Reference: https://src.fedoraproject.org/rpms/dhcp/blob/rawhide/f/0017-Send-unicast-request-release-via-correct-interface.patch
From d45463c2007a78623f2c90c250bb8e2f3e34a852 Mon Sep 17 00:00:00 2001
From: Pavel Zhukov <pzhukov@redhat.com>
Date: Thu, 21 Feb 2019 10:39:36 +0100
Subject: [PATCH 17/28] Send unicast request/release via correct interface

(#800561, #1177351)
(Submitted to dhcp-bugs@isc.org - [ISC-Bugs #30544])
---
 client/dhclient.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/client/dhclient.c b/client/dhclient.c
index 444d251..d607975 100644
--- a/client/dhclient.c
+++ b/client/dhclient.c
@@ -3417,6 +3417,14 @@ void send_request (cpp)
 #endif
 	if (destination.sin_addr.s_addr != INADDR_BROADCAST &&
 	    fallback_interface) {
+#if defined(SO_BINDTODEVICE)
+		if (setsockopt(fallback_interface -> wfdesc, SOL_SOCKET,
+			       SO_BINDTODEVICE, client->interface->name,
+			       strlen(client->interface->name)) < 0) {
+			log_error("%s:%d: Failed to bind fallback interface"
+				  " to %s: %m", MDL, client->interface->name);
+		}
+#endif
 		result = send_packet(fallback_interface, NULL, &client->packet,
 				     client->packet_length, from, &destination,
 				     NULL);
@@ -3426,6 +3434,13 @@ void send_request (cpp)
 				  client->packet_length,
 				  fallback_interface->name);
 		}
+#if defined(SO_BINDTODEVICE)
+		if (setsockopt(fallback_interface -> wfdesc, SOL_SOCKET,
+			       SO_BINDTODEVICE, NULL, 0) < 0) {
+			log_fatal("%s:%d: Failed to unbind fallback interface:"
+				  " %m", MDL);
+		}
+#endif
         }
 	else {
 		/* Send out a packet. */
@@ -3543,6 +3558,14 @@ void send_release (cpp)
 	} else
 #endif
 	if (fallback_interface) {
+#if defined(SO_BINDTODEVICE)
+		if (setsockopt(fallback_interface -> wfdesc, SOL_SOCKET,
+			       SO_BINDTODEVICE, client->interface->name,
+			       strlen(client->interface->name)) < 0) {
+			log_error("%s:%d: Failed to bind fallback interface"
+				  " to %s: %m", MDL, client->interface->name);
+		}
+#endif
 		result = send_packet(fallback_interface, NULL, &client->packet,
 				      client->packet_length, from, &destination,
 				      NULL);
@@ -3552,6 +3575,13 @@ void send_release (cpp)
 				  client->packet_length,
 				  fallback_interface->name);
 		}
+#if defined(SO_BINDTODEVICE)
+		if (setsockopt(fallback_interface -> wfdesc, SOL_SOCKET,
+			       SO_BINDTODEVICE, NULL, 0) < 0) {
+			log_fatal("%s:%d: Failed to unbind fallback interface:"
+				  " %m", MDL);
+		}
+#endif
         } else {
 		/* Send out a packet. */
 		result = send_packet(client->interface, NULL, &client->packet,
-- 
2.35.1