# Multicast forwarding for Bridge? (KVM)

## Caprisun

Hello,

I have recently begun a new amd64 build of Gentoo for a KVM Host.

Using the following config for net.br0 under /etc/conf.d/net:

```
#config_eth0="192.168.1.113 netmask 255.255.255.0 brd 192.168.1.255"

#routes_eth0="default via 192.168.1.1"

# Ethernet Bridge br0

bridge_br0="eth0"

brctl_br0="setfd 0 sethello 0 stp off"

config_br0="192.168.1.113/24"

routes_br0="default via 192.168.1.1"

# Ethernet Dummy

config_eth0="null"

```

Windows 2k3 Server and Gentoo guests can get online and perform updates etc.

However the DHCP Service on Windows seems to be deaf...

ipv4 forwarding ==1 

mc_forwarding is == 0, with denied permissions when I attempted to change via sysctl and manual manipulation.

I have multicasting options set in the kernel as built in.

```
KVM linux # more .config | grep MULTI

CONFIG_NEED_MULTIPLE_NODES=y

CONFIG_IP_MULTICAST=y

CONFIG_IP_MULTIPLE_TABLES=y

CONFIG_IP_ROUTE_MULTIPATH=y

CONFIG_IP_MROUTE_MULTIPLE_TABLES=y

# CONFIG_IPV6_MULTIPLE_TABLES is not set

# CONFIG_NET_SCH_MULTIQ is not set

# CONFIG_SCSI_MULTI_LUN is not set

CONFIG_DEVPTS_MULTIPLE_INSTANCES=y

# CONFIG_HID_MULTITOUCH is not set

```

I would greatly appreciate assistance, I'm building this setup with a diskless Dell 620 to showcase KVM for virtualized environments using diskless systems. KVM is using an Intel SSD with ext4.

It seems like this is the only traffic not behaving normally. I'm not using iptables or any other firewall.

The KVM Guest's are using Virtio drivers connected to br0. I notice that libvirt creates a vnet* interface for each one and joins it to br0.

```
KVM linux # brctl show

bridge name   bridge id      STP enabled   interfaces

br0      8000.bc305bde899c   no      eth0

                     vnet0

                     vnet1
```

----------

## _______0

use a milticast mac address.

from wikipedia:

 *Quote:*   

> Ethernet
> 
> Ethernet frames with a value of 1 in the least-significant bit of the first octet[note 2] of the destination address are treated as multicast frames and are flooded to all points on the network. While frames with ones in all bits of the destination address (FF:FF:FF:FF:FF:FF) are sometimes referred to as broadcasts, Ethernet network equipment generally does not distinguish between multicast and broadcast frames. Modern Ethernet controllers filter received packets to reduce CPU load, by looking up the hash of a multicast destination address in a table, initialized by software, which controls whether a multicast packet is dropped or fully received.

 

----------

## Caprisun

 *

----------

## _______0

what's your set up like?

router -> machine with br -> guests??

probably use dhcp on the bridge. did you double check if you have a dhcp package installed?

----------

## Caprisun

 *

----------

## Caprisun

 *Caprisun wrote:*   

>  *

----------

## Caprisun

After some configuring, the Gentoo Guest VM is handing out DHCP addresses normally.

I must assume something was wrong with the Windows Server...

----------

