[libvirt] [PATCH] nwfilter: increase pcap buffer size to be compatible with TPACKET_V3

[Christian Ehrhardt]

On Thu, Apr 26, 2018 at 8:09 AM, Christian Ehrhardt <
christian.ehrhardt at canonical.com> wrote:

>
>
> On Wed, Apr 25, 2018 at 11:25 PM, Laine Stump <laine at laine.org> wrote:
>
>> When an nwfilter rule sets the parameter CTRL_IP_LEARNING to "dhcp",
>> this turns on the "dhcpsnoop" thread, which uses libpcap to monitor
>> traffic on the domain's tap device and extract the IP address from the
>> DHCP response.
>>
>> If libpcap on the host is built with TPACKET_V3 defined, the dhcpsnoop
>> code's initialization of the libpcap socket fails with the following
>> error:
>>
>>   virNWFilterSnoopDHCPOpen:1134 : internal error: pcap_setfilter: can't
>> remove kernel filter: Bad file descriptor
>>
>> It turns out that this was because libpcap with TPACKET_V3 defined
>> requires a larger buffer size than libvirt was setting (we were
>> setting it to 128k). Changing the buffer size to 256k eliminates the
>> error, and the dhcpsnoop thread once again works properly.
>>
>> Thanks to Christian Ehrhardt <paelzer at gmail.com> for discovering that
>> buffer size was the problem.
>>
>> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1547237
>> Signed-off-by: Laine Stump <laine at laine.org>
>> ---
>>  src/nwfilter/nwfilter_dhcpsnoop.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/src/nwfilter/nwfilter_dhcpsnoop.c
>> b/src/nwfilter/nwfilter_dhcpsnoop.c
>> index 6069e70460..62eb617515 100644
>> --- a/src/nwfilter/nwfilter_dhcpsnoop.c
>> +++ b/src/nwfilter/nwfilter_dhcpsnoop.c
>> @@ -259,7 +259,7 @@ struct _virNWFilterDHCPDecodeJob {
>>   * libpcap 1.5 requires a 128kb buffer
>>   * 128 kb is bigger than (DHCP_PKT_BURST * PCAP_PBUFSIZE / 2)
>>   */
>
>
Tests completed and ok for my small testing scope of these cases:
  Tested-by: Christian Ehrhardt <christian.ehrhardt at canonical.com>

Once you updated the comment as outlined before feel free to also add
  Reviewed-by: Christian Ehrhardt <christian.ehrhardt at canonical.com>

Could you when rewriting also add this line (not required, just if you
amend anyway):
  Fixes: https://bugs.launchpad.net/libvirt/+bug/1758037
I recently see more and more Resolves: instead of "Fixes:" did we change
the recommended format for some tools and I missed it?
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