udhcpd: optional IP selection based on MAC hash

function                                             old     new   delta
find_free_or_expired_nip                             153     225     +72

Signed-off-by: Vladislav Grishenko <themiron@mail.ru>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 50 insertions, 8 deletions

diff --git a/networking/udhcp/Config.src b/networking/udhcp/Config.src
index dcd493f13..750a53a32 100644
--- a/networking/udhcp/Config.src
+++ b/networking/udhcp/Config.src
@@ -39,7 +39,21 @@ config FEATURE_UDHCPD_WRITE_LEASES_EARLY
           If selected, udhcpd will write a new file with leases every
           time a new lease has been accepted, thus eliminating the need
           to send SIGUSR1 for the initial writing or updating. Any timed
-          rewriting remains undisturbed
+          rewriting remains undisturbed.
+
+config FEATURE_UDHCPD_BASE_IP_ON_MAC
+        bool "Select IP address based on client MAC"
+        default n
+        depends on UDHCPD
+        help
+          If selected, udhcpd will base its selection of IP address to offer
+          on the client's hardware address. Otherwise udhcpd uses the next
+          consecutive free address.
+
+          This reduces the frequency of IP address changes for clients
+          which let their lease expire, and makes consecutive DHCPOFFERS
+          for the same client to (almost always) contain the same
+          IP address.
 
 config DHCPD_LEASES_FILE
         string "Absolute path to lease file"
@@ -72,7 +86,7 @@ config FEATURE_UDHCPC_ARPING
 
 config FEATURE_UDHCP_PORT
         bool "Enable '-P port' option for udhcpd and udhcpc"
-        default y
+        default n
         depends on UDHCPD || UDHCPC
         help
           At the cost of ~300 bytes, enables -P port option.
diff --git a/networking/udhcp/leases.c b/networking/udhcp/leases.c
index 7aeb37bae..c5b60b108 100644
--- a/networking/udhcp/leases.c
+++ b/networking/udhcp/leases.c
@@ -137,21 +137,42 @@ uint32_t FAST_FUNC find_free_or_expired_nip(const uint8_t *safe_mac)
         uint32_t addr;
         struct dyn_lease *oldest_lease = NULL;
 
-        addr = server_config.start_ip; /* addr is in host order here */
-        for (; addr <= server_config.end_ip; addr++) {
+#if ENABLE_FEATURE_UDHCPD_BASE_IP_ON_MAC
+        uint32_t stop;
+        unsigned i, hash;
+
+        /* hash hwaddr: use the SDBM hashing algorithm.  Seems to give good
+         * dispersal even with similarly-valued "strings".
+         */
+        hash = 0;
+        for (i = 0; i < 6; i++)
+                hash += safe_mac[i] + (hash << 6) + (hash << 16) - hash;
+
+        /* pick a seed based on hwaddr then iterate until we find a free address. */
+        addr = server_config.start_ip
+                + (hash % (1 + server_config.end_ip - server_config.start_ip));
+        stop = addr;
+#else
+        addr = server_config.start_ip;
+#define stop (server_config.end_ip + 1)
+#endif
+        do {
                 uint32_t nip;
                 struct dyn_lease *lease;
 
                 /* ie, 192.168.55.0 */
                 if ((addr & 0xff) == 0)
-                        continue;
+                        goto next_addr;
                 /* ie, 192.168.55.255 */
                 if ((addr & 0xff) == 0xff)
-                        continue;
+                        goto next_addr;
                 nip = htonl(addr);
+                /* skip our own address */
+                if (nip == server_config.server_nip)
+                        goto next_addr;
                 /* is this a static lease addr? */
                 if (is_nip_reserved(server_config.static_leases, nip))
-                        continue;
+                        goto next_addr;
 
                 lease = find_lease_by_nip(nip);
                 if (!lease) {
@@ -162,7 +183,14 @@ uint32_t FAST_FUNC find_free_or_expired_nip(const uint8_t *safe_mac)
                         if (!oldest_lease || lease->expires < oldest_lease->expires)
                                 oldest_lease = lease;
                 }
-        }
+
+ next_addr:
+                addr++;
+#if ENABLE_FEATURE_UDHCPD_BASE_IP_ON_MAC
+                if (addr > server_config.end_ip)
+                        addr = server_config.start_ip;
+#endif
+        } while (addr != stop);
 
         if (oldest_lease
          && is_expired_lease(oldest_lease)