=Hesiod=

==Background==

Hesiod is a simple string lookup service built on top of the Domain Name System.  Conceptually, the service translates a pair of strings (the "name" and "type") into a set of result strings.  This lookup is done very simply; a DNS lookup is done for name.type.ns.athena.mit.edu and the strings in the resulting TXT records are returned.

Hesiod is used in several places in Debathena:
 * Retrieving a user's entries in the passwd and group databases
  * Due to DNS limitations, the group list is often truncated.  A group must be marked as an "NFS Group" in order for group membership to be propagated to Hesiod.
 * Determining which mail system they're using and if IMAP, which server to speak to
 * Translating a locker name to an AFS path, including the user's homedir
 * Determining a workstation's "cluster" information, including whether or not the workstation is designated as a "public" cluster machine, whether it pulls from the production APT repo or the development/proposed repos
 * Determining service information (port/protocol pairs)
 * Determining servers for a specific service

Hesiod may, in some cases, be used to select a default printer for a specific workstation.  This information is ignored in the public cluster environment.

==Examples==

Hesiod can be queried with the `hesinfo` command.  

{{{
$ hesinfo jdreed passwd
jdreed:*:7263:101:Jonathan D Reed,E17-110A,6172530657,:/mit/jdreed:/bin/athena/bash
}}}

Because [wiki:NSS] uses a Hesiod backend, you can accomplish the same thing with getent(1) (assuming there is not already a local user with the same name).

{{{
$ getent passwd jdreed
jdreed:*:7263:101:Jonathan D Reed,E17-110A,6172530657,:/mit/jdreed:/bin/athena/bash
}}}

The `hes` command in the `consult` locker takes a Hesiod name and iterates over all possible Hesiod types.

{{{
$ add consult
$ hes jdreed
    PASSWD: jdreed:*:7263:101:Jonathan D Reed,E17-110A,6172530657,:/mit/jdreed:/bin/athena/bash
    FILSYS: AFS /afs/athena.mit.edu/user/j/d/jdreed w /mit/jdreed
     POBOX: POP PO14.MIT.EDU jdreed
   GRPLIST: docsourc:2567:hdops:80547:bugs-acl:967:webmeet-adm:26990:ctug-admin:23752:facdev:14008:chd-home:27063:gsipb:15001:tooltime-acl:18546:cvs-users:18719:tooltime-team:26355:minidev:691:is-archive:29512:is-www:26481:olcdev:367:mcc-www:9235:helpstaff:16886:

$ hes zephyr
    FILSYS: AFS /afs/athena.mit.edu/astaff/project/zephyr w /mit/zephyr
      SLOC: ARILINN.MIT.EDU
      SLOC: NESKAYA.MIT.EDU
   SERVICE: zephyr udp 2102
}}}