Re: [B.A.T.M.A.N.] [PATCH v3] alfred: Request MAC resolution for IPv4 address not in ARP cache

["gary" <guohuizou2000@sina.com>]

Tested-by: Gary Zou <guohuizou2000@sina.com>

-----Original Message-----
From: Jonathan Haws <jhaws@sdl.usu.edu> 
Sent: 2018年10月30日 1:58
To: guohuizou2000@sina.com; b.a.t.m.a.n@lists.open-mesh.org
Cc: Jonathan Haws <jhaws@sdl.usu.edu>
Subject: [B.A.T.M.A.N.][PATCH v3] alfred: Request MAC resolution for IPv4
address not in ARP cache


When using IPv4, if the remote server is not yet in the ARP cache, the MAC
resolution will fail and data appear to not be shared via alfred.
Add a routine (modified from batctl sources) to request MAC resolution by
simply sending a datagram to the discard port (UDP/9). This adds the remote
MAC to the ARP cache, resulting in successful MAC resolution.

Fixes: c7da798113a2 ("alfred: IPv4 multicast distribution support.")
Signed-off-by: Jonathan Haws <jhaws@sdl.usu.edu>
---
 util.c | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/util.c b/util.c
index dd3f00f..0794792 100644
--- a/util.c
+++ b/util.c
@@ -30,6 +30,7 @@
 #include <sys/ioctl.h>
 #include <sys/time.h>
 #include <time.h>
+#include <unistd.h>
 #include "alfred.h"
 
 int time_diff(struct timespec *tv1, struct timespec *tv2, @@ -80,11 +81,35
@@ bool is_valid_ether_addr(uint8_t addr[ETH_ALEN])
 	return true;
 }
 
+static void ipv4_request_mac_resolve(const alfred_addr *addr) {
+	const struct sockaddr *sockaddr;
+	struct sockaddr_in inet4;
+	size_t sockaddr_len;
+	int sock;
+	char t = 0;
+
+	sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+	if (sock < 0)
+		return;
+
+	memset(&inet4, 0, sizeof(inet4));
+	inet4.sin_family = AF_INET;
+	inet4.sin_port = htons(9);
+	inet4.sin_addr.s_addr = addr->ipv4.s_addr;
+	sockaddr = (const struct sockaddr *)&inet4;
+	sockaddr_len = sizeof(inet4);
+
+	sendto(sock, &t, sizeof(t), 0, sockaddr, sockaddr_len);
+	close(sock);
+}
+
 int ipv4_arp_request(struct interface *interface, const alfred_addr *addr,
 		     struct ether_addr *mac)
 {
 	struct arpreq arpreq;
 	struct sockaddr_in *sin;
+	int retries = 1;
 
 	memset(&arpreq, 0, sizeof(arpreq));
 	memset(mac, 0, ETH_ALEN);
@@ -96,8 +121,13 @@ int ipv4_arp_request(struct interface *interface, const
alfred_addr *addr,
 	strncpy(arpreq.arp_dev, interface->interface,
sizeof(arpreq.arp_dev));
 	arpreq.arp_dev[sizeof(arpreq.arp_dev) - 1] = '\0';
 
-	if (ioctl(interface->netsock, SIOCGARP, &arpreq) < 0)
-		return -1;
+	while ((ioctl(interface->netsock, SIOCGARP, &arpreq) < 0) ||
!(arpreq.arp_flags & ATF_COM)) {
+		ipv4_request_mac_resolve(addr);
+		usleep(200000);
+
+		if (retries-- == 0)
+			break;
+	}
 
 	if (arpreq.arp_flags & ATF_COM) {
 		memcpy(mac, arpreq.arp_ha.sa_data, sizeof(*mac));
--
2.17.1