[dpdk-dev] [PATCH 2/2] app/testpmd: compute checksum in icmpecho replies
Olivier MATZ
olivier.matz at 6wind.com
Fri May 22 14:42:08 CEST 2015
Hi Adrien,
On 05/22/2015 01:07 PM, Adrien Mazarguil wrote:
> ICMP echo replies with invalid checksums may be dropped by network nodes or
> ignored by the ping utility.
>
> Signed-off-by: Adrien Mazarguil <adrien.mazarguil at 6wind.com>
> Acked-by: Ivan Boule <ivan.boule at 6wind.com>
> ---
> app/test-pmd/icmpecho.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/app/test-pmd/icmpecho.c b/app/test-pmd/icmpecho.c
> index c5933f4..c278baf 100644
> --- a/app/test-pmd/icmpecho.c
> +++ b/app/test-pmd/icmpecho.c
> @@ -445,7 +445,8 @@ reply_to_icmp_echo_rqsts(struct fwd_stream *fs)
> * - switch IPv4 source and destinations addresses,
> * - set IP_ICMP_ECHO_REPLY in ICMP header.
> * No need to re-compute the IP header checksum.
> - * Reset ICMP checksum.
> + * ICMP checksum is computed by assuming it is valid in the
> + * echo request and not verified.
> */
> ether_addr_copy(ð_h->s_addr, ð_addr);
> ether_addr_copy(ð_h->d_addr, ð_h->s_addr);
> @@ -454,7 +455,7 @@ reply_to_icmp_echo_rqsts(struct fwd_stream *fs)
> ip_h->src_addr = ip_h->dst_addr;
> ip_h->dst_addr = ip_addr;
> icmp_h->icmp_type = IP_ICMP_ECHO_REPLY;
> - icmp_h->icmp_cksum = 0;
> + icmp_h->icmp_cksum += htons(IP_ICMP_ECHO_REQUEST << 8);
I don't think this checksum calculation method is always correct.
Example of a working case:
>>> p=Ether(str(Ether()/IP()/ICMP(seq=0)))
>>> p
<Ether dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
type=echo-request code=0 chksum=0xf7ff id=0x0 seq=0x0 |>>>
>>> del(p[ICMP].chksum)
>>> p[ICMP].type = 0
>>> p=Ether(str(p))
>>> p
<Ether dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
type=echo-reply code=0 chksum=0xffff id=0x0 seq=0x0 |>>>
We have 0xffff - 0xf7ff = 0x0800
Example of a non-working case:
>>> p=Ether(str(Ether()/IP()/ICMP(seq=0xf800)))
>>> p
<Ether dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
type=echo-request code=0 chksum=0xfffe id=0x0 seq=0xf800 |>>>
>>> del(p[ICMP].chksum)
>>> p[ICMP].type = 0
>>> p=Ether(str(p))
>>> p
<Ether dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
type=echo-reply code=0 chksum=0x7ff id=0x0 seq=0xf800 |>>>
Here, 0x7ff - 0xfffe != 0x08000
A correct solution (not tested) would be something like:
uint32_t cksum;
cksum = (~(icmp_h->icmp_cksum)) & 0xffff;
cksum += (~htons(IP_ICMP_ECHO_REQUEST << 8)) & 0xffff;
cksum += (htons(IP_ICMP_ECHO_REPLY << 8)) & 0xffff;
cksum = (cksum & 0xffff) + (cksum >> 16);
cksum = (cksum & 0xffff) + (cksum >> 16);
icmp_h->icmp_cksum = (~cksum) & 0xffff;
Regards,
Olivier
> pkts_burst[nb_replies++] = pkt;
> }
>
>
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