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alias/alias.go

95 lines
2.3 KiB
Go

package alias
import (
"context"
"math"
"github.com/coredns/coredns/plugin"
"github.com/miekg/dns"
)
// Rewrite is plugin to rewrite requests internally before being handled.
type Alias struct {
Next plugin.Handler
}
// ServeDNS implements the plugin.Handler interface.
func (al Alias) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
mw := NewResponseModifier(w)
return plugin.NextOrFailure(al.Name(), al.Next, ctx, mw, r)
}
// Name implements the Handler interface.
func (al Alias) Name() string { return "alias" }
type ResponseModifier struct {
dns.ResponseWriter
}
// Returns a dns.Msg modifier that replaces CNAME on root zones with other records.
func NewResponseModifier(w dns.ResponseWriter) *ResponseModifier {
return &ResponseModifier{
ResponseWriter: w,
}
}
func min(a, b uint32) uint32 {
if a < b {
return a
}
return b
}
// WriteMsg records the status code and calls the
// underlying ResponseWriter's WriteMsg method.
func (r *ResponseModifier) WriteMsg(res *dns.Msg) error {
// Guess zone based on authority section.
var zone string
for _, rr := range res.Ns {
if rr.Header().Rrtype == dns.TypeNS {
zone = rr.Header().Name
}
}
// Find and delete CNAME record on that zone, storing the canonical name.
var (
cname string = zone
ttl uint32 = math.MaxUint32
)
for i := 0; i < len(res.Answer); {
rr := res.Answer[i]
if rr.Header().Rrtype == dns.TypeCNAME && rr.Header().Name == cname {
cname = rr.(*dns.CNAME).Target
ttl = min(ttl, rr.(*dns.CNAME).Header().Ttl)
// Remove the CNAME record
res.Answer = append(res.Answer[:i], res.Answer[i+1:]...)
continue
}
i++
}
// Rename all the records with the above canonical name to the zone name
for _, rr := range res.Answer {
if rr.Header().Name == cname {
rr.Header().Name = zone
rr.Header().Ttl = min(ttl, rr.Header().Ttl)
}
}
return r.ResponseWriter.WriteMsg(res)
}
// Write is a wrapper that records the size of the message that gets written.
func (r *ResponseModifier) Write(buf []byte) (int, error) {
n, err := r.ResponseWriter.Write(buf)
return n, err
}
// Hijack implements dns.Hijacker. It simply wraps the underlying
// ResponseWriter's Hijack method if there is one, or returns an error.
func (r *ResponseModifier) Hijack() {
r.ResponseWriter.Hijack()
return
}