/Apache HTTP Server

Apache Module mod_proxy_ajp

Description: AJP support module for mod_proxy
Status: Extension
ModuleIdentifier: proxy_ajp_module
SourceFile: mod_proxy_ajp.c
Compatibility: Available in version 2.1 and later


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.


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.


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"

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
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 a set of headers back to the browser.
    Send a chunk of body data back to the browser.
    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).
    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.

A single byte.
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.
A number in the range of 0 to 2^16 (32768). Stored in 2 bytes with the high-order byte first.
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:

    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.


The HTTP method, encoded as a single byte:

Command Name Code
ACL 15

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.


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_nameand 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.


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.


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
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, and ssl_session 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.

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.

  prefix_code   3
  chunk_length  (integer)
  chunk        *(byte)
  chunk_terminator (byte) Ox00

  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)

  prefix_code       5
  reuse             (boolean)

  prefix_code       6
  requested_length  (integer)


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)

© 2016 The Apache Software Foundation
Licensed under the Apache License, Version 2.0.