NEW FEATURE: Add RFC4183 support to REV() (#2879)

Co-authored-by: Thomas Misilo <tmisilo@ksu.edu>
Co-authored-by: Jeffrey Cafferata <jeffrey@jcid.nl>

pkg/rfc4183/reverse.go

@@ -0,0 +1,79 @@
+package rfc4183
+
+import (
+	"fmt"
+	"net/netip"
+	"strings"
+)
+
+// ReverseDomainName implements RFC4183 for turning a CIDR block into
+// a in-addr name.  IP addresses are assumed to be /32 or /128 CIDR blocks.
+// CIDR host bits are changed to 0s.
+func ReverseDomainName(cidr string) (string, error) {
+
+	// Mask missing? Add it.
+	if !strings.Contains(cidr, "/") {
+		a, err := netip.ParseAddr(cidr)
+		if err != nil {
+			return "", fmt.Errorf("not an IP address: %w", err)
+		}
+		if a.Is4() {
+			cidr = cidr + "/32"
+		} else {
+			cidr = cidr + "/128"
+		}
+	}
+
+	// Parse the CIDR.
+	p, err := netip.ParsePrefix(cidr)
+	if err != nil {
+		return "", fmt.Errorf("not a CIDR block: %w", err)
+	}
+
+	// RFC4183 4.1 step 4: The notion of fewer than 8 mask bits is not reasonable.
+	if p.Bits() < 8 {
+		return "", fmt.Errorf("mask fewer than 8 bits is unreasonable: %s", cidr)
+	}
+
+	// Handle IPv6 separately:
+	if p.Addr().Is6() {
+		return reverseIPv6(p.Addr().AsSlice(), p.Bits())
+	}
+
+	// Zero out any host bits.
+	p = p.Masked()
+
+	// IPv4: Implement the RFC4183 process:
+
+	// 4.p Step 1
+	b := p.Addr().AsSlice()
+	x, y, z, w := b[0], b[1], b[2], b[3]
+	m := p.Bits()
+
+	if m == 8 {
+		return fmt.Sprintf("%d.in-addr.arpa", x), nil
+	}
+	if m == 16 {
+		return fmt.Sprintf("%d.%d.in-addr.arpa", y, x), nil
+	}
+	if m == 24 {
+		return fmt.Sprintf("%d.%d.%d.in-addr.arpa", z, y, x), nil
+	}
+	if m == 32 {
+		return fmt.Sprintf("%d.%d.%d.%d.in-addr.arpa", w, z, y, x), nil
+	}
+
+	// 4.1 Step 2
+	n := w // I don't understand why the RFC changes variable names at this point, but it does.
+	if m >= 24 && m <= 32 {
+		return fmt.Sprintf("%d-%d.%d.%d.%d.in-addr.arpa", n, m, z, y, x), nil
+	}
+	if m >= 16 && m < 24 {
+		return fmt.Sprintf("%d-%d.%d.%d.in-addr.arpa", z, m, y, x), nil
+	}
+	if m >= 8 && m < 16 {
+		return fmt.Sprintf("%d-%d.%d.in-addr.arpa", y, m, x), nil
+	}
+	return "", fmt.Errorf("fewer than 8 mask bits is not reasonable: %v", cidr)
+
+}