On Wed, Apr 25, 2018 at 11:25 PM, Laine Stump <laine@xxxxxxxxx> 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@xxxxxxxxx> for discovering that
buffer size was the problem.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1547237
Signed-off-by: Laine Stump <laine@xxxxxxxxx>
---
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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