file: /proc/sys/net/ipv6/conf/interface/force_tllao
variable: net.ipv6.conf.interface.force_tllao
Official reference

Enable sending the target link-layer address option even when responding to a unicast neighbor solicitation. Default: FALSE

Quoting from RFC 2461, section 4.4, Target link-layer address:

“The option MUST be included for multicast solicitations in order to avoid infinite Neighbor Solicitation “recursion” when the peer node does not have a cache entry to return a Neighbor Advertisements message. When responding to unicast solicitations, the option can be omitted since the sender of the solicitation has the correct link- layer address; otherwise it would not have be able to send the unicast solicitation in the first place. However, including the link-layer address in this case adds little overhead and eliminates a potential race condition where the sender deletes the cached link-layer address prior to receiving a response to a previous solicitation.”

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.