Based on the host name, (dns-secondary), I imagine that this machine was used as a server and quite probably does not have a GUI installed. You may wish to simply reinstall the system (you can download Debian and other distros for SPARC), If you want to get into the current system, Google and search this site for instructions on how to boot into something called "single user mode" and reset the root password.
BTW, calling the Linux command prompt a DOS prompt isn't very nice
. All the Linux shells (command line interfaces) are much more capable than anything ever designed for MS-DOS.