Copied from sane-backends.

README.linux

@@ -5,11 +5,19 @@ With Linux 2.4.* you could either use the kernel scanner module or libusb to
 access USB scanners.  In Linux 2.6.4 the kernel scanner module was removed.
 Therefore with this and later kernels libusb must be used.
 
+Permissions:
+------------
 While SANE automatically uses libusb when the library and its header file were
 present during the build of sane-backends, setting permissions will require some
 attention. So if scanimage -L lists your scanner as root but not as normal user
 read on this text.
 
+Most distributions support setting permissions without much manual
+configuration. Usually you must just add the users that are allowed to access
+the scanner to group "scanner". To make that change active, the user must login
+again. For more details, see your distribution's documentation e.g. for Debian:
+README.debian.gz. If this doesn't work you you want to know more, read on.
+
 The device files used by libusb are located in /proc/bus/usb/
 (e.g. /proc/bus/usb/001/003) or in /dev/bus/usb/ (e.g. /dev/bus/usb/001/003), if
 you use udev.  The exact file name can be found out by running sane-find-scanner
@@ -18,18 +26,22 @@ e.g. "chmod a+rw /proc/bus/usb/001/003" works, this change is not permanent.
 The permissions will be reset when the scanner is replugged or Linux is
 rebooted.
 
-One solution to set permissions on-the-fly are the Linux hot-plug tools that
+One solution to set permissions on-the-fly is Linux udev which comes with
-should come with any current distribution. Your distribution should have set up
+current distributions. SANE comes with a udev rules file in the tools/udev
-the scripts to automatically change permissions correctly. Look for
+directory which may be used by distributions or can be copied to
-"libsane.usermap" and "libusbscanner" in /etc/hotplug/usb. Usually you must just
+/etc/udev/rules.d manually. The file format is explained on top of the file
-add the users that are allowed to access the scanner to group "scanner". To make
+itself. Either you need libusb 0.1.12 or newer or USB_DEVFS_PATH=/dev/bus/usb
-that change active, the user must login again. For more details, see your
+must be exported as a system-wide environment variable.
-distribution's documentation e.g. for Debian: README.debian.gz.
 
+Older distributions may use the Linux hot-plug tools (or hotplug-ng). Your
+distribution should have set up the scripts to automatically change permissions
+correctly. Look for "libsane.usermap" and "libusbscanner" in /etc/hotplug/usb.
 If you build SANE from source you can use the hotplug script that comes with
 SANE. See the tools/hotplug/ directory in the source distribution. Please refer
 to the README in that directory for the details.
 
+Gentoo information:
+-------------------
 Gentoo users: If your USB scanner is not detected at all check that USE=usb is
 set when emerging.