What's new in the Stretch (Debian 9) release?
(Also see the official stretch release notes.)
New features
- Linux kernel series 4.9, GNU libc 2.24.
- Desktop environments: GNOME 3.22, KDE Plasma 5.8, MATE 1.16, Xfce 4.12 and others.
- Programming languages: GCC 6.3, Perl 5.24, Python 3.5, PHP 7.0 and others.
nftables is available as a replacement for iptables. See this nftables blog post for details.
If you install fresh instead of upgrading...
Network interfaces will have predictable names (predictable by software, not by humans) instead of the familiar eth0, wlan0, etc.
iproute2 (ip(8)) is installed by default, but net-tools (ifconfig(8) et al.) is not. The net-tools commands have been deprecated for some time now, and users are encouraged to switch to iproute2 commands.
Changes
The dmesg command requires superuser privileges.
The X server is no longer setuid, and may be started without root privileges. If the startx command is run as a non-root user, the Xorg.0.log (or Xorg.*.log for alternative displays) file will be written to ~/.local/share/xorg/ instead of /var/log/.
All MySQL packages have been superseded by equivalent MariaDB packages (e.g. mariadb-server-10.1). The mysql-server and default-mysql-server metapackages are transitional, and will bring in the MariaDB server.
PHP 7.0 replaces PHP 5.6. There are new metapackages without a version number in them: php-fpm, php-cli, etc. You may use these for future compatibility.
Most of the library packages with debugging symbols have been moved to a new repository. If you require these packages, you will need to add an entry to your sources.list or sources.list.d. Also note that the package names are different (ending with -dbgsym instead of -dbg).
Upgrade issues
For many Intel graphics chipsets, the Stretch X server will use the modeset driver instead of the intel driver. The modeset driver may require non-free firmware (firmware-misc-nonfree) to activate features, even on systems which did not use this firmware under Jessie.
For some older graphics chipsets, support has been relegated to "legacy drivers", which require the old setuid X server to run. Install xserver-xorg-legacy if you require one of these drivers.
Users of laptops with a touchpad, running GNOME, may find that the touchpad does not work. If this happens to you, try installing xserver-xorg-input-libinput and removing xserver-xorg-input-synaptics if it is installed. See this debian-devel post for details.