I was recently bitten by this as well.
It was particularly bad in my case: on a headless server, with the
network connection being my only way in---and the problem was not with
the main Ethernet interface, but with a separate WireGuard one that I
was not using (both marked as "auto").
The eno1 interface got a DHCP connection without issue. But the wg0
config had a post-up routing-setup bug that I'd missed in earlier
testing. As the syslog excerpt below shows, the failure to bring up wg0
caused *all* of the "Raise network interfaces" task to be walked back.
And as this only killed dhclient and left eno1 configured, I thought all
was fine... until the system dropped off the network twelve hours later.
(It's also clear that dhclient was killed uncleanly, because the next
time it runs, you get a "Removed stale PID file" message.)
The "FIB table does not exist" error is where things start to go wrong:
2024-02-05T22:19:06.343712-05:00 darkstar systemd[1]: Starting
networking.service - Raise network interfaces...
2024-02-05T22:19:06.377118-05:00 darkstar dhclient[620]: Internet Systems
Consortium DHCP Client 4.4.3-P1
2024-02-05T22:19:06.377147-05:00 darkstar ifup[620]: Internet Systems
Consortium DHCP Client 4.4.3-P1
2024-02-05T22:19:06.377154-05:00 darkstar ifup[620]: Copyright 2004-2022
Internet Systems Consortium.
2024-02-05T22:19:06.377159-05:00 darkstar ifup[620]: All rights reserved.
2024-02-05T22:19:06.377164-05:00 darkstar ifup[620]: For info, please visit
https://www.isc.org/software/dhcp/
2024-02-05T22:19:06.377178-05:00 darkstar dhclient[620]: Copyright
2004-2022 Internet Systems Consortium.
2024-02-05T22:19:06.377190-05:00 darkstar dhclient[620]: All rights
reserved.
2024-02-05T22:19:06.377201-05:00 darkstar dhclient[620]: For info, please
visit https://www.isc.org/software/dhcp/
2024-02-05T22:19:06.377213-05:00 darkstar dhclient[620]:
2024-02-05T22:19:06.675691-05:00 darkstar dhclient[620]: Listening on
LPF/eno1/XX:XX:XX:XX:XX:XX
2024-02-05T22:19:06.675765-05:00 darkstar ifup[620]: Listening on
LPF/eno1/XX:XX:XX:XX:XX:XX
2024-02-05T22:19:06.675783-05:00 darkstar ifup[620]: Sending on
LPF/eno1/XX:XX:XX:XX:XX:XX
2024-02-05T22:19:06.675796-05:00 darkstar ifup[620]: Sending on
Socket/fallback
2024-02-05T22:19:06.675831-05:00 darkstar dhclient[620]: Sending on
LPF/eno1/XX:XX:XX:XX:XX:XX
2024-02-05T22:19:06.675863-05:00 darkstar dhclient[620]: Sending on
Socket/fallback
2024-02-05T22:19:06.675919-05:00 darkstar dhclient[620]: DHCPREQUEST for
192.168.1.2 on eno1 to 255.255.255.255 port 67
2024-02-05T22:19:06.675956-05:00 darkstar ifup[620]: DHCPREQUEST for
192.168.1.2 on eno1 to 255.255.255.255 port 67
2024-02-05T22:19:09.219179-05:00 darkstar kernel: [ 6.436538] e1000e
0000:00:1f.6 eno1: NIC Link is Up 1000 Mbps Full Duplex, Flow Control: Rx/Tx
2024-02-05T22:19:09.219201-05:00 darkstar kernel: [ 6.436674] IPv6:
ADDRCONF(NETDEV_CHANGE): eno1: link becomes ready
2024-02-05T22:19:11.467192-05:00 darkstar dhclient[620]: DHCPREQUEST for
192.168.1.2 on eno1 to 255.255.255.255 port 67
2024-02-05T22:19:11.467252-05:00 darkstar ifup[620]: DHCPREQUEST for
192.168.1.2 on eno1 to 255.255.255.255 port 67
2024-02-05T22:19:11.470106-05:00 darkstar dhclient[620]: DHCPACK of
192.168.1.2 from 192.168.1.1
2024-02-05T22:19:11.470172-05:00 darkstar ifup[620]: DHCPACK of 192.168.1.2
from 192.168.1.1
2024-02-05T22:19:11.509438-05:00 darkstar dhclient[620]: bound to
192.168.1.2 -- renewal in 21284 seconds.
2024-02-05T22:19:11.509497-05:00 darkstar ifup[620]: bound to 192.168.1.2
-- renewal in 21284 seconds.
2024-02-05T22:19:11.575017-05:00 darkstar kernel: [ 8.793246] wireguard:
WireGuard 1.0.0 loaded. See www.wireguard.com for information.
2024-02-05T22:19:11.575027-05:00 darkstar kernel: [ 8.793252] wireguard:
Copyright (C) 2015-2019 Jason A. Donenfeld <ja...@zx2c4.com>. All Rights
Reserved.
2024-02-05T22:19:11.589364-05:00 darkstar ifup[689]: Error: ipv4: FIB table
does not exist.
2024-02-05T22:19:11.589373-05:00 darkstar ifup[689]: Flush terminated
2024-02-05T22:19:11.589765-05:00 darkstar ifup[583]: ifup: failed to bring
up wg0
2024-02-05T22:19:11.599118-05:00 darkstar systemd[1]: networking.service:
Main process exited, code=exited, status=1/FAILURE
2024-02-05T22:19:11.663575-05:00 darkstar systemd[1]: networking.service:
Failed with result 'exit-code'.
2024-02-05T22:19:11.664060-05:00 darkstar systemd[1]: Failed to start
networking.service - Raise network interfaces.
2024-02-05T22:19:11.664996-05:00 darkstar systemd[1]: Reached target
network.target - Network.
2024-02-05T22:19:11.665371-05:00 darkstar systemd[1]: Reached target
network-online.target - Network is Online.
