
I have a Samsung Galaxy S4 I9505.
I can't apply towelroot, once I click "make it ra1n" it stops for a while and then reboots (no messages/feedback at all).
Tried to install on stock (I9505XXUFNB9) and even flahsed to I9505XXUFNB through ODIN just in case, no dice.
Any ideas?
This thread is full of repeated questions with incorrect or incomplete answers about rooting. Let's try to answer some of those.
Can I flash TWRP, CWM, custom Recovery, etc after Towelroot?
No, unless your phone has an unlocked bootloader. If you have a locked bootloader, you can't flash a custom ROM or custom kernel other than by use of things like Safestrap.
My phone says "Custom" and an open padlock on boot!! Oh wow, is it now unlocked????
Sorry, no. The "Custom" indication means that the phone has been modified, which is detected by a program that runs periodically looking for changed files. If you bring in your phone to the carrier with "Custom" showing, they may refuse warranty. To recover that, flash a factory stock ROM, boot, then wait for an hour or two. No, that padlock doesn't indicate anything other than
How do I un-root?
Undo anything you've done first. For example, defrost frozen apps, reinstall things you deleted, uninstall Xposed, etc. Then, once you're back to stock, open SuperSU and use the "Full Unroot" option under settings. If you don't remember what you changed, or didn't keep the files around, then you'll need to re-flash factory stock.
What about Knox?
First, no, this does not "trip Knox", which is slang for "Set the Knox Warranty Void flag." (Saying "trip knox" is incorrect but commonly used.)
Knox is a security program that Samsung uses to market their phones to corporates. There's a Knox-enabled bootloader as part of that program. That Bootloader detects attempts to flash a custom kernel and when it sees you try, it blows an e-Fuse that remembers that, which shows up as the Knox Warranty Void flag. The Knox bootloader refuses to allow custom recovery to be flashed as well. Currently, there's no way to bypass that other than the Loki exploit.
Since Towelroot doesn't flash a custom kernel or custom recovery, it will not set the Knox Warranty Void flag.
Other parts of the Knox program care about the Knox Warranty Void flag - for example, the Knox Secure Container Service, which can be used to hold encrypted corporate data. The other part of the Knox program is the security enforcement on the phone. That Knox enforcement kills programs that try to use root permissions. That's what SuperSU is talking about when it offers to "Disable Knox".
The Secure Container service will not run on a phone with the Knox Warranty Void flag set. That's the only thing "tripping knox" does, to disable the containers. So far there's no evidence that this is used by carriers to reject warranty service.
Do I really need SuperSU installed?
Yes, absolutely. Without it, any program wanting root will get it, which means that the "My Verizon Mobile" spyware will be granted root and will report your phone to Verizon as being rooted. You do not want this, as it's a sure means for VZ to reject warranty claims. Install SuperSU 1.99 from the OP.so you can block that sort of application from ratting you out.
I had this "problem" too. As posted by a few others (100's of posts ago) I simply uninstalled SU > Reboot phone > Extract apk to browser (I used Dolphin cause chrome goes right to play store & you do not want the play store version > reinstall SU > Rebooted again (cause I just felt like it) and...........Bang! SU is good to go! Screenshot is below for your viewing pleasure 
During this 2nd install is when Disable Knox appeared (it didn't upon my 1st SU install) - select to disable it and the knox concerns are gone
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