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.

2 files changed, 45 insertions, 0 deletions

diff --git a/src/shared/in-addr-util.c b/src/shared/in-addr-util.c
index 457eedd6d8..5fbee6caf2 100644
--- a/src/shared/in-addr-util.c
+++ b/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;
+}
diff --git a/src/shared/in-addr-util.h b/src/shared/in-addr-util.h
index 0036acee22..8da030ceb6 100644
--- a/src/shared/in-addr-util.h
+++ b/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);