With an Ethernet tap yes, just on the same switch not necessarily (without causing at least some disruption). For example, a DHCP configured client wouldn't acc

With an Ethernet tap yes, just on the same switch not necessarily (without causing at least some disruption). For example, a DHCP configured client wouldn't accept a DHCPOFFER he didn't ask for, you'd need to do ARP spoofing which can be detected rather easily AFAIK. IPv6 is another story though, it seems like IPv6 RA (which can also contain DNS configuration) are accepted by default by all OSes. My observation so far suggested that these overrule DHCPv6 as well.