the /etc/hosts file contains a line: 127.0.0.1 localhost so localhost should be a valid hostname, but any attempt to connect to a port on localhost fails, including ping.
Connection attempts into this server from outside, using the same clients, work.
Connection attempts from this server to other servers using these clients also work.
On further investigation, it also fails to ping the loopback ip 127.0.0.1 when ping is invoked directly on that ip. In addition to pinging, I tested with 'socat TCP4..'.
Would a bad iptables prevent a localhost connection?