Re: Hostnames in pg_hba.conf - Mailing list pgsql-hackers

On 02/11/2010 08:13 AM, Bart Samwel wrote:

ISSUE #2: Reverse lookup?

There was a suggestion on the TODO list on the wiki, which basically said that maybe we could use reverse lookup to find "the" hostname and then check for that hostname in the list. I think that won't work, since IPs can go by many names and may not support reverse lookup for some hostnames (/etc/hosts anybody?). Furthermore, due to the top-to-bottom processing of pg_hba.conf, you CANNOT SKIP entries that might possibly match. For instance, if the third line is for host "foo.example.com" and the fifth line is for "bar.example.com", both lines may apply to the same IP, and you still HAVE to check the first one, even if reverse lookup turns up the second host name. So it doesn't save you any lookups, it just costs an extra one.

I don't see a need to do a reverse lookup. Reverse lookups are sometimes done as a verification check, in the sense that it's cheap to get a map from NAME -> IP, but sometimes it is much harder to get the reverse map from IP -> NAME. However, it's not a reliable check as many legitimate users have trouble getting a reverse map from IP -> NAME. It also doesn't same anything as IP -> NAME lookups are a completely different set of name servers, and these name servers are not always optimized for speed as IP -> NAME lookups are less common than NAME -> IP. Finally, if one finds a map from IP -> NAME, that doesn't prove that a map from NAME -> IP exists, so using *any* results from IP -> NAME is questionable.

I think reverse lookups are unnecessary and undesirable.

ISSUE #3: Multiple hostnames?

Currently, a pg_hba entry lists an IP / netmask combination. I would suggest allowing lists of hostnames in the entries, so that you can at least mimic the "match multiple hosts by a single rule". Any reason not to do this?

I'm mixed. In some situations, I've wanted to put multiple IP/netmask. I would say that if multiple names are supported, then multiple IP/netmask should be supported. But, this does make the lines unwieldy beyond two or three. This direction leans towards the capability to define "host classes", where the rules allows the host class, and the host class can have a list of hostnames.

2) What will you do if they specify a hostname and a netmask? This seems like a convenient way of saying "everybody on the same subnet as NAME."