private static DnsClientOptions buildDnsClientOptions(final String server) {
    final List<String> hostPort = Splitter.on(":").splitToList(server);
    final DnsClientOptions dnsClientOptions = new DnsClientOptions();
    dnsClientOptions.setHost(hostPort.get(0));
    if (hostPort.size() > 1) {
      try {
        int port = Integer.parseInt(hostPort.get(1));
        dnsClientOptions.setPort(port);
      } catch (NumberFormatException e) {
        LOG.trace("Invalid port number {}, ignoring", hostPort.get(1));
      }
    }
    return dnsClientOptions;
  }
Creates a new DNSResolver.

@param vertx Vertx instance which is used to create DNS Client
@param enrLink the ENR link to start with, of the form enrtree://PUBKEY@domain
@param seq the sequence number of the root record. If the root record seq is higher, proceed
    with visit.
@param dnsServer the DNS server to use for DNS query. If empty, the default DNS server will be
    used.
ethereum/p2p/src/main/java/org/hyperledger/besu/ethereum/p2p/discovery/dns/DNSResolver.java