support environment proxies on outbound dials (#737)

* support environment proxies on outbound dials

go.mod

@@ -50,7 +50,7 @@ require (
 	github.com/weaveworks/common v0.0.0-20200512154658-384f10054ec5
 	go.etcd.io/bbolt v1.3.4
 	golang.org/x/crypto v0.0.0-20200604202706-70a84ac30bf9
-	golang.org/x/net v0.0.0-20200602114024-627f9648deb9 // indirect
+	golang.org/x/net v0.0.0-20200602114024-627f9648deb9
 	golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543
 	google.golang.org/genproto v0.0.0-20200608115520-7c474a2e3482 // indirect
 	google.golang.org/grpc v1.29.1

net/client_grpc.go

@@ -5,6 +5,7 @@ import (
 	"crypto/tls"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"sync"
 	"time"
@@ -14,6 +15,7 @@ import (
 	"github.com/drand/drand/protobuf/drand"
 	"github.com/weaveworks/common/httpgrpc"
 	httpgrpcserver "github.com/weaveworks/common/httpgrpc/server"
+	"golang.org/x/net/proxy"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
 )
@@ -35,11 +37,13 @@ var defaultTimeout = 1 * time.Minute
 // NewGrpcClient returns an implementation of an InternalClient  and
 // ExternalClient using gRPC connections
 func NewGrpcClient(opts ...grpc.DialOption) Client {
-	return &grpcClient{
+	client := grpcClient{
 		opts:    opts,
 		conns:   make(map[string]*grpc.ClientConn),
 		timeout: defaultTimeout,
 	}
+	client.loadEnvironment()
+	return &client
 }
 
 // NewGrpcClientFromCertManager returns a Client using gRPC with the given trust
@@ -58,6 +62,13 @@ func NewGrpcClientWithTimeout(timeout time.Duration, opts ...grpc.DialOption) Cl
 	return c
 }
 
+func (g *grpcClient) loadEnvironment() {
+	opt := grpc.WithContextDialer(func(ctx context.Context, addr string) (net.Conn, error) {
+		return proxy.Dial(ctx, "tcp", addr)
+	})
+	g.opts = append([]grpc.DialOption{opt}, g.opts...)
+}
+
 func (g *grpcClient) getTimeoutContext(ctx context.Context) (context.Context, context.CancelFunc) {
 	g.Lock()
 	defer g.Unlock()