[Info-vax] Telnet problems

[glen herrmannsfeldt]

jbriggs444 <jbriggs444 at gmail.com> wrote:
(snip, someone wrote)

>> > Problem is when ever I try to telnet it takes 1 minute and 5 seconds
>> > (yes I timed it) before I get a username
>> > prompt. This is the only system that I'm  having problems with

>> The traditional reason for this is that there is no reverse
>> lookup (PTR entry) for your address in the DNS.

>> The server tries to look up your address, to find your hostname
>> (such as for the log), and waits for the DNS reply.  When the DNS
>> times out, then it does something else (like put in the IP address),
>> and that is your delay.
 
> Note that this syndrome is not caused by having no reverse dns
> entry.  It is caused by having no working reverse DNS resolution
> path to the zone containing the entry.

Yes.  Well, the usual case is no zone for the reverse, especially
within a home network.  I have had it happen for large nationwide
ISP, though.  They fixed it when I complained.

Also, there used to be a tradition that, after doing the reverse
that the host (I believe inside the resolver) would do the forward
lookup on the result.  That protects against fake or forged reverse.
(Especially with .rhosts or .shosts relying on the result.)
 
(snip)

> It is the middle case that causes problems -- a DNS lookup that
> does not succeed quickly and does not fail quickly but just sits
> there instead.
 
> A quick test would be to try the lookup yourself.  Ideally the test
> should be run prior to using telnet.  

Well, except that it is the server that is doing the lookup.
That isn't always the same as the DNS seen by the client.

> Otherwise DNS caching could
> taint the results.  [Caching is unlikely to be an issue with a
> timed-out lookup, but it's better to eliminate the possibility;
> there can be more than one flavor of cache involved]

There have been suggestions over the years to cache lookup
failure, but as far as I know it still isn't done.

-- glen