libvirt users - Mar 2014 - Questions on using bridge for KVM on ubuntu 13.10

Hi,

I have being try to connect a virtual machine to the outside through a
bridge on the host.

The following shows that I have a bridge. And I'm able to start a guest.

pengy@rigel:~$ brctl show
bridge name    bridge id        STP enabled    interfaces
br0        8000.c81f66e29049    no        em4
br1        8000.c81f66e29045    no        bond0
virbr0        8000.000000000000    yes

pengy@rigel:/mnt/nfs/pengy/kvm$ virsh create saiph_installed.xml
Domain saiph created from saiph_installed.xml


pengy@rigel:/mnt/nfs/pengy/kvm$ brctl show
bridge name    bridge id        STP enabled    interfaces
br0        8000.c81f66e29049    no        em4
                            vnet0
br1        8000.c81f66e29045    no        bond0
virbr0        8000.000000000000    yes

I assigned the static ip address (given my our network administrator)
to the guest. But I am not able to connect anywhere outside from the
guest (175.91.244.74). Do you know what might cause the problem? How
to connect the outside network from the guest? Thanks.

pengy@rigel:/mnt/nfs/pengy/kvm$ ssh 175.91.244.74
pengy@175.91.244.74's password:
Welcome to Ubuntu 13.10 (GNU/Linux 3.11.0-12-generic x86_64)

 * Documentation:  https://help.ubuntu.com/

  System information as of Tue Mar 11 14:04:07 CDT 2014

  System load:  0.0              Processes:           76
  Usage of /:   78.0% of 999MB   Users logged in:     0
  Memory usage: 1%               IP address for eth0: 175.91.244.74
  Swap usage:   0%

  Graph this data and manage this system at:
    https://landscape.canonical.com/

0 packages can be updated.
0 updates are security updates.

Last login: Tue Mar 11 14:04:07 2014
pengy@saiph:~$ ping www.google.com
^C
pengy@saiph:~$ ifconfig
eth0      Link encap:Ethernet  HWaddr 52:54:aa:00:f0:51
          inet addr:175.91.244.74  Bcast:175.91.247.255  Mask:255.255.248.0
          inet6 addr: fe80::5054:aaff:fe00:f051/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:6720 errors:0 dropped:24 overruns:0 frame:0
          TX packets:213 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:513702 (513.7 KB)  TX bytes:20976 (20.9 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)


BTW, I'm albe to follow some of the instructions at

http://wiki.libvirt.org/page/Networking#Debian.2FUbuntu_Bridging

But it is not clear to what the followings are for.

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


 *** Sample rc.local file ***
 /sbin/sysctl -p /etc/sysctl.conf
 iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS
--clamp-mss-to-pmtu
 exit 0

To verify that the changes have taken affect, please run

 tail /proc/sys/net/bridge/*
 iptables -L  FORWARD





-- 
Regards,
Peng