diff options
Diffstat (limited to 'extensions/libxt_string.man')
| -rw-r--r-- | extensions/libxt_string.man | 20 |
1 files changed, 18 insertions, 2 deletions
diff --git a/extensions/libxt_string.man b/extensions/libxt_string.man index 5f1a993c..bdeb0a62 100644 --- a/extensions/libxt_string.man +++ b/extensions/libxt_string.man @@ -7,8 +7,9 @@ Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morri Set the offset from which it starts looking for any matching. If not passed, default is 0. .TP \fB\-\-to\fP \fIoffset\fP -Set the offset up to which should be scanned. That is, byte \fIoffset\fP-1 -(counting from 0) is the last one that is scanned. +Set the offset up to which should be scanned. That is, byte \fIoffset\fP +(counting from 0) is the last one that is scanned and the maximum position of +\fIpattern\fP's last character. If not passed, default is the packet size. .TP [\fB!\fP] \fB\-\-string\fP \fIpattern\fP @@ -29,3 +30,18 @@ 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. |