Patch by Andrew Victor,

In arpping.h, fix structure alignment of "struct arpMsg".
GCC can insert padding in the structure which causes udhcpd to send an
invalid ARP packet on the network.  It will then not receive a valid
reply, which can cause it to assign an IP address that's already in use
on the network.
(With kernels before 2.4.20, the "struct ethhdr" in linux/if_ether.h
wasn't marked as packed.  This is also an issue if your toolchain was
built with a pre-2.4.20 kernel).

2 files changed, 10 insertions, 5 deletions

diff --git a/networking/udhcp/arpping.c b/networking/udhcp/arpping.c
index f2e3768c5..23c7d46b3 100644
--- a/networking/udhcp/arpping.c
+++ b/networking/udhcp/arpping.c
@@ -60,9 +60,9 @@ int arpping(uint32_t yiaddr, uint32_t ip, uint8_t *mac, char *interface)
 
 	/* send arp request */
 	memset(&arp, 0, sizeof(arp));
-	memcpy(arp.ethhdr.h_dest, MAC_BCAST_ADDR, 6);	/* MAC DA */
-	memcpy(arp.ethhdr.h_source, mac, 6);		/* MAC SA */
-	arp.ethhdr.h_proto = htons(ETH_P_ARP);		/* protocol type (Ethernet) */
+	memcpy(arp.h_dest, MAC_BCAST_ADDR, 6);		/* MAC DA */
+	memcpy(arp.h_source, mac, 6);			/* MAC SA */
+	arp.h_proto = htons(ETH_P_ARP);			/* protocol type (Ethernet) */
 	arp.htype = htons(ARPHRD_ETHER);		/* hardware type */
 	arp.ptype = htons(ETH_P_IP);			/* protocol type (ARP message) */
 	arp.hlen = 6;					/* hardware address length */
diff --git a/networking/udhcp/arpping.h b/networking/udhcp/arpping.h
index 99413aab6..6cbd2979f 100644
--- a/networking/udhcp/arpping.h
+++ b/networking/udhcp/arpping.h
@@ -11,7 +11,12 @@
 #include <netinet/in.h>
 
 struct arpMsg {
-	struct ethhdr ethhdr;	 		/* Ethernet header */
+	/* Ethernet header */
+	u_char   h_dest[6];			/* destination ether addr */
+	u_char   h_source[6];			/* source ether addr */
+	u_short  h_proto;			/* packet type ID field */
+	
+	/* ARP packet */
 	uint16_t htype;				/* hardware type (must be ARPHRD_ETHER) */
 	uint16_t ptype;				/* protocol type (must be ETH_P_IP) */
 	uint8_t  hlen;				/* hardware address length (must be 6) */
@@ -22,7 +27,7 @@ struct arpMsg {
 	uint8_t  tHaddr[6];			/* target's hardware address */
 	uint8_t  tInaddr[4];			/* target's IP address */
 	uint8_t  pad[18];			/* pad for min. Ethernet payload (60 bytes) */
-};
+} __attribute__ ((packed));
 
 /* function prototypes */
 int arpping(uint32_t yiaddr, uint32_t ip, uint8_t *arp, char *interface);