Multihomed support for client socket in snet

gateway/dataplane/BUILD.bazel

@@ -67,6 +67,7 @@ go_test(
         "//pkg/private/xtest:go_default_library",
         "//pkg/snet:go_default_library",
         "//pkg/snet/mock_snet:go_default_library",
+        "//pkg/snet/multihomed:go_default_library",
         "//pkg/snet/path:go_default_library",
         "//private/ringbuf:go_default_library",
         "@com_github_golang_mock//gomock:go_default_library",

gateway/dataplane/session_test.go

@@ -30,6 +30,7 @@ import (
 	"github.com/scionproto/scion/pkg/private/mocks/net/mock_net"
 	"github.com/scionproto/scion/pkg/snet"
 	"github.com/scionproto/scion/pkg/snet/mock_snet"
+	"github.com/scionproto/scion/pkg/snet/multihomed"
 	snetpath "github.com/scionproto/scion/pkg/snet/path"
 )
 
@@ -82,6 +83,7 @@ func TestTwoPaths(t *testing.T) {
 }
 
 func TestNoLeak(t *testing.T) {
+	multihomed.StopContinuousCheckInterfaces(t)
 	defer goleak.VerifyNone(t)
 
 	ctrl := gomock.NewController(t)

pkg/snet/BUILD.bazel

@@ -37,6 +37,7 @@ go_library(
         "//pkg/slayers/path/epic:go_default_library",
         "//pkg/slayers/path/onehop:go_default_library",
         "//pkg/slayers/path/scion:go_default_library",
+        "//pkg/snet/multihomed:go_default_library",
         "//private/topology:go_default_library",
         "//private/topology/underlay:go_default_library",
         "@com_github_gopacket_gopacket//:go_default_library",

pkg/snet/conn.go

@@ -22,7 +22,6 @@ import (
 
 	"github.com/scionproto/scion/pkg/private/common"
 	"github.com/scionproto/scion/pkg/private/ctrl/path_mgmt"
-	"github.com/scionproto/scion/pkg/private/serrors"
 	"github.com/scionproto/scion/pkg/slayers"
 )
 
@@ -58,7 +57,12 @@ type Conn struct {
 // NewCookedConn returns a "cooked" Conn. The Conn object can be used to
 // send/receive SCION traffic with the usual methods.
 // It takes as arguments a non-nil PacketConn and a non-nil Topology parameter.
-// Nil or unspecified addresses for the PacketConn object are not supported.
+// The local address of the PacketConn can be nil or unspecified, leaving the socket not bound
+// to any particular interface; however it has its limitations, namely a created Conn not
+// being able to properly react to a routing change in the OS unless the routing change is
+// accompanied by a change in the local network interfaces, in the form of changing their
+// local IP addresses, or adding/removing one or more local network interfaces.
+//
 // This is an advanced API, that allows fine-tuning of the Conn underlay functionality.
 // The general methods for obtaining a Conn object are still SCIONNetwork.Listen and
 // SCIONNetwork.Dial.
@@ -72,9 +76,7 @@ func NewCookedConn(
 		IA:   topo.LocalIA,
 		Host: pconn.LocalAddr().(*net.UDPAddr),
 	}
-	if local.Host == nil || local.Host.IP.IsUnspecified() {
-		return nil, serrors.New("nil or unspecified address is not supported.")
-	}
+
 	return &Conn{
 		conn:   pconn,
 		local:  local,

pkg/snet/multihomed/BUILD.bazel

@@ -0,0 +1,34 @@
+load("@rules_go//go:def.bzl", "go_library")
+load("//tools:go.bzl", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = [
+        "interfaces.go",
+        "outbound_addr.go",
+    ],
+    importpath = "github.com/scionproto/scion/pkg/snet/multihomed",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//pkg/log:go_default_library",
+        "//pkg/private/serrors:go_default_library",
+    ],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = [
+        "export_test.go",
+        "integrated_test.go",
+        "multihomed_test.go",
+    ],
+    embed = [":go_default_library"],
+    deps = [
+        "//pkg/addr:go_default_library",
+        "//pkg/daemon:go_default_library",
+        "//pkg/private/xtest:go_default_library",
+        "//pkg/snet:go_default_library",
+        "@com_github_stretchr_testify//require:go_default_library",
+        "@com_github_stretchr_testify//suite:go_default_library",
+    ],
+)

pkg/snet/multihomed/export_test.go

@@ -0,0 +1,49 @@
+// Copyright 2025 ETH Zurich
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package multihomed
+
+import (
+	"net/netip"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func MustGetEgressIpAddresses(t *testing.T) []netip.Addr {
+	addrs, err := egressIpAddresses()
+	require.NoError(t, err)
+	return addrs
+}
+
+func GetInternalMutex() *sync.RWMutex {
+	return &muRemoteToEgress
+}
+
+func StopTicker() {
+	stopContinuousCheckInterfaces()
+}
+
+func GetRemoteToEgressMap() map[netip.Addr]netip.Addr {
+	return remoteToEgress
+}
+
+func ReplaceRemoteToEgressMap(newMap map[netip.Addr]netip.Addr) {
+	remoteToEgress = newMap
+}
+
+func GetEgressesLastState() *[]netip.Addr {
+	return &localAddresses
+}