Subject: Zero checksum in netconsole/netdump packets

From: Chris Lalancette <[email-masked]>
Date: Mon, 06 Nov 2006 18:40:59 -0500

Hello,
      I was reading some tcpdump's of netdump traffic today, and I realized that all of the packets that go from the crashing machine to the netdump server have a zero checksum.  Looking at the code, it looks like netconsole/netdump use the function netpoll_send_udp to send out the packets.  However, in netdump_send_udp, the checksum is set to 0, and never seems to be computed.  Is this intentional, or just an oversight?  I would think that we would always want to compute the UDP checksum, but there might be something I am overlooking.  Incidentally, it seems like the only user of netpoll_send_udp is netconsole (and netdump in RedHat kernels).
     Assuming that this is just an oversight, attached is a simple patch to compute the UDP checksum in netpoll_send_udp.

Signed-off-by: Chris Lalancette <[email-masked]>
--- linux-2.6/net/core/netpoll.c.orig	2006-11-06 18:16:58.000000000 -0500
+++ linux-2.6/net/core/netpoll.c	2006-11-06 18:31:20.000000000 -0500
@@ -356,6 +356,10 @@ void netpoll_send_udp(struct netpoll *np
 	put_unaligned(htonl(np->remote_ip), &(iph->daddr));
 	iph->check    = ip_fast_csum((unsigned char *)iph, iph->ihl);
 
+	udph->check = csum_tcpudp_magic(iph->saddr, iph->daddr, udp_len,
+					IPPROTO_UDP,
+					csum_partial((unsigned char *)udph, udp_len, 0));
+
 	eth = (struct ethhdr *) skb_push(skb, ETH_HLEN);
 	skb->mac.raw = skb->data;
 	skb->protocol = eth->h_proto = htons(ETH_P_IP);