shared: in-addr-utils - add default_subnet_mask and default_prefixlen methods

These use the (deprecated) IPv4 address classes to deduce the corresponding subnet masks. This is useful when addresses
without subnet masks and prefix lengths are given.

Make use of these new functions from sd-dhcp-lease.

src/libsystemd-network/sd-dhcp-lease.c

@@ -30,6 +30,7 @@
 #include "list.h"
 #include "mkdir.h"
 #include "fileio.h"
+#include "in-addr-util.h"
 
 #include "dhcp-protocol.h"
 #include "dhcp-internal.h"
@@ -803,27 +804,20 @@ int dhcp_lease_load(const char *lease_file, sd_dhcp_lease **ret) {
 }
 
 int dhcp_lease_set_default_subnet_mask(sd_dhcp_lease *lease) {
-        uint32_t address;
+        struct in_addr address;
+        struct in_addr mask;
+        int r;
 
         assert(lease);
-        assert(lease->address != INADDR_ANY);
 
-        address = be32toh(lease->address);
+        address.s_addr = lease->address;
 
         /* fall back to the default subnet masks based on address class */
+        r = in_addr_default_subnet_mask(&address, &mask);
+        if (r < 0)
+                return r;
 
-        if ((address >> 31) == 0x0)
-                /* class A, leading bits: 0 */
-                lease->subnet_mask = htobe32(0xff000000);
-        else if ((address >> 30) == 0x2)
-                /* class B, leading bits 10 */
-                lease->subnet_mask = htobe32(0xffff0000);
-        else if ((address >> 29) == 0x6)
-                /* class C, leading bits 110 */
-                lease->subnet_mask = htobe32(0xffffff00);
-        else
-                /* class D or E, no default mask. give up */
-                return -ERANGE;
+        lease->subnet_mask = mask.s_addr;
 
         return 0;
 }

src/shared/in-addr-util.c

@@ -248,3 +248,46 @@ unsigned in_addr_netmask_to_prefixlen(const struct in_addr *addr) {
 
         return 32 - u32ctz(be32toh(addr->s_addr));
 }
+
+int in_addr_default_prefixlen(const struct in_addr *addr, unsigned char *prefixlen) {
+        uint32_t address;
+
+        assert(addr);
+        assert(addr->s_addr != INADDR_ANY);
+        assert(prefixlen);
+
+        address = be32toh(addr->s_addr);
+
+        if ((address >> 31) == 0x0)
+                /* class A, leading bits: 0 */
+                *prefixlen = 8;
+        else if ((address >> 30) == 0x2)
+                /* class B, leading bits 10 */
+                *prefixlen = 16;
+        else if ((address >> 29) == 0x6)
+                /* class C, leading bits 110 */
+                *prefixlen = 24;
+        else
+                /* class D or E, no default prefixlen */
+                return -ERANGE;
+
+        return 0;
+}
+
+int in_addr_default_subnet_mask(const struct in_addr *addr, struct in_addr *mask) {
+        unsigned char prefixlen;
+        int r;
+
+        assert(addr);
+        assert(mask);
+
+        r = in_addr_default_prefixlen(addr, &prefixlen);
+        if (r < 0)
+                return r;
+
+        assert(prefixlen > 0 && prefixlen < 32);
+
+        mask->s_addr = htobe32((0xffffffff << (32 - prefixlen)) & 0xffffffff);
+
+        return 0;
+}

src/shared/in-addr-util.h

@@ -40,6 +40,8 @@ int in_addr_to_string(int family, const union in_addr_union *u, char **ret);
 int in_addr_from_string(int family, const char *s, union in_addr_union *ret);
 int in_addr_from_string_auto(const char *s, int *family, union in_addr_union *ret);
 unsigned in_addr_netmask_to_prefixlen(const struct in_addr *addr);
+int in_addr_default_prefixlen(const struct in_addr *addr, unsigned char *prefixlen);
+int in_addr_default_subnet_mask(const struct in_addr *addr, struct in_addr *mask);
 
 static inline size_t FAMILY_ADDRESS_SIZE(int family) {
         assert(family == AF_INET || family == AF_INET6);