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// Package forward implements a forwarding proxy. It caches an upstream net.Conn for some time, so if the same
// client returns the upstream's Conn will be precached. Depending on how you benchmark this looks to be
// 50% faster than just openening a new connection for every client. It works with UDP and TCP and uses
// inband healthchecking.
package forward
import (
"crypto/tls"
"errors"
"time"
"github.com/coredns/coredns/plugin"
"github.com/coredns/coredns/request"
"github.com/miekg/dns"
ot "github.com/opentracing/opentracing-go"
"golang.org/x/net/context"
)
// Forward represents a plugin instance that can proxy requests to another (DNS) server. It has a list
// of proxies each representing one upstream proxy.
type Forward struct {
proxies []*Proxy
p Policy
from string
ignored []string
tlsConfig *tls.Config
tlsServerName string
maxfails uint32
expire time.Duration
forceTCP bool // also here for testing
hcInterval time.Duration // also here for testing
Next plugin.Handler
}
// New returns a new Forward.
func New() *Forward {
f := &Forward{maxfails: 2, tlsConfig: new(tls.Config), expire: defaultExpire, hcInterval: hcDuration, p: new(random)}
return f
}
// SetProxy appends p to the proxy list and starts healthchecking.
func (f *Forward) SetProxy(p *Proxy) {
f.proxies = append(f.proxies, p)
go p.healthCheck()
}
// Len returns the number of configured proxies.
func (f *Forward) Len() int { return len(f.proxies) }
// Name implements plugin.Handler.
func (f *Forward) Name() string { return "forward" }
// ServeDNS implements plugin.Handler.
func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
state := request.Request{W: w, Req: r}
if !f.match(state) {
return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
}
fails := 0
var span, child ot.Span
span = ot.SpanFromContext(ctx)
for _, proxy := range f.list() {
if proxy.Down(f.maxfails) {
fails++
if fails < len(f.proxies) {
continue
}
// All upstream proxies are dead, assume healtcheck is completely broken and randomly
// select an upstream to connect to.
r := new(random)
proxy = r.List(f.proxies)[0]
HealthcheckBrokenCount.Add(1)
}
if span != nil {
child = span.Tracer().StartSpan("connect", ot.ChildOf(span.Context()))
ctx = ot.ContextWithSpan(ctx, child)
}
ret, err := proxy.connect(ctx, state, f.forceTCP, true)
if child != nil {
child.Finish()
}
if err != nil {
if fails < len(f.proxies) {
continue
}
break
}
ret.Compress = true
// When using force_tcp the upstream can send a message that is too big for
// the udp buffer, hence we need to truncate the message to at least make it
// fit the udp buffer.
ret, _ = state.Scrub(ret)
w.WriteMsg(ret)
return 0, nil
}
return dns.RcodeServerFailure, errNoHealthy
}
func (f *Forward) match(state request.Request) bool {
from := f.from
if !plugin.Name(from).Matches(state.Name()) || !f.isAllowedDomain(state.Name()) {
return false
}
return true
}
func (f *Forward) isAllowedDomain(name string) bool {
if dns.Name(name) == dns.Name(f.from) {
return true
}
for _, ignore := range f.ignored {
if plugin.Name(ignore).Matches(name) {
return false
}
}
return true
}
// List returns a set of proxies to be used for this client depending on the policy in f.
func (f *Forward) list() []*Proxy { return f.p.List(f.proxies) }
var (
errInvalidDomain = errors.New("invalid domain for proxy")
errNoHealthy = errors.New("no healthy proxies")
errNoForward = errors.New("no forwarder defined")
)
// policy tells forward what policy for selecting upstream it uses.
type policy int
const (
randomPolicy policy = iota
roundRobinPolicy
)
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