Class DnsHelper

app/src/main/java/eu/faircode/email/DnsHelper.java:93–746

import javax.net.ssl.SSLSocketFactory;

public class DnsHelper {
    // https://dns.watch/
    private static final String DEFAULT_DNS4 = "84.200.69.80";
    private static final String DEFAULT_DNS6 = "2001:1608:10:25::1c04:b12f";
    private static final int CHECK_TIMEOUT = 5; // seconds
    private static final int LOOKUP_TIMEOUT = 15; // seconds

    static void init(Context context) {
        DnsClient.addDnsServerLookupMechanism(
                new AbstractDnsServerLookupMechanism("FairEmail", 1) {
                    @Override
                    public boolean isAvailable() {
                        return true;
                    }

                    @Override
                    public List<String> getDnsServerAddresses() {
                        List<String> servers = getDnsServers(context);
                        Log.i("DNS servers=" + TextUtils.join(",", servers));
                        return servers;
                    }
                });
    }

    static void checkMx(Context context, Address[] addresses) throws UnknownHostException {
        if (addresses == null)
            return;

        for (Address address : addresses) {
            String email = ((InternetAddress) address).getAddress();
            String domain = UriHelper.getEmailDomain(email);
            if (domain == null)
                continue;
            DnsRecord[] records = _lookup(context, domain, "mx", CHECK_TIMEOUT);
            if (records.length == 0)
                throw new UnknownHostException(domain);
        }
    }

    @NonNull
    static DnsRecord[] lookup(Context context, String name, String type) {
        return _lookup(context, name, type, LOOKUP_TIMEOUT);
    }

    @NonNull
    private static DnsRecord[] _lookup(Context context, String name, String type, int timeout) {
        try {
            return _lookup(context, name, type, timeout, false);
        } catch (Throwable ex) {
            if (ex instanceof MultipleIoException ||
                    ex instanceof ResolutionUnsuccessfulException ||
                    ex instanceof DnssecValidationFailedException)
                Log.i(ex);
            else
                Log.e(ex);
            return new DnsRecord[0];
        }
    }