From 736c33730f553737e9842f2699f52aa35d06baf1 Mon Sep 17 00:00:00 2001
From: Paul Stewart <pstew@chromium.org>
Date: Mon, 31 May 2021 15:45:01 +0000
Subject: [PATCH 06/19] Accept an ACK after a NAK

When a NAK is received, wait for a response interval before
acting on it.  In doing so, this allows for networks that
may have duelling DHCP servers which both consider themselves
authoritative on a network.  Since other DHCP clients also
act in a similar manner, issues of this sort end up undetected
and network administrators thus consider this situation nominal.

BUG=chromium:384897
TEST=New network_DhcpNak test subcase in CL:208214

Reviewed-on: https://chromium-review.googlesource.com/208207
---
 src/dhcp.c | 49 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 30 insertions(+), 19 deletions(-)

diff --git a/src/dhcp.c b/src/dhcp.c
index 4590958c..782eb231 100644
--- a/src/dhcp.c
+++ b/src/dhcp.c
@@ -3095,6 +3095,30 @@ dhcp_redirect_dhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
 	}
 }
 
+static void
+handle_nak(void *arg)
+{
+	struct interface *ifp = arg;
+	struct dhcp_state *state = D_STATE(ifp);
+
+	loginfox("%s: Handling deferred NAK", ifp->name);
+	if (!(ifp->ctx->options & DHCPCD_TEST)) {
+		dhcp_drop(ifp, "NAK");
+		unlink(state->leasefile);
+	}
+
+	/* If we constantly get NAKS then we should slowly back off */
+	eloop_timeout_add_sec(ifp->ctx->eloop,
+			      state->nakoff, dhcp_discover, ifp);
+	if (state->nakoff == 0)
+		state->nakoff = 1;
+	else {
+		state->nakoff *= 2;
+		if (state->nakoff > NAKOFF_MAX)
+			state->nakoff = NAKOFF_MAX;
+	}
+}
+
 static void
 dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
     const struct in_addr *from)
@@ -3266,31 +3290,17 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
 	}
 
 	if (type == DHCP_NAK) {
-		/* We should restart on a NAK */
-		LOGDHCP(logwarnx, "NAK:");
-		if ((msg = get_option_string(ifp->ctx,
-		    bootp, bootp_len, DHO_MESSAGE)))
+		if ((msg = get_option_string(ifp->ctx, bootp,
+					     bootp_len, DHO_MESSAGE)))
 		{
 			logwarnx("%s: message: %s", ifp->name, msg);
 			free(msg);
 		}
 		if (state->state == DHS_INFORM) /* INFORM should not be NAKed */
 			return;
-		if (!(ifp->ctx->options & DHCPCD_TEST)) {
-			dhcp_drop(ifp, "NAK");
-			unlink(state->leasefile);
-		}
-
-		/* If we constantly get NAKS then we should slowly back off */
-		eloop_timeout_add_sec(ifp->ctx->eloop,
-		    state->nakoff, dhcp_discover, ifp);
-		if (state->nakoff == 0)
-			state->nakoff = 1;
-		else {
-			state->nakoff *= 2;
-			if (state->nakoff > NAKOFF_MAX)
-				state->nakoff = NAKOFF_MAX;
-		}
+                LOGDHCP(logwarnx, "NAK (deferred)");
+		eloop_timeout_add_sec(ifp->ctx->eloop, DHCP_BASE,
+                                      handle_nak, ifp);
 		return;
 	}
 
@@ -3423,6 +3433,7 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
 			return;
 		}
 		eloop_timeout_delete(ifp->ctx->eloop, send_discover, ifp);
+		eloop_timeout_delete(ifp->ctx->eloop, handle_nak, ifp);
 		/* We don't request BOOTP addresses */
 		if (type) {
 			/* We used to ARP check here, but that seems to be in
-- 
2.33.0.800.g4c38ced690-goog