Re: [Qemu-devel] [PATCH 1/4] slirp: Fix restricted mode

[Gleb Natapov]

On Tue, May 24, 2011 at 02:42:55PM +0200, Jan Kiszka wrote:
> On 2011-05-24 14:37, Gleb Natapov wrote:
> > On Mon, May 23, 2011 at 04:48:16PM +0200, Jan Kiszka wrote:
> >> This aligns the code to what the documentation claims: Allow everything
> >> but requests that would have to be routed outside of the virtual LAN.
> >>
> >> So we need to drop the unneeded IP-level filter, allow TFTP requests,
> >> and add the missing protocol-level filter to ICMP.
> >>
> > May be I am missing something, but how do you disallow requests by
> > removing code that actually does filtering.
> 
> All we need to filter are the per-IP-protocol parts that do the
> forwarding via the host IP stack. That does not need to happen at IP level.
> 
> Moreover, the existing code contained some practically dead bits anyway:
> 
>         if ((ip->ip_dst.s_addr & slirp->vnetwork_mask.s_addr) ==
>             slirp->vnetwork_addr.s_addr) {
>             if (ip->ip_dst.s_addr == 0xffffffff && ip->ip_p !=
>                 IPPROTO_UDP)
>                 goto bad;
> 
> This could only trigger if vnetwork_mask.s_addr was 0 (the same applied
> to the original code before my refactoring in 2009).
> 
Not sure what do you mean by that. This checks that the ip_dst.s_addr is in
the vnetwork range. It does this by comparing net mask bits of ip_dst.s_addr 
with
vnetwork_addr.s_addr. Grep for vnetwork_mask.s_addr. This idiom is used
many times throughout the code.

--
                        Gleb.