The iPhone Dev-Team has released a new version of redsn0w that can jailbreak iOS 4.2.1 and install custom bundles independent of Cydia. Please note that this version of Redsn0w is meant for developers only and does not install the new version of Cydia. It is also tethered for newer devices.
We’ve made some updates to redsn0w to make it easier for jailbreak developers (and tinkerers) to get their programs ready for 4.2.1. As noted above, the public version of Cydia (and MobileSubstrate too!) is not 4.2.1-compatible. redsn0w will now let you install your own custom bundles independent of Cydia (the bundle can actually be Cydia if you’ve compiled it on your own). These bundles can be
up to 15MB in size, and should be in the form of a gzip-compressed tar file.
It’s very important that you get the file permissions and ownerships right in your custom redsn0w bundles. To give you a practical example of such a bundle, here’s one that includes OpenSSH, OpenSSL, and the basic apt installer programs:
redsn0w has also been updated to recognize the 4.2.1GM IPSWs. *However*, as noted above, the 4.2.x jailbreak is not yet untethered for most devices! That means until someone like @comex comes up with a way to untether it, you must use redsn0w (or a similar utility) to boot your device into a jailbroken 4.2.1 state. (The only exceptions to this are the iPhone3G, non-MC iPod touch 2G, and old-bootrom iPhone3GS. redsn0w will jailbreak those untethered!)
With the above redsn0w and SSH bundle, jailbreak developers and tinkerers can jailbreak and SSH into their 4.2.1 devices, provided they’ve done a tethered boot (using redsn0w’s “Just boot tethered right now” option).
Note: The Cydia that’s included in 0.9.6b3 is the same one as in 0.9.6b2, and so it will *not* work on 4.2.1. Don’t try installing it on 4.2.1! Instead, use the SSH bundle, or compile Cydia on your own. If you’re familiar with the apt utilities, you can use “apt-get” to install many programs from the command line. Be sure to do “apt-get update” first to refresh your sources!
Read More [via MuscleNerd]