From 3f571bdb274ad66d2a7b041f28e4a760e7be9a86 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juhani=20Krekel=C3=A4?= <juhani@krekelä.fi>
Date: Wed, 3 Jul 2019 23:09:14 +0300
Subject: [PATCH] Specify interface on the command line

---
 ethertype-dump.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/ethertype-dump.c b/ethertype-dump.c
index 4e9b528..703d5c8 100644
--- a/ethertype-dump.c
+++ b/ethertype-dump.c
@@ -7,6 +7,7 @@
 #include <net/if.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
@@ -28,7 +29,12 @@ void format_mac(const unsigned char binary_address[6], char formatted[18]) {
 	formatted[17] = '\0';
 }
 
-int main(void) {
+int main(int argc, char **argv) {
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s interface\n", argv[0]);
+		exit(1);
+	}
+
 	// Create a packet socket
 	errno = 0;
 	int packet_socket = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
@@ -38,7 +44,7 @@ int main(void) {
 
 	// Find the index of the network interface
 	struct ifreq ifr;
-	strncpy(ifr.ifr_name, "wlan0", IFNAMSIZ);
+	strncpy(ifr.ifr_name, argv[1], IFNAMSIZ);
 	errno = 0;
 	if (ioctl(packet_socket, SIOCGIFINDEX, &ifr) == -1) {
 		err(1, "ioctl");