  IP aliases configuration


  With the evolution of the _I_n_t_e_r_n_e_t and the power of the microcomput-
  ers, it is now possible and useful for a computer to act as many, by
  putting several servers in one box. The trick to achieving this is to
  let the machine respond to more than one IP number. We call these IP
  aliases.

  11..  DDeeffiinniinngg aalliiaasseess

  IP aliases are referred to this way because they are alternative IP
  numbers for a network device. Some software packages can use alternate
  configurations based on the target IP number of a request. Web (httpd)
  servers and the virtual POP e-mail server distributed with Linuxconf
  are the most notable examples.

  An alias is associated with a network device. Linuxconf provides you
  with a list of active network devices (aliases can only be defined for
  active devices). Although any active device may be chosen, it is often
  simpler to pick one of the Ethernet devices (eth0) of the computer.
  Furthermore, using IP numbers originating from the same network number
  as the primary IP number of the device will simplify routing.


       You don't have to tell any other machine on the network
       about what you're doing. It just works.


  If you pick IP numbers unrelated to the network of the Ethernet
  device, you will need to define special routing on the other hosts of
  the network. Your server will look like a gateway to a "virtual
  network." This is often done in practice when you need a huge amount
  of aliases, exceeding the available numbers on the local network.

  Once you have selected a network device, you are taken into a simple
  dialog with many empty lines. Just enter the aliases (IP numbers) you
  wish to associate with this device. If you need more empty lines, use
  the _A_d_d button.


  22..  UUssiinngg nnaammeess

  _L_i_n_u_x_c_o_n_f initializes the IP aliases well after the basic networking,
  so at this point, the DNS is fully reachable.  You can safely use
  names to define aliases instead of just IP numbers. This is useful if
  you have a few disparate (unrelated) aliases to define.


  33..  DDeeffiinniinngg rraannggeess

  Big Internet _W_e_b servers host several "virtual domains." While we can
  define one alias per virtual Web server, it is much easier to allocate
  a range of IP numbers and map the virtual servers into that range. See
  the "IP allocation space" of the _D_N_S _c_o_n_f_i_g_u_r_a_t_o_r section of
  _L_i_n_u_x_c_o_n_f.

  You can define, for example, fifty IP aliases just by entering a
  single line in the dialog, such as the following entry:


               192.168.1.1-50


  44..  HHooww mmaannyy aalliiaasseess ccaann LLiinnuuxx  hhaannddllee??

  Linux 2.0 handles 256 aliases per device. Since Linux 2.0.31 there has
  been and continues to be  a soft limit on the kernel table used to
  store the aliases. The limit is set to 256 at boot time.  This limit
  may be changed by writing a new value in the file


               /proc/sys/net/core/net_alias_max


  But there is a small catch -- this new limit only becomes active when
  you set the first alias of a device. If there are already IP aliases
  on a network device, they will have to be unset.  When there is no
  alias on a device, the table is deleted and it will be recreated with
  the new limit the next time you set the aliases.

  _L_i_n_u_x_c_o_n_f handles all this for you and will unset some aliases before
  attempting to grow the table. Some messages are sent to the log
  explaining this.

  So Linux can handle a lot of aliases.


  55..  AA sstteepp bbyy sstteepp eexxaammppllee

  Here is a example of how to set up one IP alias and help to create a
  new one easily.


  55..11..  FFiirrsstt,, aallllooccaattee aa bbuunncchh ooff aalliiaasseess

  In the IP alias dialog, define a full range of IP aliases. This will
  likely be sufficient for quite some time. Suppose your eth0 network is
  192.168.1.0 and there is an unused section. Just enter

  192.168.1.50-100

  which will allocate 51 IP aliases for future use.


  55..22..  TTeellll tthhee DDNNSS aabboouutt tthhiiss

  Now, in the "Configure domain name server" menu, you select the "IP
  allocation space" entry. Enter the range and a small description such
  as "Virtual domains." The DNS is not always on the same machine where
  you define the IP aliases.


  55..33..  EEaacchh ttiimmee yyoouu aallllooccaattee aa nneeww vviirrttuuaall WWWWWW ddoommaaiinn

  Go into the DNS and add a host for the domain, generally
  www.new_domain. For the IP number, hit ctrl-X and Linuxconf will
  compute the first unused IP number in the IP aliases range.  Pick this
  number.

  Then go into the httpd configuration and add a section for the new www
  domain. Repeat this step for each new domain until you are out of IP
  aliases.

  Using this strategy (defining a bunch of aliases first, telling the
  DNS about them next, and using them last), you will avoid many errors.