networking is not working on qemu guest (Malta Mips)

Question

I am trying to configure networking on QEMU malta mips, which is running on vmware host (ubuntu) using tap/tun device and bridge interface. My qemu guest is unable to retrieve ip address from DHCP server. If i give it manually, it can just connect with its host. Using tcpdump i came to know that outgoing traffic is working perfectly but incoming is not working.

Can anyone suggest me how to solve this kind of issue? Thank You

Answer 1

If you use NAT mode, then your host machine will act as a router for your guest VM. This means you must enable routing on your host.

Assuming you start qemu and link it to tap0 interface and your outgoing internet interface is eth0, then you should:

1. Create the tap0 virtual interface:

   tunctl -t tap0
   ifconfig tap0 192.168.0.1 netmask 255.255.255.0 up
   

2. Activate routing

   # activate ip forwarding
   echo 1 > /proc/sys/net/ipv4/ip_forward
   
   # Create forwarding rules, where
   # tap0 - virtual interface
   # eth0 - net connected interface
   
   iptables -A FORWARD -i tap0 -o eth0 -j ACCEPT
   iptables -A FORWARD -i eth0 -o tap0 -m state --state ESTABLISHED,RELATED -j ACCEPT
   iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
   

3. Start your VM with somenthing like this:

   qemu [..] -net nic,model=e1000,vlan=0 -net tap,ifname=tap0,vlan=0,script=no
   

4. In your VM, configure an interface with ip 192.168.0.2/24 and default gateway 192.168.0.1