Networking/Ideas/NetworkManagerMethods

Current status

Features

• Automatic link-local addresses
• Manual addresses
• Automatic global address
• Gateway/DNS for other hosts (IPv4 only)

Feature dependencies

• IPv6: Automatic global address needs link-local address
• Gateway needs a manual address (at least a default one), conflicts with dynamic address

IPv4/IPv6 methods

Method	Protocol	Link-local	Manual	Automatic	Gateway
disabled	IPv4	none	forbidden	none	none
ignore	IPv6	kernel	forbidden	kernel	none
link-local	IPv4	userspace	forbidden	none	none
link-local	IPv6	kernel	forbidden	none	none
manual	IPv4	none	required	none	none
manual	IPv6	kernel	required	none	none
auto	IPv4	none	allowed	dhcp	none
auto	IPv6	kernel	allowed	ra+dhcp	none
dhcp	IPv6	kernel	allowed	dhcp	none
shared	IPv4	none	?	none	dhcp+dns+NAT

Notes

• Even when IPv4 is in the disabled state, the network interface communicates on the L2 level.
• There's no disable method for IPv6 and its semantics is unclear (whether disable means the same as link-local, or it also applies to the link-local address which is unaffected in all other methods).
• The usage of IPv4 and IPv6 link-local addresses substatially differs. While IPv6 uses link-local addresses for *all* methods, IPv4 only uses it for the special link-local method.
• The IPv4 disable and IPv4/manual methods only differ in the number of addresses (0 versus >=1), therefore one of them is redundant.
• The IPv6 link-local and IPv6/manual methods only differ in the number of addresses (0 versus >=1), therefore one of them is redundant.
• The IPv6 dhcp method is currently defunct.
• There's no IPv6 shared method, yet.

Feature combinations

L	M	A	G	IPv4	IPv6	notes
no	no	no	no	disabled	unsupported	kernel's disable_ipv6 buggy
no	no	no	yes	invalid	invalid	missing manual address
no	no	yes	no	auto	invalid	missing IPv6 link-local address
no	no	yes	yes	invalid	invalid	missing manual address
no	yes	no	no	method=manual	unsupported	missing kernel feature
no	yes	no	yes	method=shared	unsupported	missing kernel feature
no	yes	yes	no	method=auto	unsupported	missing kernel feature
no	yes	yes	yes	invalid	invalid	both sides of dynamic configuration
yes	no	no	no	method=link-local	method=link-local
yes	no	no	yes	invalid	invalid	missing manual address
yes	no	yes	no	unsupported	method=auto
yes	no	yes	yes	invalid	invalid	missing manual address
yes	yes	no	no	unsupported	method=manual
yes	yes	no	yes	method=shared	unsupported
yes	yes	yes	no	unsupported	method=auto
yes	yes	yes	yes	invalid	invalid	both sides of dynamic configuration

• L: Automatic link-local addresses
• M: Manual addresses
• A: Automatic global address
• G: Gateway/DNS for other hosts

Notes

• There's a bug in the kernel (found by Dan Winship) that once setting IFF_UP with disable_ipv6=1 affects future state. But we shouldn't need disable_ipv6 just as we don't need disable_ipv4, see the next note.
• Kernel doesn't support turning link-local addressing on/off (without affecting other features).

Proposal

Use one configuration directives per feature whenever possible, use multiple configuration directives per feature where further tweaking is needed.

Configuration directives

name	value	meaning
link-local	enabled	configure LL address as soon as possible
	fallback	configure a LL address when other configuration methods failed
	disabled	default for IPv4
	undefined	default for IPv6, use system default
rdisc	enabled	default, IPv6 only
	disabled	skip directly to DHCP (if enabled)
dhcp	enabled	default
	disabled	never use DHCP, only useful for IPv6
sharing	enabled	provide sharing facilities
	disabled	default

Consequences on IPv6 configuration

LL	RD	DHCP	current status	notes
no	no	no	unsupported
no	no	yes	invalid
no	yes	no	invalid
no	yes	yes	invalid
yes	no	no	method=link-local/manual	link-local
yes	no	yes	broken	link-local and DHCPv6 (without rdisc)
yes	yes	no	unsupported	link-local and rdisc only
yes	yes	yes	method=auto	link-local and rdisc, DHCPv6 if requested by router

Notes

• For any valid combination of the above directives, you can optionally add one or more manually specified addresses.
• The DHCP only method is not properly implemented and it never worked.

Consequences on IPv4 configuration

LL	RD	DHCP	current status	notes
no	no	method=disabled/manual
no	yes	method=auto	classic DHCP configuration
yes	no	unsupported	link-local method doesn't allow manual addresses
yes	yes	unsupported	immediate link-local, follow-up DHCP
fallback	no	method=manual/link-local	not so useful
fallback	yes	unsupported	DHCP with fallback to link-local}