forked from coredns/coredns
/
forward.go
200 lines (160 loc) · 4.7 KB
/
forward.go
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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 (
"context"
"crypto/tls"
"errors"
"time"
"github.com/coredns/coredns/plugin"
"github.com/coredns/coredns/request"
"github.com/miekg/dns"
ot "github.com/opentracing/opentracing-go"
)
// 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
hcInterval time.Duration
from string
ignored []string
tlsConfig *tls.Config
tlsServerName string
maxfails uint32
expire time.Duration
forceTCP bool // 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, p: new(random), from: ".", hcInterval: hcDuration}
return f
}
// SetProxy appends p to the proxy list and starts healthchecking.
func (f *Forward) SetProxy(p *Proxy) {
f.proxies = append(f.proxies, p)
p.start(f.hcInterval)
}
// 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
var upstreamErr error
span = ot.SpanFromContext(ctx)
i := 0
list := f.list()
deadline := time.Now().Add(defaultTimeout)
for time.Now().Before(deadline) {
if i >= len(list) {
// reached the end of list, reset to begin
i = 0
fails = 0
}
proxy := list[i]
i++
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)
}
var (
ret *dns.Msg
err error
)
for {
ret, err = proxy.connect(ctx, state, f.forceTCP, true)
if err != nil && err == errCachedClosed { // Remote side closed conn, can only happen with TCP.
continue
}
break
}
if child != nil {
child.Finish()
}
ret, err = truncated(state, ret, err)
upstreamErr = err
if err != nil {
// Kick off health check to see if *our* upstream is broken.
if f.maxfails != 0 && err != errStopped {
proxy.Healthcheck()
}
if fails < len(f.proxies) {
continue
}
break
}
// Check if the reply is correct; if not return FormErr.
if !state.Match(ret) {
formerr := state.ErrorMessage(dns.RcodeFormatError)
w.WriteMsg(formerr)
return 0, nil
}
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
}
if upstreamErr != nil {
return dns.RcodeServerFailure, upstreamErr
}
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 forward")
errNoHealthy = errors.New("no healthy proxies")
errNoForward = errors.New("no forwarder defined")
errCachedClosed = errors.New("cached connection was closed by peer")
errStopped = errors.New("proxy has been stopped")
)
// policy tells forward what policy for selecting upstream it uses.
type policy int
const (
randomPolicy policy = iota
roundRobinPolicy
sequentialPolicy
)
const defaultTimeout = 5 * time.Second