netutil: GetDefaultHost for getting the default IP of the host machine

pkg/netutil/routes.go

@@ -0,0 +1,28 @@
+// Copyright 2016 The etcd Authors
+//
+// 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.
+
+// +build !linux !386,!amd64
+
+package netutil
+
+import (
+	"fmt"
+	"runtime"
+)
+
+// GetDefaultHost fetches the a resolvable name that corresponds
+// to the machine's default routable interface
+func GetDefaultHost() (string, error) {
+	return "", fmt.Errorf("default host not supported on %s_%s", runtime.GOOS, runtime.GOARCH)
+}

pkg/netutil/routes_linux.go

@@ -0,0 +1,133 @@
+// Copyright 2016 The etcd Authors
+//
+// 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.
+
+// +build linux
+// +build 386 amd64
+
+// TODO support native endian but without using "unsafe"
+
+package netutil
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"net"
+	"syscall"
+)
+
+var errNoDefaultRoute = fmt.Errorf("could not find default route")
+
+func GetDefaultHost() (string, error) {
+	rmsg, rerr := getDefaultRoute()
+	if rerr != nil {
+		return "", rerr
+	}
+
+	attrs, aerr := syscall.ParseNetlinkRouteAttr(rmsg)
+	if aerr != nil {
+		return "", aerr
+	}
+
+	oif := uint32(0)
+	for _, attr := range attrs {
+		if attr.Attr.Type == syscall.RTA_PREFSRC {
+			return net.IP(attr.Value).String(), nil
+		}
+		if attr.Attr.Type == syscall.RTA_OIF {
+			oif = binary.LittleEndian.Uint32(attr.Value)
+		}
+	}
+
+	if oif == 0 {
+		return "", errNoDefaultRoute
+	}
+
+	// prefsrc not detected, fall back to getting address from iface
+
+	ifmsg, ierr := getIface(oif)
+	if ierr != nil {
+		return "", ierr
+	}
+
+	attrs, aerr = syscall.ParseNetlinkRouteAttr(ifmsg)
+	if aerr != nil {
+		return "", aerr
+	}
+
+	for _, attr := range attrs {
+		if attr.Attr.Type == syscall.RTA_SRC {
+			return net.IP(attr.Value).String(), nil
+		}
+	}
+
+	return "", errNoDefaultRoute
+}
+
+func getDefaultRoute() (*syscall.NetlinkMessage, error) {
+	dat, err := syscall.NetlinkRIB(syscall.RTM_GETROUTE, syscall.AF_UNSPEC)
+	if err != nil {
+		return nil, err
+	}
+
+	msgs, msgErr := syscall.ParseNetlinkMessage(dat)
+	if msgErr != nil {
+		return nil, msgErr
+	}
+
+	rtmsg := syscall.RtMsg{}
+	for _, m := range msgs {
+		if m.Header.Type != syscall.RTM_NEWROUTE {
+			continue
+		}
+		buf := bytes.NewBuffer(m.Data[:syscall.SizeofRtMsg])
+		if rerr := binary.Read(buf, binary.LittleEndian, &rtmsg); rerr != nil {
+			continue
+		}
+		if rtmsg.Dst_len == 0 {
+			// zero-length Dst_len implies default route
+			return &m, nil
+		}
+	}
+
+	return nil, errNoDefaultRoute
+}
+
+func getIface(idx uint32) (*syscall.NetlinkMessage, error) {
+	dat, err := syscall.NetlinkRIB(syscall.RTM_GETADDR, syscall.AF_UNSPEC)
+	if err != nil {
+		return nil, err
+	}
+
+	msgs, msgErr := syscall.ParseNetlinkMessage(dat)
+	if msgErr != nil {
+		return nil, msgErr
+	}
+
+	ifaddrmsg := syscall.IfAddrmsg{}
+	for _, m := range msgs {
+		if m.Header.Type != syscall.RTM_NEWADDR {
+			continue
+		}
+		buf := bytes.NewBuffer(m.Data[:syscall.SizeofIfAddrmsg])
+		if rerr := binary.Read(buf, binary.LittleEndian, &ifaddrmsg); rerr != nil {
+			continue
+		}
+		if ifaddrmsg.Index == idx {
+			return &m, nil
+		}
+	}
+
+	return nil, errNoDefaultRoute
+}