• June 12, 2024


mod_proxy_ajp - Apache HTTP Server Version 2.4

mod_proxy_ajp – Apache HTTP Server Version 2.4

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”
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 optionProxyPass “/app” “ajp” secret=YOUR_AJP_SECRET
Balancers may also be used:
Balancer Reverse Proxy
BalancerMember “ajp” loadfactor=1
BalancerMember “ajp” loadfactor=2
ProxySet lbmethod=bytraffic

ProxyPass “/app” “balancercluster/app”
Note that usually no
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
Rewriting Proxied PathProxyPass “/apps/foo” “ajp”
ProxyPassReverse “/apps/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
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. HTTP headers, etc) is sent over the connection in
a highly condensed form (e. 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.
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 ‘’). Note that the encoded length does
not include the trailing ‘’ — 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 character, the
C code can pass around references into a single buffer, without copying.
if the 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)
4… (n+3)
Data Length (n)
Packet Format (Container->Server)
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
Type of Packet
Forward Request
Begin the request-processing cycle with the following data
The web server asks the container to shut itself down.
The web server asks the container to take control
(secure login phase).
The web server asks the container to respond quickly with a CPong.
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
Send Body Chunk
Send a chunk of the body from the servlet container to the web
server (and presumably, onto the browser).
Send Headers
Send the response headers from the servlet container to the web
End Response
Marks the end of the response (and thus the request-handling cycle).
Get Body Chunk
Get further data from the request if it hasn’t all been
transferred yet.
CPong Reply
The reply to a CPing request
Each of the above messages has a different internal structure, detailed
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:
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
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
The HTTP method, encoded as a single byte:
Command NameCode
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):
NameCode valueCode name
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:
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? secret0x0CStringSupported since 2. 42
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. 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.
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.
prefix_code 3
chunk_length (integer)
chunk *(byte)
chunk_terminator (byte) Ox00
prefix_code 4
_status_code (integer)
_status_msg (string)
response_headers *(res_header_name header_value)
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)
The chunk is basically binary data, and is sent directly back to the
The status code and message are the usual HTTP things
(e. 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
After the code or the string header name, the header value is
immediately encoded.
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.
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. 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)
How to configure mod_proxy_ajp with Tomcat ? - Zeitoun.net

How to configure mod_proxy_ajp with Tomcat ? – Zeitoun.net

mod_proxy_ajp is an Apache module which can be used to forward a client HTTP request to an internal Tomcat application server using the AJP protocol.
To respond to the question “Why should I use mod_proxy_ajp rather than a classic mod_proxy? “, here is a small recap:
You can gain a lot of flexibility (lot of the apache modules/features can be used especially “name-based virtual hosting”)
Practical for those who need to support Java applications along with PHP / Perl … (only one apache server is needed)
Certificates management is easier in apache configuration (this argument is a lot subjective)
It’s not Tomcat’s main objective to serve static resources (not optimized for that)
Load balancing/cluster management is easier with an apache frontend
Tomcat configuration
You just have to create the AJP connector in the conf/ file like that:

This line will enable AJP connections to the 8009 port of your tomcat server (localhost for example).
Apache2 configuration
One way (useful if this apache is a global front end) is to create a virtual host for this application.
Listen 1979
NameVirtualHost *:1979

ServerName localhost
ErrorLog /var/log/apache2/
CustomLog /var/log/apache2/ combined

AddDefaultCharset Off
Order deny, allow
Allow from all

ProxyPass / ajplocalhost:8009/
ProxyPassReverse / ajplocalhost:8009/

ProxyPass and ProxyPassReverse are classic reverse proxy directives used to forward the stream to another location.
ajp… is the AJP connector location (your tomcat’s server host/port)
What’s the result?
A web client will connect through HTTP to localhost:1979/ (supposing your apache2 server is running on localhost), the mod_proxy_ajp will forward you request transparently using the AJP protocol to the tomcat application server on localhost:8009.
apache to tomcat: mod_jk vs mod_proxy - Stack Overflow

apache to tomcat: mod_jk vs mod_proxy – Stack Overflow

What are the advantages and disadvantages of using mod_jk and mod_proxy for fronting a tomcat instance with apache?
I’ve been using mod_jk in production for years but I’ve heard that it’s “the old way” of fronting tomcat. Should I consider changing? Would there be any benefits?
asked Jul 4 ’09 at 10:07
cherouvimcherouvim30. 8k14 gold badges101 silver badges145 bronze badges
A pros/cons comparison for those modules exists on
* Pros:
o No need for a separate module compilation and maintenance. mod_proxy,
mod_proxy_, mod_proxy_ajp and mod_proxy_balancer comes as part of
standard Apache 2. 2+ distribution
o Ability to use or AJP protocols, even within the same
* Cons:
o mod_proxy_ajp does not support large 8K+ packet sizes.
o Basic load balancer
o Does not support Domain model clustering
o Advanced load balancer
o Advanced node failure detection
o Support for large AJP packet sizes
o Need to build and maintain a separate module
answered Jul 6 ’09 at 18:27
If you wish to stay in Apache land, you can also try the newer mod_proxy_ajp, which uses the AJP protocol to communicate with Tomcat instead of plain old HTTP, but which leverages mod_proxy to do the work.
answered Jul 4 ’09 at 10:27
Vinko Vrsalovic♦Vinko Vrsalovic248k51 gold badges321 silver badges365 bronze badges
answered Mar 31 ’10 at 13:57
When using mod_jk, you are using the AJP. When using mod_proxy you will use HTTP or HTTPS. And this is essentially what makes all the difference.
The Apache JServ Protocol (AJP)
The Apache JServ Protocol (AJP) is a binary protocol that can proxy inbound requests from a web server through to an application server that sits behind the web server. AJP is a highly trusted protocol and should never be exposed to untrusted clients, which could use it to gain access to sensitive information or execute code on the application server.
Easy to set up as the correct forwarding of HTTP headers is not required.
It is less resource intensive because the TCP packets are forwarded in binary format instead of doing a costly HTTP exchange.
Transferred data is not encrypted. It should only be used within trusted networks.
Hypertext Transfer Protocol (HTTP)
HTTP functions as a request–response protocol in the client–server computing model. A web browser, for example, may be the client and an application running on a computer hosting a website may be the server. The client submits an HTTP request message to the server. The server, which provides resources such as HTML files and other content, or performs other functions on behalf of the client, returns a response message to the client. The response contains completion status information about the request and may also contain requested content in its message body.
Can be encrypted with SSL/TLS making it suitable for traffic across untrusted networks.
It is flexible as it allows to modify the request before forwarding. For example, setting custom headers.
More overhead as the correct forwarding of the HTTP headers has to be ensured.
More resource intensive as the request is fully parsed before forwarding.
answered Oct 25 ’20 at 11:00
The FoolThe Fool4, 1503 gold badges16 silver badges35 bronze badges
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Frequently Asked Questions about mod_proxy_ajp

What is Mod_proxy_ajp?

mod_proxy_ajp is an Apache module which can be used to forward a client HTTP request to an internal Tomcat application server using the AJP protocol […] mod_proxy_ajp is an Apache module which can be used to forward a client HTTP request to an internal Tomcat application server using the AJP protocol.

What is Mod_jk and mod_proxy?

When using mod_jk , you are using the AJP . When using mod_proxy you will use HTTP or HTTPS . And this is essentially what makes all the difference.Jul 4, 2009

What is ProxyPass and ProxyPassReverse?

ProxyPassReverse will intercept those headers, and rewrite them to match the Apache proxy server. ProxyPass will create a reverse proxy. A reverse proxy (or gateway), appears to the client just like an ordinary web server. The client makes ordinary requests for content in the namespace of the reverse proxy.Jun 12, 2012

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