network/address: acquire address from DHCP server lease file

If the DHCP server on an interface is configured with its server
address is null, then let's reuse the previous server address if
possible.

Otherwise, if networkd is restarted or the host is rebooted, then
possibly new subnet is picked, and clients that already have addresses
in the previous subnet cannot access the server.

Prompted by https://github.com/systemd/systemd/pull/30021#discussion_r1518478633.

src/network/networkd-address.c

@@ -1495,6 +1495,10 @@ static int address_acquire(Link *link, const Address *address, union in_addr_uni
         assert(address);
         assert(ret);
 
+        r = address_acquire_from_dhcp_server_leases_file(link, address, ret);
+        if (r != -ENOENT)
+                return r;
+
         r = address_pool_acquire(link->manager, address->family, address->prefixlen, &a);
         if (r < 0)
                 return r;

src/network/networkd-address.h

@@ -57,6 +57,7 @@ struct Address {
         bool scope_set:1;
         bool ip_masquerade_done:1;
         bool requested_as_null:1;
+        bool used_by_dhcp_server:1;
 
         /* duplicate_address_detection is only used by static or IPv4 dynamic addresses.
          * To control DAD for IPv6 dynamic addresses, set IFA_F_NODAD to flags. */

src/network/networkd-dhcp-server.c

@@ -7,6 +7,7 @@
 #include "sd-dhcp-server.h"
 
 #include "dhcp-protocol.h"
+#include "dhcp-server-lease-internal.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "network-common.h"
@@ -82,6 +83,7 @@ int network_adjust_dhcp_server(Network *network, Set **addresses) {
                         /* TODO: check if the prefix length is small enough for the pool. */
 
                         network->dhcp_server_address = address;
+                        address->used_by_dhcp_server = true;
                         break;
                 }
                 if (!network->dhcp_server_address) {
@@ -128,6 +130,7 @@ int network_adjust_dhcp_server(Network *network, Set **addresses) {
                 a->prefixlen = network->dhcp_server_address_prefixlen;
                 a->in_addr.in = network->dhcp_server_address_in_addr;
                 a->requested_as_null = !in4_addr_is_set(&network->dhcp_server_address_in_addr);
+                a->used_by_dhcp_server = true;
 
                 r = address_section_verify(a);
                 if (r < 0)
@@ -144,6 +147,49 @@ int network_adjust_dhcp_server(Network *network, Set **addresses) {
         return 0;
 }
 
+int address_acquire_from_dhcp_server_leases_file(Link *link, const Address *address, union in_addr_union *ret) {
+        struct in_addr a;
+        uint8_t prefixlen;
+        int r;
+
+        assert(link);
+        assert(link->manager);
+        assert(address);
+        assert(ret);
+
+        /* If the DHCP server address is configured as a null address, reuse the server address of the
+         * previous instance. */
+        if (address->family != AF_INET)
+                return -ENOENT;
+
+        if (!address->used_by_dhcp_server)
+                return -ENOENT;
+
+        if (!link_dhcp4_server_enabled(link))
+                return -ENOENT;
+
+        if (link->manager->persistent_storage_fd < 0)
+                return -EBUSY; /* The persistent storage is not ready, try later again. */
+
+        _cleanup_free_ char *lease_file = path_join("dhcp-server-lease", link->ifname);
+        if (!lease_file)
+                return -ENOMEM;
+
+        r = dhcp_server_leases_file_get_server_address(
+                        link->manager->persistent_storage_fd,
+                        lease_file,
+                        &a,
+                        &prefixlen);
+        if (r < 0)
+                return r;
+
+        if (prefixlen != address->prefixlen)
+                return -ENOENT;
+
+        ret->in = a;
+        return 0;
+}
+
 int link_start_dhcp4_server(Link *link) {
         int r;
 

src/network/networkd-dhcp-server.h

@@ -2,14 +2,16 @@
 #pragma once
 
 #include "conf-parser.h"
+#include "in-addr-util.h"
 #include "set.h"
 
+typedef struct Address Address;
 typedef struct Link Link;
 typedef struct Manager Manager;
 typedef struct Network Network;
 
 int network_adjust_dhcp_server(Network *network, Set **addresses);
-
+int address_acquire_from_dhcp_server_leases_file(Link *link, const Address *address, union in_addr_union *ret);
 int link_request_dhcp_server(Link *link);
 
 int link_start_dhcp4_server(Link *link);