Util: Add a function to calculate the gateway

1 files changed, 33 insertions, 0 deletions

diff --git a/util/util.go b/util/util.go
new file mode 100644
index 0000000..c7816df
--- /dev/null
+++ b/util/util.go
@@ -0,0 +1,33 @@
+// package util defines public utility functions to be used by applications
+// these are outside of the client package as they can be used even if a client hasn't been created yet
+package util
+
+import (
+	"net"
+
+	"github.com/eduvpn/eduvpn-common/i18nerr"
+)
+
+// CalculateGateway takes a CIDR encoded subnet `cidr` and returns the gateway and an error
+func CalculateGateway(cidr string) (string, error) {
+	_, ipn, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return "", i18nerr.WrapInternalf(err, "failed to parse CIDR for calculating gateway: %v", cidr)
+	}
+
+	ret := make(net.IP, len(ipn.IP))
+	copy(ret, ipn.IP)
+
+	for i := len(ret) - 1; i >= 0; i-- {
+		ret[i]++
+		if ret[i] > 0 {
+			break
+		}
+	}
+
+	if !ipn.Contains(ret) {
+		return "", i18nerr.Newf("IP network does not contain incremented IP: %v", ret)
+	}
+
+	return ret.String(), nil
+}