308 lines
7.5 KiB
Go
308 lines
7.5 KiB
Go
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package mdns
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import (
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"fmt"
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"net"
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"os"
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"strings"
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"github.com/miekg/dns"
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)
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const (
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// defaultTTL is the default TTL value in returned DNS records in seconds.
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defaultTTL = 120
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)
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// Zone is the interface used to integrate with the server and
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// to serve records dynamically
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type Zone interface {
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// Records returns DNS records in response to a DNS question.
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Records(q dns.Question) []dns.RR
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}
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// MDNSService is used to export a named service by implementing a Zone
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type MDNSService struct {
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Instance string // Instance name (e.g. "hostService name")
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Service string // Service name (e.g. "_http._tcp.")
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Domain string // If blank, assumes "local"
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HostName string // Host machine DNS name (e.g. "mymachine.net.")
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Port int // Service Port
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IPs []net.IP // IP addresses for the service's host
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TXT []string // Service TXT records
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serviceAddr string // Fully qualified service address
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instanceAddr string // Fully qualified instance address
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enumAddr string // _services._dns-sd._udp.<domain>
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}
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// validateFQDN returns an error if the passed string is not a fully qualified
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// hdomain name (more specifically, a hostname).
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func validateFQDN(s string) error {
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if len(s) == 0 {
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return fmt.Errorf("FQDN must not be blank")
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}
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if s[len(s)-1] != '.' {
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return fmt.Errorf("FQDN must end in period: %s", s)
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}
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// TODO(reddaly): Perform full validation.
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return nil
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}
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// NewMDNSService returns a new instance of MDNSService.
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//
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// If domain, hostName, or ips is set to the zero value, then a default value
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// will be inferred from the operating system.
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//
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// TODO(reddaly): This interface may need to change to account for "unique
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// record" conflict rules of the mDNS protocol. Upon startup, the server should
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// check to ensure that the instance name does not conflict with other instance
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// names, and, if required, select a new name. There may also be conflicting
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// hostName A/AAAA records.
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func NewMDNSService(instance, service, domain, hostName string, port int, ips []net.IP, txt []string) (*MDNSService, error) {
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// Sanity check inputs
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if instance == "" {
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return nil, fmt.Errorf("missing service instance name")
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}
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if service == "" {
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return nil, fmt.Errorf("missing service name")
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}
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if port == 0 {
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return nil, fmt.Errorf("missing service port")
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}
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// Set default domain
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if domain == "" {
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domain = "local."
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}
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if err := validateFQDN(domain); err != nil {
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return nil, fmt.Errorf("domain %q is not a fully-qualified domain name: %v", domain, err)
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}
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// Get host information if no host is specified.
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if hostName == "" {
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var err error
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hostName, err = os.Hostname()
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if err != nil {
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return nil, fmt.Errorf("could not determine host: %v", err)
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}
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hostName = fmt.Sprintf("%s.", hostName)
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}
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if err := validateFQDN(hostName); err != nil {
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return nil, fmt.Errorf("hostName %q is not a fully-qualified domain name: %v", hostName, err)
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}
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if len(ips) == 0 {
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var err error
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ips, err = net.LookupIP(hostName)
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if err != nil {
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// Try appending the host domain suffix and lookup again
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// (required for Linux-based hosts)
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tmpHostName := fmt.Sprintf("%s%s", hostName, domain)
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ips, err = net.LookupIP(tmpHostName)
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if err != nil {
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return nil, fmt.Errorf("could not determine host IP addresses for %s", hostName)
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}
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}
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}
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for _, ip := range ips {
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if ip.To4() == nil && ip.To16() == nil {
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return nil, fmt.Errorf("invalid IP address in IPs list: %v", ip)
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}
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}
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return &MDNSService{
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Instance: instance,
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Service: service,
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Domain: domain,
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HostName: hostName,
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Port: port,
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IPs: ips,
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TXT: txt,
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serviceAddr: fmt.Sprintf("%s.%s.", trimDot(service), trimDot(domain)),
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instanceAddr: fmt.Sprintf("%s.%s.%s.", instance, trimDot(service), trimDot(domain)),
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enumAddr: fmt.Sprintf("_services._dns-sd._udp.%s.", trimDot(domain)),
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}, nil
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}
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// trimDot is used to trim the dots from the start or end of a string
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func trimDot(s string) string {
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return strings.Trim(s, ".")
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}
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// Records returns DNS records in response to a DNS question.
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func (m *MDNSService) Records(q dns.Question) []dns.RR {
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switch q.Name {
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case m.enumAddr:
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return m.serviceEnum(q)
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case m.serviceAddr:
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return m.serviceRecords(q)
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case m.instanceAddr:
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return m.instanceRecords(q)
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case m.HostName:
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if q.Qtype == dns.TypeA || q.Qtype == dns.TypeAAAA {
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return m.instanceRecords(q)
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}
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fallthrough
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default:
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return nil
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}
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}
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func (m *MDNSService) serviceEnum(q dns.Question) []dns.RR {
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switch q.Qtype {
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case dns.TypeANY:
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fallthrough
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case dns.TypePTR:
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rr := &dns.PTR{
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Hdr: dns.RR_Header{
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Name: q.Name,
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Rrtype: dns.TypePTR,
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Class: dns.ClassINET,
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Ttl: defaultTTL,
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},
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Ptr: m.serviceAddr,
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}
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return []dns.RR{rr}
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default:
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return nil
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}
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}
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// serviceRecords is called when the query matches the service name
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func (m *MDNSService) serviceRecords(q dns.Question) []dns.RR {
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switch q.Qtype {
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case dns.TypeANY:
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fallthrough
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case dns.TypePTR:
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// Build a PTR response for the service
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rr := &dns.PTR{
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Hdr: dns.RR_Header{
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Name: q.Name,
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Rrtype: dns.TypePTR,
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Class: dns.ClassINET,
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Ttl: defaultTTL,
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},
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Ptr: m.instanceAddr,
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}
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servRec := []dns.RR{rr}
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// Get the instance records
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instRecs := m.instanceRecords(dns.Question{
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Name: m.instanceAddr,
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Qtype: dns.TypeANY,
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})
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// Return the service record with the instance records
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return append(servRec, instRecs...)
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default:
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return nil
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}
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}
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// serviceRecords is called when the query matches the instance name
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func (m *MDNSService) instanceRecords(q dns.Question) []dns.RR {
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switch q.Qtype {
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case dns.TypeANY:
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// Get the SRV, which includes A and AAAA
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recs := m.instanceRecords(dns.Question{
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Name: m.instanceAddr,
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Qtype: dns.TypeSRV,
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})
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// Add the TXT record
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recs = append(recs, m.instanceRecords(dns.Question{
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Name: m.instanceAddr,
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Qtype: dns.TypeTXT,
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})...)
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return recs
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case dns.TypeA:
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var rr []dns.RR
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for _, ip := range m.IPs {
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if ip4 := ip.To4(); ip4 != nil {
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rr = append(rr, &dns.A{
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Hdr: dns.RR_Header{
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Name: m.HostName,
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Rrtype: dns.TypeA,
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Class: dns.ClassINET,
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Ttl: defaultTTL,
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},
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A: ip4,
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})
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}
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}
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return rr
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case dns.TypeAAAA:
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var rr []dns.RR
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for _, ip := range m.IPs {
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if ip.To4() != nil {
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// TODO(reddaly): IPv4 addresses could be encoded in IPv6 format and
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// putinto AAAA records, but the current logic puts ipv4-encodable
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// addresses into the A records exclusively. Perhaps this should be
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// configurable?
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continue
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}
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if ip16 := ip.To16(); ip16 != nil {
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rr = append(rr, &dns.AAAA{
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Hdr: dns.RR_Header{
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Name: m.HostName,
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Rrtype: dns.TypeAAAA,
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Class: dns.ClassINET,
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Ttl: defaultTTL,
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},
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AAAA: ip16,
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})
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}
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}
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return rr
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case dns.TypeSRV:
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// Create the SRV Record
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srv := &dns.SRV{
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Hdr: dns.RR_Header{
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Name: q.Name,
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Rrtype: dns.TypeSRV,
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Class: dns.ClassINET,
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Ttl: defaultTTL,
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},
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Priority: 10,
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Weight: 1,
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Port: uint16(m.Port),
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Target: m.HostName,
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}
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recs := []dns.RR{srv}
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// Add the A record
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recs = append(recs, m.instanceRecords(dns.Question{
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Name: m.instanceAddr,
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Qtype: dns.TypeA,
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})...)
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// Add the AAAA record
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recs = append(recs, m.instanceRecords(dns.Question{
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Name: m.instanceAddr,
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Qtype: dns.TypeAAAA,
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})...)
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return recs
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case dns.TypeTXT:
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txt := &dns.TXT{
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Hdr: dns.RR_Header{
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Name: q.Name,
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Rrtype: dns.TypeTXT,
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Class: dns.ClassINET,
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Ttl: defaultTTL,
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},
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Txt: m.TXT,
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}
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return []dns.RR{txt}
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}
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return nil
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}
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