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mod_proxy_ajp - Apache HTTP Server Version 2.4









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Apache HTTP Server Version 2.4



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Apache Module mod_proxy_ajp

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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
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 checklisthttpd changelogKnown issuesReport a bugSee also

mod_proxy
Environment Variable documentation
Comments


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 ProxyProxyPass "/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
</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 PathProxyPass "/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.
    
    ByteA 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 NameCode
      OPTIONS1
      GET2
      HEAD3
      POST4
      PUT5
      DELETE6
      TRACE7
      PROPFIND8
      PROPPATCH9
      MKCOL10
      COPY11
      MOVE12
      LOCK13
      UNLOCK14
      ACL15
      REPORT16
      VERSION-CONTROL17
      CHECKIN18
      CHECKOUT19
      UNCHECKOUT20
      SEARCH21
      MKWORKSPACE22
      UPDATE23
      LABEL24
      MERGE25
      BASELINE_CONTROL26
      MKACTIVITY27
    
    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_nameand their codes
    is as follows (all are case-sensitive):
    
      NameCode valueCode name
      accept0xA001SC_REQ_ACCEPT
      accept-charset0xA002SC_REQ_ACCEPT_CHARSET
      
      accept-encoding0xA003SC_REQ_ACCEPT_ENCODING
      
      accept-language0xA004SC_REQ_ACCEPT_LANGUAGE
      
      authorization0xA005SC_REQ_AUTHORIZATION
      
      connection0xA006SC_REQ_CONNECTION
      content-type0xA007SC_REQ_CONTENT_TYPE
      
      content-length0xA008SC_REQ_CONTENT_LENGTH
      
      cookie0xA009SC_REQ_COOKIE
      cookie20xA00ASC_REQ_COOKIE2
      host0xA00BSC_REQ_HOST
      pragma0xA00CSC_REQ_PRAGMA
      referer0xA00DSC_REQ_REFERER
      user-agent0xA00ESC_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:
    
      InformationCode ValueType Of ValueNote
      ?context0x01-Not currently implemented
      
      ?servlet_path0x02-Not currently implemented
      
      ?remote_user0x03String
      ?auth_type0x04String
      ?query_string0x05String
      ?jvm_route0x06String
      ?ssl_cert0x07String
      ?ssl_cipher0x08String
      ?ssl_session0x09String
      ?req_attribute0x0AStringName (the name of the
      attribute follows)
      ?ssl_key_size0x0BInteger
      are_done0xFF-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.
    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:
    
      NameCode value
      Content-Type0xA001
      Content-Language0xA002
      Content-Length0xA003
      Date0xA004
      Last-Modified0xA005
      Location0xA006
      Set-Cookie0xA007
      Set-Cookie20xA008
      Servlet-Engine0xA009
      Status0xA00A
      WWW-Authenticate0xA00B
    
     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)
  



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