On Wed, Feb 13, 2019 at 03:52:54PM +0000, Dominic Raferd wrote:
> > With hostnames the [] are optional and usually not used.
>
> OT: is this true generally or just for milter lookups?
Generally, yes, but naturally not for the SMTP nexthop syntax.
After all, MX lookups are an SMTP-only feature, and are are not
applicable in locating milters, policy services, ...
One obvious difference is that milters require an "inet:" or "unix:"
prefix, but the SMTP nexthop syntax does not.
LMTP (implemented in the same underlying executable as SMTP) also
does not use MX lookups, and its nexthop syntax is closer to that
of milters:
http://www.postfix.org/lmtp.8.html
LMTP DESTINATION SYNTAX
LMTP destinations have the following form:
unix:pathname
Connect to the local UNIX-domain server that is bound to the
specified pathname. If the process runs chrooted, an absolute
pathname is interpreted relative to the Postfix queue directory.
inet:hostname
inet:hostname:port
inet:[address]
inet:[address]:port
Connect to the specified TCP port on the specified local or
remote host. If no port is specified, connect to the port
defined as lmtp in services(4). If no such service is found,
the lmtp_tcp_port configuration parameter (default value of 24)
will be used. An IPv6 address must be formatted as
[ipv6:address].
> I thought that for hostname-based relay transports (for example),
> including in transport_maps, it was recommended to use [] to
> prevent repeated unnecessary MX/DNS lookups? e.g.
>
> # postconf relayhost
> relayhost = [smtp.talktalk.net]
This is a matter of *correctness* not efficiency. If, with SMTP,
you mean to connect to a specific host, then say so, by enclosing
the hostname in []. If you mean to connect to the MX hosts of a
particular domain, then leave the [] out. Whichover one is the
correct way to reach the destination, the other one is liable to
wrong in many cases. Some destinations will resolve to the same
set of IP addresses either way, but you should not depend on that.
--
Viktor.
