pathalias(1) pathalias(1)
NAME [Toc] [Back]
pathalias - electronic address router
SYNOPSIS [Toc] [Back]
pathalias [-ivcDf] [-l host] [-d link] [-t link] [files]
DESCRIPTION [Toc] [Back]
pathalias computes the shortest paths and corresponding routes from
one host (computer system) to all other known, reachable hosts.
pathalias reads host-to-host connectivity information on standard
input or in the named files, and writes a list of host-route pairs on
the standard output.
Options [Toc] [Back]
pathalias recognizes the following options and command-line arguments:
-i Ignore case: map all host names to lowercase. By
default, case is significant.
-c Print costs. Print the path cost (see below) before
each host-route pair.
-v Verbose. Report some statistics on the standard error
output.
-D Terminal domains. Domain members are terminal.
-f First hop cost. The printed cost is the cost to the
first relay in a path instead of the cost of the path
itself; implies (and overrides) the -c option.
-l host Set local host name to host. By default, pathalias
discovers the local host name in a system-dependent
way.
-d link Declare a dead link, host, or network (see below). If
link is of the form host1!host2, the link from host1 to
host2 is treated as an extremely high cost (i.e., DEAD)
link. If link is a single host name, that host is
treated as dead and is used as an intermediate host of
last resort on any path. If link is a network name,
the network requires a gateway.
-t link Trace input for link, host, or network on the standard
error output. The form of link is as above.
The public domain version of pathalias includes two undocumented
options that are briefly described in the Special Options section
below.
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Input Format [Toc] [Back]
A line beginning with white space continues the preceding line.
Anything following # on an input line is ignored.
A list of host-to-host connections consists of a ``from'' host in
column 1, followed by white space, followed by a comma-separated list
of ``to' hosts, called links. A link may be preceded or followed by a
network character to use in the route. Valid network characters are !
(default), @, :, and %. A link (and network character, if present)
may be followed by a ``cost'' enclosed in parentheses. Costs can be
arbitrary arithmetic expressions involving numbers, parentheses, +, -,
*, and /. Negative costs are prohibited. The following symbolic
costs are recognized:
LOCAL 25 (local-area network connection)
DEDICATED 100 (high speed dedicated link)
DIRECT 200 (toll-free call)
DEMAND 300 (long-distance call)
HOURLY 500 (hourly poll)
EVENING 2000 (time restricted call)
DAILY 5000 (daily poll, also called POLLED)
WEEKLY 30000 (irregular poll)
In addition, DEAD is a very large number (effectively infinite), and
HIGH and LOW are -5 and +5 respectively, for baud-rate or quality
bonuses/penalties, and FAST is -80, for adjusting costs of links that
use high-speed (9.6 Kbaud or more) modems. These symbolic costs
represent an imperfect measure of bandwidth, monetary cost, and
frequency of connections. For most mail traffic, it is important to
minimize the number of hosts in a route, thus, e.g., HOURLY is far
greater than DAILY divided by 24. If no cost is given, a default of
4000 is used.
For the most part, arithmetic expressions that mix symbolic constants
other than HIGH, LOW, and FAST make no sense. For example, if a host
calls a local neighbor whenever there is work, and additionally polls
every evening, the cost is DIRECT, not DIRECT+EVENING.
Some examples:
down princeton!(DEDICATED), tilt,
%thrash(LOCAL)
princeton topaz!(DEMAND+LOW)
topaz @rutgers(LOCAL+1)
If a link is encountered more than once, the least-cost occurrence
dictates the cost and network character. Links are treated as
bidirectional but asymmetric: for each link declared in the input, a
DEAD reverse link is assumed.
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If the ``to'' host in a link is surrounded by angle brackets, the link
is considered terminal, and further links beyond this one are heavily
penalized. For example, with input
seismo <research>(10), research(100), ihnp4(10)
research allegra(10)
ihnp4 allegra(50)
the path from seismo to research is direct, but the path from seismo
to allegra uses ihnp4 as a relay; not research.
The set of names by which a host is known by its neighbors is called
its aliases. Aliases are declared as follows:
name=alias, alias ...
The name used in the route to or through aliased hosts is the name by
which the host is known to its predecessor in the route.
Fully connected networks, such as the ARPANET or a local-area network,
are declared as follows:
net = {host, host, ...}
The host-list can be preceded or followed by a routing character (!
by default), and can be followed by a cost (4000 by default). The
network name is optional; if not given, pathalias creates one.
etherhosts = {rahway, milan, joliet}!(LOCAL)
ringhosts = @{gimli, alida, almo}(DEDICATED)
= {etherhosts, ringhosts}(0)
The routing character used in a route to a network member is the one
encountered when ``entering'' the network. See also the sections on
gateways and domains.
Connection data can be given while hiding host names by declaring
private {host, host, ...}
pathalias does not generate routes for private hosts, but can produce
routes through them. The scope of a private declaration extends from
the declaration to the end of the input file in which it appears, or
to a private declaration with an empty host list, whichever comes
first. The latter scope rule offers a way to retain the semantics of
private declarations when reading from the standard input.
Dead hosts, links, or networks can be presented in the input stream by
declaring
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dead {arg, ...}
where arg has the same form as the argument to the -d option.
To force a specific cost for a link, delete all prior declarations
with
delete {host1!host2}
and declare the link as desired. To delete a host and all its links,
use
delete {host}
Error diagnostics refer to the file in which the error was found. To
alter the file name, use
file {filename}
Fine-tuning is possible by adjusting the weights of all links from a
given host, as in
adjust {host1, host-2(LOW), host3(-1)}
If no cost is given, a default of 4000 is used.
Input from compressed (and uncompressed) files can be piped into
pathalias with the following script.
for i in $*; do
case $i in
*.Z) echo "file {`expr $i : ').Z'`}
zcat $i ;;
*) echo "file {$i}"
cat $i ;;
esac
echo "private {}"
done
Output Format [Toc] [Back]
A list of host-route pairs is written to the standard output, where
route is a string appropriate for use with printf() (see printf(3S)),
suchas
rutgers princeton!topaz!%s@rutgers
The %s in the route string should be replaced by the user name at the
destination host (this task is normally performed by a mailer).
Except for domains (see below), the name of a network is never used in
routes. Thus, in the earlier example, the path from rahway to milan
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would be milan!%s, not etherhosts!milan!%s.
Gateways [Toc] [Back]
A network is represented by a pseudo-host and a set of network
members. Links from the members to the network have the weight given
in the input, while the cost from the network to the members is zero.
If a network is declared dead, the member-to-network links are marked
dead, which effectively prohibits access to the network from its
members.
However, if the input also shows an explicit link from any host to the
network, then that host can be used as a gateway (in particular, the
gateway need not be a network member).
For example, suppose CSNET is declared dead on the command line and
the input contains
CSNET = {...}
csnet-relay CSNET
Then routes to CSNET hosts will use csnet-relay as a gateway.
Domains [Toc] [Back]
A network whose name begins with . is called a domain. Domains are
presumed to require gateways; i.e., they are DEAD. The route given by
a path through a domain is similar to that for a network, but here the
domain name is appended to the end of the name of the next host.
Subdomains are permitted. For example:
harvard .EDU # harvard is gateway to .EDU domain
.EDU = {.BERKELEY, .UMICH}
.BERKELEY = {ernie}
yields
ernie ...!harvard!ernie.BERKELEY.EDU!%s
Output is given for the nearest gateway to a domain; e.g., the example
above gives
.EDU ...!harvard!%s
Output is given for a subdomain if it has a different route than its
parent domain, or if all its ancestor domains are private.
If the -D option is given on the command line, pathalias treats a link
from a domain to a host member of that domain as terminal. This
property extends to host members of subdomains, etc., and discourages
routes that use any domain member as a relay.
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Special Options [Toc] [Back]
The public domain version of pathalias includes two undocumented
options that rewrite named files with intermediate data of limited
usage. Here are brief descriptions:
-g file Dump graph edges into file in the form host>host
for simple connections and host@<tab>host for
network connections (from hosts to networks only).
-s file Dump shortest path tree into file in the form host
<tab>[@]host[!](cost), including both connections
from hosts to networks and from networks to hosts.
This data may be useful for generating lists of
one-way connections.
BUGS [Toc] [Back]
The -i option should be the default.
The order of arguments is significant. In particular, -i and -t
should appear early in the command line.
pathalias can generate hybrid (i.e., ambiguous) routes, which are
abhorrent and most certainly should not be given as examples in a
manual entry. Experienced mappers largely shun '@' when preparing
input; this is historical, but also reflects UUCP's simplistic syntax
for source routes.
Mixed-mode paths are ambiguous because the precedence of @ versus ! is
not specified, varies from host to host, and is configurable. They
should rarely be used.
Multiple @s in routes are prohibited by many mailers. To circumvent
this restriction, mailers instead support the ``magic %'' rule,
described below. When pathalias would otherwise generate a path
containing multiple @s, it instead generates a path to which the
``magic %'' rule can be correctly applied.
Basically, the ``magic %'' rule for generating paths is ``when
constructing a path that would require multiple @s, replace all but
the right-most @ with %.
When a mailer that supports the ``magic %'' rule receives a message
that was routed to it via ..path..@host, it processes the route as
follows:
1. Remove the trailing "@host" part of the route.
2. Examine the remaining route from right to left, proceeding to
the next step when a "!" is seen. If a `%' is seen, change
it to `@' and proceed to the next step immediately.
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3. Continue processing the message using the modified route. If
the modified route contains both `!' and `@' characters, the
exact selection of the next host to route the message is
governed by the specific precedence of `!' vs. `@' at this
host.
For example, if a host, jazz.nonesuch.com, received a message
with a path foo!joe%[email protected], the
mailer would convert the path to foo![email protected],
and then forward it appropriately. If the host were
configured such that `!' were of higher precedence than `@',
the message would be forwarded to host foo, which would then
deliver the message to [email protected]. If instead
jazz.nonesuch.com were configured with `@' as higher in
precedence, it would forward the message to host
castle.hrh.gov.uk, which would then deliver it to foo!joe.
(Clearly, pathalias could only correctly generate such a path
if it knew the precedence at host jazz.nonesuch.com; since
the database does not contain that information, such paths
from pathalias should be viewed with suspicion.)
The -D option suppresses insignificant routes to domain members.
This is benign, perhaps even beneficial, but confusing, since the
behavior is undocumented and somewhat unpredictable.
AUTHOR [Toc] [Back]
pathalias was developed by Peter Honeyman and Steven M. Bellovin.
FILES [Toc] [Back]
newsgroup comp.mail.maps Likely location of some input
files.
SEE ALSO [Toc] [Back]
P.Honeyman and S.M. Bellovin, PATHALIAS or The Care and Feeding of
Relative Addresses, in Proc. Summer USENIX Conf., Atlanta, 1986.
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