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I just set up an ARM64 image using QEMU and I'm trying to access the internet from within the binary. The host OS(Ubuntu) receives internet from the eth0 interface. I have the tap0 interface set up. I've added tap0 and eth0 to virbr0.

$ brctl show virbr0
bridge name bridge id       STP enabled interfaces
virbr0      8000.080027c9997e   yes     eth0
                                        tap0

Now, I try to start the VM as follows :-

sudo qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic -smp 1 -m 512 \
    -global virtio-blk-device.scsi=off -device virtio-scsi-device,id=scsi \
    -drive file=ubuntu-core-14.04.1-core-arm64.img,id=coreimg,cache=unsafe,if=none -device scsi-hd,drive=coreimg \
    -kernel vmlinuz-3.13.0-55-generic \
    -initrd initrd.img-3.13.0-55-generic \
    -netdev user,id=unet -device virtio-net-device,netdev=unet \
    -net nic -net tap,ifname=tap0,script=no,downscript=no \
    --append "console=ttyAMA0 root=/dev/sda"

The output of ifconfig on host is as follows :-

$ ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:c9:99:7e
          inet6 addr: fe80::a00:27ff:fec9:997e/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2942 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7889 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:239227 (239.2 KB)  TX bytes:557156 (557.1 KB)

tap0      Link encap:Ethernet  HWaddr 8e:3d:c3:0e:58:36
          inet6 addr: fe80::8c3d:c3ff:fe0e:5836/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:412 errors:0 dropped:427 overruns:0 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:0 (0.0 B)  TX bytes:21432 (21.4 KB)

virbr0    Link encap:Ethernet  HWaddr 08:00:27:c9:99:7e
          inet addr:10.0.2.15  Bcast:0.0.0.0  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2928 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2940 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:195163 (195.1 KB)  TX bytes:236587 (236.5 KB)

Once I start the VM, the output of ifconfig is as follows.

root@localhost:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 52:54:00:12:34:57
          inet addr:10.0.2.15  Bcast:10.0.2.255  Mask:255.255.255.0
          inet6 addr: fe80::5054:ff:fe12:3457/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:5 errors:0 dropped:0 overruns:0 frame:0
          TX packets:38 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:846 (846.0 B)  TX bytes:3320 (3.3 KB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

However, I don't seem to be able to ping any remote addresses(eg: 8.8.8.8). Why does this happen? How can I fix this?

[EDIT] When starting the VM I get the following warnings.

W: /etc/qemu-ifup: no bridge for guest interface found
Warning: hub port hub0port0 has no peer
Warning: vlan 0 with no nics
Warning: netdev hub0port0 has no peer
Warning: requested NIC (anonymous, model unspecified) was not created (not supported by this machine?)
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Your virbr0 uses 10.0.2.25 as an address, that is private. So what's your route to 8.8.8.8? – ikrabbe Jul 5 '15 at 21:53
    
@ikrabbe Sorry, I'm not sure I understand. I have not added in any routes using iptables. – user3655 Jul 5 '15 at 21:55
    
When you have an interface on your host with an address 10.0.2.25 and nothing else, how is this private IP routed to access the internet? Show us your ip route on the host, would be a first approach. It also seems that your virtual host through tap0 has the same address as the virbr0. How do you ping from the host to the client and vice versa? – ikrabbe Jul 6 '15 at 6:13
    
You need -net nic -net tap,ifname=tap0 not -net user. You'd use the user stuff if you were not configuring your own bridge. – mikeserv Jul 6 '15 at 6:22
    
@mikeserv I'm not using "-net user". I've tried using both "-netdev user" and "-netdev tap". Is that what you meant? – user3655 Jul 6 '15 at 9:50
up vote 1 down vote

I was getting same warnings while I used the obsolete -net option. Then I changed it to this:

-netdev tap,ifname=tap0,script=no,downscript=no,id=my_net_id \
-device driver=virtio-net,netdev=my_net_id

It fixed the problem.

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