Re: BUG #9518: temporary login failure - "missing pg_hba entry"

I wrote:
> IOW, it looks to me like intermittent failures in the reverse DNS lookup
> could disable matching by hostname, and nothing would be said in the
> postmaster log. Why is there no complaint if check_hostname's call to
> pg_getnameinfo_all (line 600 in HEAD) fails?

After sleeping on it, I think probably the reason it is like that is a
desire to not clutter the postmaster log if there are some legitimate
clients without rDNS entries. That is, suppose pg_hba.conf has

host foo.bar.com ...
host 192.168.168.1 ...

and you've not bothered to create a reverse-DNS entry for 192.168.168.1.
We will try (and fail) to look up the rDNS entry while considering the
foo.bar.com line. We certainly don't want a failure there to prevent us
from reaching the 192.168.168.1 line, and we don't really want to clutter
the postmaster log with a bleat about it, either. Hence the lack of any
error logging in the existing code. (The later cross-check on whether
the forward DNS matches does have an error report, which maybe isn't such
a great thing either from this standpoint.)

The problem of course is that if the rDNS failure prevents us from
matching to *any* line, we exit with no error more helpful than
"missing pg_hba entry", which is not very desirable in this case.

I guess we could do something like remember the fact that we tried and
failed to do an rDNS lookup, and report it as DETAIL in the eventual
"missing pg_hba entry" report. Not quite sure if it's worth the trouble
--- any thoughts?

Another objection to the code as it stands is that if there are multiple
pg_hba lines containing hostnames, we'll repeat the failing rDNS lookup
at each one. This is at best a huge waste of cycles (multiple network
roundtrips, if the DNS server isn't local), and at worst inconsistent
if things actually are intermittent and a later lookup attempt succeeds.
I think we want to fix it to be sure that there's exactly one rDNS lookup
attempt, occurring at the first line with a hostname.

regards, tom lane