Re: Warning on enabling ip6 protochain 6

[ramkumar p <ramkumar.paranandi () gmail com>]

Hi,

Thanks good info.

If we specify "ip6 tcp port 25 " does this also filter the traffic with IPv6
and extension headers like Routing, Fragment,hop and destination options
etc...  and tcp port 25  or it filters only ipv6 traffic without extension
headers and tcp port 25

Thanks,
./Ram


On Thu, Aug 4, 2011 at 7:46 PM, Guy Harris <guy () alum mit edu> wrote:

> On Aug 2, 2011, at 4:42 PM, ramkumar p wrote:
>
> > I am receiving warning that kernel filter failed: invalid argument  when
>  I enable ip6 protochain 6 to filter tcp traffic.
>
> That warning means that the filter code generated for "ip6 protochain 6"
> was rejected by the Linux kernel socket filter code.  It's only a warning;
> libpcap will do the filtering in user mode (without any filtering being done
> in the kernel, so there is a greater risk of dropped packets).
>
> The kernel is correct to reject the filter code.  "ip6 protochain" requires
> parsing through an indefinite number of extension headers, so the BPF code
> has a loop.  No kernel implementation of BPF I know of (*BSD, Mac OS X,
> Linux, WinPcap, Tru64 UNIX, etc.) allows BPF programs that loop, as, if
> somebody has sufficient privilege to give the kernel a filter program (and
> often systems are set up to allow at least some users to capture network
> traffic), that would give them sufficient privilege to make at least one
> kernel thread or process or interrupt routine loop infinitely; the intent of
> BPF is to allow only "safe" filters to be given to the kernel.
>
> That could, in theory, be fixed - for example, BSD/OS's BPF interpreter had
> an instruction that would do IPv6 extension header parsing - but the Linux
> kernel's BPF interpreter doesn't have any instructions to handle that, and
> nobody's implemented, for example, the proof-carrying code mechanism
> mentioned in the BPF+ paper mentioned on the Related Projects page:
>
>        http://www.tcpdump.org/related.html
>
> to allow the kernel to determine whether a given looping BPF program is
> "safe".-
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