What does “421 machines not registered with their own name server not allow in” mean?
In order to block access to the U of U facilities to domains that have caused us problems in the past, we do a lookup of each TCP/IP address that connects to us, and verify that they are not one of the bad guys. If the address is in a bad domain, or if the domain can not be determined, we block access to the domain. The lookup done is (as is true for most modern security packages) to check DNS for the PTR record for the address. If that maps to a name that maps back to at least the number then we are sure that we’ve correctly identified the domain. If your address doesn’t appear to have a PTR record, software has no way to mechanically determine it is part of your domain instead of some known hacker domain. Therefore we don’t let it in. (Note also that having no name is going to cause it other problems. Sendmail 8.9.x, for example, won’t accept mail from it…) Note that the name of the machine need not be anything more than, for example, line128.domain.com. Our software, like other si
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- What does "421 machines not registered with their own name server not allow in" mean?
