[Snort-devel] Flags keyword still doesn't treat rserved bits as ECE and CWR

Joel Esler jesler at ...402...
Mon Apr 11 21:55:38 EDT 2011


We have this slated for fix in the next release.

On Monday, April 11, 2011,  <Joshua.Kinard at ...3108...> wrote:
>
> Hi snort-devel,
>
> Back in the December timeframe, I sent in a patch to relabel '1' and '2'
> to become 'C' and 'E' in the 'flags', as these bits are now official,
> per RFC 3168.
>
> In the ChangeLog for snort-2.9.0.4, I see this:
>
>   * src/detection-plugins/sp_tcp_flag_check.c:
>     Changed the reserved bits flags "1, 2" to "C, E". The old values can
> still
>     be used for backwards compatability.
>
> Yet, as of snort-2.9.0.5, if I look in
> src/detection-plugins/sp_tcp_flag_check.c::ParseTCPFlags(), I see only
> this:
>
>             case '1': /* reserved bit flags */
>                 idx->tcp_flags |= R_RES1;
>                 break;
>
>             case '2': /* reserved bit flags */
>                 idx->tcp_flags |= R_RES2;
>                 break;
>
> The the patch I submitted should have changed that area to consider 'C'
> and 'E' (while keeping '1' and '2' as well).  The manual's TeX code was
> also not updated.
>
> Was this patch missed by accident?
>
>
> It seems one additional bit of the reserved field in the TCP header has
> a use now as the 'NONCE', or 'N', flag.  I see references to it in the
> Snort source, but uncertain of how well supported it is.  Flags does not
> currently check for this bit.  Is this of interest?  Might the remaining
> two reserved bits be worth checking incase they contain invalid bits
> (kind of like fragbits checking the 'R' or "evil bit")?
>
> Also curious, I see references to TCP options, such as SACK, MSS, etc,
> but there does not appear to be a dedicated rule option to parsing and
> checking the TCP options field.  Has this ever been considered?  I know
> it's a fairly complicated field from looking it up (SACK especially
> deals with some variable data), so checking this might need multiple new
> rule options.
>
> Cheers!,
>
> --J
>
>
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-- 
Joel Esler | http://blog.snort.org | http://vrt-blog.snort.org |
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