Add Debian upgrade docs

debian9.md

@@ -0,0 +1,40 @@
+# Upgrading Debian 8 -> 9
+
+## Steps
+
+1. Create backup (`/etc`, `/home`, `/root`, `/var/lib/mysql`, mysqldump, ...)
+2. Update sources in `/etc/apt/sources.list` and `/etc/apt/sources.list.d/` to stretch
+3. `apt update`
+4. `apt upgrade`
+5. `apt dist-upgrade`
+6. Fix errors and warnings and _only then_ `reboot`
+7. `dpkg -l | grep -v ^ii` should be empty or packages should be removed
+8. `reboot`
+9. test
+
+## Known problems
+
+### fail2ban
+
+Some configuration changes, e.g. multiple `ignore` directives are no longer allowed, so
+
+```
+ignore = 127.0.0.1/8
+ignore = 10.0.0.1/8
+```
+
+becomes
+
+```
+ignore = 127.0.0.1/8 10.0.0.1/8
+```
+
+### IPv6
+
+`/etc/rc.local`
+
+```
+/sbin/ip -6 addr add <ipv6-address>/64 dev eth0
+/sbin/ip -f inet6 route add <ipv6-gateway> dev eth0		# this one is new
+/sbin/route -A inet6 add default gw <ipv6-gateway> dev eth0
+```