reverseproxy: Support repeated --to flags in command (#4693)

* feat: Multiple 'to' upstreams in reverse-proxy cmd

* Repeat --to for multiple upstreams, rather than comma-separating in a single flag

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/reverseproxy/command.go

@@ -41,6 +41,7 @@ A simple but production-ready reverse proxy. Useful for quick deployments,
 demos, and development.
 
 Simply shuttles HTTP(S) traffic from the --from address to the --to address.
+Multiple --to addresses may be specified by repeating the flag.
 
 Unless otherwise specified in the addresses, the --from address will be
 assumed to be HTTPS if a hostname is given, and the --to address will be
@@ -57,7 +58,7 @@ default, all incoming headers are passed through unmodified.)
 		Flags: func() *flag.FlagSet {
 			fs := flag.NewFlagSet("reverse-proxy", flag.ExitOnError)
 			fs.String("from", "localhost", "Address on which to receive traffic")
-			fs.String("to", "", "Upstream address to which traffic should be sent")
+			fs.Var(&reverseProxyCmdTo, "to", "Upstream address(es) to which traffic should be sent")
 			fs.Bool("change-host-header", false, "Set upstream Host header to address of upstream")
 			fs.Bool("insecure", false, "Disable TLS verification (WARNING: DISABLES SECURITY BY NOT VERIFYING SSL CERTIFICATES!)")
 			fs.Bool("internal-certs", false, "Use internal CA for issuing certs")
@@ -70,7 +71,6 @@ func cmdReverseProxy(fs caddycmd.Flags) (int, error) {
 	caddy.TrapSignals()
 
 	from := fs.String("from")
-	to := fs.String("to")
 	changeHost := fs.Bool("change-host-header")
 	insecure := fs.Bool("insecure")
 	internalCerts := fs.Bool("internal-certs")
@@ -78,7 +78,7 @@ func cmdReverseProxy(fs caddycmd.Flags) (int, error) {
 	httpPort := strconv.Itoa(caddyhttp.DefaultHTTPPort)
 	httpsPort := strconv.Itoa(caddyhttp.DefaultHTTPSPort)
 
-	if to == "" {
+	if len(reverseProxyCmdTo) == 0 {
 		return caddy.ExitCodeFailedStartup, fmt.Errorf("--to is required")
 	}
 
@@ -106,9 +106,18 @@ func cmdReverseProxy(fs caddycmd.Flags) (int, error) {
 	}
 
 	// set up the upstream address; assume missing information from given parts
-	toAddr, toScheme, err := parseUpstreamDialAddress(to)
+	// mixing schemes isn't supported, so use first defined (if available)
-	if err != nil {
+	toAddresses := make([]string, len(reverseProxyCmdTo))
-		return caddy.ExitCodeFailedStartup, fmt.Errorf("invalid upstream address %s: %v", to, err)
+	var toScheme string
+	for i, toLoc := range reverseProxyCmdTo {
+		addr, scheme, err := parseUpstreamDialAddress(toLoc)
+		if err != nil {
+			return caddy.ExitCodeFailedStartup, fmt.Errorf("invalid upstream address %s: %v", toLoc, err)
+		}
+		if scheme != "" && toScheme != "" {
+			toScheme = scheme
+		}
+		toAddresses[i] = addr
 	}
 
 	// proceed to build the handler and server
@@ -120,9 +129,16 @@ func cmdReverseProxy(fs caddycmd.Flags) (int, error) {
 		}
 	}
 
+	upstreamPool := UpstreamPool{}
+	for _, toAddr := range toAddresses {
+		upstreamPool = append(upstreamPool, &Upstream{
+			Dial: toAddr,
+		})
+	}
+
 	handler := Handler{
 		TransportRaw: caddyconfig.JSONModuleObject(ht, "protocol", "http", nil),
-		Upstreams:    UpstreamPool{{Dial: toAddr}},
+		Upstreams:    upstreamPool,
 	}
 
 	if changeHost {
@@ -187,7 +203,15 @@ func cmdReverseProxy(fs caddycmd.Flags) (int, error) {
 		return caddy.ExitCodeFailedStartup, err
 	}
 
-	fmt.Printf("Caddy proxying %s -> %s\n", fromAddr.String(), toAddr)
+	for _, to := range toAddresses {
+		fmt.Printf("Caddy proxying %s -> %s\n", fromAddr.String(), to)
+	}
+	if len(toAddresses) > 1 {
+		fmt.Println("Load balancing policy: random")
+	}
 
 	select {}
 }
+
+// reverseProxyCmdTo holds the parsed values from repeated use of the --to flag.
+var reverseProxyCmdTo caddycmd.StringSlice