Add option to specify local source ip when connecting to udp backend

config/gobetween.toml

@@ -169,6 +169,7 @@ protocol = "udp"
 #  [servers.default.udp]             # (optional)
 #  max_requests  = 0                 # (optional) if > 0 accepts no more requests than max_requests and closes session
 #  max_responses = 0                 # (optional) if > 0 accepts no more responses than max_responses from backend and closes session
+#  source = 192.0.2.1                # (optional) Specify source address. This option sets the laddr (local address) when calling DialUDP().
 #  transparent = false               # (optional, not supported in Windows)
 #                                    #            if true - work in transparent mode, when forwarded udp packets have client source address.
 #                                    #            (requires additional host configuration)
@@ -210,7 +211,7 @@ protocol = "udp"
 #  ## Probe sends string to backend and expects specific string in response to check if
 #  ## backend is alive. Only the beginning of received buffer will be compared with expected value, so that
 #  ## backend response can contain extra data, not being checked. probe_send and probe_recv are
-#  ## configured as strings and support escape sequences: \n \r or \xNN for hex. First probe_recv_len bytes of server response 
+#  ## configured as strings and support escape sequences: \n \r or \xNN for hex. First probe_recv_len bytes of server response
 #  ## could be also matched using regular expression.
 #  ##
 #  kind = "probe"

src/config/config.go

@@ -187,6 +187,7 @@ type Udp struct {
 	MaxRequests  uint64 `toml:"max_requests" json:"max_requests"`
 	MaxResponses uint64 `toml:"max_responses" json:"max_responses"`
 	Transparent  bool   `toml:"transparent" json:"transparent"`
+	Source       string `toml:"source" json:"source"`
 }
 
 /**

src/server/udp/server.go

@@ -232,6 +232,7 @@ func (this *Server) serve() {
 		ClientIdleTimeout:  utils.ParseDurationOrDefault(*this.cfg.ClientIdleTimeout, 0),
 		BackendIdleTimeout: utils.ParseDurationOrDefault(*this.cfg.BackendIdleTimeout, 0),
 		Transparent:        this.cfg.Udp.Transparent,
+		Source:             this.cfg.Udp.Source,
 	}
 
 	var cp *connPool
@@ -331,6 +332,16 @@ func (this *Server) electAndConnect(pool *connPool, clientAddr *net.UDPAddr) (ne
 		if err != nil {
 			return nil, nil, fmt.Errorf("Could not dial UDP addr %v from %v: %v", addr, clientAddr, err)
 		}
+	} else if this.cfg.Udp.Source != "" {
+		laddrStr := this.cfg.Udp.Source + ":0" // 0 means "pick a random source port"
+		laddr, err := net.ResolveUDPAddr("udp", laddrStr)
+		if err != nil {
+			return nil, nil, fmt.Errorf("Could not resolve udp local address %s: %v", laddrStr, err)
+		}
+		conn, err = net.DialUDP("udp", laddr, addr)
+		if err != nil {
+			return nil, nil, fmt.Errorf("Could not dial UDP addr %v from %v: %v", addr, laddr, err)
+		}
 	} else {
 		conn, err = net.DialUDP("udp", nil, addr)
 		if err != nil {

src/server/udp/session/config.go

@@ -8,4 +8,5 @@ type Config struct {
 	ClientIdleTimeout  time.Duration
 	BackendIdleTimeout time.Duration
 	Transparent        bool
+	Source             string
 }