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Hello,
On Mon, 4 Feb 2013, Daniel Borkmann wrote:
> From: root Daniel Borkmann <dborkman@xxxxxxxxxx>
>
> In our test lab, we have a simple SCTP client connecting to a SCTP
> server via an IPVS load balancer. On some machines, load balancing
> works, but on others the initial handshake just fails, thus no
> SCTP connection whatsoever can be established.
>
> We observed that the SCTP INIT-ACK handshake reply from the IPVS
> machine to the client had a correct IP checksum, but corrupt SCTP
> checksum when forwarded, thus on the client-side the packet was
> dropped and an intial handshake retriggered until all attempts
> run into the void.
Hm, may be packet is received with CHECKSUM_PARTIAL
value? Is it a virtual NIC or a real NIC with the
forwarding problem?
> This patch fixes the problem by taking hardware offload features
> of the NIC into account (which was just ignored here), similar as
So, if NIC has no such support the bug remains?
Why it works for some boxes? Where is the difference in
the RX devices? GRO? skb->ip_summed?
> done in the SCTP checksumming code itself. Also, while at it,
> the checksum is in little-endian format (as fixed in commit 458f04c:
> sctp: Clean up sctp checksumming code). Tested by myself.
I don't feel such optimization can work in all cases.
skb->dev is NULL in LOCAL_OUT hook (NAT via loopback).
For dnat_handler skb->dev can be present but it is an old
value. The new skb->dst value is prepared by ip_vs_nat_xmit
but is still not set. For snat_handler the ip_vs_route_me_harder()
can reroute, for example, when traffic from different VIPs
use their own ISP link (another device).
Even if we provide the new dst to IPVS nat handlers,
later a netfilter rule can change the path and reroute the
packet to different device. As result, I'm not sure
CHECKSUM_PARTIAL is handled properly for SCTP in
dev_hard_start_xmit and skb_checksum_help for the
case where we are rerouted to device without hw csum
support. skb_checksum_help supports only csum in 16 bits.
Also, skb_transport_header is not updated yet,
IPVS runs before such protocols, i.e. in LOCAL_IN, while
ip_local_deliver_finish (where iphdr is skipped) is
called after all handlers in this hook. It is not
valid for FORWARD hook (for the snat_handler case).
Still, the sctphoff value is present and can be used
instead.
May be you can instead test a change that adds a
missing skb->ip_summed = CHECKSUM_UNNECESSARY in
both handlers, including a change for the
mentioned commit for endian format. May be crc32 should
be changed from __be32 to __u32 and using the
sctph->checksum = sctp_end_cksum(crc32) variant?
I hope it will fix the wrong checksum in forwarding path
if it is caused by the wrong ip_summed value.
> Cc: Venkata Mohan Reddy <mohanreddykv@xxxxxxxxx>
> Cc: Simon Horman <horms@xxxxxxxxxxxx>
> Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> ---
> net/netfilter/ipvs/ip_vs_proto_sctp.c | 41
> ++++++++++++++++++++---------------
> 1 file changed, 23 insertions(+), 18 deletions(-)
>
> diff --git a/net/netfilter/ipvs/ip_vs_proto_sctp.c
> b/net/netfilter/ipvs/ip_vs_proto_sctp.c
> index 746048b..dc41622 100644
> --- a/net/netfilter/ipvs/ip_vs_proto_sctp.c
> +++ b/net/netfilter/ipvs/ip_vs_proto_sctp.c
> @@ -61,14 +61,33 @@ sctp_conn_schedule(int af, struct sk_buff *skb, struct
> ip_vs_proto_data *pd,
> return 1;
> }
>
> +static void sctp_nat_csum(struct sk_buff *skb, sctp_sctphdr_t *sctph,
> + unsigned int sctphoff)
> +{
> + struct sk_buff *iter;
> +
> + /* Calculate the checksum */
> + if (!(skb->dev->features & NETIF_F_SCTP_CSUM)) {
> + __u32 crc32 = sctp_start_cksum((__u8 *)sctph,
> + skb_headlen(skb) - sctphoff);
> + skb_walk_frags(skb, iter)
> + crc32 = sctp_update_cksum((u8 *) iter->data,
> + skb_headlen(iter), crc32);
> + sctph->checksum = sctp_end_cksum(crc32);
> + } else {
> + /* no need to seed pseudo checksum for SCTP */
> + skb->ip_summed = CHECKSUM_PARTIAL;
> + skb->csum_start = (skb_transport_header(skb) - skb->head);
> + skb->csum_offset = offsetof(struct sctphdr, checksum);
> + }
> +}
> +
> static int
> sctp_snat_handler(struct sk_buff *skb, struct ip_vs_protocol *pp,
> struct ip_vs_conn *cp, struct ip_vs_iphdr *iph)
> {
> sctp_sctphdr_t *sctph;
> unsigned int sctphoff = iph->len;
> - struct sk_buff *iter;
> - __be32 crc32;
>
> #ifdef CONFIG_IP_VS_IPV6
> if (cp->af == AF_INET6 && iph->fragoffs)
> @@ -92,13 +111,7 @@ sctp_snat_handler(struct sk_buff *skb, struct
> ip_vs_protocol *pp,
> sctph = (void *) skb_network_header(skb) + sctphoff;
> sctph->source = cp->vport;
>
> - /* Calculate the checksum */
> - crc32 = sctp_start_cksum((u8 *) sctph, skb_headlen(skb) - sctphoff);
> - skb_walk_frags(skb, iter)
> - crc32 = sctp_update_cksum((u8 *) iter->data, skb_headlen(iter),
> - crc32);
> - crc32 = sctp_end_cksum(crc32);
> - sctph->checksum = crc32;
> + sctp_nat_csum(skb, sctph, sctphoff);
>
> return 1;
> }
> @@ -109,8 +122,6 @@ sctp_dnat_handler(struct sk_buff *skb, struct
> ip_vs_protocol *pp,
> {
> sctp_sctphdr_t *sctph;
> unsigned int sctphoff = iph->len;
> - struct sk_buff *iter;
> - __be32 crc32;
>
> #ifdef CONFIG_IP_VS_IPV6
> if (cp->af == AF_INET6 && iph->fragoffs)
> @@ -134,13 +145,7 @@ sctp_dnat_handler(struct sk_buff *skb, struct
> ip_vs_protocol *pp,
> sctph = (void *) skb_network_header(skb) + sctphoff;
> sctph->dest = cp->dport;
>
> - /* Calculate the checksum */
> - crc32 = sctp_start_cksum((u8 *) sctph, skb_headlen(skb) - sctphoff);
> - skb_walk_frags(skb, iter)
> - crc32 = sctp_update_cksum((u8 *) iter->data, skb_headlen(iter),
> - crc32);
> - crc32 = sctp_end_cksum(crc32);
> - sctph->checksum = crc32;
> + sctp_nat_csum(skb, sctph, sctphoff);
>
> return 1;
> }
> --
> 1.7.11.7
Regards
--
Julian Anastasov <ja@xxxxxx>
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