networkd: reduce the IPv4 DAD timeout to 200ms

The original timeout of 7 seconds is very long for today's networks. Reduce it
to 200ms. Note that this change also affects IPv4 link-local addressing.

NEWS

@@ -109,6 +109,13 @@ CHANGES WITH 258 in spe:
         * The meson option 'integration-tests' has been deprecated, and will be
           removed in a future release.
 
+        systemd-networkd and networkctl:
+
+        * systemd-networkd now supports configuring the timeout for IPv4
+          Duplicate Address Detection via a new setting
+          IPv4DuplicateAddressDetectionTimeoutSec=. The default timeout value
+          has been changed from 7 seconds to 200 milliseconds.
+
         — <place>, <date>
 
 CHANGES WITH 257:

man/systemd.network.xml

@@ -996,7 +996,7 @@ DuplicateAddressDetection=none</programlisting></para>
         <listitem>
           <para>Configures the maximum timeout for IPv4 Duplicate Address Detection (RFC 5227). Must be a
           value between 1 millisecond and 60 seconds. If set, Duplicate Address Detection takes a randomized
-          time between 57% (4/7) and 100% of the given value. If unset, defaults to 7 seconds.</para>
+          time between 57% (4/7) and 100% of the given value. If unset, defaults to 200 milliseconds.</para>
 
           <xi:include href="version-info.xml" xpointer="v258"/>
         </listitem>

src/libsystemd-network/sd-ipv4acd.c

@@ -43,7 +43,7 @@
 #define MAX_CONFLICTS 10U
 
 /* Default timeout from the RFC */
-#define DEFAULT_ACD_TIMEOUT_USEC (7 * USEC_PER_SEC)
+#define DEFAULT_ACD_TIMEOUT_USEC (200 * USEC_PER_MSEC)
 
 typedef enum IPv4ACDState {
         IPV4ACD_STATE_INIT,