plugin/federation: Add upstream option to federation (#2177)

* add upstream

* add upstream

* debug ci

* debug ci

* set context

* update readme

* update readme

* remove empty if

3 files changed, 28 insertions, 3 deletions

diff --git a/plugin/federation/README.md b/plugin/federation/README.md
index 021921cb2..1bcd23af6 100644
--- a/plugin/federation/README.md
+++ b/plugin/federation/README.md
@@ -17,11 +17,16 @@ Enabling *federation* without also having *kubernetes* is a noop.
 ~~~
 federation [ZONES...] {
     NAME DOMAIN
+    upstream [ADDRESS...]
 }
 ~~~
 
 * Each **NAME** and **DOMAIN** defines federation membership. One entry for each. A duplicate
   **NAME** will silently overwrite any previous value.
+* `upstream` [**ADDRESS**...] defines the upstream resolvers used for resolving the `CNAME` target
+  produced by this plugin.  If no **ADDRESS** is given, CoreDNS
+  will resolve External Services against itself. **ADDRESS** can be an IP, an IP:port, or a path
+  to a file structured like resolv.conf.
 
 ## Examples
 
@@ -33,6 +38,7 @@ Here we handle all service requests in the `prod` and `stage` federations.
     federation cluster.local {
         prod prod.feddomain.com
         staging staging.feddomain.com
+        upstream
     }
 }
 ~~~
@@ -45,6 +51,7 @@ cluster.local {
     federation {
         prod prod.feddomain.com
         staging staging.feddomain.com
+        upstream
     }
 }
 ~~~
diff --git a/plugin/federation/federation.go b/plugin/federation/federation.go
index 779636b6a..daf470891 100644
--- a/plugin/federation/federation.go
+++ b/plugin/federation/federation.go
@@ -20,6 +20,7 @@ import (
 	"github.com/coredns/coredns/plugin/etcd/msg"
 	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	"github.com/coredns/coredns/plugin/pkg/nonwriter"
+	"github.com/coredns/coredns/plugin/pkg/upstream"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -27,8 +28,9 @@ import (
 
 // Federation contains the name to zone mapping used for federation in kubernetes.
 type Federation struct {
-	f     map[string]string
-	zones []string
+	f        map[string]string
+	zones    []string
+	Upstream *upstream.Upstream
 
 	Next        plugin.Handler
 	Federations Func
@@ -49,7 +51,8 @@ func (f *Federation) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.
 		return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
 	}
 
-	state := request.Request{W: w, Req: r}
+	state := request.Request{W: w, Req: r, Context: ctx}
+
 	zone := plugin.Zones(f.zones).Matches(state.Name())
 	if zone == "" {
 		return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
@@ -105,6 +108,13 @@ func (f *Federation) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.
 
 	m.Answer = []dns.RR{service.NewCNAME(state.QName(), service.Host)}
 
+	if f.Upstream != nil {
+		aRecord, err := f.Upstream.Lookup(state, service.Host, state.QType())
+		if err == nil && aRecord != nil && len(aRecord.Answer) > 0 {
+			m.Answer = append(m.Answer, aRecord.Answer...)
+		}
+	}
+
 	w.WriteMsg(m)
 	return dns.RcodeSuccess, nil
 }
diff --git a/plugin/federation/setup.go b/plugin/federation/setup.go
index 06daaf4e1..84e9aba16 100644
--- a/plugin/federation/setup.go
+++ b/plugin/federation/setup.go
@@ -6,6 +6,7 @@ import (
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/kubernetes"
+	"github.com/coredns/coredns/plugin/pkg/upstream"
 	"github.com/miekg/dns"
 
 	"github.com/mholt/caddy"
@@ -62,6 +63,13 @@ func federationParse(c *caddy.Controller) (*Federation, error) {
 		for c.NextBlock() {
 			x := c.Val()
 			switch x {
+			case "upstream":
+				args := c.RemainingArgs()
+				u, err := upstream.New(args)
+				if err != nil {
+					return nil, err
+				}
+				fed.Upstream = &u
 			default:
 				args := c.RemainingArgs()
 				if x := len(args); x != 1 {