They are not really boolean, because we have both ipv4 and ipv6, but for each protocol we have either unset, no, and yes. From https://github.com/systemd/systemd/issues/13316#issuecomment-582906817: LinkLocalAddressing must be a boolean option, at least for ipv4: - LinkLocalAddressing=no => no LL at all. - LinkLocalAddressing=yes + Static Address => invalid configuration, warn and interpret as LinkLocalAddressing=no, no LL at all. (we check that during parsing and reject) - LinkLocalAddressing=yes + DHCP => LL process should be subordinated to the DHCP one, an LL address must be acquired at start or after a short N unsuccessful DHCP attemps, and must not stop DHCP to keeping trying. When a DHCP address is acquired, drop the LL address. If the DHCP address is lost, re-adquire a new LL address. (next patch will move in this direction) - LinkLocalAddressing=fallback has no reason to exist, because LL address must always be allocated as a fallback option when using DHCP. Having both DHCP and LL address at the same time is an RFC violation, so LinkLocalAdressing=yes correctly implemented is already the "fallback" behavior. The fallback option must be deprecated and if present in older configs must be interpreted as LinkLocalAddressing=yes. (removed) - And for IPv6, the LinkLocalAddress option has any sense at all? IPv6-LL address aren't required to be always set for every IPv6 enabled interface (in this case, coexisting with static or dynamic address if any)? Shouldn't be always =yes? (good question) This effectively reverts 29e81083bd2fcb2dbf83f67ef358c7d25adf7e9d. There is no special "fallback" mode now, so the check doesn't make sense anymore.
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