Apache HTTP Server Version 2.4

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# Apache Module mod_proxy_ajp

Description: | AJP support module for `mod_proxy`
Status: | Extension
Module Identifier: | proxy_ajp_module
Source File: | mod_proxy_ajp.c
Compatibility: | Available in version 2.1 and later

### Summary

This module requires the service of `mod_proxy`. It provides support for the `Apache JServ Protocol version 1.3` (hereafter AJP13).

Thus, in order to get the ability of handling `AJP13` protocol, `mod_proxy` and `mod_proxy_ajp` have to be present in the server.

### Warning

Do not enable proxying until you have secured your server. Open proxy servers are dangerous both to your network and to the Internet at large.

### Topics

- Usage
- Environment Variables
- Overview of the protocol
- Basic Packet Structure
- Request Packet Structure
- Response Packet Structure

### Directives

This module provides no directives.

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

### See also

- `mod_proxy`
- Environment Variable documentation

## Usage

This module is used to reverse proxy to a backend application server (e.g. Apache Tomcat) using the AJP13 protocol. The usage is similar to an HTTP reverse proxy, but uses the `ajp://` prefix:

### Simple Reverse Proxy

    ProxyPass "/app" "ajp://backend.example.com:8009/app"

Options such as the `secret` option of Tomcat (required by default since Tomcat 8.5.51 and 9.0.31) can just be added as a separate parameter at the end of `ProxyPass` or `BalancerMember`. This parameter is available in Apache HTTP Server 2.4.42 and later:

### Simple Reverse Proxy with `secret` option

    ProxyPass "/app" "ajp://backend.example.com:8009/app" secret=YOUR_AJP_SECRET

Balancers may also be used:

### Balancer Reverse Proxy

    <Proxy "balancer://cluster">
        BalancerMember "ajp://app1.example.com:8009" loadfactor=1
        BalancerMember "ajp://app2.example.com:8009" loadfactor=2
        ProxySet lbmethod=bytraffic
    </Proxy>
    ProxyPass "/app" "balancer://cluster/app"

Note that usually no `ProxyPassReverse` directive is necessary. The AJP request includes the original host header given to the proxy, and the application server can be expected to generate self-referential headers relative to this host, so no rewriting is necessary.

The main exception is when the URL path on the proxy differs from that on the backend. In this case, a redirect header can be rewritten relative to the original host URL (not the backend `ajp://` URL), for example:

### Rewriting Proxied Path

    ProxyPass "/apps/foo" "ajp://backend.example.com:8009/foo"
    ProxyPassReverse "/apps/foo" "http://www.example.com/foo"

However, it is usually better to deploy the application on the backend server at the same path as the proxy rather than to take this approach.

## Environment Variables

Environment variables whose names have the prefix `AJP_` are forwarded to the origin server as AJP request attributes (with the `AJP_` prefix removed from the name of the key).

## Overview of the protocol

The `AJP13` protocol is packet-oriented. A binary format was presumably chosen over the more readable plain text for reasons of performance. The web server communicates with the servlet container over TCP connections. To cut down on the expensive process of socket creation, the web server will attempt to maintain persistent TCP connections to the servlet container, and to reuse a connection for multiple request/response cycles.

Once a connection is assigned to a particular request, it will not be used for any others until the request-handling cycle has terminated. In other words, requests are not multiplexed over connections. This makes for much simpler code at either end of the connection, although it does cause more connections to be open at once.

Once the web server has opened a connection to the servlet container, the connection can be in one of the following states:

- Idle
  No request is being handled over this connection.
- Assigned
  The connection is handling a specific request.

Once a connection is assigned to handle a particular request, the basic request information (e.g. HTTP headers, etc) is sent over the connection in a highly condensed form (e.g. common strings are encoded as integers). Details of that format are below in Request Packet Structure. If there is a body to the request `(content-length > 0)`, that is sent in a separate packet immediately after.

At this point, the servlet container is presumably ready to start processing the request. As it does so, it can send the following messages back to the web server:

- SEND_HEADERS
  Send a set of headers back to the browser.
- SEND_BODY_CHUNK
  Send a chunk of body data back to the browser.
- GET_BODY_CHUNK
  Get further data from the request if it hasn't all been transferred yet. This is necessary because the packets have a fixed maximum size and arbitrary amounts of data can be included the body of a request (for uploaded files, for example). (Note: this is unrelated to HTTP chunked transfer).
- END_RESPONSE
  Finish the request-handling cycle.

Each message is accompanied by a differently formatted packet of data. See Response Packet Structures below for details.

## Basic Packet Structure

There is a bit of an XDR heritage to this protocol, but it differs in lots of ways (no 4 byte alignment, for example).

AJP13 uses network byte order for all data types.

There are four data types in the protocol: bytes, booleans, integers and strings.

Byte
  A single byte.
Boolean
  A single byte, `1 = true`, `0 = false`. Using other non-zero values as true (i.e. C-style) may work in some places, but it won't in others.
Integer
  A number in the range of `0 to 2^16 (32768)`. Stored in 2 bytes with the high-order byte first.
String
  A variable-sized string (length bounded by 2^16). Encoded with the length packed into two bytes first, followed by the string (including the terminating '\0'). Note that the encoded length does not include the trailing '\0' -- it is like `strlen`. This is a touch confusing on the Java side, which is littered with odd autoincrement statements to skip over these terminators. I believe the reason this was done was to allow the C code to be extra efficient when reading strings which the servlet container is sending back -- with the terminating \0 character, the C code can pass around references into a single buffer, without copying. if the \0 was missing, the C code would have to copy things out in order to get its notion of a string.

### Packet Size

According to much of the code, the max packet size is `8 * 1024 bytes (8K)`. The actual length of the packet is encoded in the header.

### Packet Headers

Packets sent from the server to the container begin with `0x1234`. Packets sent from the container to the server begin with `AB` (that's the ASCII code for A followed by the ASCII code for B). After those first two bytes, there is an integer (encoded as above) with the length of the payload. Although this might suggest that the maximum payload could be as large as 2^16, in fact, the code sets the maximum to be 8K.

Packet Format (Server->Container)
Byte | 0 | 1 | 2 | 3 | 4...(n+3)
Contents | 0x12 | 0x34 | Data Length (n) | Data

Packet Format (Container->Server)
Byte | 0 | 1 | 2 | 3 | 4...(n+3)
Contents | A | B | Data Length (n) | Data

For most packets, the first byte of the payload encodes the type of message. The exception is for request body packets sent from the server to the container -- they are sent with a standard packet header (`0x1234` and then length of the packet), but without any prefix code after that.

The web server can send the following messages to the servlet container:

Code | Type of Packet | Meaning
2 | Forward Request | Begin the request-processing cycle with the following data
7 | Shutdown | The web server asks the container to shut itself down.
8 | Ping | The web server asks the container to take control (secure login phase).
10 | CPing | The web server asks the container to respond quickly with a CPong.
none | Data | Size (2 bytes) and corresponding body data.

To ensure some basic security, the container will only actually do the `Shutdown` if the request comes from the same machine on which it's hosted.

The first `Data` packet is send immediately after the `Forward Request` by the web server.

The servlet container can send the following types of messages to the webserver:

Code | Type of Packet | Meaning
3 | Send Body Chunk | Send a chunk of the body from the servlet container to the web server (and presumably, onto the browser).
4 | Send Headers | Send the response headers from the servlet container to the web server (and presumably, onto the browser).
5 | End Response | Marks the end of the response (and thus the request-handling cycle).
6 | Get Body Chunk | Get further data from the request if it hasn't all been transferred yet.
9 | CPong Reply | The reply to a CPing request

Each of the above messages has a different internal structure, detailed below.

## Request Packet Structure

For messages from the server to the container of type Forward Request:

    AJP13_FORWARD_REQUEST :=
        prefix_code      (byte) 0x02 = JK_AJP13_FORWARD_REQUEST
        method           (byte)
        protocol         (string)
        req_uri          (string)
        remote_addr      (string)
        remote_host      (string)
        server_name      (string)
        server_port      (integer)
        is_ssl           (boolean)
        num_headers      (integer)
        request_headers *(req_header_name req_header_value)
        attributes      *(attribut_name attribute_value)
        request_terminator (byte) OxFF

The `request_headers` have the following structure:

    req_header_name :=
        sc_req_header_name | (string)  [see below for how this is parsed]

    sc_req_header_name := 0xA0xx (integer)

    req_header_value := (string)

The `attributes` are optional and have the following structure:

    attribute_name := sc_a_name | (sc_a_req_attribute string)

    attribute_value := (string)

Not that the all-important header is `content-length`, because it determines whether or not the container looks for another packet immediately.

### Detailed description of the elements of Forward Request

### Request prefix

For all requests, this will be 2. See above for details on other Prefix codes.

### Method

The HTTP method, encoded as a single byte:

Command Name | Code
OPTIONS | 1
GET | 2
HEAD | 3
POST | 4
PUT | 5
DELETE | 6
TRACE | 7
PROPFIND | 8
PROPPATCH | 9
MKCOL | 10
COPY | 11
MOVE | 12
LOCK | 13
UNLOCK | 14
ACL | 15
REPORT | 16
VERSION-CONTROL | 17
CHECKIN | 18
CHECKOUT | 19
UNCHECKOUT | 20
SEARCH | 21
MKWORKSPACE | 22
UPDATE | 23
LABEL | 24
MERGE | 25
BASELINE_CONTROL | 26
MKACTIVITY | 27

Later version of ajp13, will transport additional methods, even if they are not in this list.

### protocol, req_uri, remote_addr, remote_host, server_name, server_port, is_ssl

These are all fairly self-explanatory. Each of these is required, and will be sent for every request.

### Headers

The structure of `request_headers` is the following: First, the number of headers `num_headers` is encoded. Then, a series of header name `req_header_name` / value `req_header_value` pairs follows. Common header names are encoded as integers, to save space. If the header name is not in the list of basic headers, it is encoded normally (as a string, with prefixed length). The list of common headers `sc_req_header_name`and their codes is as follows (all are case-sensitive):

Name | Code value | Code name
accept | 0xA001 | SC_REQ_ACCEPT
accept-charset | 0xA002 | SC_REQ_ACCEPT_CHARSET
accept-encoding | 0xA003 | SC_REQ_ACCEPT_ENCODING
accept-language | 0xA004 | SC_REQ_ACCEPT_LANGUAGE
authorization | 0xA005 | SC_REQ_AUTHORIZATION
connection | 0xA006 | SC_REQ_CONNECTION
content-type | 0xA007 | SC_REQ_CONTENT_TYPE
content-length | 0xA008 | SC_REQ_CONTENT_LENGTH
cookie | 0xA009 | SC_REQ_COOKIE
cookie2 | 0xA00A | SC_REQ_COOKIE2
host | 0xA00B | SC_REQ_HOST
pragma | 0xA00C | SC_REQ_PRAGMA
referer | 0xA00D | SC_REQ_REFERER
user-agent | 0xA00E | SC_REQ_USER_AGENT

The Java code that reads this grabs the first two-byte integer and if it sees an `'0xA0'` in the most significant byte, it uses the integer in the second byte as an index into an array of header names. If the first byte is not `0xA0`, it assumes that the two-byte integer is the length of a string, which is then read in.

This works on the assumption that no header names will have length greater than `0x9FFF (==0xA000 - 1)`, which is perfectly reasonable, though somewhat arbitrary.

### Note:

The `content-length` header is extremely important. If it is present and non-zero, the container assumes that the request has a body (a POST request, for example), and immediately reads a separate packet off the input stream to get that body.

### Attributes

The attributes prefixed with a `?` (e.g. `?context`) are all optional. For each, there is a single byte code to indicate the type of attribute, and then its value (string or integer). They can be sent in any order (though the C code always sends them in the order listed below). A special terminating code is sent to signal the end of the list of optional attributes. The list of byte codes is:

Information | Code Value | Type Of Value | Note
?context | 0x01 | - | Not currently implemented
?servlet_path | 0x02 | - | Not currently implemented
?remote_user | 0x03 | String |
?auth_type | 0x04 | String |
?query_string | 0x05 | String |
?jvm_route | 0x06 | String |
?ssl_cert | 0x07 | String |
?ssl_cipher | 0x08 | String |
?ssl_session | 0x09 | String |
?req_attribute | 0x0A | String | Name (the name of the attribute follows)
?ssl_key_size | 0x0B | Integer |
?secret | 0x0C | String | Supported since 2.4.42
are_done | 0xFF | - | request_terminator

The `context` and `servlet_path` are not currently set by the C code, and most of the Java code completely ignores whatever is sent over for those fields (and some of it will actually break if a string is sent along after one of those codes). I don't know if this is a bug or an unimplemented feature or just vestigial code, but it's missing from both sides of the connection.

The `remote_user` and `auth_type` presumably refer to HTTP-level authentication, and communicate the remote user's username and the type of authentication used to establish their identity (e.g. Basic, Digest).

The `query_string`, `ssl_cert`, `ssl_cipher`, `ssl_session` and `ssl_key_size` refer to the corresponding pieces of HTTP and HTTPS.

The `jvm_route`, is used to support sticky sessions -- associating a user's sesson with a particular Tomcat instance in the presence of multiple, load-balancing servers.

The `secret` is sent when the `secret=secret_keyword` parameter is used in `ProxyPass` or `BalancerMember` directives. The backend needs to support secret and the values must match. `request.secret` or `requiredSecret` are documented in the AJP configuration of the Apache Tomcat.

Beyond this list of basic attributes, any number of other attributes can be sent via the `req_attribute` code `0x0A`. A pair of strings to represent the attribute name and value are sent immediately after each instance of that code. Environment values are passed in via this method.

Finally, after all the attributes have been sent, the attribute terminator, `0xFF`, is sent. This signals both the end of the list of attributes and also then end of the Request Packet.

## Response Packet Structure

for messages which the container can send back to the server.

    AJP13_SEND_BODY_CHUNK :=
      prefix_code   3
      chunk_length  (integer)
      chunk        *(byte)
      chunk_terminator (byte) Ox00

    AJP13_SEND_HEADERS :=
      prefix_code       4
      http_status_code  (integer)
      http_status_msg   (string)
      num_headers       (integer)
      response_headers *(res_header_name header_value)

    res_header_name :=
        sc_res_header_name | (string)   [see below for how this is parsed]

    sc_res_header_name := 0xA0 (byte)

    header_value := (string)

    AJP13_END_RESPONSE :=
      prefix_code       5
      reuse             (boolean)

    AJP13_GET_BODY_CHUNK :=
      prefix_code       6
      requested_length  (integer)

### Details:

### Send Body Chunk

The chunk is basically binary data, and is sent directly back to the browser.

### Send Headers

The status code and message are the usual HTTP things (e.g. `200` and `OK`). The response header names are encoded the same way the request header names are. See header_encoding above for details about how the codes are distinguished from the strings.
The codes for common headers are:

Name | Code value
Content-Type | 0xA001
Content-Language | 0xA002
Content-Length | 0xA003
Date | 0xA004
Last-Modified | 0xA005
Location | 0xA006
Set-Cookie | 0xA007
Set-Cookie2 | 0xA008
Servlet-Engine | 0xA009
Status | 0xA00A
WWW-Authenticate | 0xA00B

After the code or the string header name, the header value is immediately encoded.

### End Response

Signals the end of this request-handling cycle. If the `reuse` flag is true `(anything other than 0 in the actual C code)`, this TCP connection can now be used to handle new incoming requests. If `reuse` is false (==0), the connection should be closed.

### Get Body Chunk

The container asks for more data from the request (If the body was too large to fit in the first packet sent over or when the request is chunked). The server will send a body packet back with an amount of data which is the minimum of the `request_length`, the maximum send body size `(8186 (8 Kbytes - 6))`, and the number of bytes actually left to send from the request body.
If there is no more data in the body (i.e. the servlet container is trying to read past the end of the body), the server will send back an empty packet, which is a body packet with a payload length of 0. `(0x12,0x34,0x00,0x00)`