gdhcp: Allow client to work on veth devices

veth devices do not compute a checksum over the entire message.  They
simply pre-compute the pseudo-header portion and include this in the
checksum field.  Support such devices by computing the partial checksum.
If the partial checksum matches, accept the packet.

1 files changed, 7 insertions, 2 deletions

diff --git a/gdhcp/client.c b/gdhcp/client.c
index c58c7be91..2afa19e6e 100644
--- a/gdhcp/client.c
+++ b/gdhcp/client.c
@@ -1355,8 +1355,13 @@ static int dhcp_recv_l2_packet(struct dhcp_packet *dhcp_pkt, int fd,
 	packet.ip.tot_len = packet.udp.len; /* yes, this is needed */
 	check = packet.udp.check;
 	packet.udp.check = 0;
-	if (check && check != dhcp_checksum(&packet, bytes))
-		return -1;
+
+	if (check) {
+		uint16_t partial = ~dhcp_checksum(&packet, sizeof(struct iphdr));
+
+		if (check != partial && check != dhcp_checksum(&packet, bytes))
+			return -1;
+	}
 
 	memcpy(dhcp_pkt, &packet.data, bytes - (sizeof(packet.ip) +
 							sizeof(packet.udp)));