Installed Classic on Linux and it does not work?
Most probably neither xinetd or inetd services aren't installed or configured on your system. If you run Slackware it's inetd, other distributions use xinetd. You most probably already have it installed as most distributions install it by default - but it is probably not running. To check, you can use one of these commands:
# chkconfig --list xinetd
xinetd 0:off 1:off 2:off 3:on 4:on 5:on 6:off
# ps ax | grep inetd
2331 ? Ss 0:31 xinetd -stayalive...
The chkconfig command is used mostly on RPM based distributions. This command shows you for which init runlevels is xinetd active. Runlevels 3, 4 and 5 are standard multiuser runlevels.
If xinetd is running, you can try restarting it to load new configuration after Firebird is installed. It is done with one of the following commands (depends on Linux distribution you are using):
# /etc/init.d/xinetd restart
# /etc/rc.d/rc.inetd restart
# service xinetd restart
It's also good idea is to check the privileges in /etc/hosts.allow and /etc/hosts.deny files.
Some distributions (like Gentoo for example) are known to lock down everything and you need to explicitly allow it to work. You should check the xinetd configuration file for Firebird service (/etc/xinetd.d/firebird) and see if it has a line like this:
"only_from = 192.168.0.0/24"