I updated from openwrt 21 to 24 - used squashfs-sysupgrade.bin
After update it boot but - no internet - not even DHCP - can not get IP.
Setting IP to static on PC like 192.168.1.153
Can not ping 192.168.1.1 - ssh obviously also does not work.
... lots of internet search
then I run Wireshark - boot router - and noticed
192.168.1.1 send message to 192.168.1.255
"Please press button now to enter failsafe"
I press(dont hold) reset-button on router load - after power up
(I clicked many times till it started blinking fast one led)
and set IP to static on PC 192.168.1.153 mask 255.255.255.0
I seasrch - "openwrt failsafe"
finally I can ping 192.168.1.1
but ssh does not work
Bad server host key: Invalid key length
Solution:
file ~/.ssh/config Host 192.168.1.1 User root UserKnownHostsFile /dev/null
then ssh connect ssh root@192.168.1.1
after connect (look openwrt failsafe wiki link above) firstboot or jffs2reset
reboot - and it works finally in normal boot
(return PC network configuration to dhcp)
I tried to load my "backup" config to new 24 openwrt after "failsafe restore".
After reboot it returned to "dead" state - no dhcp nothing - I repeated failsafe again.
Il restore my configs manually without backup.
This thread for search-bots to make this more visible in search.
There no information about “openwrt failsafe” when “openwrt no ping 192.168.1.1” and “openwrt no dhcp after update”.
Although you don't even mention your device, this is not unexpected - you're skipping two major releases.
That is even less supported. During a sysupgrade, you might be lucky that there could be migration scripts in place holding your hand (only in one direction though) - restoring a backup from an older versions does just that, replacing everything with the broken/ incompatible backup. So this result was guaranteed to happen.
It does matter, a lot, as many targets had a swconfig --> DSA migration with totally different switch semantics within those five years, not your ath79 target (yet!) though. While these 'should' have been prevented/ clearly rejected by the compat_version, there are still some other things that have the potential to wreak havoc.
…then there's the small problem that there never was an "openwrt 20" to begin with (only 19.07.x or 21.02.x), so whatever you had installed before might not have been vanilla OpenWrt to begin with (and 3rd party firmwares 'based on' OpenWrt tend to need different configs for their changes, often proprietary drivers - in which case breakage is guaranteed if you retain the configs)).
Either way, ignoring all of the above, testing of config compatibility only happens between subsequent (major-)releases - skipping (major-)releases is explicitly not supported (and you can be lucky that a simple config reset fixed it for you). Yes, it is attempted to avoid surprises (via compat_version) and to warn the user in case of known issues (e.g. swconfig --> DSA, changes in the partition split requiring manual config changes to the bootloader, etc.), but still - there is no testing between arbitrary versions (but the last major), nor is it expected to just work.
(The ssh complaints are expected as well, old release, old keys using now-obsolete encryption methods, config ~= obsolete keys retained, those just have to be redone from time to time).