fastboot not finding SPH-L720 on Linux This is with Ubuntu 13.10 on the host and a Sprint Samsung Galaxy S4 SPH-L720.

When in normal mode, 'adb devices' finds my phone.

I do 'adb reboot bootloader' to reboot my phone into bootloader mode.

'fastboot devices' comes back empty. 'sudo fastboot devices' also comes back empty.

I have setup the udev rules file accordingly (replaced my real login with myuser)

~  cat /etc/udev/rules.d/51-android.rules     
SUBSYSTEM=="usb", ATTR{idVendor}=="04e8", MODE="0666", GROUP="myuser"


lsusb returns the following (fyi, the device is an S4, not an S2)

Bus 002 Device 006: ID 04e8:685d Samsung Electronics Co., Ltd GT-I9100 Phone [Galaxy S II] (Download mode)


I've read a ton of articles on this and I can't get this working. Any help is appreciated. Thanks!
Accordingly to the answer to this question, Samsung devices do not support fastboot.
This thread on XDA Forums states the same.