dhcp: T3841: add ping-check option

docs/configuration/service/dhcp-server.rst

@@ -39,6 +39,21 @@ Configuration
 
    Multiple DNS servers can be defined.
 
+.. cfgcmd:: set service dhcp-server shared-network-name <name> ping-check
+
+   When the DHCP server is considering dynamically allocating an IP address to a
+   client, it first sends an ICMP Echo request (a ping) to the address being
+   assigned. It waits for a second, and if no ICMP Echo response has been heard,
+   it assigns the address.
+
+   If a response is heard, the lease is abandoned, and the server does not
+   respond to the client. The lease will remain abandoned for a minimum of
+   abandon-lease-time seconds (defaults to 24 hours).
+
+   If a there are no free addressses but there are abandoned IP addresses, the
+   DHCP server will attempt to reclaim an abandoned IP address regardless of the
+   value of abandon-lease-time.
+
 Individual Client Subnet
 -------------------------
 
@@ -105,6 +120,22 @@ Individual Client Subnet
    request where no full FQDN is passed. This option can be given multiple times
    if you need multiple search domains (DHCP Option 119).
 
+.. cfgcmd:: set service dhcp-server shared-network-name <name> subnet <subnet>
+   ping-check
+
+   When the DHCP server is considering dynamically allocating an IP address to a
+   client, it first sends an ICMP Echo request (a ping) to the address being
+   assigned. It waits for a second, and if no ICMP Echo response has been heard,
+   it assigns the address.
+
+   If a response is heard, the lease is abandoned, and the server does not
+   respond to the client. The lease will remain abandoned for a minimum of
+   abandon-lease-time seconds (defaults to 24 hours).
+
+   If a there are no free addressses but there are abandoned IP addresses, the
+   DHCP server will attempt to reclaim an abandoned IP address regardless of the
+   value of abandon-lease-time.
+
 Failover
 --------