diff options
| author | Jeremy Sowden <jeremy@azazel.net> | 2023-06-11 12:34:29 +0100 |
|---|---|---|
| committer | Phil Sutter <phil@nwl.cc> | 2023-06-16 13:31:25 +0200 |
| commit | 3513f2dd8b9c101dcecee6145a7eac62297ca1c8 (patch) | |
| tree | a33f13ebd027dbf44bbf19dc61d44955aea245c8 /extensions/libxt_string.man | |
| parent | f6d6ad24354ecd2997a48ba51b12e7dc34addd15 (diff) | |
man: string: document BM false negatives
For non-linear skb's there's a possibility that the kernel's Boyer-Moore
text-search implementation may miss matches. There's a warning about
this in the kernel source. Include that warning in the man-page.
Link: https://bugzilla.netfilter.org/show_bug.cgi?id=1390
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Diffstat (limited to 'extensions/libxt_string.man')
| -rw-r--r-- | extensions/libxt_string.man | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/extensions/libxt_string.man b/extensions/libxt_string.man index 5f1a993c..2a470ece 100644 --- a/extensions/libxt_string.man +++ b/extensions/libxt_string.man @@ -29,3 +29,18 @@ iptables \-A INPUT \-p tcp \-\-dport 80 \-m string \-\-algo bm \-\-string 'GET / # The hex string pattern can be used for non-printable characters, like |0D 0A| or |0D0A|. .br iptables \-p udp \-\-dport 53 \-m string \-\-algo bm \-\-from 40 \-\-to 57 \-\-hex\-string '|03|www|09|netfilter|03|org|00|' +.P +Note: Since Boyer-Moore (BM) performs searches for matches from right to left and +the kernel may store a packet in multiple discontiguous blocks, it's possible +that a match could be spread over multiple blocks, in which case this algorithm +won't find it. +.P +If you wish to ensure that such thing won't ever happen, use the +Knuth-Pratt-Morris (KMP) algorithm instead. In conclusion, choose the proper +string search algorithm depending on your use-case. +.P +For example, if you're using the module for filtering, NIDS or any similar +security-focused purpose, then choose KMP. On the other hand, if you really care +about performance \(em for example, you're classifying packets to apply Quality +of Service (QoS) policies \(em and you don't mind about missing possible matches +spread over multiple fragments, then choose BM. |