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@xxxxxxxxxx> --- Since v1: * Adapt the text better to the context * Add `Link:` to the commit message extensions/libxt_string.man | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/extensions/libxt_string.man b/extensions/libxt_string.man index 5f1a993c57eb..0822ffdb7870 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 +NB 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. -- 2.39.2
