Rtt min/avg/max/mdev = 0.199/0.199/0.199/0.000 msĪnd the following is the corresponding output from tcpdump running on same host at the same time: $ sudo tcpdump -eni any icmp $ sudo sysctl -w _echo_ignore_broadcasts=0 ( src is the preferred source address, which will be used as the source address for non-replying traffics with the corresponding destination when you are not specifying one, e.g. Local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1 It appears to cause a difference for certain broadcast destination though, like 255.255.255.255, probably because it is used to determine the broadcasting link/domain in such case.) $ ip r show table local type local
In the unicast case the specified interface of a local route does not seem to matter.
Those routes determine what addresses you can bind/use as source address and what traffics will be "trapped" in the host itself (instead of actually being sent / forwarded out to another host). All of the combiantions could be used for a loopback test, at least in Linux, since in what cases will loopback occur pretty much entirely depends on the type local routes in the route table local (ID 255).
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