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I always root my android devices, but there's something called obtaining S-OFF for certain devices, and I have no idea what that means. What exactly does the term S-OFF relate to? Why would I want to obtain S-OFF for my device?

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As of note S-OFF/S-ON (S stands for Security) only officially applies to HTC devices, but many use it to describe any sort of locked/unlocked bootloader. –  Broam May 14 '12 at 15:54
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up vote 6 down vote accepted

If you have a locked bootloader, it's S-ON. If you fully unlock the bootloader it's S-OFF. As eldarerathis notes below there's an intermediate state, where it's unlocked but still S-ON, where you can flash recoveries and ROMs but not radio firmware and so on.

If you don't unlock the bootloader, you can't flash unsigned firmware and most often can't alter your ROM (i.e., no permanent rooting). The locked bootloader generally restores a known good system image on boot if it sees that changes have been made.

For some devices there's no known method of unlocking the bootloader; most of the time you need something official like HTC's unlocker tool.

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Minor niggle: On HTC devices (I guess maybe others) your bootloader can be unlocked and still S-ON. In such a case, I believe you can flash system images and boot images, but you cannot flash radios or bootloaders (I'm not completely positive on the details, though, as my EVO has been S-OFF for a long time). Best example is that the HTCdev unlock leaves you S-ON (source for that). Their unlocker tool actually does not give you S-OFF status on any device I know of, unfortunately. –  eldarerathis Mar 23 '12 at 21:09
    
@eldarerathis I had no idea about that, thanks. HTC does weird things with their phones. –  Matthew Read Mar 23 '12 at 21:11
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Yeah, it's kind of unfortunate, really. I was planning to upgrade to a newer HTC phone with an unlockable bootloader but decided against it when I dug around XDA and found that it would still be S-ON, so that's the only reason I thought to add that comment. It's not very well publicized, IMHO. –  eldarerathis Mar 23 '12 at 21:12
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What's unfortunate is the confusion about S-ON and S-OFF. Believe me you want to leave security on (S-ON). Yes you can upgrade to newer HTC phones and use the official unlocker to install custom roms. Even with S-ON it's still unlocked for that purpose. Meaning yes you can install custom system, kernel (boot) and recovery partitions. What you can't do is mess with the radio which you really shouldn't be for the sake of everyone on your carrier. Any other argument is for specific unnecessary tools for installing roms. –  user171593 Apr 3 '13 at 8:10
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