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23 <modulesynopsis metafile="mod_proxy_ajp.xml.meta">
25 <name>mod_proxy_ajp</name>
26 <description>AJP support module for
27 <module>mod_proxy</module></description>
28 <status>Extension</status>
29 <sourcefile>mod_proxy_ajp.c</sourcefile>
30 <identifier>proxy_ajp_module</identifier>
31 <compatibility>Available in version 2.1 and later</compatibility>
34 <p>This module <em>requires</em> the service of <module
35 >mod_proxy</module>. It provides support for the
36 <code>Apache JServ Protocol version 1.3</code> (hereafter
39 <p>Thus, in order to get the ability of handling <code>AJP13</code>
40 protocol, <module>mod_proxy</module> and
41 <module>mod_proxy_ajp</module> have to be present in the server.</p>
43 <note type="warning"><title>Warning</title>
44 <p>Do not enable proxying until you have <a
45 href="mod_proxy.html#access">secured your server</a>. Open proxy
46 servers are dangerous both to your network and to the Internet at
51 <seealso><module>mod_proxy</module></seealso>
53 <section id="overviewprotocol"><title>Overview of the protocol</title>
54 <p>The <code>AJP13</code> protocol is packet-oriented. A binary format
55 was presumably chosen over the more readable plain text for reasons of
56 performance. The web server communicates with the servlet container over
57 TCP connections. To cut down on the expensive process of socket creation,
58 the web server will attempt to maintain persistent TCP connections to the
59 servlet container, and to reuse a connection for multiple request/response
61 <p>Once a connection is assigned to a particular request, it will not be
62 used for any others until the request-handling cycle has terminated. In
63 other words, requests are not multiplexed over connections. This makes
64 for much simpler code at either end of the connection, although it does
65 cause more connections to be open at once.</p>
66 <p>Once the web server has opened a connection to the servlet container,
67 the connection can be in one of the following states:</p>
69 <li> Idle <br/> No request is being handled over this connection. </li>
70 <li> Assigned <br/> The connecton is handling a specific request.</li>
72 <p>Once a connection is assigned to handle a particular request, the basic
73 request informaton (e.g. HTTP headers, etc) is sent over the connection in
74 a highly condensed form (e.g. common strings are encoded as integers).
75 Details of that format are below in Request Packet Structure. If there is a
76 body to the request <code>(content-length > 0)</code>, that is sent in a
77 separate packet immediately after.</p>
78 <p>At this point, the servlet container is presumably ready to start
79 processing the request. As it does so, it can send the
80 following messages back to the web server:</p>
82 <li>SEND_HEADERS <br/>Send a set of headers back to the browser.</li>
83 <li>SEND_BODY_CHUNK <br/>Send a chunk of body data back to the browser.
85 <li>GET_BODY_CHUNK <br/>Get further data from the request if it hasn't all
86 been transferred yet. This is necessary because the packets have a fixed
87 maximum size and arbitrary amounts of data can be included the body of a
88 request (for uploaded files, for example). (Note: this is unrelated to
89 HTTP chunked tranfer).</li>
90 <li>END_RESPONSE <br/> Finish the request-handling cycle.</li>
92 <p>Each message is accompanied by a differently formatted packet of data.
93 See Response Packet Structures below for details.</p>
96 <section id="basppacketstruct"><title>Basic Packet Structure</title>
97 <p>There is a bit of an XDR heritage to this protocol, but it differs
98 in lots of ways (no 4 byte alignment, for example).</p>
99 <p>Byte order: I am not clear about the endian-ness of the individual
100 bytes. I'm guessing the bytes are little-endian, because that's what
101 XDR specifies, and I'm guessing that sys/socket library is magically
102 making that so (on the C side). If anyone with a better knowledge of
103 socket calls can step in, that would be great.</p>
104 <p>There are four data types in the protocol: bytes, booleans,
105 integers and strings.</p>
107 <dt><strong>Byte</strong></dt><dd>A single byte.</dd>
108 <dt><strong>Boolean</strong></dt>
109 <dd>A single byte, <code>1 = true</code>, <code>0 = false</code>.
110 Using other non-zero values as true (i.e. C-style) may work in some places,
111 but it won't in others.</dd>
112 <dt><strong>Integer</strong></dt>
113 <dd>A number in the range of <code>0 to 2^16 (32768)</code>. Stored in
114 2 bytes with the high-order byte first.</dd>
115 <dt><strong>String</strong></dt>
116 <dd>A variable-sized string (length bounded by 2^16). Encoded with
117 the length packed into two bytes first, followed by the string
118 (including the terminating '\0'). Note that the encoded length does
119 <strong>not</strong> include the trailing '\0' -- it is like
120 <code>strlen</code>. This is a touch confusing on the Java side, which
121 is littered with odd autoincrement statements to skip over these
122 terminators. I believe the reason this was done was to allow the C
123 code to be extra efficient when reading strings which the servlet
124 container is sending back -- with the terminating \0 character, the
125 C code can pass around references into a single buffer, without copying.
126 if the \0 was missing, the C code would have to copy things out in order
127 to get its notion of a string.</dd>
130 <section><title>Packet Size</title>
131 <p>According to much of the code, the max packet size is <code>
132 8 * 1024 bytes (8K)</code>. The actual length of the packet is encoded in
135 <section><title>Packet Headers</title>
136 <p>Packets sent from the server to the container begin with
137 <code>0x1234</code>. Packets sent from the container to the server
138 begin with <code>AB</code> (that's the ASCII code for A followed by the
139 ASCII code for B). After those first two bytes, there is an integer
140 (encoded as above) with the length of the payload. Although this might
141 suggest that the maximum payload could be as large as 2^16, in fact, the
142 code sets the maximum to be 8K.</p>
145 <td colspan="6"><em>Packet Format (Server->Container)</em></td>
159 <td colspan="2">Data Length (n)</td>
165 <td colspan="6"><em>Packet Format (Container->Server)</em></td>
179 <td colspan="2">Data Length (n)</td>
183 <p>For most packets, the first byte of the payload encodes the type of
184 message. The exception is for request body packets sent from the server to
185 the container -- they are sent with a standard packet header (<code>
186 0x1234</code> and then length of the packet), but without any prefix code
188 <p>The web server can send the following messages to the servlet
193 <td>Type of Packet</td>
198 <td>Forward Request</td>
199 <td>Begin the request-processing cycle with the following data</td>
204 <td>The web server asks the container to shut itself down.</td>
209 <td>The web server asks the container to take control
210 (secure login phase).</td>
215 <td>The web server asks the container to respond quickly with a CPong.
221 <td>Size (2 bytes) and corresponding body data.</td>
224 <p>To ensure some basic security, the container will only actually do the
225 <code>Shutdown</code> if the request comes from the same machine on which
227 <p>The first <code>Data</code> packet is send immediatly after the
228 <code>Forward Request</code> by the web server.</p>
229 <p>The servlet container can send the following types of messages to the
234 <td>Type of Packet</td>
239 <td>Send Body Chunk</td>
240 <td>Send a chunk of the body from the servlet container to the web
241 server (and presumably, onto the browser). </td>
245 <td>Send Headers</td>
246 <td>Send the response headers from the servlet container to the web
247 server (and presumably, onto the browser).</td>
251 <td>End Response</td>
252 <td>Marks the end of the response (and thus the request-handling cycle).
257 <td>Get Body Chunk</td>
258 <td>Get further data from the request if it hasn't all been
259 transferred yet.</td>
264 <td>The reply to a CPing request</td>
267 <p>Each of the above messages has a different internal structure, detailed
271 <section id="rpacetstruct"><title>Request Packet Structure</title>
272 <p>For messages from the server to the container of type
273 <em>Forward Request</em>:</p>
275 AJP13_FORWARD_REQUEST :=
276 prefix_code (byte) 0x02 = JK_AJP13_FORWARD_REQUEST
283 server_port (integer)
285 num_headers (integer)
286 request_headers *(req_header_name req_header_value)
287 attributes *(attribut_name attribute_value)
288 request_terminator (byte) OxFF
290 <p>The <code>request_headers</code> have the following structure:
293 sc_req_header_name | (string) [see below for how this is parsed]
295 sc_req_header_name := 0xA0xx (integer)
297 req_header_value := (string)
299 <p>The <code>attributes</code> are optional and have the following
302 attribute_name := sc_a_name | (sc_a_req_attribute string)
304 attribute_value := (string)
307 <p>Not that the all-important header is <code>content-length</code>,
308 because it determines whether or not the container looks for another
309 packet immediately.</p>
310 <section><title>Detailed description of the elements of Forward Request
312 <section><title>Request prefix</title>
313 <p>For all requests, this will be 2. See above for details on other Prefix
316 <section><title>Method</title>
317 <p>The HTTP method, encoded as a single byte:</p>
319 <tr><td>Command Name</td><td>Code</td></tr>
320 <tr><td>OPTIONS</td><td>1</td></tr>
321 <tr><td>GET</td><td>2</td></tr>
322 <tr><td>HEAD</td><td>3</td></tr>
323 <tr><td>POST</td><td>4</td></tr>
324 <tr><td>PUT</td><td>5</td></tr>
325 <tr><td>DELETE</td><td>6</td></tr>
326 <tr><td>TRACE</td><td>7</td></tr>
327 <tr><td>PROPFIND</td><td>8</td></tr>
328 <tr><td>PROPPATCH</td><td>9</td></tr>
329 <tr><td>MKCOL</td><td>10</td></tr>
330 <tr><td>COPY</td><td>11</td></tr>
331 <tr><td>MOVE</td><td>12</td></tr>
332 <tr><td>LOCK</td><td>13</td></tr>
333 <tr><td>UNLOCK</td><td>14</td></tr>
334 <tr><td>ACL</td><td>15</td></tr>
335 <tr><td>REPORT</td><td>16</td></tr>
336 <tr><td>VERSION-CONTROL</td><td>17</td></tr>
337 <tr><td>CHECKIN</td><td>18</td></tr>
338 <tr><td>CHECKOUT</td><td>19</td></tr>
339 <tr><td>UNCHECKOUT</td><td>20</td></tr>
340 <tr><td>SEARCH</td><td>21</td></tr>
341 <tr><td>MKWORKSPACE</td><td>22</td></tr>
342 <tr><td>UPDATE</td><td>23</td></tr>
343 <tr><td>LABEL</td><td>24</td></tr>
344 <tr><td>MERGE</td><td>25</td></tr>
345 <tr><td>BASELINE_CONTROL</td><td>26</td></tr>
346 <tr><td>MKACTIVITY</td><td>27</td></tr>
348 <p>Later version of ajp13, will transport
349 additional methods, even if they are not in this list.</p>
351 <section><title>protocol, req_uri, remote_addr, remote_host, server_name,
352 server_port, is_ssl</title>
353 <p>These are all fairly self-explanatory. Each of these is required, and
354 will be sent for every request.</p>
356 <section><title>Headers</title>
357 <p>The structure of <code>request_headers</code> is the following:
358 First, the number of headers <code>num_headers</code> is encoded.
359 Then, a series of header name <code>req_header_name</code> / value
360 <code>req_header_value</code> pairs follows.
361 Common header names are encoded as integers,
362 to save space. If the header name is not in the list of basic headers,
363 it is encoded normally (as a string, with prefixed length). The list of
364 common headers <code>sc_req_header_name</code>and their codes
365 is as follows (all are case-sensitive):</p>
367 <tr><td>Name</td><td>Code value</td><td>Code name</td></tr>
368 <tr><td>accept</td><td>0xA001</td><td>SC_REQ_ACCEPT</td></tr>
369 <tr><td>accept-charset</td><td>0xA002</td><td>SC_REQ_ACCEPT_CHARSET
371 <tr><td>accept-encoding</td><td>0xA003</td><td>SC_REQ_ACCEPT_ENCODING
373 <tr><td>accept-language</td><td>0xA004</td><td>SC_REQ_ACCEPT_LANGUAGE
375 <tr><td>authorization</td><td>0xA005</td><td>SC_REQ_AUTHORIZATION</td>
377 <tr><td>connection</td><td>0xA006</td><td>SC_REQ_CONNECTION</td></tr>
378 <tr><td>content-type</td><td>0xA007</td><td>SC_REQ_CONTENT_TYPE</td>
380 <tr><td>content-length</td><td>0xA008</td><td>SC_REQ_CONTENT_LENGTH</td>
382 <tr><td>cookie</td><td>0xA009</td><td>SC_REQ_COOKIE</td></tr>
383 <tr><td>cookie2</td><td>0xA00A</td><td>SC_REQ_COOKIE2</td></tr>
384 <tr><td>host</td><td>0xA00B</td><td>SC_REQ_HOST</td></tr>
385 <tr><td>pragma</td><td>0xA00C</td><td>SC_REQ_PRAGMA</td></tr>
386 <tr><td>referer</td><td>0xA00D</td><td>SC_REQ_REFERER</td></tr>
387 <tr><td>user-agent</td><td>0xA00E</td><td>SC_REQ_USER_AGENT</td></tr>
389 <p>The Java code that reads this grabs the first two-byte integer and if
390 it sees an <code>'0xA0'</code> in the most significant
391 byte, it uses the integer in the second byte as an index into an array of
392 header names. If the first byte is not <code>0xA0</code>, it assumes that
393 the two-byte integer is the length of a string, which is then read in.</p>
394 <p>This works on the assumption that no header names will have length
395 greater than <code>0x9999 (==0xA000 - 1)</code>, which is perfectly
396 reasonable, though somewhat arbitrary.</p>
397 <note><title>Note:</title>
398 The <code>content-length</code> header is extremely
399 important. If it is present and non-zero, the container assumes that
400 the request has a body (a POST request, for example), and immediately
401 reads a separate packet off the input stream to get that body.
404 <section><title>Attributes</title>
405 <p>The attributes prefixed with a <code>?</code>
406 (e.g. <code>?context</code>) are all optional. For each, there is a
407 single byte code to indicate the type of attribute, and then its value
408 (string or integer). They can be sent in any order (though the C code
409 always sends them in the order listed below). A special terminating code
410 is sent to signal the end of the list of optional attributes. The list of
413 <tr><td>Information</td><td>Code Value</td><td>Type Of Value</td><td>Note</td></tr>
414 <tr><td>?context</td><td>0x01</td><td>-</td><td>Not currently implemented
416 <tr><td>?servlet_path</td><td>0x02</td><td>-</td><td>Not currently implemented
418 <tr><td>?remote_user</td><td>0x03</td><td>String</td><td></td></tr>
419 <tr><td>?auth_type</td><td>0x04</td><td>String</td><td></td></tr>
420 <tr><td>?query_string</td><td>0x05</td><td>String</td><td></td></tr>
421 <tr><td>?jvm_route</td><td>0x06</td><td>String</td><td></td></tr>
422 <tr><td>?ssl_cert</td><td>0x07</td><td>String</td><td></td></tr>
423 <tr><td>?ssl_cipher</td><td>0x08</td><td>String</td><td></td></tr>
424 <tr><td>?ssl_session</td><td>0x09</td><td>String</td><td></td></tr>
425 <tr><td>?req_attribute</td><td>0x0A</td><td>String</td><td>Name (the name of the
426 attribute follows)</td></tr>
427 <tr><td>?ssl_key_size</td><td>0x0B</td><td>Integer</td><td></td></tr>
428 <tr><td>are_done</td><td>0xFF</td><td>-</td><td>request_terminator</td></tr>
430 <p>The <code>context</code> and <code>servlet_path</code> are not
431 currently set by the C code, and most of the Java code completely ignores
432 whatever is sent over for those fields (and some of it will actually break
433 if a string is sent along after one of those codes). I don't know if this
434 is a bug or an unimplemented feature or just vestigial code, but it's
435 missing from both sides of the connection.</p>
436 <p>The <code>remote_user</code> and <code>auth_type</code> presumably
437 refer to HTTP-level authentication, and communicate the remote user's
438 username and the type of authentication used to establish their identity
439 (e.g. Basic, Digest).</p>
440 <p>The <code>query_string</code>, <code>ssl_cert</code>,
441 <code>ssl_cipher</code>, and <code>ssl_session</code> refer to the
442 corresponding pieces of HTTP and HTTPS.</p>
443 <p>The <code>jvm_route</code>, is used to support sticky
444 sessions -- associating a user's sesson with a particular Tomcat instance
445 in the presence of multiple, load-balancing servers.</p>
446 <p>Beyond this list of basic attributes, any number of other attributes
447 can be sent via the <code>req_attribute</code> code <code>0x0A</code>.
448 A pair of strings to represent the attribute name and value are sent
449 immediately after each instance of that code. Environment values are passed
450 in via this method.</p>
451 <p>Finally, after all the attributes have been sent, the attribute
452 terminator, <code>0xFF</code>, is sent. This signals both the end of the
453 list of attributes and also then end of the Request Packet.</p>
457 <section id="resppacketstruct"><title>Response Packet Structure</title>
458 <p>for messages which the container can send back to the server.</p>
460 AJP13_SEND_BODY_CHUNK :=
462 chunk_length (integer)
464 chunk_terminator (byte) Ox00
467 AJP13_SEND_HEADERS :=
469 http_status_code (integer)
470 http_status_msg (string)
471 num_headers (integer)
472 response_headers *(res_header_name header_value)
475 sc_res_header_name | (string) [see below for how this is parsed]
477 sc_res_header_name := 0xA0 (byte)
479 header_value := (string)
481 AJP13_END_RESPONSE :=
486 AJP13_GET_BODY_CHUNK :=
488 requested_length (integer)
490 <section><title>Details:</title></section>
491 <section><title>Send Body Chunk</title>
492 <p>The chunk is basically binary data, and is sent directly back to the
495 <section><title>Send Headers</title>
496 <p>The status code and message are the usual HTTP things
497 (e.g. <code>200</code> and <code>OK</code>). The response header names are
498 encoded the same way the request header names are. See header_encoding above
499 for details about how the codes are distinguished from the strings.<br />
500 The codes for common headers are:</p>
502 <tr><td>Name</td><td>Code value</td></tr>
503 <tr><td>Content-Type</td><td>0xA001</td></tr>
504 <tr><td>Content-Language</td><td>0xA002</td></tr>
505 <tr><td>Content-Length</td><td>0xA003</td></tr>
506 <tr><td>Date</td><td>0xA004</td></tr>
507 <tr><td>Last-Modified</td><td>0xA005</td></tr>
508 <tr><td>Location</td><td>0xA006</td></tr>
509 <tr><td>Set-Cookie</td><td>0xA007</td></tr>
510 <tr><td>Set-Cookie2</td><td>0xA008</td></tr>
511 <tr><td>Servlet-Engine</td><td>0xA009</td></tr>
512 <tr><td>Status</td><td>0xA00A</td></tr>
513 <tr><td>WWW-Authenticate</td><td>0xA00B</td></tr>
515 <p> After the code or the string header name, the header value is
516 immediately encoded.</p>
518 <section><title>End Response</title>
519 <p>Signals the end of this request-handling cycle. If the
520 <code>reuse</code> flag is true <code>(==1)</code>, this TCP connection can
521 now be used to handle new incoming requests. If <code>reuse</code> is false
522 (anything other than 1 in the actual C code), the connection should
525 <section><title>Get Body Chunk</title>
526 <p>The container asks for more data from the request (If the body was
527 too large to fit in the first packet sent over or when the request is
528 chuncked). The server will send a body packet back with an amount of data
529 which is the minimum of the <code>request_length</code>, the maximum send
530 body size <code>(8186 (8 Kbytes - 6))</code>, and the number of bytes
531 actually left to send from the request body.<br/>
532 If there is no more data in the body (i.e. the servlet container is
533 trying to read past the end of the body), the server will send back an
534 <em>empty</em> packet, which is a body packet with a payload length of 0.
535 <code>(0x12,0x34,0x00,0x00)</code></p>