Linux Network Administrator Guide, Second Edition

cost is an arithmetic expression made up of numbers and symbolic expressions like DAILY or WEEKLY. Lines beginning with a hash sign are ignored.

As an example, consider moria, which polls swim.twobirds.com twice a day and bert.sesame.com once per week. The link to bert uses a slow 2,400 bps modem. moria would publish the following maps entry:

moria.orcnet.org
        bert.sesame.com(DAILY/2),
        swim.twobirds.com(WEEKLY+LOW)
moria.orcnet.org = moria

The last line makes moria known under its UUCP name, as well. Note that its cost must be specified as DAILY/2 because calling twice a day actually halves the cost for this link.

Using the information from such map files, pathalias is able to calculate optimal routes to any destination site listed in the paths file and produce a pathalias database from this which can then be used for routing to these sites.

pathalias provides a couple of other features like site-hiding (i.e., making sites accessible only through a gateway). See the pathalias manual page for details and a complete list of link costs.

Comments in the map file generally contain additional information on the sites described in it. There is a rigid format in which to specify this information so that it can be retrieved from the maps. For instance, a program called uuwho uses a database created from the map files to display this information in a nicely formatted way. When you register your site with an organization that distributes map files to its members, you generally have to fill out such a map entry. Below is a sample map entry (in fact, it's the one for Olaf's site):

#N      monad, monad.swb.de, monad.swb.sub.org
#S      AT 486DX50; Linux 0.99
#O      private
#C      Olaf Kirch
#E      [email protected]
#P      Kattreinstr. 38, D-64295 Darmstadt, FRG
#L      49 52 03 N / 08 38 40 E
#U      brewhq
#W      [email protected] (Olaf Kirch); Sun Jul 25 16:59:32 MET DST 1993
#
monad   brewhq(DAILY/2)
# Domains
monad = monad.swb.de
monad = monad.swb.sub.org

The whitespace after the first two characters is a tab. The meaning of most of the fields is pretty obvious; you will receive a detailed description from whichever domain you register with. The L field is the most fun to find out: it gives your geographical position in latitude/longitude and is used to draw the PostScript maps that show all sites for each country, as well as worldwide. [111]

Configuring elm

elm stands for 'electronic mail' and is one of the more reasonably named Unix tools. It provides a full- screen interface with a good help feature. We won't discuss how to use elm here, but only dwell on its configuration options.

Theoretically, you can run elm unconfigured, and everything works well - if you are lucky. But there are a few options that must be set, although they are required only on occasion.

When it starts, elm reads a set of configuration variables from the elm.rc file in /etc/elm. Then it attempts to read the file .elm/elmrc in your home directory. You don't usually write this file yourself. It is created when you choose 'Save new options' from elm 's options menu.

The set of options for the private elmrc file is also available in the global elm.rc file. Most settings in your private elmrc file override those of the global file.

Global elm Options

In the global elm.rc file, you must set the options that pertain to your host's name. For example, at the Virtual Brewery, the file for vlager contains the following:

#
# The local hostname
hostname = vlager
#
# Domain name
hostdomain =.vbrew.com
#
# Fully qualified domain name
hostfullname = vlager.vbrew.com

These options set elm 's idea of the local hostname. Although this information is rarely used, you should set the options. Note that these particular options only take effect when giving them in the global configuration file; when found in your private elmrc, they will be ignored.

National Character Sets

A set of standards and RFCs have been developed that amend the RFC-822 standard to support various types of messages, such as plain text, binary data, PostScript files, etc. These standards are commonly referred to as MIME, or Multipurpose Internet Mail Extensions. Among other things, MIME also lets the recipient know if a character set other than standard ASCII has been used when writing the message, for example, using French accents or German umlauts. elm supports these characters to some extent.

The character set used by Linux internally to represent characters is usually referred to as ISO-8859-1, which is the name of the standard it conforms to. It is also known as Latin-1. Any message using characters from this character set should have the following line in its header:

Content-Type: text/plain; charset=iso-8859-1

The receiving system should recognize this field and take appropriate measures when displaying the message. The default for text/plain messages is a charset value of us-ascii.

To be able to display messages with character sets other than ASCII, elm must know how to print these