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

310 lines
7.7 KiB
Go

package mdns
import (
"fmt"
"net"
"os"
"strings"
"sync/atomic"
"github.com/miekg/dns"
)
const (
// defaultTTL is the default TTL value in returned DNS records in seconds.
defaultTTL = 120
)
// Zone is the interface used to integrate with the server and
// to serve records dynamically
type Zone interface {
// Records returns DNS records in response to a DNS question.
Records(q dns.Question) []dns.RR
}
// MDNSService is used to export a named service by implementing a Zone
type MDNSService struct {
Instance string // Instance name (e.g. "hostService name")
Service string // Service name (e.g. "_http._tcp.")
Domain string // If blank, assumes "local"
HostName string // Host machine DNS name (e.g. "mymachine.net.")
Port int // Service Port
IPs []net.IP // IP addresses for the service's host
TXT []string // Service TXT records
TTL uint32
serviceAddr string // Fully qualified service address
instanceAddr string // Fully qualified instance address
enumAddr string // _services._dns-sd._udp.<domain>
}
// validateFQDN returns an error if the passed string is not a fully qualified
// hdomain name (more specifically, a hostname).
func validateFQDN(s string) error {
if len(s) == 0 {
return fmt.Errorf("FQDN must not be blank")
}
if s[len(s)-1] != '.' {
return fmt.Errorf("FQDN must end in period: %s", s)
}
// TODO(reddaly): Perform full validation.
return nil
}
// NewMDNSService returns a new instance of MDNSService.
//
// If domain, hostName, or ips is set to the zero value, then a default value
// will be inferred from the operating system.
//
// TODO(reddaly): This interface may need to change to account for "unique
// record" conflict rules of the mDNS protocol. Upon startup, the server should
// check to ensure that the instance name does not conflict with other instance
// names, and, if required, select a new name. There may also be conflicting
// hostName A/AAAA records.
func NewMDNSService(instance, service, domain, hostName string, port int, ips []net.IP, txt []string) (*MDNSService, error) {
// Sanity check inputs
if instance == "" {
return nil, fmt.Errorf("missing service instance name")
}
if service == "" {
return nil, fmt.Errorf("missing service name")
}
if port == 0 {
return nil, fmt.Errorf("missing service port")
}
// Set default domain
if domain == "" {
domain = "local."
}
if err := validateFQDN(domain); err != nil {
return nil, fmt.Errorf("domain %q is not a fully-qualified domain name: %v", domain, err)
}
// Get host information if no host is specified.
if hostName == "" {
var err error
hostName, err = os.Hostname()
if err != nil {
return nil, fmt.Errorf("could not determine host: %v", err)
}
hostName = fmt.Sprintf("%s.", hostName)
}
if err := validateFQDN(hostName); err != nil {
return nil, fmt.Errorf("hostName %q is not a fully-qualified domain name: %v", hostName, err)
}
if len(ips) == 0 {
var err error
ips, err = net.LookupIP(trimDot(hostName))
if err != nil {
// Try appending the host domain suffix and lookup again
// (required for Linux-based hosts)
tmpHostName := fmt.Sprintf("%s%s", hostName, domain)
ips, err = net.LookupIP(trimDot(tmpHostName))
if err != nil {
return nil, fmt.Errorf("could not determine host IP addresses for %s", hostName)
}
}
}
for _, ip := range ips {
if ip.To4() == nil && ip.To16() == nil {
return nil, fmt.Errorf("invalid IP address in IPs list: %v", ip)
}
}
return &MDNSService{
Instance: instance,
Service: service,
Domain: domain,
HostName: hostName,
Port: port,
IPs: ips,
TXT: txt,
TTL: defaultTTL,
serviceAddr: fmt.Sprintf("%s.%s.", trimDot(service), trimDot(domain)),
instanceAddr: fmt.Sprintf("%s.%s.%s.", instance, trimDot(service), trimDot(domain)),
enumAddr: fmt.Sprintf("_services._dns-sd._udp.%s.", trimDot(domain)),
}, nil
}
// trimDot is used to trim the dots from the start or end of a string
func trimDot(s string) string {
return strings.Trim(s, ".")
}
// Records returns DNS records in response to a DNS question.
func (m *MDNSService) Records(q dns.Question) []dns.RR {
switch q.Name {
case m.enumAddr:
return m.serviceEnum(q)
case m.serviceAddr:
return m.serviceRecords(q)
case m.instanceAddr:
return m.instanceRecords(q)
case m.HostName:
if q.Qtype == dns.TypeA || q.Qtype == dns.TypeAAAA {
return m.instanceRecords(q)
}
fallthrough
default:
return nil
}
}
func (m *MDNSService) serviceEnum(q dns.Question) []dns.RR {
switch q.Qtype {
case dns.TypeANY:
fallthrough
case dns.TypePTR:
rr := &dns.PTR{
Hdr: dns.RR_Header{
Name: q.Name,
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: atomic.LoadUint32(&m.TTL),
},
Ptr: m.serviceAddr,
}
return []dns.RR{rr}
default:
return nil
}
}
// serviceRecords is called when the query matches the service name
func (m *MDNSService) serviceRecords(q dns.Question) []dns.RR {
switch q.Qtype {
case dns.TypeANY:
fallthrough
case dns.TypePTR:
// Build a PTR response for the service
rr := &dns.PTR{
Hdr: dns.RR_Header{
Name: q.Name,
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: atomic.LoadUint32(&m.TTL),
},
Ptr: m.instanceAddr,
}
servRec := []dns.RR{rr}
// Get the instance records
instRecs := m.instanceRecords(dns.Question{
Name: m.instanceAddr,
Qtype: dns.TypeANY,
})
// Return the service record with the instance records
return append(servRec, instRecs...)
default:
return nil
}
}
// serviceRecords is called when the query matches the instance name
func (m *MDNSService) instanceRecords(q dns.Question) []dns.RR {
switch q.Qtype {
case dns.TypeANY:
// Get the SRV, which includes A and AAAA
recs := m.instanceRecords(dns.Question{
Name: m.instanceAddr,
Qtype: dns.TypeSRV,
})
// Add the TXT record
recs = append(recs, m.instanceRecords(dns.Question{
Name: m.instanceAddr,
Qtype: dns.TypeTXT,
})...)
return recs
case dns.TypeA:
var rr []dns.RR
for _, ip := range m.IPs {
if ip4 := ip.To4(); ip4 != nil {
rr = append(rr, &dns.A{
Hdr: dns.RR_Header{
Name: m.HostName,
Rrtype: dns.TypeA,
Class: dns.ClassINET,
Ttl: atomic.LoadUint32(&m.TTL),
},
A: ip4,
})
}
}
return rr
case dns.TypeAAAA:
var rr []dns.RR
for _, ip := range m.IPs {
if ip.To4() != nil {
// TODO(reddaly): IPv4 addresses could be encoded in IPv6 format and
// putinto AAAA records, but the current logic puts ipv4-encodable
// addresses into the A records exclusively. Perhaps this should be
// configurable?
continue
}
if ip16 := ip.To16(); ip16 != nil {
rr = append(rr, &dns.AAAA{
Hdr: dns.RR_Header{
Name: m.HostName,
Rrtype: dns.TypeAAAA,
Class: dns.ClassINET,
Ttl: atomic.LoadUint32(&m.TTL),
},
AAAA: ip16,
})
}
}
return rr
case dns.TypeSRV:
// Create the SRV Record
srv := &dns.SRV{
Hdr: dns.RR_Header{
Name: q.Name,
Rrtype: dns.TypeSRV,
Class: dns.ClassINET,
Ttl: atomic.LoadUint32(&m.TTL),
},
Priority: 10,
Weight: 1,
Port: uint16(m.Port),
Target: m.HostName,
}
recs := []dns.RR{srv}
// Add the A record
recs = append(recs, m.instanceRecords(dns.Question{
Name: m.instanceAddr,
Qtype: dns.TypeA,
})...)
// Add the AAAA record
recs = append(recs, m.instanceRecords(dns.Question{
Name: m.instanceAddr,
Qtype: dns.TypeAAAA,
})...)
return recs
case dns.TypeTXT:
txt := &dns.TXT{
Hdr: dns.RR_Header{
Name: q.Name,
Rrtype: dns.TypeTXT,
Class: dns.ClassINET,
Ttl: atomic.LoadUint32(&m.TTL),
},
Txt: m.TXT,
}
return []dns.RR{txt}
}
return nil
}