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