Use net.ParseCIDR instead of custom-built parsers (#3055)

Removes avoidable conversion to and from net.IP to string.

cmd/interface-ips.go

@@ -0,0 +1,69 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * 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 cmd
+
+import (
+	"fmt"
+	"net"
+	"sort"
+)
+
+// byLastOctetValue implements sort.Interface used in sorting a list
+// of ip address by their last octet value.
+type byLastOctetValue []net.IP
+
+func (n byLastOctetValue) Len() int      { return len(n) }
+func (n byLastOctetValue) Swap(i, j int) { n[i], n[j] = n[j], n[i] }
+func (n byLastOctetValue) Less(i, j int) bool {
+	return []byte(n[i].To4())[3] < []byte(n[j].To4())[3]
+}
+
+// getInterfaceIPv4s is synonymous to net.InterfaceAddrs()
+// returns net.IP IPv4 only representation of the net.Addr.
+// Additionally the returned list is sorted by their last
+// octet value.
+//
+// [The logic to sort by last octet is implemented to
+// prefer CIDRs with higher octects, this in-turn skips the
+// localhost/loopback address to be not preferred as the
+// first ip on the list. Subsequently this list helps us print
+// a user friendly message with appropriate values].
+func getInterfaceIPv4s() ([]net.IP, error) {
+	addrs, err := net.InterfaceAddrs()
+	if err != nil {
+		return nil, fmt.Errorf("Unable to determine network interface address. %s", err)
+	}
+	// Go through each return network address and collate IPv4 addresses.
+	var nips []net.IP
+	for _, addr := range addrs {
+		if addr.Network() == "ip+net" {
+			var nip net.IP
+			// Attempt to parse the addr through CIDR.
+			nip, _, err = net.ParseCIDR(addr.String())
+			if err != nil {
+				return nil, fmt.Errorf("Unable to parse addrss %s, error %s", addr, err)
+			}
+			// Collect only IPv4 addrs.
+			if nip.To4() != nil {
+				nips = append(nips, nip)
+			}
+		}
+	}
+	// Sort the list of IPs by their last octet value.
+	sort.Sort(sort.Reverse(byLastOctetValue(nips)))
+	return nips, nil
+}