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

[Christian Ehrhardt]

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)
>   */
>

I just started building with the change for a few tests on this - no
results yet.

But we are all puzzled/unsure enough on the size that I'd already ask to
modify the comment above to explain the new size.

Maybe we should explain:
- why 128 isn't enough
- why you chose "only" 256
- why the default size might be too big
- your size considerations for many guest scenarios

That will help the next one stumbling over this code.



> -# define PCAP_BUFFERSIZE        (128 * 1024)
> +# define PCAP_BUFFERSIZE        (256 * 1024)
>
>  # define MAX_QUEUED_JOBS        (DHCP_PKT_BURST + 2 * DHCP_PKT_RATE)
>
> --
> 2.14.3
>
> --
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>



-- 
Christian Ehrhardt
Software Engineer, Ubuntu Server
Canonical Ltd
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