[Qemu-discuss] bridged ICMP with no replies for tap0 on linux host

[Kate F]

Hi,

I'm bridging to tap0 on a linux host, and i can see pings arrive from
my VM (with tshark -i tap), but I see no ICMP reply.

I suspect this to be a problem with the host. I was using Debian
wheezy, with qemu (1.1.2+dfsg-6a+deb7u3). My configuration worked
there.

I upgraded to Jessie (qemu 2.1.2, Debian 2.1+dfsg-5), and that's when
my configuration stopped working. I've also tried installing the qemu
package I was using before (1.1.2+dfsg-6a+deb7u3) and my problem
persists.
So I believe this is not qemu's fault.

My VM is FreeBSD 10.0, amd64. Started thus:

  ; nice qemu-system-x86_64 -nographic -enable-kvm -m 768 -smp 2 \
      -hda freebsd.img -hdb swap.img -hdc home.img \
      -netdev tap,id=tap0,ifname=tap0,script=no,downscript=no \
          -device rtl8139,netdev=tap0

The host is also amd64. I gave tap0 an IP so that I can try to
diagnose this problem. Here's the traffic I see over tap0 on the host:

  ; ifconfig tap0
  tap0      Link encap:Ethernet  HWaddr aa:26:97:82:c6:f1
            inet addr:100.64.0.57  Bcast:100.64.0.255  Mask:255.255.255.0
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:523 errors:0 dropped:0 overruns:0 frame:0
            TX packets:18 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:500
            RX bytes:50062 (48.8 KiB)  TX bytes:1068 (1.0 KiB)
  ; tshark -i tap0
  Capturing on 'tap0'
    1   0.000000 RealtekU_12:34:57 -> Broadcast    ARP 42 Who has
100.64.0.57?  Tell 100.64.0.5
    2   0.000033 Apple_31:63:da -> RealtekU_12:34:57 ARP 42
100.64.0.57 is at 34:15:9e:31:63:da
    3   0.000351   100.64.0.5 -> 100.64.0.57  ICMP 98 Echo (ping)
request  id=0x1304, seq=0/0, ttl=64
    4   1.034739   100.64.0.5 -> 100.64.0.57  ICMP 98 Echo (ping)
request  id=0x1304, seq=1/256, ttl=64
    5   2.083052   100.64.0.5 -> 100.64.0.57  ICMP 98 Echo (ping)
request  id=0x1304, seq=2/512, ttl=64
    6   3.155970   100.64.0.5 -> 100.64.0.57  ICMP 98 Echo (ping)
request  id=0x1304, seq=3/768, ttl=64
    ...

And on the VM:

  # ifconfig re0
  re0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
          
options=8009b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,LINKSTATE>
          ether 52:54:00:12:34:56
          inet 100.64.0.5 netmask 0xffffff00 broadcast 100.64.0.255
          inet6 fe80::5054:ff:fe12:3456%re0 prefixlen 64 scopeid 0x1
          nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
          media: Ethernet autoselect (100baseTX <full-duplex>)
          status: active
  # ping 100.64.0.57
  PING 100.64.0.57 (100.64.0.57): 56 data bytes
  ^C
  --- 100.64.0.57 ping statistics ---
  4 packets transmitted, 0 packets received, 100.0% packet loss
  #
  # arp -a
  ? (100.64.0.5) at 52:54:00:12:34:56 on re0 permanent [ethernet]
  ? (100.64.0.57) at 34:15:9e:31:63:da on re0 expires in 1065 seconds [ethernet]
  #

The host has no firewall (as far as I'm aware!) and iptables accepts everything:

  # iptables -L
  Chain INPUT (policy ACCEPT)
  target     prot opt source               destination

  Chain FORWARD (policy ACCEPT)
  target     prot opt source               destination

  Chain OUTPUT (policy ACCEPT)
  target     prot opt source               destination
  #

And some sysctls on the host:

  net.ipv4.ip_forward = 1

  net.bridge.bridge-nf-call-ip6tables = 0
  net.bridge.bridge-nf-call-iptables = 0
  net.bridge.bridge-nf-call-arptables = 0

Now I'm at a loss for what to do next.

Why am I seeing no ICMP replies on the host?
Where can I look next in order to diagnose that?

Thanks,

-- 
Kate