networkd tries only once to send IPv6 router solicitation, then never tries again

["Kevin P. Fleming" <kevin@xxxxxxx>]

I've just setup a new system running Debian Bookworm, so it has the
Debian systemd 251.4-3 packages installed. There's a bonded network
interface setup with systemd-networkd.

During boot, networkd attempts to send a router solicitation but fails
with "No buffer space available". I don't know why it is getting that
error, but I'd expect that to be treated as a non-permanent error, but
networkd never attempts to send a router solicitation again (unless I
use 'networkctl down <if> && networkctl up <if>', in which case the
solicitation is send and the DHCPv6 client starts and gets an
address).

Here's a portion of the log showing the behavior:

Oct 05 07:45:57 edge22-c systemd-networkd[521]: core: Received new
foreign address (configured): fe80::c71:3fff:fef0:c1fb/64 (valid
forever, preferred forever), flags: permanent, scope: link
Oct 05 07:45:57 edge22-c systemd-networkd[521]: core: Gained IPv6LL
Oct 05 07:45:57 edge22-c systemd-networkd[521]: core: Discovering IPv6 routers
Oct 05 07:45:57 edge22-c systemd-networkd[521]: core: NDISC: Started
IPv6 Router Solicitation client
Oct 05 07:45:57 edge22-c systemd-networkd[521]: core:
link_check_ready(): DHCPv4, DHCPv6, DHCP-PD or IPv4LL is enabled but
no dynamic address is assigned yet.
Oct 05 07:45:57 edge22-c systemd-networkd[521]: Sent message
type=signal sender=n/a destination=n/a
path=/org/freedesktop/network1/link/_34
interface=org.freedesktop.DBus.Properties member=PropertiesChanged
cookie=4 reply_cookie=0 signature=sa{sv}as error-name=n/a
error-message=n/a
Oct 05 07:45:57 edge22-c systemd-networkd[521]: core:
link_check_ready(): DHCPv4, DHCPv6, DHCP-PD or IPv4LL is enabled but
no dynamic address is assigned yet.
Oct 05 07:45:57 edge22-c systemd-networkd[521]: Sent message
type=signal sender=n/a destination=n/a path=/org/freedesktop/network1
interface=org.freedesktop.DBus.Properties member=PropertiesChanged
cookie=5 reply_cookie=0 signature=sa{sv}as error-name=n/a
error-message=n/a
Oct 05 07:45:57 edge22-c systemd-networkd[521]: core: Received new
foreign route (configured): dst: fe80::c71:3fff:fef0:c1fb/128, src:
n/a, gw: n/a, prefsrc: n/a, scope: global, table: local(255), proto:
kernel, type: local, nexthop: 0, priority: 0, flags: n/a
Oct 05 07:45:58 edge22-c systemd-networkd[521]: core: NDISC: Error
sending Router Solicitation: No buffer space available
Oct 05 07:45:58 edge22-c systemd-networkd[521]: core: NDISC: Stopping
IPv6 Router Solicitation client
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client: DISCOVER
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client:
Received message from RAW socket, processing.
Oct 05 07:45:59 edge22-c systemd-networkd[521]: Ignoring DHCP option
61 while parsing.
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client: OFFER
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client:
REQUEST (requesting)
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client:
Received message from RAW socket, processing.
Oct 05 07:45:59 edge22-c systemd-networkd[521]: Ignoring DHCP option
61 while parsing.
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client: ACK
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client:
lease expires in 23h 59min 59s
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client:
T2 expires in 20h 59min 59s
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 client:
T1 expires in 12h

...

Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: DHCPv4 address
and routes set.
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core:
link_check_ready(): IPv4LL:no DHCPv4:yes DHCPv6:no DHCP-PD:no NDisc:no
Oct 05 07:45:59 edge22-c systemd-networkd[521]: core: State changed:
configuring -> configured

At the same time that the router solicitation fails the DHCPv4 client
is able to obtain an address, so the network link is fully
operational.

I'd certainly like to find out why the initial packet got "No buffer
space available", but I think it would make sense for networkd to not
treat that as a permanent condition and never try again to obtain IPv6
information.