Re: Nameserver calls - libesmtp problem?

[Brian Stafford <brian stafford uklinux net>]

On Thu, 21 June 11:23 Carlos Morgado wrote:
| 
| On 2001.06.21 08:02:54 +0100 Albrecht Dreß wrote:
|
| > Anyway, just curious: why *does* libesmtp call the nameserver?
| Transferring
| > mails to a MTA would need a lookup of its address once (or never, if
| it is
| > localhost), and then we can rest in peace. Or is this a too
| simplified view?
| > 
| 
| tip, if you're using dial-on-demand you should setup a local
| nameserver and
| fake local zone and make all your resolv go through it to avoid this 
| situations.

All applications will benefit from the use of the local name server.

| i suspect libESMTP is doing MX lookups for 'localhost'
| (tcpdump or ethereal should expose it)

No not MX.  IMO this is the wrong thing to do for mail submission.
For example, using MX lookups behind a firewall which blocks access
to port 25 on the internet will cause all connections to the MXed
hostnames to fail.  In a situation like this, a sysadmin would expect
outgoing mail to use one or more MTAs which sit astride the firewall.
This scenario is becoming increasingly common in corporate environments,
usually in conjunction with a requirement to authenticate to the
submission server.

Additionally, if multiple recipients in different domains are specified,
multiple SMTP servers must be contacted thus penalising users on a slow
connection to their ISP (especially for large messages).  Better to
copy the message once to the ISP's MTA and let that distribute the
message by performing MX lookup.

libESMTP looks up A records (or AAAA if your hosts implementation
of gethostbyname_r supports it).  In the future it may gain the ability
to use SRV records though I have no plans to do this in the short term.

Brian