Tomas Korbar:
> Hi,
>
> > Is "_submission._tcp" special only in the SMTP client? What happens
> > with "_submission._tcp.$mydomain in other Postfix programs?
>
> _submission._tcp is special only to SMTP client. Behaviour of all other
> programs remains the same.
>
> > The patch changes the global proto_info structure. That is unnecessary,
> > and I have changed Postfix 3.6 to use "const INET_PROTO_INFO *"
> > pointers everywhere, to prevent such unnecessary changes in the future.
>
> You are right. Fixed in the new patch posted on
> https://tkorbar.fedorapeople.org/postfix-3.5.4-relayhost.patch
>
> > What is the difference in behavior between
> >
> > [_submission._tcp.$mydomain]:587
> >
> > and
> >
> > _submission._tcp.$mydomain:587
>
> In the first case, smtp client will perform directly lookup for SRV record
> of submission
> service of domain specified with $mydomain.
>
> In the second case, smtp client will at first try to perform lookup for MX
> record of
> "_submission._tcp.$mydomain" domain, which will most likely fail and in
> response
> to that, smtp_host_addr function will be called and that will cause the
> name to be
> resolved the same way as in the previous case.
Maybe it is better for the SMTP client to not look up MX records
if the domain starts with _submission._tcp. Then the decision to
look up SRV could be made higher-up, at the point where the SMTP
client decides if it wants to do MX lookups or non-MX lookups.
Thwn we end up with a hierarchy like:
smtp_connect_remote
calls smtp_domain_addr if it really wants to do MX lookup
calls smtp_host_addr if MX record does not exist
calls smtp_srv_ddr if it really wants to do SRV lookup
calls smtp_host_addr with the SRV lookup result
calls smtp_host_addr if it wants neither MX nor SRV lookup
If it turns out that smtp_srv_addr() and smtp_host_addr() are almost
the same then that code can be factored out into a helper function.
I wrote the above before I realized that SRV lookup can return more
than one record, and that those records specify a TCP or UDP
port. It basically means that SRV returns someothing similar to
MX lookup except that it also returns a port to connect to.
This means that we'd have to extend the DNS_RR with at least a
field for the port. Maybe we can encode priority in one number.
BTW I think that your code has a memory leak - it does not free the
address returned from SRV lookup.
> > Where is the documentation that describes how this works?
>
> Please point me where I should write it.
If we can identify the behavior than I can add that.
> >The SRV handler in libpostfix-dns should parse the entire record.
>
> Ok. What is your preference on how this should be done? Should i
> leave it with hostname within the buffer that gets passed to
> dns_rr_create function and then handle it inside smtp_addr_one
> function or do you have something else in mind?
Whoa. SRV specifies a lot of things that your patch is ignoring.
- TCP or UDP port
- priority and weight
- there can be more than one record
Is that kosher?
Wietse
