doc: add documentation for SSH keywords

doc/userguide/rules/index.rst

@@ -13,6 +13,7 @@ Suricata Rules
    file-keywords
    dns-keywords
    tls-keywords
+   ssh-keywords
    ja3-keywords
    modbus-keyword
    dnp3-keywords

doc/userguide/rules/ssh-keywords.rst

@@ -0,0 +1,60 @@
+SSH Keywords
+============
+
+Suricata comes with several rule keywords to match on SSH connections.
+
+ssh_proto
+---------
+
+Match on the version of the SSH protocol used.
+
+Example::
+
+  alert ssh any any -> any any (msg:"match SSH protocol version"; \
+      ssh_proto; content:"2.0"; sid:1000010;)
+
+The example above matches on SSH connections with SSH version 2.
+
+``ssh_proto`` is a 'Sticky buffer'.
+
+``ssh_proto`` can be used as ``fast_pattern``.
+
+ssh_version
+-----------
+
+Match on the software string from the SSH banner.
+
+Example::
+
+  alert ssh any any -> any any (msg:"match SSH software string"; \
+      ssh_software: content:"openssh"; nocase; sid:1000020;)
+
+The example above matches on SSH connections where the software string contains "openssh".
+
+``ssh_software`` is a 'Sticky buffer'.
+
+``ssh_software`` can be used as ``fast_pattern``.
+
+ssh.protoversion
+----------------
+
+This is a legacy keyword. Use ``ssh_proto`` instead!
+
+Match on the version of the SSH protocol used.
+
+Example::
+
+  alert ssh any any -> any any (msg:"match SSH protocol version"; \
+      ssh.protoversion:"2.0"; sid:1000030;)
+
+ssh.softwareversion
+-------------------
+
+This is a legacy keyword. Use ``ssh_software`` instead!
+
+Match on the software string from the SSH banner.
+
+Example::
+
+  alert ssh any any -> any any (msg:"match SSH software string"; \
+      ssh.softwareversion:"OpenSSH"; sid:10000040;)