/*
For fragmentation, we use a naive algorithm.
We use the same header for every fragment, and include the same EDNS0
section in every additional section.
We add one RR at a time, until our fragment is larger than 512 bytes,
then we remove the last RR so that it fits in the 512 byte size limit.
If we discover that one of the fragments ends up with 0 RR in it (for
example because a single RR is too big), then we return a single
truncated response instead of the set of fragments.
We could perhaps make the process of building fragments faster by
bisecting the set of RR that we include in an answer. So, if we have 8
RR we could try all, then if that is too big, 4 RR, and if that fits
then 6 RR, until an optimal set of RR is found.
We could also possibly produce a smaller set of responses by
optimizing how we combine RR. Just taking account the various sizes is
the same as the bin packing problem, which is NP-hard:
https://en.wikipedia.org/wiki/Bin_packing_problem
While some non-optimal but reasonable heuristics exist, in the case of
DNS we would have to use some sophisticated algorithm to also consider
name compression.
*/
func frag(reply *dns.Msg) []dns.Msg {
// create a return value
all_frags := []dns.Msg{}
HasEdns0 := true
// get each RR section and save a copy out
remaining_answer := make([]dns.RR, len(reply.Answer))
copy(remaining_answer, reply.Answer)
remaining_ns := make([]dns.RR, len(reply.Ns))
copy(remaining_ns, reply.Ns)
remaining_extra := make([]dns.RR, len(reply.Extra))
copy(remaining_extra, reply.Extra)
// if we don't have EDNS0 in the packet, add it now
if reply.IsEdns0() == nil {
reply.SetEdns0(512, false)
}
// the EDNS option for later use
var edns0_rr dns.RR = nil
// remove the EDNS0 option from our additional ("extra") section
// (we will include it separately on every fragment)
for ofs, r := range remaining_extra {
// found the EDNS option
if r.Header().Rrtype == dns.TypeOPT {
// save the EDNS option
edns0_rr = r
// remove from the set of extra RR
remaining_extra = append(remaining_extra[0:ofs], remaining_extra[ofs+1:]...)
// in principle we should only have one EDNS0 section
break
}
}
if edns0_rr == nil {
log.Printf("Server reply missing EDNS0 option")
return []dns.Msg{}
//HasEdns0 = false
}
// now build fragments
for {
// make a shallow copy of our reply packet, and prepare space for our RR
frag := *reply
frag.Answer = []dns.RR{}
frag.Ns = []dns.RR{}
frag.Extra = []dns.RR{}
// add our custom EDNS0 option (needed in every fragment)
local_opt := new(dns.EDNS0_LOCAL)
local_opt.Code = dns.EDNS0LOCALSTART + 1
local_opt.Data = []byte{0, 0}
if HasEdns0 == true {
edns0_rr_copy := dns.Copy(edns0_rr)
edns0_rr_copy.(*dns.OPT).Option = append(edns0_rr_copy.(*dns.OPT).Option, local_opt)
frag.Extra = append(frag.Extra, edns0_rr_copy)
}
//if HasEdns0 == false {
// frag.Extra = append(frag.Extra, local_opt)
//}
// add as many RR to the answer as we can
for len(remaining_answer) > 0 {
frag.Answer = append(frag.Answer, remaining_answer[0])
if frag.Len() <= 512 {
// if the new answer fits, then remove it from our remaining list
remaining_answer = remaining_answer[1:]
} else {
// otherwise we are full, remove it from our fragment and stop
frag.Answer = frag.Answer[0 : len(frag.Answer)-1]
break
}
}
for len(remaining_ns) > 0 {
frag.Ns = append(frag.Ns, remaining_ns[0])
if frag.Len() <= 512 {
// if the new answer fits, then remove it from our remaining list
remaining_ns = remaining_ns[1:]
} else {
// otherwise we are full, remove it from our fragment and stop
frag.Ns = frag.Ns[0 : len(frag.Ns)-1]
break
}
}
for len(remaining_extra) > 0 {
frag.Extra = append(frag.Extra, remaining_extra[0])
if frag.Len() <= 512 {
// if the new answer fits, then remove it from our remaining list
remaining_extra = remaining_extra[1:]
} else {
// otherwise we are full, remove it from our fragment and stop
frag.Extra = frag.Extra[0 : len(frag.Extra)-1]
break
}
}
// check to see if we didn't manage to add any RR
if (len(frag.Answer) == 0) && (len(frag.Ns) == 0) && (len(frag.Extra) == 1) {
// TODO: test this :)
// return a single truncated fragment without any RR
frag.MsgHdr.Truncated = true
frag.Extra = []dns.RR{}
return []dns.Msg{frag}
}
// add to our list of fragments
all_frags = append(all_frags, frag)
// if we have finished all remaining sections, we are done
if (len(remaining_answer) == 0) && (len(remaining_ns) == 0) && (len(remaining_extra) == 0) {
break
}
}
// fix up our fragments so they have the correct sequence and length values
for n, frag := range all_frags {
frag_edns0 := frag.IsEdns0()
for _, opt := range frag_edns0.Option {
if opt.Option() == dns.EDNS0LOCALSTART+1 {
opt.(*dns.EDNS0_LOCAL).Data = []byte{byte(len(all_frags)), byte(n)}
}
}
}
// return our fragments
return all_frags
}
"strconv"
"strings"
"github.com/miekg/dns"
"github.com/skynetservices/skydns/msg"
)
const ednsStubCode = dns.EDNS0LOCALSTART + 10
// ednsStub is the EDNS0 record we add to stub queries. Queries which have this record are
// not forwarded again.
var ednsStub = func() *dns.OPT {
o := new(dns.OPT)
o.Hdr.Name = "."
o.Hdr.Rrtype = dns.TypeOPT
e := new(dns.EDNS0_LOCAL)
e.Code = ednsStubCode
e.Data = []byte{1}
o.Option = append(o.Option, e)
return o
}()
// Look in .../dns/stub/<domain>/xx for msg.Services. Loop through them
// extract <domain> and add them as forwarders (ip:port-combos) for
// the stub zones. Only numeric (i.e. IP address) hosts are used.
func (s *server) UpdateStubZones() {
stubmap := make(map[string][]string)
services, err := s.backend.Records("stub.dns."+s.config.Domain, false, net.IP{})
if err != nil {
logf("stub zone update failed: %s", err)
func (this ClientProxy) ServeDNS(w dns.ResponseWriter, request *dns.Msg) {
// if we don't have EDNS0 in the packet, add it now
// TODO: in principle we should check packet size here, since we have made it bigger,
// but for this demo code we will just rely on most queries being really small
proxy_req := *request
opt := proxy_req.IsEdns0()
var client_buf_size uint16
if opt == nil {
proxy_req.SetEdns0(512, false)
client_buf_size = 512
_D("%s QID:%d adding EDNS0 to packet", w.RemoteAddr(), request.Id)
opt = proxy_req.IsEdns0()
} else {
client_buf_size = opt.UDPSize()
}
// add our custom EDNS0 option
local_opt := new(dns.EDNS0_LOCAL)
local_opt.Code = dns.EDNS0LOCALSTART
opt.Option = append(opt.Option, local_opt)
// create a connection to the server
// XXX: for now we will only handle UDP - this will break in unpredictable ways in production!
conn, err := dns.DialTimeout("udp", this.SERVERS[rand.Intn(len(this.SERVERS))], this.timeout)
if err != nil {
_D("%s QID:%d error setting up UDP socket: %s", w.RemoteAddr(), request.Id, err)
SRVFAIL(w, request)
return
}
defer conn.Close()
// set our timeouts
// TODO: we need to insure that our timeouts work like we expect
conn.SetReadDeadline(time.Now().Add(this.timeout))
conn.SetWriteDeadline(time.Now().Add(this.timeout))
// send our query
err = conn.WriteMsg(&proxy_req)
if err != nil {
_D("%s QID:%d error writing message: %s", w.RemoteAddr(), request.Id, err)
SRVFAIL(w, request)
return
}
// wait for our reply
response := wait_for_response(w, conn, request)
if response == nil {
return
}
// get fragment information from first response (if any)
num_frags, sequence_num := get_fragment_info(response)
// if we did not have a fragmented response, send it to the client
if num_frags == -1 {
w.WriteMsg(response)
return
}
// build a map to hold the fragments that we have received
frags := map[int]dns.Msg{sequence_num: *response}
// wait for all fragments to arrive
// duplicates overwrite previous packet, missing packets eventually timeout
for len(frags) < num_frags {
response := wait_for_response(w, conn, request)
if response == nil {
return
}
_, sequence_num := get_fragment_info(response)
// TODO: remove the extra EDNS0 option
frags[sequence_num] = *response
}
// rebuild our original packet
rebuilt_reply, ok := frags[0]
if !ok {
_D("%s QID:%d missing fragment 0", w.RemoteAddr(), request.Id)
SRVFAIL(w, request)
return
}
for n := 1; n < num_frags; n++ {
frag, ok := frags[n]
if !ok {
_D("%s QID:%d missing fragment %d", w.RemoteAddr(), request.Id, n)
SRVFAIL(w, request)
return
}
rebuilt_reply.Answer = append(rebuilt_reply.Answer, frag.Answer...)
rebuilt_reply.Ns = append(rebuilt_reply.Ns, frag.Ns...)
for _, r := range frag.Extra {
// remove EDNS0 present in fragments from final answer
if r.Header().Rrtype != dns.TypeOPT {
rebuilt_reply.Extra = append(rebuilt_reply.Extra, r)
}
}
}
// verify that we don't exceed the client buffer size
if rebuilt_reply.Len() > int(client_buf_size) {
// truncate if we need to
// TODO: test this
rebuilt_reply.MsgHdr.Truncated = true
rebuilt_reply.Answer = []dns.RR{}
rebuilt_reply.Ns = []dns.RR{}
rebuilt_reply.Extra = []dns.RR{}
}
// send our rebuilt reply
w.WriteMsg(&rebuilt_reply)
}
