reverseproxy: allow user to define source address

caddytest/integration/caddyfile_adapt/reverse_proxy_localaddr.caddyfiletest

@@ -0,0 +1,57 @@
+https://example.com {
+	reverse_proxy http://localhost:54321 {
+		transport http {
+			local_address 192.168.0.1
+		}
+	}
+}
+
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":443"
+					],
+					"routes": [
+						{
+							"match": [
+								{
+									"host": [
+										"example.com"
+									]
+								}
+							],
+							"handle": [
+								{
+									"handler": "subroute",
+									"routes": [
+										{
+											"handle": [
+												{
+													"handler": "reverse_proxy",
+													"transport": {
+														"local_address": "192.168.0.1",
+														"protocol": "http"
+													},
+													"upstreams": [
+														{
+															"dial": "localhost:54321"
+														}
+													]
+												}
+											]
+										}
+									]
+								}
+							],
+							"terminal": true
+						}
+					]
+				}
+			}
+		}
+	}
+}

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -1326,7 +1326,11 @@ func (h *HTTPTransport) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				return d.Err("cannot specify \"tls_trust_pool\" twice in caddyfile")
 			}
 			h.TLS.CARaw = caddyconfig.JSONModuleObject(ca, "provider", modStem, nil)
-
+		case "local_address":
+			if !d.NextArg() {
+				return d.ArgErr()
+			}
+			h.LocalAddress = d.Val()
 		default:
 			return d.Errf("unrecognized subdirective %s", d.Val())
 		}

modules/caddyhttp/reverseproxy/httptransport.go

@@ -132,6 +132,10 @@ type HTTPTransport struct {
 	// to change or removal while experimental.
 	Versions []string `json:"versions,omitempty"`
 
+	// Specify the address to bind to when connecting to an upstream. In other words,
+	// it is the address the upstream sees as the remote address.
+	LocalAddress string `json:"local_address,omitempty"`
+
 	// The pre-configured underlying HTTP transport.
 	Transport *http.Transport `json:"-"`
 
@@ -185,6 +189,31 @@ func (h *HTTPTransport) NewTransport(caddyCtx caddy.Context) (*http.Transport, e
 		FallbackDelay: time.Duration(h.FallbackDelay),
 	}
 
+	if h.LocalAddress != "" {
+		netaddr, err := caddy.ParseNetworkAddressWithDefaults(h.LocalAddress, "tcp", 0)
+		if err != nil {
+			return nil, err
+		}
+		if netaddr.PortRangeSize() > 1 {
+			return nil, fmt.Errorf("local_address must be a single address, not a port range")
+		}
+		switch netaddr.Network {
+		case "tcp", "tcp4", "tcp6":
+			dialer.LocalAddr, err = net.ResolveTCPAddr(netaddr.Network, netaddr.JoinHostPort(0))
+			if err != nil {
+				return nil, err
+			}
+		case "unix", "unixgram", "unixpacket":
+			dialer.LocalAddr, err = net.ResolveUnixAddr(netaddr.Network, netaddr.JoinHostPort(0))
+			if err != nil {
+				return nil, err
+			}
+		case "udp", "udp4", "udp6":
+			return nil, fmt.Errorf("local_address must be a TCP address, not a UDP address")
+		default:
+			return nil, fmt.Errorf("unsupported network")
+		}
+	}
 	if h.Resolver != nil {
 		err := h.Resolver.ParseAddresses()
 		if err != nil {