Add option to parse resolv.conf for proxy upstreams (#353)

* Add option to parse resolv.conf for proxy upstreams

* Add test and README update for resolv.conf proxy

* Run gofmt

3 files changed, 125 insertions, 4 deletions

diff --git a/middleware/proxy/README.md b/middleware/proxy/README.md
index f621781d1..ab530a0a9 100644
--- a/middleware/proxy/README.md
+++ b/middleware/proxy/README.md
@@ -30,7 +30,7 @@ proxy FROM TO... {
 ~~~
 
 * **FROM** is the name to match for the request to be proxied.
-* **TO** is the destination endpoint to proxy to. At least one is required, but multiple may be specified.
+* **TO** is the destination endpoint to proxy to. At least one is required, but multiple may be specified. To may be an IP:Port pair, or may reference a file in resolv.conf format
 * `policy` is the load balancing policy to use; applies only with multiple backends. May be one of random, least_conn, or round_robin. Default is random.
 * `fail_timeout` specifies how long to consider a backend as down after it has failed. While it is down, requests will not be routed to that backend. A backend is "down" if CoreDNS fails to communicate with it. The default value is 10 seconds ("10s").
 * `max_fails` is the number of failures within fail_timeout that are needed before considering a backend to be down. If 0, the backend will never be marked as down. Default is 1.
@@ -86,3 +86,11 @@ proxy . backend:1234 {
 	except miek.nl example.org
 }
 ~~~
+
+Proxy everything except example.org using the host resolv.conf nameservers:
+
+~~~
+proxy . /etc/resolv.conf {
+	except miek.nl example.org
+}
+~~~
diff --git a/middleware/proxy/upstream.go b/middleware/proxy/upstream.go
index 1571486e5..02575d1a9 100644
--- a/middleware/proxy/upstream.go
+++ b/middleware/proxy/upstream.go
@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"os"
 	"strconv"
 	"strings"
 	"sync/atomic"
@@ -65,13 +66,27 @@ func NewStaticUpstreams(c *caddyfile.Dispenser) ([]Upstream, error) {
 		if len(to) == 0 {
 			return upstreams, c.ArgErr()
 		}
+
+		// process the host list, substituting in any nameservers in files
+		var toHosts []string
 		for _, host := range to {
 			h, _, err := net.SplitHostPort(host)
 			if err != nil {
 				h = host
 			}
 			if x := net.ParseIP(h); x == nil {
-				return upstreams, fmt.Errorf("not an IP address: `%s'", h)
+				// it's a file, parse as resolv.conf
+				c, err := dns.ClientConfigFromFile(host)
+				if err == os.ErrNotExist {
+					return upstreams, fmt.Errorf("not an IP address or file: `%s'", h)
+				} else if err != nil {
+					return upstreams, err
+				}
+				for _, s := range c.Servers {
+					toHosts = append(toHosts, net.JoinHostPort(s, c.Port))
+				}
+			} else {
+				toHosts = append(toHosts, host)
 			}
 		}
 
@@ -81,8 +96,8 @@ func NewStaticUpstreams(c *caddyfile.Dispenser) ([]Upstream, error) {
 			}
 		}
 
-		upstream.Hosts = make([]*UpstreamHost, len(to))
-		for i, host := range to {
+		upstream.Hosts = make([]*UpstreamHost, len(toHosts))
+		for i, host := range toHosts {
 			uh := &UpstreamHost{
 				Name:        defaultHostPort(host),
 				Conns:       0,
diff --git a/middleware/proxy/upstream_test.go b/middleware/proxy/upstream_test.go
index c48b4446b..bbc51f59a 100644
--- a/middleware/proxy/upstream_test.go
+++ b/middleware/proxy/upstream_test.go
@@ -1,6 +1,10 @@
 package proxy
 
 import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -78,6 +82,19 @@ func TestAllowedPaths(t *testing.T) {
 	}
 }
 
+func writeTmpFile(t *testing.T, data string) (string, string) {
+	tempDir, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatalf("tempDir: %v", err)
+	}
+
+	path := filepath.Join(tempDir, "resolv.conf")
+	if err := ioutil.WriteFile(path, []byte(data), 0644); err != nil {
+		t.Fatalf("writeFile: %v", err)
+	}
+	return tempDir, path
+}
+
 func TestProxyParse(t *testing.T) {
 	tests := []struct {
 		inputUpstreams string
@@ -143,6 +160,11 @@ proxy . 8.8.8.8:53 {
 }`,
 			true,
 		},
+		{
+			`
+proxy . some_bogus_filename`,
+			true,
+		},
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.inputUpstreams)
@@ -152,3 +174,79 @@ proxy . 8.8.8.8:53 {
 		}
 	}
 }
+
+func TestResolvParse(t *testing.T) {
+	tests := []struct {
+		inputUpstreams string
+		filedata       string
+		shouldErr      bool
+		expected       []string
+	}{
+		{
+			`
+proxy . FILE
+`,
+			`
+nameserver 1.2.3.4
+nameserver 4.3.2.1
+`,
+			false,
+			[]string{"1.2.3.4:53", "4.3.2.1:53"},
+		},
+		{
+			`
+proxy example.com 1.1.1.1:5000
+proxy . FILE
+proxy example.org 2.2.2.2:1234
+`,
+			`
+nameserver 1.2.3.4
+`,
+			false,
+			[]string{"1.1.1.1:5000", "1.2.3.4:53", "2.2.2.2:1234"},
+		},
+		{
+			`
+proxy example.com 1.1.1.1:5000
+proxy . FILE
+proxy example.org 2.2.2.2:1234
+`,
+			`
+junky resolve.conf
+`,
+			false,
+			[]string{"1.1.1.1:5000", "2.2.2.2:1234"},
+		},
+	}
+	for i, test := range tests {
+		tempDir, path := writeTmpFile(t, test.filedata)
+		defer os.RemoveAll(tempDir)
+		config := strings.Replace(test.inputUpstreams, "FILE", path, -1)
+		c := caddy.NewTestController("dns", config)
+		upstreams, err := NewStaticUpstreams(&c.Dispenser)
+		if (err != nil) != test.shouldErr {
+			t.Errorf("Test %d expected no error, got %v", i+1, err)
+		}
+		var hosts []string
+		for _, u := range upstreams {
+			for _, h := range u.(*staticUpstream).Hosts {
+				hosts = append(hosts, h.Name)
+			}
+		}
+		if !test.shouldErr {
+			if len(hosts) != len(test.expected) {
+				t.Errorf("Test %d expected %d hosts got %d", i+1, len(test.expected), len(upstreams))
+			} else {
+				ok := true
+				for i, v := range test.expected {
+					if v != hosts[i] {
+						ok = false
+					}
+				}
+				if !ok {
+					t.Errorf("Test %d expected %v got %v", i+1, test.expected, upstreams)
+				}
+			}
+		}
+	}
+}