Zero checksum in netconsole/netdump packets
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 PROTECTED]> --- linux-2.6/net/core/netpoll.c.orig 2006-11-06 18:16:58.0 -0500 +++ linux-2.6/net/core/netpoll.c 2006-11-06 18:31:20.0 -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);
Re: Zero checksum in netconsole/netdump packets
From: Chris Lalancette <[EMAIL PROTECTED]> Date: Mon, 06 Nov 2006 18:40:59 -0500 > Assuming that this is just an oversight, attached is a simple > patch to compute the UDP checksum in netpoll_send_udp. If the resulting checksum is zero, you should set it to all 1's, like the real UDP code does. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Zero checksum in netconsole/netdump packets
Quoting Chris Lalancette: | Hello, | I realized that all of the packets that go from the crashing machine to the netdump server have a zero checksum. | 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 PROTECTED]> | RFC 768 allows to not compute the checksum by leaving uh->check at 0 - hence it is not illegal. But without David's suggestion the code is not valid, since otherwise there is no way of distinguishing a computed `0' from an ignored `0' field: if ( udph->check == 0 ) udph->check = -1; - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Zero checksum in netconsole/netdump packets
On Tue, 7 Nov 2006, Gerrit Renker wrote: Quoting Chris Lalancette: | Hello, | I realized that all of the packets that go from the crashing machine to the netdump server have a zero checksum. | 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 PROTECTED]> | RFC 768 allows to not compute the checksum by leaving uh->check at 0 - hence it is not illegal. BTW: leaving UDP checksum at 0 is only valid for IPv4, with IPv6 we _have to_ compute a checksum. Best regards, Krzysztof Olędzki
Re: Zero checksum in netconsole/netdump packets
David Miller wrote: From: Chris Lalancette <[EMAIL PROTECTED]> Date: Mon, 06 Nov 2006 18:40:59 -0500 Assuming that this is just an oversight, attached is a simple patch to compute the UDP checksum in netpoll_send_udp. If the resulting checksum is zero, you should set it to all 1's, like the real UDP code does. David, Ah, thanks. Forgot about that. I re-spun the patch with the change (attached). I also moved the UDP checksum calculation up to where the rest of the UDP header setup is, to make it more consistent. Thanks again for the comments! Signed-off-by: Chris Lalancette <[EMAIL PROTECTED]> --- linux-2.6/net/core/netpoll.c.orig 2006-11-06 18:16:58.0 -0500 +++ linux-2.6/net/core/netpoll.c 2006-11-07 08:16:29.0 -0500 @@ -340,6 +340,12 @@ void netpoll_send_udp(struct netpoll *np udph->dest = htons(np->remote_port); udph->len = htons(udp_len); udph->check = 0; + udph->check = csum_tcpudp_magic(htonl(np->local_ip), + htonl(np->remote_ip), + udp_len, IPPROTO_UDP, + csum_partial((unsigned char *)udph, udp_len, 0)); + if (udph->check == 0) + udph->check = -1; skb->nh.iph = iph = (struct iphdr *)skb_push(skb, sizeof(*iph));
Re: Zero checksum in netconsole/netdump packets
From: Chris Lalancette <[EMAIL PROTECTED]> Date: Tue, 07 Nov 2006 08:33:00 -0500 > David Miller wrote: > > From: Chris Lalancette <[EMAIL PROTECTED]> > > Date: Mon, 06 Nov 2006 18:40:59 -0500 > > > >> Assuming that this is just an oversight, attached is a simple > >> patch to compute the UDP checksum in netpoll_send_udp. > > > > If the resulting checksum is zero, you should set it to > > all 1's, like the real UDP code does. > > David, > Ah, thanks. Forgot about that. I re-spun the patch with the change > (attached). I also moved the UDP checksum calculation up to where the rest > of the UDP header setup is, to make it more consistent. > > Thanks again for the comments! > > Signed-off-by: Chris Lalancette <[EMAIL PROTECTED]> Looks good, applied, thanks Chris. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
