[GLLUG] IPv6 in IPv4 sit tunnel configuration

[Chris Bell]

Hello,
I am trying to configure a replacement firewall, this time based on Debian 10, 
Buster, with shorewall and shorewall6, including an IPv6 in IPv4 tunnel 
provided by Hurricane Electric. HE provided the following:

modprobe ipv6
ip tunnel add he-ipv6 mode sit remote IPv4_address local IPv4_address ttl 255
ip link set he-ipv6 up
ip addr add 2001:470:1f08:2ea::2/64 dev he-ipv6
ip route add ::/0 dev he-ipv6
ip -f inet6 addr

together with
Server IPv6 Address
Client IPv6 Address
Routed IPv6 Prefix

which mainly works except that:
add tunnel "sit0" failed: No buffer space available

There are several often historical packages related to networking, and now 
systemd has added more possible complications which may or may not work.
I added /etc/systemd/network/25-sit.netdev based on an example:

[NetDev]
Name=he-ipv6
Kind=sit
MTUBytes=1480

[Tunnel]
Local=[IPv4_address]
Remote=[IPv4_address]

which did not help with buffers so I am now trying to figure out whether 
devlink-sb is what I need to configure, and if so what I need to do. The man 
pages have references to PCI devices, but sit just uses port 41, while there 
is no suggestion about required buffer size. Is there any information about 
requirements?
My /etc/network/interfaces file has full information about the several wired 
connections but no reference to the sit device.

Thanks for any help.
-- 
Chris Bell
Website http://chrisbell.org.uk