iPhone dev team has also released the OpenSSH package in .tgz format, to make Redsn0w able to read the jailbreak apps. And these are steps to use Redsn0w to jailbreak apps on iOS 4.2.1 without the existence of Cydia.
jailbreak iOS 4.2.1 using Redsn0w. Afterward, launch Redsn0w and select “Install Custom Bundle” and “Just boot tethered right now” options. It will allow you to choose the package you wish to install.
Step 2: After selecting the process step, Redsn0w will then install the package. Then continue the process by clicking “Next” and proceed with the normal instructions which involves entering into DFU mode.
Step 3: Your device will reboot with SSH package once the above process done. If you are using Windows you can use WinSCP, if you are on Mac use Cyberduck to continue installing to SSH on your device.
Step 4: Now from here onwards, whenever you want to SSH into your device, you must first run your device in the so called “jailbroken state” on every reboot by using “Just boot tethered right now” option in Redsn0w.
Step 5: well done.
Note: Do not apply the above instructions using Redsn0w if you are relied on unlock, the Redsn0w 0.9.6b3 only supports tethered jailbreak.