libsystemd-dhcp: Add functions for sending unicast UDP messages

Create a helper functions setting up an unicast DHCP UDP socket and
sending data. Add function stubs for the test program.

[tomegun: initialize structs when allocating, and drop unneccesary 'err']

src/libsystemd-dhcp/dhcp-internal.h

@@ -29,8 +29,11 @@
 #include "dhcp-protocol.h"
 
 int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link);
+int dhcp_network_bind_udp_socket(int index, be32_t client_address);
 int dhcp_network_send_raw_socket(int s, const union sockaddr_union *link,
                                  const void *packet, size_t len);
+int dhcp_network_send_udp_socket(int s, be32_t server_address,
+                                 const void *packet, size_t len);
 
 int dhcp_option_append(uint8_t **buf, size_t *buflen, uint8_t code,
                        size_t optlen, const void *optval);

src/libsystemd-dhcp/dhcp-network.c

@@ -53,6 +53,27 @@ int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link)
         return s;
 }
 
+int dhcp_network_bind_udp_socket(int index, be32_t client_address)
+{
+        int s;
+        union sockaddr_union src = {
+                .in.sin_family = AF_INET,
+                .in.sin_port = htobe16(DHCP_PORT_CLIENT),
+                .in.sin_addr.s_addr = client_address,
+        };
+
+        s = socket(AF_INET, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
+        if (s < 0)
+                return -errno;
+
+        if (bind(s, &src.sa, sizeof(src.in)) < 0) {
+                close_nointr_nofail(s);
+                return -errno;
+        }
+
+        return s;
+}
+
 int dhcp_network_send_raw_socket(int s, const union sockaddr_union *link,
                                  const void *packet, size_t len)
 {
@@ -61,3 +82,18 @@ int dhcp_network_send_raw_socket(int s, const union sockaddr_union *link,
 
         return 0;
 }
+
+int dhcp_network_send_udp_socket(int s, be32_t server_address,
+                                 const void *packet, size_t len)
+{
+        union sockaddr_union dest = {
+                .in.sin_family = AF_INET,
+                .in.sin_port = htobe16(DHCP_PORT_SERVER),
+                .in.sin_addr.s_addr = server_address,
+        };
+
+        if (sendto(s, packet, len, 0, &dest.sa, sizeof(dest.in)) < 0)
+                return -errno;
+
+        return 0;
+}

src/libsystemd-dhcp/test-dhcp-client.c

@@ -184,6 +184,17 @@ int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link)
         return test_fd[0];
 }
 
+int dhcp_network_bind_udp_socket(int index, be32_t client_address)
+{
+        return 0;
+}
+
+int dhcp_network_send_udp_socket(int s, be32_t server_address,
+                                 const void *packet, size_t len)
+{
+        return 0;
+}
+
 static void test_discover_message(sd_event *e)
 {
         sd_dhcp_client *client;