PMDF System Manager's Guide
PMDF-REF-6.0

The process of rewriting an address starts by extracting the first host/domain specification from the address. (Readers who are not familiar with RFC 822 address conventions are advised to read that standard, at least in a cursory fashion, at this point in order to understand the following discussion.) The order in which host/domain specifications in the address are scanned is as follows:

• Hosts in source routes (read from left to right).
• Hosts appearing to the right of the at sign.
• Hosts appearing to the right of the last singleton percent sign.
• Hosts appearing to the left of the first exclamation point. The order of the last two items are switched if the bangoverpercent keyword is in effect on the channel that is doing the address rewriting. That is, if the channel which is attempting to enqueue the message is itself marked with the bangoverpercent channel keyword.⁹

  Some highly hypothetical examples of addresses and the host name that would be extracted first are shown below:

  Address	First host/domain specification	Comments
  user@a	a	a is a "short-form" domain name
  user@a.b.c	a.b.c	a.b.c is a "fully-qualified" domain name (FQDN)
  user@[0.1.2.3]	[0.1.2.3]	[0.1.2.3] is a "domain literal"
  @a:user@b.c.d	a	This is a source-routed address with a a short-form domain name, the "route"
  @a.b.c:user@d.e.f	a.b.c	Source routed address, route part is fully-qualified
  @[0.1.2.3]:user@d.e.f	[0.1.2.3]	Source-routed address, route part is a domain literal
  @a,@b,@c:user@d.e.f	a	Source-routed address with an a to b to c routing
  @a,@[0.1.2.3]:user@b	a	Source-routed address with a domain literal in the route part
  user%A@B	B	This non-standard form of routing is called a "percent hack"
  user%A%B%C@D	D	A built up percent hack
  user%A	A
  user%A%B	B
  user%%A%B	B
  user%A%%B	A%%B	Of questionable value
  @A:user%B@C	A
  A!user	A	"Bang-style" addressing; commonly used for UUCP
  A!user@B	B
  A!user%B@C	C
  A!user%B	B	nobangoverpercent keyword active; the default
  A!user%B	A	bangoverpercent keyword active
  @A:B!user@C	A
  @A,@B:C!user%D@E	A	Too grotesque to consider, really

  Note that RFC 822 does not say anything about the interpretation of exclamation points, !, and percent signs, %, in addresses. It is customary to interpret percent signs in the same manner as at signs, @, if no at sign is present, so this convention is adopted by PMDF.

  The special interpretation of repeated percent signs is used to allow percent signs as part of local usernames, which is used in handling PSIMail and other foreign mail system addresses. The interpretation of exclamation points conforms to RFC 976's "bang-style" address conventions and makes it possible to use UUCP addresses with PMDF.

  The order of these interpretations is not specified by either RFC 822 or RFC 976, so the bangoverpercent and nobangoverpercent keywords can be used to control the order in which they are applied by the channel doing the rewriting. Note that the default is more "standard", although the alternate setting may be useful under some circumstances.

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  Note

  ⁹ For instance, if this is a message being sent from a user agent, then the enqueuing channel --- the channel doing the rewriting --- would be the local, l, channel. If it is a message coming in from Message Router via PMDF-MR then usually it will be the mr_local channel doing the rewriting. And, if it is a message coming in from DECUS UUCP, then the vn_gateway channel will be doing the rewriting.

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