I realized that the path without:
<parameter name='CTRL_IP_LEARNING' value='dhcp'/>
works but is totally different.
It uses pcap_open_live
So I compared our code in virNWFilterSnoopDHCPOpen with pcap_open_live.
We use different buffer sizes and don't set promisc and timeout.
But the minimal change I found that seems to make it work was:
--- a/src/nwfilter/nwfilter_dhcpsnoop.c
+++ b/src/nwfilter/nwfilter_dhcpsnoop.c
@@ -1115,7 +1115,6 @@ virNWFilterSnoopDHCPOpen(const char *ifname, virMacAddr
*mac,
}
if (pcap_set_snaplen(handle, PCAP_PBUFSIZE) < 0 ||
- pcap_set_buffer_size(handle, PCAP_BUFFERSIZE) < 0 ||
pcap_activate(handle) < 0) {
virReportError(VIR_ERR_INTERNAL_ERROR,
_("setup of pcap handle failed: %s"),
The size for this was defined as:
/*
* libpcap 1.5 requires a 128kb buffer
* 128 kb is bigger than (DHCP_PKT_BURST * PCAP_PBUFSIZE / 2)
*/
# define PCAP_BUFFERSIZE (128 * 1024)
This is from [1], does all that from a libpcap experts POV make sense?
Would it be reasonable to drop this call these days or change the size?
[1]:
https://libvirt.org/git/?p=libvirt.git;a=commit;h=49b59a151f60b0a178b023b727bac30f80bd6000
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https://bugs.launchpad.net/bugs/1758037
Title:
LTC Test- Ubuntu18.04: Starting the guest with network filter defined
will fail with "cause is unknown".
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