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

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

  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 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 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)
-# define PCAP_BUFFERSIZE        (128 * 1024)
+# define PCAP_BUFFERSIZE        (256 * 1024)

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