dhcp: delay restarts after NAKs

The server might answer to a DHCPREQUEST with a NAK and currently the
client restarts the configuration process immediately.  It was
observed that this can easily generate loops in which the network is
flooded with DISCOVER,OFFER,REQUEST,NAK sequences.

RFC 2131 only states that "if the client receives a DHCPNAK message,
the client restarts the configuration process" without further
details.

Add a delay with exponential backoff between retries after NAKs to
limit the number of requests and cap the delay to 30 minutes.

1 files changed, 25 insertions, 4 deletions

diff --git a/src/libsystemd-network/sd-dhcp-client.c b/src/libsystemd-network/sd-dhcp-client.c
index d3ad6b7717..cad1a52c09 100644
--- a/src/libsystemd-network/sd-dhcp-client.c
+++ b/src/libsystemd-network/sd-dhcp-client.c
@@ -43,6 +43,9 @@
 #define MAX_CLIENT_ID_LEN (sizeof(uint32_t) + MAX_DUID_LEN)  /* Arbitrary limit */
 #define MAX_MAC_ADDR_LEN CONST_MAX(INFINIBAND_ALEN, ETH_ALEN)
 
+#define RESTART_AFTER_NAK_MIN_USEC (1 * USEC_PER_SEC)
+#define RESTART_AFTER_NAK_MAX_USEC (30 * USEC_PER_MINUTE)
+
 struct sd_dhcp_client {
         unsigned n_ref;
 
@@ -101,6 +104,7 @@ struct sd_dhcp_client {
         sd_dhcp_client_cb_t cb;
         void *userdata;
         sd_dhcp_lease *lease;
+        usec_t start_delay;
 };
 
 static const uint8_t default_req_opts[] = {
@@ -945,6 +949,7 @@ error:
 }
 
 static int client_initialize_time_events(sd_dhcp_client *client) {
+        uint64_t usec = 0;
         int r;
 
         assert(client);
@@ -952,10 +957,15 @@ static int client_initialize_time_events(sd_dhcp_client *client) {
 
         client->timeout_resend = sd_event_source_unref(client->timeout_resend);
 
+        if (client->start_delay) {
+                sd_event_now(client->event, clock_boottime_or_monotonic(), &usec);
+                usec += client->start_delay;
+        }
+
         r = sd_event_add_time(client->event,
                               &client->timeout_resend,
                               clock_boottime_or_monotonic(),
-                              0, 0,
+                              usec, 0,
                               client_timeout_resend, client);
         if (r < 0)
                 goto error;
@@ -985,7 +995,7 @@ static int client_initialize_events(sd_dhcp_client *client,
         return 0;
 }
 
-static int client_start(sd_dhcp_client *client) {
+static int client_start_delayed(sd_dhcp_client *client) {
         int r;
 
         assert_return(client, -EINVAL);
@@ -1013,6 +1023,11 @@ static int client_start(sd_dhcp_client *client) {
         return client_initialize_events(client, client_receive_message_raw);
 }
 
+static int client_start(sd_dhcp_client *client) {
+        client->start_delay = 0;
+        return client_start_delayed(client);
+}
+
 static int client_timeout_expire(sd_event_source *s, uint64_t usec,
                                  void *userdata) {
         sd_dhcp_client *client = userdata;
@@ -1362,6 +1377,7 @@ static int client_set_lease_timeouts(sd_dhcp_client *client) {
 static int client_handle_message(sd_dhcp_client *client, DHCPMessage *message,
                                  int len) {
         DHCP_CLIENT_DONT_DESTROY(client);
+        char time_string[FORMAT_TIMESPAN_MAX];
         int r = 0, notify_event = 0;
 
         assert(client);
@@ -1409,6 +1425,7 @@ static int client_handle_message(sd_dhcp_client *client, DHCPMessage *message,
 
                 r = client_handle_ack(client, message, len);
                 if (r >= 0) {
+                        client->start_delay = 0;
                         client->timeout_resend =
                                 sd_event_source_unref(client->timeout_resend);
                         client->receive_message =
@@ -1458,11 +1475,15 @@ static int client_handle_message(sd_dhcp_client *client, DHCPMessage *message,
                         if (r < 0)
                                 goto error;
 
-                        r = client_start(client);
+                        r = client_start_delayed(client);
                         if (r < 0)
                                 goto error;
 
-                        log_dhcp_client(client, "REBOOTED");
+                        log_dhcp_client(client, "REBOOT in %s", format_timespan(time_string, FORMAT_TIMESPAN_MAX,
+                                                                                client->start_delay, USEC_PER_SEC));
+
+                        client->start_delay = CLAMP(client->start_delay * 2,
+                                                    RESTART_AFTER_NAK_MIN_USEC, RESTART_AFTER_NAK_MAX_USEC);
 
                         return 0;
                 } else if (r == -ENOMSG)