This really solved my problem.
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If you get this error message:
Starting proftpd: hostname - Fatal: unable to determine IP address of 'hostname'
there's a problem with your DNS (name resolution). The most common cause for this problem is a hostname assigned to a machine which gets its IP via DHCP on a network without a DNS server. A quick fix for this problem is adding the hostname to the '/etc/hosts' file while using the same IP used for 'localhost', i.e. '127.0.0.1':
127.0.0.1 localhost.localdomain localhost
127.0.0.1 hostname of machine short name
If you don't know the hostname of your machine, run the hostname command.
Having made that change, restart the network as 'root' with
service network restart
and try starting the FTP server again.