Sun Microsystems Sun Workstation 100U System Manager's Manual page 348

SendmaU installation and Opel'atloD Gulcle
18
addresses; this allows mailer-speei8c customizatioa. Finally, ruleset lour is applied to
do any default conversion to extemallorm.
The third purpose is to map addresses into the actual set 01 instructions necessary
to get the message delivered. Ruleset zero must resolve to the internallorm, which is
in turn used as a pointer to a mailer descriptor. The mailer descriptor describes the
interrace requirements 01 the mailer.
&.3.1. Phlloaopil7
The particular philoeophy you choose will depend heavily on the size and struc-
ture of your organization. I wiD present a lew possible philosophies here.
One general point applies to aD 01 thelle philosophies: it is almost always a
mis-
take to try to do lull name resolution. For example, il you are trying to let names 01
the form "userOhost" to the Arpanet, it does not pay to route them to
"xyzvax!decvax!ucbvax!c70:userOhost" since you then depend on several links not
under your control. The best approach to this problem is to simply lorward to
"xyzvax!userOhost" and let xyzvax worry about it Irom there.
In
summary, just let
the message closer to the destination, rather thaa determining the lull path.
&.3.1.1. Larp alt., DlaIl7 hoat. - minimum Intol'matloD
VersloD 4.1
Berkeley
is
an example 01 a large site; it has more than two or three hosts.
Berkeley has decided that the only reasonable philosophy tor their environment
is
to
designate one host
38
site guru. It must be able to resolve any piece or mail it
receives. The other sites should have the minimum amount 01 inlormation they can
get away with, and even this minimum should be hints rather than solid inlorma-
tion.
For example, a typical site on the Berkeley local ether network is "monet."
Monet has a list 01 known ethemet hosts; it it receives mail lor any 01 them, it can
do direct delivery. II it receives mail lor any unknown host, it just passes it directly
to "ucbvax," the Berkeley master host. Ucbvax may determine that the host name
is illegal and reject the message, or may
be
able to do delivery. However, it is
important to note that when a new ethernet host is added, the only host that mru'
have its tables updated is ucbvax; the others
mtJlI
be updated as convenient, but this
is not critical.
This picture is slightly muddied due to network connections that are not actu-
ally located on ucbvax. For example, the Berkeley TCP connection is currently on
"ucbarpa." However, monet doe, no' know about this; the inlormation
is
hidden
between ucbvu and ucbarpa. Mail loin, Irom monet to a TCP host
is
trBllsrerred
via the ethernet Irom monet to ucbvax, then via the ethemet Irom ucbvax to
ucbarpa, and then is submitted to the Arpanet. Although this involves some extra
hops, Berkeley feels this is an acceptable tradeol.
An interesting point
is
that it would be possible to update monet to send TCP
mail directly to ucbarpa if the load got too high: it monet failed to note a host as a
TCP host, the mail would go via ucbvax
88
berore; and it monet incorrectly sent a
message to ucbarpa, it would stiD be sent by ucbarpa to ucbvax as before. The only
problem that might occur in this scenario is 'looping': it ucbarpa thought that
ucbvax had the TCP connection and vice versa. For this reason, updates should
GlwG1I8
happen to the master host 8rat.
This philosophy results as much lrom the need to have a single lOurce lor the
configuration files (typically built using mI(l) or some similar tool) as any logical
need. Maintainin& more thaD three separate tables by hand is essentially
aD
impQ&o
sible job.
Last Mod 8/11/83