At least once a day a poster on this board asks what a tethered jailbreak is. This thread aims to provide a central location to which such posters can be directed, with clear and concise answers to these questions.
So, what's a tethered jailbreak?
A tethered jailbreak is a jailbreak which leaves the jailbroken phone unable to boot without the help of a computer. Any time such a phone is completely powered off, restarted (for instance after installing certain jailbreak apps) or its battery dies, it must be plugged into a computer and the jailbreaking application must be re-run to get it past the Apple logo boot screen. Such a phone does not lose its jailbreak, and all customizations will be preserved, but the phone will not be able to boot past the Apple logo screen on its own.
Why do some phones only support a tethered jailbreak?
That requires a little more explanation. Every iPhone has a chip in it which contains a tiny program called iBoot. iBoot is the phone's bootloader, the program that tells the phone how to load the operating system. During this process, iBoot checks the OS to make sure that it is the official version provided by Apple. If it is not the correct version (for example, because it has been modified by a jailbreak), it will normally refuse to boot your phone.
Clever hackers discovered a weakness in iBoot (which they called the 24kpwn hack) that allowed them to inject their own code into iBoot to bypass this security check and boot a jailbroken version of the OS that iBoot normally would not boot. Basically, by crashing iBoot at a certain point in the boot process, they could inject 24kb of their own code and iBoot would think it was perfectly normal. This code disabled the security check, and it was the only known way of getting a jailbroken iPhone to boot itself.
As of sometime in October, Apple started shipping iPhones with a new version of iBoot. This new version patched the 24kpwn hack, meaning we lost our only known way of modifying iBoot so that it can boot a modified version of the OS. It is still possible to use a computer to bypass iBoot, but one can't convince iBoot to bypass itself. This is why the new iPhones can have only a tethered jailbreak.
Does my phone have the new version of iBoot?
There are ways to check. If your iBoot version is 359.3, then you can have an untethered jailbreak (whether on 3.1.2 or any other version of the OS); if it is 359.3.2 or 359.3-2 you cannot, at least not now.
If you are considering buying a new or used 3GS and cannot find out the iBoot version, you should at least check the 4th and 5th digits of the serial number, which represent the week of manufacture. For our purposes, the lower this number is, the better. Because different plants began using the new version of iBoot at different times, there is no firm rule for which phones will be safe; in general, however, a phone manufactured in week 36 or earlier will have an untethered jailbreak, 37-40 is iffy, and 41 and higher will almost certainly be tethered. These rules do not apply to re-manufactured phones, which are given new serial numbers but which have older components and may have the older version of iBoot.
Can I still jailbreak my phone?
Yes, all iPhones are jailbreakable. The tethered jailbreak is more a problem of inconvenience, since you might occasionally be stuck without a phone until you can reach a computer. The good news is that the iPhone is designed to run without rebooting or completely powering off under normal use; some people go weeks without the need to restart their phone. Only you can decide whether the advantages of a jailbreak outweigh the inconvenience of being tethered, however.
Will there ever be an untethered jailbreak again?
Ever is a very long time and iPhone hackers are brilliant, so it is always possible that a solution will be found. Right now, though, there is no prospect for a new iBoot hack, and it is therefore impossible to predict when or even if one will be found. For now you should assume that any phone with the new iBoot is not untetherable.
So, what's a tethered jailbreak?
A tethered jailbreak is a jailbreak which leaves the jailbroken phone unable to boot without the help of a computer. Any time such a phone is completely powered off, restarted (for instance after installing certain jailbreak apps) or its battery dies, it must be plugged into a computer and the jailbreaking application must be re-run to get it past the Apple logo boot screen. Such a phone does not lose its jailbreak, and all customizations will be preserved, but the phone will not be able to boot past the Apple logo screen on its own.
Why do some phones only support a tethered jailbreak?
That requires a little more explanation. Every iPhone has a chip in it which contains a tiny program called iBoot. iBoot is the phone's bootloader, the program that tells the phone how to load the operating system. During this process, iBoot checks the OS to make sure that it is the official version provided by Apple. If it is not the correct version (for example, because it has been modified by a jailbreak), it will normally refuse to boot your phone.
Clever hackers discovered a weakness in iBoot (which they called the 24kpwn hack) that allowed them to inject their own code into iBoot to bypass this security check and boot a jailbroken version of the OS that iBoot normally would not boot. Basically, by crashing iBoot at a certain point in the boot process, they could inject 24kb of their own code and iBoot would think it was perfectly normal. This code disabled the security check, and it was the only known way of getting a jailbroken iPhone to boot itself.
As of sometime in October, Apple started shipping iPhones with a new version of iBoot. This new version patched the 24kpwn hack, meaning we lost our only known way of modifying iBoot so that it can boot a modified version of the OS. It is still possible to use a computer to bypass iBoot, but one can't convince iBoot to bypass itself. This is why the new iPhones can have only a tethered jailbreak.
Does my phone have the new version of iBoot?
There are ways to check. If your iBoot version is 359.3, then you can have an untethered jailbreak (whether on 3.1.2 or any other version of the OS); if it is 359.3.2 or 359.3-2 you cannot, at least not now.
If you are considering buying a new or used 3GS and cannot find out the iBoot version, you should at least check the 4th and 5th digits of the serial number, which represent the week of manufacture. For our purposes, the lower this number is, the better. Because different plants began using the new version of iBoot at different times, there is no firm rule for which phones will be safe; in general, however, a phone manufactured in week 36 or earlier will have an untethered jailbreak, 37-40 is iffy, and 41 and higher will almost certainly be tethered. These rules do not apply to re-manufactured phones, which are given new serial numbers but which have older components and may have the older version of iBoot.
Can I still jailbreak my phone?
Yes, all iPhones are jailbreakable. The tethered jailbreak is more a problem of inconvenience, since you might occasionally be stuck without a phone until you can reach a computer. The good news is that the iPhone is designed to run without rebooting or completely powering off under normal use; some people go weeks without the need to restart their phone. Only you can decide whether the advantages of a jailbreak outweigh the inconvenience of being tethered, however.
Will there ever be an untethered jailbreak again?
Ever is a very long time and iPhone hackers are brilliant, so it is always possible that a solution will be found. Right now, though, there is no prospect for a new iBoot hack, and it is therefore impossible to predict when or even if one will be found. For now you should assume that any phone with the new iBoot is not untetherable.
