prefer users to use instead of the canonical name of the service host. This gives service administrators more flexibility in deploying services. For example, a shell login server might be named “long-vanity-hostname.example.com”, but users will naturally prefer to type something like “login.example.com”. Hostname aliases also allow for administrators to set up load balancing for some sorts of services based on rotating CNAME records in DNS.
Service principal canonicalization¶
MIT Kerberos clients currently always do forward resolution (looking up the IPv4 and possibly IPv6 addresses using getaddrinfo()) of the hostname part of a host-based service principal to canonicalize the hostname. They obtain the “canonical” name of the host when doing so. By default, MIT Kerberos clients will also then do reverse DNS resolution (looking up the hostname associated with the IPv4 or IPv6 address using getnameinfo()) of the hostname. Using the krb5.conf setting
[libdefaults] rdns = false
will disable reverse DNS lookup on clients. The default setting is “true”.
Operating system bugs may prevent a setting of rdns = false from disabling reverse DNS lookup. Some versions of GNU libc have a bug in getaddrinfo() that cause them to look up PTR records even when not required. MIT Kerberos releases krb5-1.10.2 and newer have a workaround for this problem, as does the krb5-1.9.x series as of release krb5-1.9.4.
Reverse DNS mismatches¶
Sometimes, an enterprise will have control over its forward DNS but not its reverse DNS. The reverse DNS is sometimes under the control of the Internet service provider of the enterprise, and the enterprise may not have much influence in setting up reverse DNS records for its address space. If there are difficulties with getting forward and reverse DNS to match, it is best to set rdns = false on client machines.
Overriding application behavior¶
Applications can choose to use a default hostname component in their service principal name when accepting authentication, which avoids some sorts of hostname mismatches. Because not all relevant applications do this yet, using the krb5.conf setting
[libdefaults] ignore_acceptor_hostname = true
will allow the Kerberos library to override the application’s choice of service principal hostname and will allow a server program to accept incoming authentications using any key in its keytab that matches the service name and realm name (if given). This setting defaults to “false” and is available in releases krb5-1.10 and later.
One service principal entry that should be in the keytab is a principal whose hostname component is the canonical hostname that getaddrinfo() reports for all known aliases for the host. If the reverse DNS information does not match this canonical hostname, an additional service principal entry should be in the keytab for this different hostname.
Specific application advice¶
Secure shell (ssh)¶
Setting GSSAPIStrictAcceptorCheck = no in the configuration file of modern versions of the openssh daemon will allow the daemon to try any key in its keytab when accepting a connection, rather than looking for the keytab entry that matches the host’s own idea of its name (typically the name that gethostname() returns). This requires krb5-1.10 or later.