"""Usage: python3 check_test_ips.py IP|HOSTNAME [...]

Check if the given IPs, or IPs associated with given hostnames, are
routable. Under the assumption that those IPs are local a positive
result means they should be usable for tests.

The script prints the first two (in order of arguments) usable
addresses to sys.stdout. IPv6 addresses in the output are enclosed in
square brackets.

"""
import socket
import sys


def try_connect(sockaddr):
    """Try to connect a UDP socket to the given address. "Connecting" a
    UDP socket means only setting the default destination for packets,
    so nothing is actually sent. Return True if the socket.connect()
    call was successful, False otherwise.

    For loopback this effectively tests if the address is really
    configured, to detect e.g. situations where a system may have "::1
    localhost" in /etc/hosts, but actually has IPv6 disabled.

    """
    af, socktype, proto, canonname, sa = sockaddr
    try:
        s = socket.socket(af, socktype, proto)
        s.connect(sa)
    except:
        return False
    finally:
        s.close()
    return True


if __name__ == "__main__":
    import argparse
    parser = argparse.ArgumentParser(
        description='Check if IPs/hostnames are routable')
    parser.add_argument('hosts', metavar='HOST', nargs='+',
                        help='the hostnames/IPs to check')
    parser.add_argument('--quiet', '-q', action='store_true',
                        help='disable debug output')
    args = parser.parse_args()

    test_ips = []
    for name in args.hosts:
        addrs = list(map(lambda t: t[-1][0],
                         filter(try_connect,
                                socket.getaddrinfo(name, 12345,
                                                   proto=socket.IPPROTO_UDP))))
        if not args.quiet:
            print(f'{name}: {addrs}', file=sys.stderr)
        test_ips += addrs

    print(' '.join(f'[{i}]' if ':' in i else i for i in test_ips[:2]))