From 2135a4f84555cff27a2aedc000a6fa8d710c831a Mon Sep 17 00:00:00 2001
From: Denis Kenzior <denkenz@gmail.com>
Date: Wed, 13 Oct 2021 17:49:18 -0500
Subject: [PATCH] netconfig: Try to put gateway mac into ARP cache

In certain situations, it is possible for us to know the MAC of the
default gateway when DHCP finishes.  This is quite typical on many home
network and small network setups.  It is thus possible to pre-populate
the ARP cache with the gateway MAC address to save an extra round trip
at connection time.

Another advantage is during roaming.  After version 4.20, linux kernel
flushes ARP caches by default whenever netdev encounters a no carrier
condition (as is the case during roaming).  This can prevent packets
from going out after a roam for a significant amount of time due to
lost/delayed ARP responses.
---
 src/netconfig.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/src/netconfig.c b/src/netconfig.c
index 21281154..c51a1ec2 100644
--- a/src/netconfig.c
+++ b/src/netconfig.c
@@ -574,6 +574,39 @@ static struct l_rtnl_address *netconfig_get_dhcp4_address(
 	return ret;
 }
 
+static void netconfig_gateway_to_arp(struct netconfig *netconfig)
+{
+	const struct l_dhcp_lease *lease;
+	_auto_(l_free) char *server_id = NULL;
+	_auto_(l_free) char *gw = NULL;
+	const uint8_t *server_mac;
+	struct in_addr in_gw;
+
+	/* Can only do this for DHCP in certain network setups */
+	if (netconfig->rtm_protocol != RTPROT_DHCP)
+		return;
+
+	lease = l_dhcp_client_get_lease(netconfig->dhcp_client);
+	if (!lease)
+		return;
+
+	server_id = l_dhcp_lease_get_server_id(lease);
+	gw = l_dhcp_lease_get_gateway(lease);
+	server_mac = l_dhcp_lease_get_server_mac(lease);
+
+	if (strcmp(server_id, gw) || !server_mac)
+		return;
+
+	l_debug("Gateway MAC is known, setting into ARP cache");
+	in_gw.s_addr = l_dhcp_lease_get_gateway_u32(lease);
+
+	if (!l_rtnl_neighbor_set_hwaddr(rtnl, netconfig->ifindex, AF_INET,
+					&in_gw, server_mac, ETH_ALEN,
+					netconfig_set_neighbor_entry_cb, NULL,
+					NULL))
+		l_debug("l_rtnl_neighbor_set_hwaddr failed");
+}
+
 static void netconfig_ifaddr_added(struct netconfig *netconfig,
 					const struct ifaddrmsg *ifa,
 					uint32_t len)
@@ -861,6 +894,8 @@ static void netconfig_ipv4_ifaddr_add_cmd_cb(int error, uint16_t type,
 		return;
 	}
 
+	netconfig_gateway_to_arp(netconfig);
+
 	if (!netconfig_ipv4_routes_install(netconfig)) {
 		l_error("netconfig: Failed to install IPv4 routes.");
 		return;
@@ -1372,6 +1407,15 @@ bool netconfig_configure(struct netconfig *netconfig,
 
 bool netconfig_reconfigure(struct netconfig *netconfig)
 {
+	/*
+	 * Starting with kernel 4.20, ARP cache is flushed when the netdev
+	 * detects NO CARRIER.  This can result in unnecessarily long delays
+	 * (about 1 second on some networks) due to ARP query response being
+	 * lost or delayed.  Try to force the gateway into the ARP cache
+	 * to alleviate this
+	 */
+	netconfig_gateway_to_arp(netconfig);
+
 	if (netconfig->rtm_protocol == RTPROT_DHCP) {
 		/* TODO l_dhcp_client sending a DHCP inform request */
 	}