Fix NewUDPMuxDefault panic

Author: FrantaBOT

error_conn.go

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: 2026 The Pion community <https://pion.ly>
+// SPDX-License-Identifier: MIT
+
+package ice
+
+import (
+	"net"
+	"time"
+)
+
+// ErrorConn for net.PacketConn when the value is nil.
+type ErrorConn struct{}
+
+func (e *ErrorConn) ReadFrom([]byte) (n int, addr net.Addr, err error) {
+	var dummyAddr *net.IPAddr
+
+	return 0, dummyAddr, ErrPacketConnNil
+}                                                                //nolint:revive
+func (e *ErrorConn) WriteTo([]byte, net.Addr) (n int, err error) { return 0, ErrPacketConnNil } //nolint:revive
+func (e *ErrorConn) Close() error                                { return ErrPacketConnNil }    //nolint:revive
+func (e *ErrorConn) LocalAddr() net.Addr {
+	var dummyAddr *net.IPAddr
+
+	return dummyAddr
+}                                                     //nolint:revive
+func (e *ErrorConn) SetDeadline(time.Time) error      { return ErrPacketConnNil } //nolint:revive
+func (e *ErrorConn) SetReadDeadline(time.Time) error  { return ErrPacketConnNil } //nolint:revive
+func (e *ErrorConn) SetWriteDeadline(time.Time) error { return ErrPacketConnNil } //nolint:revive

errors.go

@@ -172,6 +172,9 @@ var (
 	// ErrAgentOptionNotUpdatable indicates an option cannot be updated after construction.
 	ErrAgentOptionNotUpdatable = errors.New("option can only be set during agent construction")
 
+	// ErrAgentOptionNotUpdatable indicates an net.PacketConn value was nil.
+	ErrPacketConnNil = errors.New("net.PacketConn is nil")
+
 	errAttributeTooShortICECandidate = errors.New("attribute not long enough to be ICE candidate")
 	errClosingConnection             = errors.New("failed to close connection")
 	errConnectionAddrAlreadyExist    = errors.New("connection with same remote address already exists")

udp_mux.go

@@ -67,7 +67,11 @@ func NewUDPMuxDefault(params UDPMuxParams) *UDPMuxDefault { //nolint:cyclop
 	}
 
 	var localAddrsForUnspecified []net.Addr
-	if udpAddr, ok := params.UDPConn.LocalAddr().(*net.UDPAddr); !ok { //nolint:nestif
+	//nolint:nestif
+	if params.UDPConn == nil {
+		params.Logger.Errorf("UDPConn is nil")
+		params.UDPConn = &ErrorConn{}
+	} else if udpAddr, ok := params.UDPConn.LocalAddr().(*net.UDPAddr); !ok { //nolint:nestif
 		params.Logger.Errorf("LocalAddr is not a net.UDPAddr, got %T", params.UDPConn.LocalAddr())
 	} else if ok && udpAddr.IP.IsUnspecified() {
 		// For unspecified addresses, the correct behavior is to return errListenUnspecified, but