While the alias is available after enabling the unit, we don't
actually do that in our testing environment (adding a symlink manually
would work too, then again, why not just use the proper name?).
The client identifier serves as unique identifier just like a unique MAC
address would, so even with identity_leases disabled some DHCP servers
might assign unique leases per identity.
DHCP servers will respond to port 67 if giaddr is non-zero, which we set
if we are not broadcasting. While such messages are received fine via
RAW socket the kernel will respond with an ICMP port unreachable if no
socket is bound to that port. Instead of opening a dummy socket on port
67 just to avoid the ICMPs we can also just operate with a single
socket, bind it to port 67 and send our requests from that port.
Since SO_REUSEADDR behaves on Linux like SO_REUSEPORT does on other
systems we can bind that port even if a DHCP server is running on the
same host as the daemon (this might have to be adapted to make this work
on other systems, but due to the raw socket the plugin is not that portable
anyway).
We could make the same change for charon (actually setting it for charon
in strongswan.conf.testing would work for charon-systemd too), however,
there are dozens of test cases that currently set charondebug in
ipsec.conf.
The main difference is that ping now reports icmp_seq instead of
icmp_req, so we match for icmp_.eq, which works with both releases.
tcpdump now also reports port 4500 as ipsec-nat-t.
Tobias Brunner