  Secondary specification


  One key feature of the _D_N_S system is its ability to maintain backup
  copies of information about domains. The information is entered on the
  _p_r_i_m_a_r_y _D_N_S of a domain and copied to secondary (backup) DNSs.

  11..  PPrriinncciippllee


  11..11..  WWhhoo ddooeess wwhhaatt??

  The information is not copied by the primary, but instead is pulled by
  the secondaries. The rules however, are set by the primary (see the
  section about secondaries requirements in the primary specification
  screen).

  Clearly, the secondary DNS administrator has to tell his DNS to act as
  a secondary for a given domain, but the primary DNS will tell it how!


  11..22..  SSeerriiaall nnuummbbeerr

  The secondary will regularly talk (as told by the primary) to the
  primary and check the serial number (revision number) of the domain.
  If the serial number of the primary is the same as the copy of the
  secondary, or is smaller, no copy will be made.

  A common error is to forget to update the serial number (SOA record).
  Secondaries won't copy the new information.  This may lead to strange
  behavior. _L_i_n_u_x_c_o_n_f manages serial numbers automatically!


  22..  TTaasskk

  The secondary specification is simple. You give a domain name and the
  IP number of the primary DNS for that domain, followed (optionally) by
  alternative IP numbers, just in case the first one fails. That's it.
  _L_i_n_u_x_c_o_n_f takes care of setting a file name so the backup information
  will be stored.


  33..  SShhoouulldd II tteellll MMoomm??

  Someone has to tell others that your _D_N_S is the secondary of a given
  domain. If it's not done, then your _D_N_S is possibly working for
  nothing: it won't receive any query for that domain.

  This job must be done in the primary specification (see DNS
  advertising). So, if your DNS is a secondary, it is not your job :-)
  The primary must explicitly name the secondaries.  Further, it has to
  register this information in the DNS of its parent domain. This is
  often forgotten.