Add net.InterfaceAddrs() conversion functions to netutils

net.InterfaceAddrs() has a weird and useless return value that various
pieces of code parse in variously correct ways. Add functions to
reliably convert its return values to net.IP or netip.Addr.

Author: danwinship

net/v2/convert.go

@@ -19,6 +19,7 @@ package net
 import (
 	"net"
 	"net/netip"
+	"strings"
 )
 
 // AddrFromIP converts a net.IP to a netip.Addr. Given valid input this will always
@@ -113,6 +114,40 @@ func IPFromAddr(addr netip.Addr) net.IP {
 	return net.IP(addr.AsSlice())
 }
 
+// IPFromInterfaceAddr can be used to extract the underlying IP address value (as a
+// net.IP) from the return values of net.InterfaceAddrs(), net.Interface.Addrs(), or
+// net.Interface.MulticastAddrs(). (net.Addr is also used in some other APIs, but this
+// function should not be used on net.Addrs that are not "interface addresses".)
+func IPFromInterfaceAddr(ifaddr net.Addr) net.IP {
+	// On both Linux and Windows, the values returned from the "interface address"
+	// methods are currently *net.IPNet for unicast addresses or *net.IPAddr for
+	// multicast addresses.
+	if ipnet, ok := ifaddr.(*net.IPNet); ok {
+		return ipnet.IP
+	} else if ipaddr, ok := ifaddr.(*net.IPAddr); ok {
+		return ipaddr.IP
+	}
+
+	// Try to deal with other similar types... in particular, this is needed for
+	// some existing unit tests...
+	addrStr := ifaddr.String()
+	// If it has a subnet length (like net.IPNet) or optional zone identifier (like
+	// net.IPAddr), trim that away.
+	if end := strings.IndexAny(addrStr, "/%"); end != -1 {
+		addrStr = addrStr[:end]
+	}
+	// What's left is either an IP address, or something we can't parse.
+	return ParseIPSloppy(addrStr)
+}
+
+// AddrFromInterfaceAddr can be used to extract the underlying IP address value (as a
+// netip.Addr) from the return values of net.InterfaceAddrs(), net.Interface.Addrs(), or
+// net.Interface.MulticastAddrs(). (net.Addr is also used in some other APIs, but this
+// function should not be used on net.Addrs that are not "interface addresses".)
+func AddrFromInterfaceAddr(ifaddr net.Addr) netip.Addr {
+	return AddrFromIP(IPFromInterfaceAddr(ifaddr))
+}
+
 // PrefixFromIPNet converts a *net.IPNet to a netip.Prefix. Given valid input this will
 // always succeed; it will return the invalid netip.Prefix on nil or garbage input.
 func PrefixFromIPNet(ipnet *net.IPNet) netip.Prefix {

net/v2/convert_test.go

@@ -126,6 +126,91 @@ func TestIPFromAddr(t *testing.T) {
 	}
 }
 
+type dummyNetAddr string
+
+func (d dummyNetAddr) Network() string {
+	return "dummy"
+}
+func (d dummyNetAddr) String() string {
+	return string(d)
+}
+
+func TestIPFromInterfaceAddr_AddrFromInterfaceAddr(t *testing.T) {
+	testCases := []struct {
+		desc   string
+		ifaddr net.Addr
+		out    string
+	}{
+		{
+			desc:   "net.IPNet",
+			ifaddr: &net.IPNet{IP: net.IP{192, 168, 1, 1}, Mask: net.CIDRMask(24, 32)},
+			out:    "192.168.1.1",
+		},
+		{
+			desc:   "net.IPAddr",
+			ifaddr: &net.IPAddr{IP: net.IP{192, 168, 1, 2}},
+			out:    "192.168.1.2",
+		},
+		{
+			desc:   "net.IPAddr with zone",
+			ifaddr: &net.IPAddr{IP: net.IP{192, 168, 1, 3}, Zone: "eth0"},
+			out:    "192.168.1.3",
+		},
+		{
+			desc:   "net.TCPAddr",
+			ifaddr: &net.TCPAddr{IP: net.IP{192, 168, 1, 4}, Port: 80},
+			out:    "",
+		},
+		{
+			desc:   "unknown plain IP",
+			ifaddr: dummyNetAddr("192.168.1.5"),
+			out:    "192.168.1.5",
+		},
+		{
+			desc:   "unknown CIDR",
+			ifaddr: dummyNetAddr("192.168.1.6/24"),
+			out:    "192.168.1.6",
+		},
+		{
+			desc:   "unknown IP with zone",
+			ifaddr: dummyNetAddr("192.168.1.7%eth0"),
+			out:    "192.168.1.7",
+		},
+		{
+			desc:   "unknown sockaddr",
+			ifaddr: dummyNetAddr("192.168.1.8:80"),
+			out:    "",
+		},
+		{
+			desc:   "unknown junk",
+			ifaddr: dummyNetAddr("junk"),
+			out:    "",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			ip := IPFromInterfaceAddr(tc.ifaddr)
+			addr := AddrFromInterfaceAddr(tc.ifaddr)
+			if tc.out == "" {
+				if ip != nil {
+					t.Errorf("expected IPFromInterfaceAddr to return nil but got %q", ip.String())
+				}
+				if addr.IsValid() {
+					t.Errorf("expected AddrFromInterfaceAddr to return zero but got %q", addr.String())
+				}
+			} else {
+				if ip.String() != tc.out {
+					t.Errorf("expected IPFromInterfaceAddr to return %q but got %q", tc.out, ip.String())
+				}
+				if addr.String() != tc.out {
+					t.Errorf("expected AddrFromInterfaceAddr to return %q but got %q", tc.out, addr.String())
+				}
+			}
+		})
+	}
+}
+
 func TestPrefixFromIPNet(t *testing.T) {
 	// See test cases in ips_test.go
 	for _, tc := range goodTestCIDRs {