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feat: added support for Legacy Unicast Responses #11

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Sep 20, 2023
Merged

feat: added support for Legacy Unicast Responses #11

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46 changes: 33 additions & 13 deletions mdns/mdns.go
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Original file line number Diff line number Diff line change
Expand Up @@ -207,28 +207,48 @@ func (c *connector) mainloop() {
go c.readloop(in)
for {
msg := <-in
msg.MsgHdr.Response = true // convert question to response
msg.MsgHdr.Authoritative = true
msg.MsgHdr.Response = true // convert question to response
msg.MsgHdr.Authoritative = true // answer should be authoritative otherwise it may be discarded

// https://datatracker.ietf.org/doc/html/rfc6762#section-6.7
// if source port is not 5353 then it's "One-Shot Multicast DNS Query" and we should send unicast response
isLegacyUnicast := msg.UDPAddr.Port != 5353

msg.Answer = make([]dns.RR, 0) // some queries already have an answer, we should not answer them
for _, result := range c.query(msg.Question) {
// Set Cache-Flush bit
result.RR.Header().Class = result.RR.Header().Class | 0x8000
if isLegacyUnicast == true {
// https://datatracker.ietf.org/doc/html/rfc6762#section-6.7
// The resource record TTL given in a legacy unicast response SHOULD NOT be greater than ten seconds
result.RR.Header().Ttl = 10
} else {
// Set Cache-Flush bit
result.RR.Header().Class = result.RR.Header().Class | 0x8000
}
msg.Answer = append(msg.Answer, result.RR)
}
msg.Extra = append(msg.Extra, c.findExtra(msg.Answer...)...)

if len(msg.Answer) > 0 {
addr := c.UDPAddr
// check unicast-response bit https://tools.ietf.org/html/rfc6762#section-5.4
//if msg.Question[0].Qclass & 32768 > 0 {
// log.Println("using unicast")
// addr = msg.UDPAddr
//}
var addr *net.UDPAddr
// https://tools.ietf.org/html/rfc6762#section-5.4
// Check if unicast-response bit set
isQueryUnicast := msg.Question[0].Qclass&32768 > 0

if isLegacyUnicast || isQueryUnicast {
addr = msg.UDPAddr
} else {
// https://datatracker.ietf.org/doc/html/rfc6762#section-11
// A host sending Multicast DNS queries to a link-local destination
// address MUST only accept responses to that query that originate
// from the local link, and silently discard any other response packets.
addr = c.UDPAddr
}
msg.UDPAddr = addr

// nuke questions
msg.Question = nil

msg.UDPAddr = addr
if isLegacyUnicast == false {
msg.Question = nil
}

if err := c.writeMessage(msg.Msg, addr); err != nil {
log.Println("Cannot send: ", err)
Expand Down