micro/util/mdns/dns_sd.go
2020-04-11 13:02:53 +01:00

85 lines
3.3 KiB
Go

package mdns
import "github.com/miekg/dns"
// DNSSDService is a service that complies with the DNS-SD (RFC 6762) and MDNS
// (RFC 6762) specs for local, multicast-DNS-based discovery.
//
// DNSSDService implements the Zone interface and wraps an MDNSService instance.
// To deploy an mDNS service that is compliant with DNS-SD, it's recommended to
// register only the wrapped instance with the server.
//
// Example usage:
// service := &mdns.DNSSDService{
// MDNSService: &mdns.MDNSService{
// Instance: "My Foobar Service",
// Service: "_foobar._tcp",
// Port: 8000,
// }
// }
// server, err := mdns.NewServer(&mdns.Config{Zone: service})
// if err != nil {
// log.Fatalf("Error creating server: %v", err)
// }
// defer server.Shutdown()
type DNSSDService struct {
MDNSService *MDNSService
}
// Records returns DNS records in response to a DNS question.
//
// This function returns the DNS response of the underlying MDNSService
// instance. It also returns a PTR record for a request for "
// _services._dns-sd._udp.<Domain>", as described in section 9 of RFC 6763
// ("Service Type Enumeration"), to allow browsing of the underlying MDNSService
// instance.
func (s *DNSSDService) Records(q dns.Question) []dns.RR {
var recs []dns.RR
if q.Name == "_services._dns-sd._udp."+s.MDNSService.Domain+"." {
recs = s.dnssdMetaQueryRecords(q)
}
return append(recs, s.MDNSService.Records(q)...)
}
// dnssdMetaQueryRecords returns the DNS records in response to a "meta-query"
// issued to browse for DNS-SD services, as per section 9. of RFC6763.
//
// A meta-query has a name of the form "_services._dns-sd._udp.<Domain>" where
// Domain is a fully-qualified domain, such as "local."
func (s *DNSSDService) dnssdMetaQueryRecords(q dns.Question) []dns.RR {
// Intended behavior, as described in the RFC:
// ...it may be useful for network administrators to find the list of
// advertised service types on the network, even if those Service Names
// are just opaque identifiers and not particularly informative in
// isolation.
//
// For this purpose, a special meta-query is defined. A DNS query for PTR
// records with the name "_services._dns-sd._udp.<Domain>" yields a set of
// PTR records, where the rdata of each PTR record is the two-abel
// <Service> name, plus the same domain, e.g., "_http._tcp.<Domain>".
// Including the domain in the PTR rdata allows for slightly better name
// compression in Unicast DNS responses, but only the first two labels are
// relevant for the purposes of service type enumeration. These two-label
// service types can then be used to construct subsequent Service Instance
// Enumeration PTR queries, in this <Domain> or others, to discover
// instances of that service type.
return []dns.RR{
&dns.PTR{
Hdr: dns.RR_Header{
Name: q.Name,
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: defaultTTL,
},
Ptr: s.MDNSService.serviceAddr,
},
}
}
// Announcement returns DNS records that should be broadcast during the initial
// availability of the service, as described in section 8.3 of RFC 6762.
// TODO(reddaly): Add this when Announcement is added to the mdns.Zone interface.
//func (s *DNSSDService) Announcement() []dns.RR {
// return s.MDNSService.Announcement()
//}