HTTP Keywords
=============
.. role:: example-rule-emphasis

Using the HTTP specific sticky buffers provides a way to efficiently
inspect specific fields of the HTTP protocol. After specifying a
sticky buffer in a rule it should be followed by one or more :doc:`payload-keywords`.

Many of the sticky buffers have legacy variants in the older "content modifier"
notation. See :ref:`rules-modifiers` for more information. As a
refresher:

* **'sticky buffers'** are placed first and all keywords following it apply to that buffer, for instance::

      alert http any any -> any any (http.response_line; content:"403 Forbidden"; sid:1;)

  Sticky buffers apply to all "payload" keywords following it. E.g. `content`, `isdataat`, `byte_test`, `pcre`.

* **'content modifiers'** look back in the rule, e.g.::

      alert http any any -> any any (content:"index.php"; http_uri; sid:1;)

  Content modifiers only apply to the preceding `content` keyword.

The following **request** keywords are available:

============================== ======================== ==================
Keyword                        Legacy Content Modifier  Direction
============================== ======================== ==================
http.uri                       http_uri                 Request
http.uri.raw                   http_raw_uri             Request
http.method                    http_method              Request
http.request_line              http_request_line (*)    Request
http.request_body              http_client_body         Request
http.header                    http_header              Both
http.header.raw                http_raw_header          Both
http.cookie                    http_cookie              Both
http.user_agent                http_user_agent          Request
http.host                      http_host                Request
http.host.raw                  http_raw_host            Request
http.accept                    http_accept (*)          Request
http.accept_lang               http_accept_lang (*)     Request
http.accept_enc                http_accept_enc (*)      Request
http.referer                   http_referer (*)         Request
http.connection                http_connection (*)      Request
http.content_type              http_content_type (*)    Both
http.content_len               http_content_len (*)     Both
http.start                     http_start (*)           Both
http.protocol                  http_protocol (*)        Both
http.header_names              http_header_names (*)    Both
============================== ======================== ==================

\*) sticky buffer

The following **response** keywords are available:

============================== ======================== ==================
Keyword                        Legacy Content Modifier  Direction
============================== ======================== ==================
http.stat_msg                  http_stat_msg            Response
http.stat_code                 http_stat_code           Response
http.response_line             http_response_line (*)   Response
http.header                    http_header              Both
http.header.raw                http_raw_header          Both
http.cookie                    http_cookie              Both
http.response_body             http_server_body         Response
http.server                    N/A                      Response
http.location                  N/A                      Response
file.data                      file_data (*)            Response
http.content_type              http_content_type (*)    Both
http.content_len               http_content_len (*)     Both
http.start                     http_start (*)           Both
http.protocol                  http_protocol (*)        Both
http.header_names              http_header_names (*)    Both
============================== ======================== ==================

\*) sticky buffer

HTTP Primer
-----------
It is important to understand the structure of HTTP requests and
responses. A simple example of a HTTP request and response follows:

**HTTP request**

::

   GET /index.html HTTP/1.0\r\n

GET is the request **method**.  Examples of methods are: GET, POST, PUT,
HEAD, etc. The URI path is ``/index.html`` and the HTTP version is
``HTTP/1.0``. Several HTTP versions have been used over the years; of
the versions 0.9, 1.0 and 1.1, 1.0 and 1.1 are the most commonly used
today.

Example request with keywords:

+--------------------------------+------------------+
| HTTP                           | Keyword          |
+--------------------------------+------------------+
| GET /index.html HTTP/1.1\\r\\n | http.request_line|
+--------------------------------+------------------+
| Host: www.oisf.net\\r\\n       | http.header      |
+--------------------------------+------------------+
| Cookie: **<cookie data>**      | http.cookie      |
+--------------------------------+------------------+

Example request with finer grained keywords:

+------------------------------------------+---------------------+
| HTTP                                     | Keyword             |
+------------------------------------------+---------------------+
| **GET** */index.html* **HTTP/1.1**\\r\\n | **http.method**     |
|                                          | *http.uri*          |
|                                          | **http.protocol**   |
+------------------------------------------+---------------------+
| Host: **www.oisf.net**\\r\\n             | **http.host**       |
|                                          +---------------------+
| User-Agent: **Mozilla/5.0**\\r\\n        | **http.user_agent** |
+------------------------------------------+---------------------+
| Cookie: **<cookie data>**                | **http.cookie**     |
+------------------------------------------+---------------------+

**HTTP response**

::

   HTTP/1.0 200 OK\r\n
   <html>
   <title> some page </title>
   </HTML>

In this example, HTTP/1.0 is the HTTP version, 200 the response status
code and OK the response status message.

Although cookies are sent in an HTTP header, you can not match on them
with the ``http.header`` keyword. Cookies are matched with their own
keyword, namely ``http.cookie``.

Each part of the table belongs to a so-called *buffer*. The HTTP
method belongs to the method buffer, HTTP headers to the header buffer
etc. A buffer is a specific portion of the request or response that
Suricata extracts in memory for inspection.

All previous described keywords can be used in combination with a
buffer in a signature. The keywords ``distance`` and ``within`` are
relative modifiers, so they may only be used within the same
buffer. You can not relate content matches against different buffers
with relative modifiers.

http.method
-----------

With the ``http.method`` content modifier, it is possible to match
specifically and only on the HTTP method buffer. The keyword can be
used in combination with all previously mentioned content modifiers
such as: ``depth``, ``distance``, ``offset``, ``nocase`` and ``within``.

Examples of methods are: **GET**, **POST**, **PUT**, **HEAD**,
**DELETE**, **TRACE**, **OPTIONS**, **CONNECT** and **PATCH**.

Example of a method in a HTTP request:

.. image:: http-keywords/method2.png

Example of the purpose of method:

.. image:: http-keywords/method.png

.. image:: http-keywords/Legenda_rules.png

.. image:: http-keywords/method1.png

.. _rules-http-uri-normalization:

http.uri and http.uri.raw
-------------------------

With the ``http.uri`` and the ``http.uri.raw`` content modifiers, it
is possible to match specifically and only on the request URI
buffer. The keyword can be used in combination with all previously
mentioned content modifiers like ``depth``, ``distance``, ``offset``,
``nocase`` and ``within``.

The uri has two appearances in Suricata: the uri.raw and the
normalized uri. The space for example can be indicated with the
heximal notation %20. To convert this notation in a space, means
normalizing it. It is possible though to match specific on the
characters %20 in a uri. This means matching on the uri.raw. The
uri.raw and the normalized uri are separate buffers. So, the uri.raw
inspects the uri.raw buffer and can not inspect the normalized buffer.

.. note:: uri.raw never has any spaces in it.
          With this request line ``GET /uid=0(root) gid=0(root) HTTP/1.1``,
          the ``http.uri.raw`` will match ``/uid=0(root)``
          and ``http.protocol`` will match ``gid=0(root) HTTP/1.1``
          Reference: `https://redmine.openinfosecfoundation.org/issues/2881 <https://redmine.openinfosecfoundation.org/issues/2881>`_

Example of the URI in a HTTP request:

.. image:: http-keywords/uri1.png

Example of the purpose of ``http.uri``:

.. image:: http-keywords/uri.png

uricontent
----------

The ``uricontent`` keyword has the exact same effect as the
``http.uri`` content modifier. ``uricontent`` is a deprecated
(although still supported) way to match specifically and only on the
request URI buffer.

Example of ``uricontent``:

.. container:: example-rule

    alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"ET TROJAN Possible Vundo Trojan Variant reporting to Controller"; flow:established,to_server; content:"POST "; depth:5; :example-rule-emphasis:`uricontent:"/frame.html?";` urilen: > 80; classtype:trojan-activity; reference:url,doc.emergingthreats.net/2009173; reference:url,www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/VIRUS/TROJAN_Vundo; sid:2009173; rev:2;)

The difference between ``http.uri`` and ``uricontent`` is the syntax:

.. image:: http-keywords/uricontent1.png

.. image:: http-keywords/http_uri.png

When authoring new rules, it is recommended that the ``http.uri``
content sticky buffer be used rather than the deprecated ``uricontent``
keyword.

urilen
------

The ``urilen`` keyword is used to match on the length of the request
URI. It is possible to use the ``<`` and ``>`` operators, which
indicate respectively *smaller than* and *larger than*.

The format of ``urilen`` is::

  urilen:3;

Other possibilities are::

  urilen:1;
  urilen:>1;
  urilen:<10;
  urilen:10<>20;	(bigger than 10, smaller than 20)

Example:

.. image:: http-keywords/urilen.png

Example of ``urilen`` in a signature:

.. container:: example-rule

    alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"ET TROJAN Possible Vundo Trojan Variant reporting to Controller"; flow:established,to_server; content:"POST "; depth:5; uricontent:"/frame.html?"; :example-rule-emphasis:`urilen: > 80;` classtype:trojan-activity; reference:url,doc.emergingthreats.net/2009173; reference:url,www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/VIRUS/TROJAN_Vundo; sid:2009173; rev:2;)

You can also append ``norm`` or ``raw`` to define what sort of buffer you want
to use (normalized or raw buffer).

http.protocol
-------------

The ``http.protocol`` inspects the protocol field from the HTTP request or
response line. If the request line is 'GET / HTTP/1.0\r\n', then this buffer
will contain 'HTTP/1.0'.

Example::

    alert http any any -> any any (flow:to_server; http.protocol; content:"HTTP/1.0"; sid:1;)

``http.protocol`` replaces the previous keyword name: ```http_protocol``. You may continue to use the previous name, but it's recommended that rules be converted to use the new name.

Example::

    alert http any any -> any any (flow:to_server; http.protocol; content:"HTTP/1.0"; sid:1;)

http.request_line
-----------------

The ``http.request_line`` forces the whole HTTP request line to be inspected.

Example::

    alert http any any -> any any (http.request_line; content:"GET / HTTP/1.0"; sid:1;)

http.header and http.header.raw
-------------------------------

With the ``http.header`` content modifier, it is possible to match
specifically and only on the HTTP header buffer. This contains all of
the extracted headers in a single buffer, except for those indicated
in the documentation that are not able to match by this buffer and
have their own content modifier (e.g. ``http.cookie``). The modifier
can be used in combination with all previously mentioned content
modifiers, like ``depth``, ``distance``, ``offset``, ``nocase`` and
``within``.

    **Note**: the header buffer is *normalized*. Any trailing
    whitespace and tab characters are removed. See:
    https://lists.openinfosecfoundation.org/pipermail/oisf-users/2011-October/000935.html.
    If there are multiple values for the same header name, they are
    concatenated with a comma and space (", ") between each of them.
    See RFC 2616 4.2 Message Headers.
    To avoid that, use the ``http.header.raw`` keyword.

Example of a header in a HTTP request:

.. image:: http-keywords/header.png

Example of the purpose of ``http.header``:

.. image:: http-keywords/header1.png

http.cookie
-----------

With the ``http.cookie`` content modifier, it is possible to match
specifically and only on the cookie buffer. The keyword can be used in
combination with all previously mentioned content modifiers like
``depth``, ``distance``, ``offset``, ``nocase`` and ``within``.

Note that cookies are passed in HTTP headers, but are extracted to a
dedicated buffer and matched using their own specific content
modifier.

Example of a cookie in a HTTP request:

.. image:: http-keywords/cookie.png

Example of the purpose of ``http.cookie``:

.. image:: http-keywords/cookie1.png

http.user_agent
---------------

The ``http.user_agent`` content modifier is part of the HTTP request
header. It makes it possible to match specifically on the value of the
User-Agent header. It is normalized in the sense that it does not
include the _"User-Agent: "_ header name and separator, nor does it
contain the trailing carriage return and line feed (CRLF). The keyword
can be used in combination with all previously mentioned content
modifiers like ``depth``, ``distance``, ``offset``, ``nocase`` and
``within``. Note that the ``pcre`` keyword can also inspect this
buffer when using the ``/V`` modifier.

Normalization: leading spaces **are not** part of this buffer. So
"User-Agent: \r\n" will result in an empty ``http.user_agent`` buffer.

Example of the User-Agent header in a HTTP request:

.. image:: http-keywords/user_agent.png

Example of the purpose of ``http.user_agent``:

.. image:: http-keywords/user_agent_match.png

Notes
~~~~~

-  The ``http.user_agent`` buffer will NOT include the header name,
   colon, or leading whitespace.  i.e. it will not include
   "User-Agent: ".

-  The ``http.user_agent`` buffer does not include a CRLF (0x0D
   0x0A) at the end.  If you want to match the end of the buffer, use a
   relative ``isdataat`` or a PCRE (although PCRE will be worse on
   performance).

-  If a request contains multiple "User-Agent" headers, the values will
   be concatenated in the ``http.user_agent`` buffer, in the order
   seen from top to bottom, with a comma and space (", ") between each
   of them.

   Example request::

          GET /test.html HTTP/1.1
          User-Agent: SuriTester/0.8
          User-Agent: GGGG

   ``http.user_agent`` buffer contents::

          SuriTester/0.8, GGGG

-  Corresponding PCRE modifier: ``V``

-  Using the ``http.user_agent`` buffer is more efficient when it
   comes to performance than using the ``http.header`` buffer (~10%
   better).

-  `https://blog.inliniac.net/2012/07/09/suricata-http\_user\_agent-vs-http\_header/ <https://blog.inliniac.net/2012/07/09/suricata-http_user_agent-vs-http_header/>`_

http.accept
-----------

Sticky buffer to match on the HTTP Accept header. Only contains the header
value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (http.accept; content:"image/gif"; sid:1;)

http.accept_enc
---------------

Sticky buffer to match on the HTTP Accept-Encoding header. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (http.accept_enc; content:"gzip"; sid:1;)


http.accept_lang
----------------

Sticky buffer to match on the HTTP Accept-Language header. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (http.accept_lang; content:"en-us"; sid:1;)


http.connection
---------------

Sticky buffer to match on the HTTP Connection header. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (http.connection; content:"keep-alive"; sid:1;)


http.content_type
-----------------

Sticky buffer to match on the HTTP Content-Type headers. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Use flow:to_server or flow:to_client to force inspection of request or response.

Examples::

    alert http any any -> any any (flow:to_server; \
            http.content_type; content:"x-www-form-urlencoded"; sid:1;)

    alert http any any -> any any (flow:to_client; \
            http.content_type; content:"text/javascript"; sid:2;)


http.content_len
----------------

Sticky buffer to match on the HTTP Content-Length headers. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Use flow:to_server or flow:to_client to force inspection of request or response.

Examples::

    alert http any any -> any any (flow:to_server; \
            http.content_len; content:"666"; sid:1;)

    alert http any any -> any any (flow:to_client; \
            http.content_len; content:"555"; sid:2;)

To do a numeric inspection of the content length, ``byte_test`` can be used.

Example, match if C-L is equal to or bigger than 8079::

    alert http any any -> any any (flow:to_client; \
            http.content_len; byte_test:0,>=,8079,0,string,dec; sid:3;)

http.referer
---------------

Sticky buffer to match on the HTTP Referer header. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (http.referer; content:".php"; sid:1;)

http.start
----------

Inspect the start of a HTTP request or response. This will contain the
request/response line plus the request/response headers. Use flow:to_server
or flow:to_client to force inspection of request or response.

Example::

    alert http any any -> any any (http.start; content:"HTTP/1.1|0d 0a|User-Agent"; sid:1;)

The buffer contains the normalized headers and is terminated by an extra
\\r\\n to indicate the end of the headers.

http.header_names
-----------------

Inspect a buffer only containing the names of the HTTP headers. Useful
for making sure a header is not present or testing for a certain order
of headers.

Buffer starts with a \\r\\n and ends with an extra \\r\\n.

Example buffer::

    \\r\\nHost\\r\\n\\r\\n

Example rule::

    alert http any any -> any any (http.header_names; content:"|0d 0a|Host|0d 0a|"; sid:1;)

Example to make sure *only* Host is present::

    alert http any any -> any any (http.header_names; \
            content:"|0d 0a|Host|0d 0a 0d 0a|"; sid:1;)

Example to make sure *User-Agent* is directly after *Host*::

    alert http any any -> any any (http.header_names; \
            content:"|0d 0a|Host|0d 0a|User-Agent|0d 0a|"; sid:1;)

Example to make sure *User-Agent* is after *Host*, but not necessarily directly after::

    alert http any any -> any any (http.header_names; \
            content:"|0d 0a|Host|0d 0a|"; content:"|0a 0d|User-Agent|0d 0a|"; \
            distance:-2; sid:1;)

http.request_body
-----------------

With the ``http.request_body`` content modifier, it is possible to
match specifically and only on the HTTP request body. The keyword can
be used in combination with all previously mentioned content modifiers
like ``distance``, ``offset``, ``nocase``, ``within``, etc.

Example of ``http.request_body`` in a HTTP request:

.. image:: http-keywords/client_body.png

Example of the purpose of ``http.client_body``:

.. image:: http-keywords/client_body1.png

Note: how much of the request/client body is inspected is controlled
in the :ref:`libhtp configuration section
<suricata-yaml-configure-libhtp>` via the ``request-body-limit``
setting.

``http.request_body`` replaces the previous keyword name: ```http_client_body``. You may continue
+to use the previous name, but it's recommended that rules be converted to use
+the new name.

http.stat_code
--------------

With the ``http.stat_code`` content modifier, it is possible to match
specifically and only on the HTTP status code buffer. The keyword can
be used in combination with all previously mentioned content modifiers
like ``distance``, ``offset``, ``nocase``, ``within``, etc.

Example of ``http.stat_code`` in a HTTP response:

.. image:: http-keywords/stat_code.png

Example of the purpose of ``http.stat_code``:

.. image:: http-keywords/stat-code1.png

http.stat_msg
-------------

With the ``http.stat_msg`` content modifier, it is possible to match
specifically and only on the HTTP status message buffer. The keyword
can be used in combination with all previously mentioned content
modifiers like ``depth``, ``distance``, ``offset``, ``nocase`` and
``within``.

Example of ``http.stat_msg`` in a HTTP response:

.. image:: http-keywords/stat_msg.png

Example of the purpose of ``http.stat_msg``:

.. image:: http-keywords/stat_msg_1.png

http.response_line
------------------

The ``http.response_line`` forces the whole HTTP response line to be inspected.

Example::

    alert http any any -> any any (http.response_line; content:"HTTP/1.0 200 OK"; sid:1;)

http.response_body
------------------

With the ``http.response_body`` content modifier, it is possible to
match specifically and only on the HTTP response body. The keyword can
be used in combination with all previously mentioned content modifiers
like ``distance``, ``offset``, ``nocase``, ``within``, etc.

Note: how much of the response/server body is inspected is controlled
in your :ref:`libhtp configuration section
<suricata-yaml-configure-libhtp>` via the ``response-body-limit``
setting.

Notes
~~~~~

-  Using ``http.response_body`` is similar to having content matches
   that come after ``file.data`` except that it doesn't permanently
   (unless reset) set the detection pointer to the beginning of the
   server response body. i.e. it is not a sticky buffer.

-  ``http.response_body`` will match on gzip decoded data just like
   ``file.data`` does.

-  Since ``http.response_body`` matches on a server response, it
   can't be used with the ``to_server`` or ``from_client`` flow
   directives.

-  Corresponding PCRE modifier: ``Q``

-  further notes at the ``file.data`` section below.

``http.response_body`` replaces the previous keyword name: ```http_server_body``. You may continue
+to use the previous name, but it's recommended that rules be converted to use
+the new name.

http.server
-----------

Sticky buffer to match on the HTTP Server headers. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (flow:to_client; \
            http.server; content:"Microsoft-IIS/6.0"; sid:1;)

http.location
-------------

Sticky buffer to match on the HTTP Location headers. Only contains the
header value. The \\r\\n after the header are not part of the buffer.

Example::

    alert http any any -> any any (flow:to_client; \
            http.location; content:"http://www.google.com"; sid:1;)


http.host and http.host.raw
---------------------------

With the ``http.host`` content modifier, it is possible to
match specifically and only the normalized hostname.
The ``http.host.raw`` inspects the raw hostname.

The keyword can be used in combination with most of the content modifiers
like ``distance``, ``offset``, ``within``, etc.

The ``nocase`` keyword is not allowed anymore. Keep in mind that you need
to specify a lowercase pattern.

Notes
~~~~~

-  ``http.host`` does not contain the port associated with
   the host (i.e. abc.com:1234). To match on the host and port
   or negate a host and port use ``http.host.raw``.

-  The ``http.host`` and ``http.host.raw`` buffers are populated
   from either the URI (if the full URI is present in the request like
   in a proxy request) or the HTTP Host header. If both are present, the
   URI is used.

-  The ``http.host`` and ``http.host.raw`` buffers will NOT
   include the header name, colon, or leading whitespace if populated
   from the Host header.  i.e. they will not include "Host: ".

-  The ``http.host`` and ``http.host.raw`` buffers do not
   include a CRLF (0x0D 0x0A) at the end.  If you want to match the end
   of the buffer, use a relative 'isdataat' or a PCRE (although PCRE
   will be worse on performance).

-  The ``http.host`` buffer is normalized to be all lower case.

-  The content match that ``http.host`` applies to must be all lower
   case or have the ``nocase`` flag set.

-  ``http.host.raw`` matches the unnormalized buffer so matching
   will be case-sensitive (unless ``nocase`` is set).

-  If a request contains multiple "Host" headers, the values will be
   concatenated in the ``http.host`` and ``http.host.raw``
   buffers, in the order seen from top to bottom, with a comma and space
   (", ") between each of them.

   Example request::

          GET /test.html HTTP/1.1
          Host: ABC.com
          Accept: */*
          Host: efg.net

   ``http.host`` buffer contents::

          abc.com, efg.net

   ``http.host.raw`` buffer contents::

          ABC.com, efg.net

-  Corresponding PCRE modifier (``http_host``): ``W``
-  Corresponding PCRE modifier (``http_raw_host``): ``Z``

file.data
---------

With ``file.data``, the HTTP response body is inspected, just like
with ``http.response_body``. The ``file.data`` keyword is a sticky buffer.

Example::

  alert http any any -> any any (file.data; content:"abc"; content:"xyz";)

.. image:: http-keywords/file_data.png

The ``file.data`` keyword affects all following content matches, until
the ``pkt_data`` keyword is encountered or it reaches the end of the
rule. This makes it a useful shortcut for applying many content
matches to the HTTP response body, eliminating the need to modify each
content match individually.

As the body of a HTTP response can be very large, it is inspected in
smaller chunks.

How much of the response/server body is inspected is controlled
in your :ref:`libhtp configuration section
<suricata-yaml-configure-libhtp>` via the ``response-body-limit``
setting.

If the HTTP body is a flash file compressed with 'deflate' or 'lzma',
it can be decompressed and ``file.data`` can match on the decompress data.
Flash decompression must be enabled under ``libhtp`` configuration:

::

    # Decompress SWF files.
    # 2 types: 'deflate', 'lzma', 'both' will decompress deflate and lzma
    # compress-depth:
    # Specifies the maximum amount of data to decompress,
    # set 0 for unlimited.
    # decompress-depth:
    # Specifies the maximum amount of decompressed data to obtain,
    # set 0 for unlimited.
    swf-decompression:
      enabled: yes
      type: both
      compress-depth: 0
      decompress-depth: 0

Notes
~~~~~

-  file.data is the preferred notation, however, file_data is still
   recognized by the engine and works as well.

-  If a HTTP body is using gzip or deflate, ``file.data`` will match
   on the decompressed data.

-  Negated matching is affected by the chunked inspection. E.g.
   'content:!"<html";' could not match on the first chunk, but would
   then possibly match on the 2nd. To avoid this, use a depth setting.
   The depth setting takes the body size into account.
   Assuming that the ``response-body-minimal-inspect-size`` is bigger
   than 1k, 'content:!"<html"; depth:1024;' can only match if the
   pattern '<html' is absent from the first inspected chunk.

-  ``file.data`` can also be used with SMTP
