[Swan-dev] libswan/ttoaddress.c: use getaddrinfo(3) instead of gethostbyname2(3)

[D. Hugh Redelmeier]

| From: Andrew Cagney <andrew.cagney at gmail.com>

| On Mon, 3 May 2021 at 13:54, D. Hugh Redelmeier <hugh at vault.libreswan.fi>
| wrote:
| 
| > commit 379929c054bbe6022abbc456f5c1fd9bd453470d

| >     We always accept the first result from getaddrinfo(3).  This may
| >     change prioritization of IPv4 vs IPv6, but at least it matches RFC
| >     3484 (according to the man page).

| I'm not sure how much we want to trust getaddrinfo() to Do_The_Right_Thing
| here.

Yeah.

getaddrinfo(3) says:

	Normally, the application should try using the addresses in
	the order in which they are returned.  The sorting function
	used within getaddrinfo() is defined in RFC 3484; the order
	can be tweaked for a particular system by editing
	/etc/gai.conf (available since glibc 2.5).

The RFC itself is obsoleted by rfc 6724 but I don't know if anyone told
getaddrinfo.

I haven't understood 3484's ordering so I don't know if it does what
we want.  But it does conform to a standard!

| For instance, given a choice between IPv4 and IPv6 which will it return
| first?  The documentation I'm reading states:
|      By default IPv6 address entries are ordered before IPv4 ones, but the
|      order of the entries in the list can be controlled using ip6addrctl(8).

Which documention?  It's not in getaddrinfo(3) on my system

| yet Pluto, at least for now, wants IPv4 to trump IPv6.  This so that
| existing configs don't magically switch protocols and wanders into into the
| suspect IPv6 tunneling IPv4 code path.
| 
| (my best guess at RFC 3484 is that it should prefer IPv4 because
| ::ffff:N.N.N.N will sort earlier, but I'm guessing and no documentation for
| getaddrinfo() even hits at it complying with that RFC?)

I hope that works out.  I'm too lazy to test until we see a problem.
This seems like something the ipcheck unit testing should check.

I don't even know how often pluto resolves without specifyig an AF.

We actually want both hosts of a connection to use the same AF and both 
subnets too.  I don't know how to say "any AF, but the same for these two 
domain names".

As you say, we also want "same as last time we resolved this".  I
would guess that getaddrinfo has a stable ordering so that this is not
a worse problem than we had.

This topic may deserve more thought.