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>

diff --git a/extensions/libxt_string.man b/extensions/libxt_string.man
index 5f1a993c57ebf6a2660483b0a1e941f096fecfd9..2a470ece19c9ddde3293bce97ce9f78ccc96f562 100644 (file)
--- 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.