CentOS - May 2018 - New mail server slow connecting to web server

Having successfully killed my (Centos 7) mail server I have had to build a new 
one. 

Thankfully I had a server build that I was about to start configuring as my 
new web server.

I appear to have set everything up okay, Exim, SpamAssassin, ClamAv, Dovecot, 
Postgresql, httpd etc.

Everything appears fine apart from when I try to connect to my web server.  
Whatever tried to connect suffers from a delay, and either times out or works 
eventually.

I started off with problems connecting Exim to Postgresql on the web server. I 
then found that I couldn't connect using psql, or using Perl DBD-Pg.  I then
found I couldn't SSH or SCP, dispite them both being on the same subnet.

The more I use the new system, better it appears to be, but the delay is still 
there.

name resolution isn't the problem as I can nslookup both ways without 
problems, and specifying IP addresses makes no difference.  For testing I 
have disabled the firewall and selinux and that made no difference.

If I try to connect the other way there is no delay, but I cannot access the 
Postgresql server on my new mail server from the web server.

Anyone got any ideas?

On 05/24/2018 09:10 PM, Gary Stainburn wrote:
> name resolution isn't the problem as I can nslookup both ways without
> problems

Can you provide more detail here regarding what tests you performed?? 
Long delays are usually a timeout on a reverse DNS lookup (that is, a 
PTR lookup) on the connecting IP address.

Connect to the server, and get the address that the server sees as its 
peer either from the log or using "ss -o state established -t".? Use 
"time host $ip" to test the reverse lookup.? Send the output to the
list.