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package etcd
import (
"context"
"errors"
"github.com/coredns/coredns/plugin/pkg/log"
"github.com/coredns/coredns/request"
"github.com/miekg/dns"
)
// Stub wraps an Etcd. We have this type so that it can have a ServeDNS method.
type Stub struct {
*Etcd
Zone string // for what zone (and thus what nameservers are we called)
}
// ServeDNS implements the plugin.Handler interface.
func (s Stub) ServeDNS(ctx context.Context, w dns.ResponseWriter, req *dns.Msg) (int, error) {
if hasStubEdns0(req) {
log.Warningf("Forwarding cycle detected, refusing msg: %s", req.Question[0].Name)
return dns.RcodeRefused, errors.New("stub forward cycle")
}
req = addStubEdns0(req)
proxy, ok := (*s.Etcd.Stubmap)[s.Zone]
if !ok { // somebody made a mistake..
return dns.RcodeServerFailure, nil
}
state := request.Request{W: w, Req: req}
m, e := proxy.Forward(state)
if e != nil {
return dns.RcodeServerFailure, e
}
m.RecursionAvailable, m.Compress = true, true
state.SizeAndDo(m)
w.WriteMsg(m)
return dns.RcodeSuccess, nil
}
// hasStubEdns0 checks if the message is carrying our special edns0 zero option.
func hasStubEdns0(m *dns.Msg) bool {
option := m.IsEdns0()
if option == nil {
return false
}
for _, o := range option.Option {
if o.Option() == ednsStubCode && len(o.(*dns.EDNS0_LOCAL).Data) == 1 &&
o.(*dns.EDNS0_LOCAL).Data[0] == 1 {
return true
}
}
return false
}
// addStubEdns0 adds our special option to the message's OPT record.
func addStubEdns0(m *dns.Msg) *dns.Msg {
option := m.IsEdns0()
// Add a custom EDNS0 option to the packet, so we can detect loops when 2 stubs are forwarding to each other.
if option != nil {
option.Option = append(option.Option, &dns.EDNS0_LOCAL{Code: ednsStubCode, Data: []byte{1}})
return m
}
m.Extra = append(m.Extra, ednsStub)
return m
}
const (
ednsStubCode = dns.EDNS0LOCALSTART + 10
stubDomain = "stub.dns"
)
var ednsStub = func() *dns.OPT {
o := new(dns.OPT)
o.Hdr.Name = "."
o.Hdr.Rrtype = dns.TypeOPT
o.SetUDPSize(4096)
e := new(dns.EDNS0_LOCAL)
e.Code = ednsStubCode
e.Data = []byte{1}
o.Option = append(o.Option, e)
return o
}()
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