Linux

sshd try to resolve client`s ip-address via DNS, but it can`t access DNS-server.
As you describe, there is problem to make ip-connection between sshd-computer and your DNS-server.
Does you ping your DNS-server by name or by ip-address ? If by name, then check /etc/hosts and /etc/resolv.conf files.
If pings fail when pinging bye ip-address, then you need to check route table on sshd-computer (add some routes or default route).
Use traceroute <your DNS> to locate problem.

With best regards,
Vlad.

There are some files that need to edited. These are some that I do.
/etc/nsswitch.conf: The hosts line reads host: files dns
It will first look in the local files then check DNS

/etc/resolv.conf:
domain yourdomain.com
search yourdomain.com
nameserver ip.address.of.nameserver

/etc/defaultrouter:
the ip address of the default router for you subnet needs to be here

Since we have nameservers I hon't use the /etc/host file to handle ip address conversions to computer names.
But if you do it should be like this
ip.address computername