On 05/02/18 00:12, Viktor Dukhovni wrote: > > >> On Feb 4, 2018, at 5:46 PM, J Doe <gene...@nativemethods.com> wrote: >> >> Feb 4 15:05:46 server postfix/smptd[718]: warning: hostname >> 1-2-3-4.dyn.isp.net does not resolve to address 1.2.3.4: Name or service not >> known >> >> Does this mean that: >> >> 1. smtpd receives a connection from an smtp client and does a reverse DNS >> lookup >> 2. smtpd performs a forward DNS lookup on the result and compares the >> resulting IP address to the initial IP >> 3. If the IP addresses don’t match it reports this error >> >> ... or is some other logic used to generate the error message? > > The message happens when the hostname obtained from 1 fails to resolve > to an IP address that can be compared in 2. The error is a hard error > (NXDomain). >
Is this a reliable bad-host detector? The last three instances in my log were subsequently rejected by a DNSBL Allen C
