CentOS - Dec 2021 - Qemu - enabling "bridge mode" for primary physical interface for VMs

Once upon a time, Lists <lists at benjamindsmith.com>
said:
> I understand that it's possible to allow the 4 VM guest systems to each
have a
> "direct" fixed IP address and access the addresses \via the host
network
> adapter, while the host retains its fixed IP. 
If you are running NetworkManager (the default), it's not too hard.
Here's an example step-by-step for changing an existing interface
"em1" to
be a bridge "br0":


# Create a bridge interface
nmcli con add type bridge ifname br0 bridge.stp no

# Copy all the IPv4/IPv6 config from an existing interface
nmcli con mod bridge-br0 $(nmcli -f
ipv4.method,ipv4.addresses,ipv4.gateway,ipv6.method,ipv6.addresses,ipv6.gateway
con show em1 | grep -v -- -- | sed 's/:  */ /')
# -or- just set an IPv4 address/gateway to known values
nmcli con mod bridge-br0 ipv4.method manual ipv4.address 10.1.1.2/24
ipv4.gateway 10.1.1.1 ipv6.method ignore

# Make a connection for the physical ethernet em1 to be part of the bridge
nmcli con add type ethernet ifname em1 master bridge-br0

# Switch from the "regular" em1 to the bridge
nmcli con down em1; nmcli con up bridge-br0; nmcli con up bridge-slave-em1

# Disable the original config
nmcli con mod em1 autoconnect 0


Then you set your VMs to use the bridge - in the libvirt XML for
example, you'd have something like:

    <interface type='bridge'>
      <mac address='52:54:00:12:34:56'/>
      <source bridge='br0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00'
slot='0x03' function='0x0'/>
    </interface>


Inside the VM, configure the interface just as if it was a physical system
on that subnet.

-- 
Chris Adams <linux at cmadams.net>

Thank you, I'll be trying this on a spare machine here before I try it in 
production. Carefully reading the directions, although I see where bridge-br0 
is created, I don't see where bridge-slave-em1 is defined? 



On Tuesday, December 7, 2021 8:25:37 PM PST Chris Adams
wrote:
> Once upon a time, Lists <lists at benjamindsmith.com> said:
> > I understand that it's possible to allow the 4 VM guest systems to
each
> > have a "direct" fixed IP address and access the addresses
\via the host
> > network adapter, while the host retains its fixed IP.
> 
> If you are running NetworkManager (the default), it's not too hard.
> Here's an example step-by-step for changing an existing interface
"em1" to
> be a bridge "br0":
> 
> 
> # Create a bridge interface
> nmcli con add type bridge ifname br0 bridge.stp no
> 
> # Copy all the IPv4/IPv6 config from an existing interface
> nmcli con mod bridge-br0 $(nmcli -f
> ipv4.method,ipv4.addresses,ipv4.gateway,ipv6.method,ipv6.addresses,ipv6.gat
> eway con show em1 | grep -v -- -- | sed 's/:  */ /') # -or- just
set an IPv4
> address/gateway to known values
> nmcli con mod bridge-br0 ipv4.method manual ipv4.address 10.1.1.2/24
> ipv4.gateway 10.1.1.1 ipv6.method ignore
> 
> # Make a connection for the physical ethernet em1 to be part of the bridge
> nmcli con add type ethernet ifname em1 master bridge-br0
> 
> # Switch from the "regular" em1 to the bridge
> nmcli con down em1; nmcli con up bridge-br0; nmcli con up bridge-slave-em1
> 
> # Disable the original config
> nmcli con mod em1 autoconnect 0
> 
> 
> Then you set your VMs to use the bridge - in the libvirt XML for
> example, you'd have something like:
> 
>     <interface type='bridge'>
>       <mac address='52:54:00:12:34:56'/>
>       <source bridge='br0'/>
>       <model type='virtio'/>
>       <address type='pci' domain='0x0000'
bus='0x00' slot='0x03'
> function='0x0'/> </interface>
> 
> 
> Inside the VM, configure the interface just as if it was a physical system
> on that subnet.
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Once upon a time, Lists <lists at benjamindsmith.com>
said:
> Thank you, I'll be trying this on a spare machine here before I try it
in
> production. Carefully reading the directions, although I see where
bridge-br0
> is created, I don't see where bridge-slave-em1 is defined? 
This part:
> > # Make a connection for the physical ethernet em1 to be part of the
bridge
> > nmcli con add type ethernet ifname em1 master bridge-br0
does it.  If you don't specify a connection name, NM names a new bridge
member connection profile as "bridge-slave-<interface name>".

-- 
Chris Adams <linux at cmadams.net>

Thank you Chris Adams for excellent information! It worked, see below. 

On Tuesday, December 7, 2021 8:25:37 PM PST Chris Adams
wrote:
> Once upon a time, Lists <lists at benjamindsmith.com> said:
> > I understand that it's possible to allow the 4 VM guest systems to
each
> > have a "direct" fixed IP address and access the addresses
\via the host
> > network adapter, while the host retains its fixed IP.
> 
> If you are running NetworkManager (the default), it's not too hard.
> Here's an example step-by-step for changing an existing interface
"em1" to
> be a bridge "br0":
> 
> 
> # Create a bridge interface
> nmcli con add type bridge ifname br0 bridge.stp no
> 
> # Copy all the IPv4/IPv6 config from an existing interface
> nmcli con mod bridge-br0 $(nmcli -f
> ipv4.method,ipv4.addresses,ipv4.gateway,ipv6.method,ipv6.addresses,ipv6.gat
> eway con show em1 | grep -v -- -- | sed 's/:  */ /') # -or- just
set an IPv4
> address/gateway to known values
> nmcli con mod bridge-br0 ipv4.method manual ipv4.address 10.1.1.2/24
> ipv4.gateway 10.1.1.1 ipv6.method ignore
> 
> # Make a connection for the physical ethernet em1 to be part of the bridge
> nmcli con add type ethernet ifname em1 master bridge-br0
> 
> # Switch from the "regular" em1 to the bridge
> nmcli con down em1; nmcli con up bridge-br0; nmcli con up bridge-slave-em1
> 
> # Disable the original config
> nmcli con mod em1 autoconnect 0
> 
> 
> Then you set your VMs to use the bridge - in the libvirt XML for
> example, you'd have something like:
> 
>     <interface type='bridge'>
>       <mac address='52:54:00:12:34:56'/>
>       <source bridge='br0'/>
>       <model type='virtio'/>
>       <address type='pci' domain='0x0000'
bus='0x00' slot='0x03'
> function='0x0'/> </interface>
> 
> 
> Inside the VM, configure the interface just as if it was a physical system
> on that subnet.
I just wanted to say that this worked, exactly as shown, and after trying it 
once on development hardward, I tried it on a production server and it also 
worked with minimal hassle. 

I ran into an oddity because the software image of the production server was 
from a disk image, not a fresh install and I had to wrestle with some
"Network
Adapters" that no longer really existed. My advice to anybody trying this 
would be to go into Network Manager and ensure the following before starting: 

1) Set up the server to manual IP addresses, not DHCP. 
2) Remove all duplicitous or inactive network adapters and interfaces. 
3) Make sure that in nmtui, that all editable connection(s) also show in the 
interface that lets you activate them.
4) Make sure you are not dependent on the network adapter you are trying to 
use to contact the server! In my case I ended up falling back to IPMI. Minor 
issue because I had it available, could have been far worse if I hadn't. 

... and then do the above. 

Ben S
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