doc: add multiple buffer matching documentation

Signed-off-by: jason taylor <jtfas90@gmail.com>

doc/userguide/rules/index.rst

@@ -42,3 +42,4 @@ Suricata Rules
    datasets
    lua-detection
    differences-from-snort
+   multi-buffer-matching

doc/userguide/rules/multi-buffer-matching.rst

@@ -0,0 +1,91 @@
+Multiple Buffer Matching
+========================
+
+Suricata 7 and newer now supports matching contents in multiple
+buffers within the same transaction.
+
+For example a single DNS transaction that has two queries in it:
+
+query 1: example.net
+query 2: something.com
+
+Example rule:
+
+.. container:: example-rule
+
+    `alert dns $HOME_NET any -> $EXTERNAL_NET any (msg:"DNS Multiple Question Example Rule"; dns.query; content:"example"; dns.query; content:".com"; classtype:misc-activity; sid:1; rev:1;)`
+
+Within the single DNS query transaction, there are two queries
+and Suricata will set up two instances of a dns.query buffer.
+
+The first ``dns.query`` buffer will look for content:"example";
+
+The second ``dns.query`` buffer will look for content:".com";
+
+The example rule will alert on the example query since all the
+content matches are satisfied for the rule.
+
+For matching multiple headers in HTTP2 traffic a rule using the
+new functionality would look like:
+
+.. container:: example-rule
+
+    `alert http2 any any -> any any (msg:"HTTP2 Multiple Header Buffer Example"; flow:established,to_server; http.request_header; content:"method|3a 20|GET"; http.request_header; content:"authority|3a 20|example.com"; classtype:misc-activity; sid:1; rev:1;)`
+
+With HTTP2 there are multiple headers seen in the same flow record.
+We now have a way to write a rule in a more efficient way using the
+multiple buffer capability.
+
+
+**Note** Existing behavior when using sticky buffers still applies:
+
+Example rule:
+
+.. container:: example-rule
+
+   `alert dns $HOME_NET any -> $EXTERNAL_NET any (msg:"DNS Query Sticky Buffer Classic Example Rule"; dns.query; content:"example"; content:".net"; classtype:misc-activity; sid:1; rev:1;)`
+
+The above rule will alert on a single dns query containing
+"example.net" or "example.domain.net" since the rule content
+matches are within a single ``dns.query`` buffer and all 
+content match requirements of the rule are met.
+
+
+**Note:** This is new behavior. In versions of Suricata prior to
+version 7 multiple statements of the same sticky buffer did not
+make a second instance of the buffer. For example:
+
+dns.query; content:"example"; dns.query; content:".com";
+
+would be equivalent to:
+
+dns.query; content:"example"; content:".com";
+
+Using our example from above, the first query is for example.net
+which matches content:"example"; but does not match content:".com";
+
+The second query is for something.com which would match on the
+content:".com"; but not the content:"example"; 
+
+So with the Suricata behavior prior to Suricata 7, the signature
+would not fire in this case since both content conditions will
+not be met.
+
+Multiple buffer matching is currently enabled for use with the
+following keywords:
+
+``dns.query``
+``file.data``
+``file.magic``
+``file.name``
+``http2.header``
+``http2.header_name``
+``ike.vendor``
+``krb5_cname``
+``krb5_sname``
+``mqtt.subscribe.topic``
+``mqtt.unsubscribe.topic``
+``quic.cyu.hash``
+``quic.cyu.string``
+``tls.certs``
+``tls.cert_subject``