Re: [Qemu-discuss] Wget working but ping is not

[Mike Lovell]

On 08/10/2012 05:26 AM, Ottavio Caruso wrote:
> On 10 August 2012 12:13, Jerzy Grzegorek <address@hidden> wrote:
> > I built a little linux system for arm versatilepb machine, based on Busybox.
> > I've run it on qemu and after booting I've launched udhcpc. My host is
> > connecting to internet. Wget is working fine but ping is working only with
> > localnet.
> >
> > I used kernel 3.2.26 and qemu 1.1.1 (I tried 0.15 and 1.0 versions tool)
> >
> >
> >
> > sudo qemu-system-arm -M versatilepb -m 128M -nographic \
> >    -kernel uImage-3.2.26-initramfs \
> >    -append "console=ttyAMA0 ip=10.0.2.15:10.0.2.2:10.0.2.2" \
> >    -net nic,model=smc91c111 -net nic,vlan=0 -net user,vlan=0
> This is a limitation of Qemu is you use "-net user". I think you can
> ping outside the virtual Qemu network only with tunnelled or bridged
> networking, but I have never bothered trying.

yeah. this is a limitation of the user network backend. the problem is 
that the IP stack that the user backend uses doesn't support ICMP. all 
the network backends for qemu pass ethernet frames back and forth. the 
user backend, in addition to passing ethernet frames between the guest 
and the qemu process, also sets up a IP stack based on SLIRP and 
provides some basic IP services such as NAT, DHCP, and an SMB server. 
since this IP stack is somewhat limited, ping doesn't work and there 
isn't support for other things like ipv6.

the other network backends should work as expected. they just pass the 
ethernet frames between the guest and the other end of the network 
backend, such as a tap device. these network backend aren't looking at 
the IP layer at all and don't care about it. you could use a networking 
protocol other than IP when using them as well.

hope that adds some clarification.

mike