sd-ipv4ll/networkd: generate predictable addresses

Increase the chance of using the same link local address between reboots. The
pseudo random sequence of addresses we attempt is now seeded with data that is
very likely to stay the same between reboots, but at the same time be unique
to the specific machine/nic.

First we try to use the ID_NET_NAME_* data from the udev db combined with the
machin-id, which is guaranteed to be unique and persistent, if available. If
that is not possible (e.g., in containers where we don't have access to the
udev db) we fallback to using the MAC address of the interface, which is
guaranteed to be unique, and likely to be persistent.

[tomegun: three minor changes:

 - don't expose HASH_KEY in the siphash24 header
 - get rid of some compile-warnings (and some casts at the same time),
   by using uint8_t[8] rather than uint64_t in the api
 - added commit message]

1 files changed, 1 insertions, 28 deletions

diff --git a/src/udev/net/link-config.c b/src/udev/net/link-config.c
index 62439c0c71..472a4150c3 100644
--- a/src/udev/net/link-config.c
+++ b/src/udev/net/link-config.c
@@ -294,44 +294,17 @@ static bool mac_is_permanent(struct udev_device *device) {
         return type == 0;
 }
 
-#define HASH_KEY SD_ID128_MAKE(d3,1e,48,fa,90,fe,4b,4c,9d,af,d5,d7,a1,b1,2e,8a)
-
 static int get_mac(struct udev_device *device, bool want_random, struct ether_addr *mac) {
         int r;
 
         if (want_random)
                 random_bytes(mac->ether_addr_octet, ETH_ALEN);
         else {
-                const char *name;
                 uint8_t result[8];
-                size_t l, sz;
-                uint8_t *v;
-
-                /* fetch some persistent data unique (on this machine) to this device */
-                name = udev_device_get_property_value(device, "ID_NET_NAME_ONBOARD");
-                if (!name) {
-                        name = udev_device_get_property_value(device, "ID_NET_NAME_SLOT");
-                        if (!name) {
-                                name = udev_device_get_property_value(device, "ID_NET_NAME_PATH");
-                                if (!name)
-                                        return -ENOENT;
-                        }
-                }
 
-                l = strlen(name);
-                sz = sizeof(sd_id128_t) + l;
-                v = alloca(sz);
-
-                /* fetch some persistent data unique to this machine */
-                r = sd_id128_get_machine((sd_id128_t*) v);
+                r = net_get_unique_predictable_data(device, result);
                 if (r < 0)
                         return r;
-                memcpy(v + sizeof(sd_id128_t), name, l);
-
-                /* Let's hash the machine ID plus the device name. We
-                 * use a fixed, but originally randomly created hash
-                 * key here. */
-                siphash24(result, v, sz, HASH_KEY.bytes);
 
                 assert_cc(ETH_ALEN <= sizeof(result));
                 memcpy(mac->ether_addr_octet, result, ETH_ALEN);