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-<title>Connections in the FIN_WAIT_2 state and Apache - Apache HTTP Server</title>
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-<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
-<p class="apache">Apache HTTP Server Version 2.0</p>
-<img alt="" src="../images/feather.gif" /></div>
-<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
-<div id="path">
-<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.0</a> &gt; <a href="./">Miscellaneous Documentation</a></div><div id="page-content"><div id="preamble"><h1>Connections in the FIN_WAIT_2 state and Apache</h1>
-<div class="toplang">
-<p><span>Available Languages: </span><a href="../en/misc/fin_wait_2.html" title="English">&nbsp;en&nbsp;</a></p>
-</div>
-
-
- <div class="warning"><h3>Warning:</h3>
- <p>This document has not been fully updated
- to take into account changes made in the 2.0 version of the
- Apache HTTP Server. Some of the information may still be
- relevant, but please use it with care.</p>
- </div>
-
- <p>Starting with the Apache 1.2 betas, people are reporting
- many more connections in the FIN_WAIT_2 state (as reported
- by <code>netstat</code>) than they saw using older
- versions. When the server closes a TCP connection, it sends
- a packet with the FIN bit set to the client, which then
- responds with a packet with the ACK bit set. The client
- then sends a packet with the FIN bit set to the server,
- which responds with an ACK and the connection is closed.
- The state that the connection is in during the period
- between when the server gets the ACK from the client and
- the server gets the FIN from the client is known as
- FIN_WAIT_2. See the <a href="ftp://ds.internic.net/rfc/rfc793.txt">TCP RFC</a> for
- the technical details of the state transitions.</p>
-
- <p>The FIN_WAIT_2 state is somewhat unusual in that there
- is no timeout defined in the standard for it. This means
- that on many operating systems, a connection in the
- FIN_WAIT_2 state will stay around until the system is
- rebooted. If the system does not have a timeout and too
- many FIN_WAIT_2 connections build up, it can fill up the
- space allocated for storing information about the
- connections and crash the kernel. The connections in
- FIN_WAIT_2 do not tie up an httpd process.</p>
-
- </div>
-<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#why">Why Does It Happen?</a></li>
-<li><img alt="" src="../images/down.gif" /> <a href="#what">What Can I Do About it?</a></li>
-<li><img alt="" src="../images/down.gif" /> <a href="#appendix">Appendix</a></li>
-</ul></div>
-<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
-<div class="section">
-<h2><a name="why" id="why">Why Does It Happen?</a></h2>
-
- <p>There are numerous reasons for it happening, some of them
- may not yet be fully clear. What is known follows.</p>
-
- <h3><a name="buggy" id="buggy">Buggy Clients and Persistent
- Connections</a></h3>
-
- <p>Several clients have a bug which pops up when dealing with
- persistent connections (aka
- keepalives). When the connection is idle and the server
- closes the connection (based on the <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>),
- the client is programmed so that the client does not send
- back a FIN and ACK to the server. This means that the
- connection stays in the FIN_WAIT_2 state until one of the
- following happens:</p>
-
- <ul>
- <li>The client opens a new connection to the same or a
- different site, which causes it to fully close the older
- connection on that socket.</li>
-
- <li>The user exits the client, which on some (most?)
- clients causes the OS to fully shutdown the
- connection.</li>
-
- <li>The FIN_WAIT_2 times out, on servers that have a
- timeout for this state.</li>
- </ul>
-
- <p>If you are lucky, this means that the buggy client will
- fully close the connection and release the resources on
- your server. However, there are some cases where the socket
- is never fully closed, such as a dialup client
- disconnecting from their provider before closing the
- client. In addition, a client might sit idle for days
- without making another connection, and thus may hold its
- end of the socket open for days even though it has no
- further use for it. <strong>This is a bug in the browser or
- in its operating system's TCP implementation.</strong></p>
-
- <p>The clients on which this problem has been verified to
- exist:</p>
-
- <ul>
- <li>Mozilla/3.01 (X11; I; FreeBSD 2.1.5-RELEASE
- i386)</li>
-
- <li>Mozilla/2.02 (X11; I; FreeBSD 2.1.5-RELEASE
- i386)</li>
-
- <li>Mozilla/3.01Gold (X11; I; SunOS 5.5 sun4m)</li>
-
- <li>MSIE 3.01 on the Macintosh</li>
-
- <li>MSIE 3.01 on Windows 95</li>
- </ul>
-
- <p>This does not appear to be a problem on:</p>
-
- <ul>
- <li>Mozilla/3.01 (Win95; I)</li>
- </ul>
-
- <p>It is expected that many other clients have the same
- problem. What a client <strong>should do</strong> is
- periodically check its open socket(s) to see if they have
- been closed by the server, and close their side of the
- connection if the server has closed. This check need only
- occur once every few seconds, and may even be detected by a
- OS signal on some systems (<em>e.g.</em>, Win95 and NT
- clients have this capability, but they seem to be ignoring
- it).</p>
-
- <p>Apache <strong>cannot</strong> avoid these FIN_WAIT_2
- states unless it disables persistent connections for the
- buggy clients, just like we recommend doing for Navigator
- 2.x clients due to other bugs. However, non-persistent
- connections increase the total number of connections needed
- per client and slow retrieval of an image-laden web page.
- Since non-persistent connections have their own resource
- consumptions and a short waiting period after each closure,
- a busy server may need persistence in order to best serve
- its clients.</p>
-
- <p>As far as we know, the client-caused FIN_WAIT_2 problem
- is present for all servers that support persistent
- connections, including Apache 1.1.x and 1.2.</p>
-
-
-
- <h3><a name="code" id="code">A necessary bit of code
- introduced in 1.2</a></h3>
-
- <p>While the above bug is a problem, it is not the whole
- problem. Some users have observed no FIN_WAIT_2 problems
- with Apache 1.1.x, but with 1.2b enough connections build
- up in the FIN_WAIT_2 state to crash their server. The most
- likely source for additional FIN_WAIT_2 states is a
- function called <code>lingering_close()</code> which was
- added between 1.1 and 1.2. This function is necessary for
- the proper handling of persistent connections and any
- request which includes content in the message body
- (<em>e.g.</em>, PUTs and POSTs). What it does is read any
- data sent by the client for a certain time after the server
- closes the connection. The exact reasons for doing this are
- somewhat complicated, but involve what happens if the
- client is making a request at the same time the server
- sends a response and closes the connection. Without
- lingering, the client might be forced to reset its TCP
- input buffer before it has a chance to read the server's
- response, and thus understand why the connection has
- closed. See the <a href="#appendix">appendix</a> for more
- details.</p>
-
- <p>The code in <code>lingering_close()</code> appears to
- cause problems for a number of factors, including the
- change in traffic patterns that it causes. The code has
- been thoroughly reviewed and we are not aware of any bugs
- in it. It is possible that there is some problem in the BSD
- TCP stack, aside from the lack of a timeout for the
- FIN_WAIT_2 state, exposed by the
- <code>lingering_close</code> code that causes the observed
- problems.</p>
-
-
- </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
-<div class="section">
-<h2><a name="what" id="what">What Can I Do About it?</a></h2>
-
- <p>There are several possible workarounds to the problem, some
- of which work better than others.</p>
-
- <h3><a name="add_timeout" id="add_timeout">Add a timeout for FIN_WAIT_2</a></h3>
-
- <p>The obvious workaround is to simply have a timeout for the
- FIN_WAIT_2 state. This is not specified by the RFC, and
- could be claimed to be a violation of the RFC, but it is
- widely recognized as being necessary. The following systems
- are known to have a timeout:</p>
-
- <ul>
- <li><a href="http://www.freebsd.org/">FreeBSD</a>
- versions starting at 2.0 or possibly earlier.</li>
-
- <li><a href="http://www.netbsd.org/">NetBSD</a> version
- 1.2(?)</li>
-
- <li><a href="http://www.openbsd.org/">OpenBSD</a> all
- versions(?)</li>
-
- <li><a href="http://www.bsdi.com/">BSD/OS</a> 2.1, with
- the <a href="ftp://ftp.bsdi.com/bsdi/patches/patches-2.1/K210-027">
- K210-027</a> patch installed.</li>
-
- <li><a href="http://www.sun.com/">Solaris</a> as of
- around version 2.2. The timeout can be tuned by using
- <code>ndd</code> to modify
- <code>tcp_fin_wait_2_flush_interval</code>, but the
- default should be appropriate for most servers and
- improper tuning can have negative impacts.</li>
-
- <li><a href="http://www.linux.org/">Linux</a> 2.0.x and
- earlier(?)</li>
-
- <li><a href="http://www.hp.com/">HP-UX</a> 10.x defaults
- to terminating connections in the FIN_WAIT_2 state after
- the normal keepalive timeouts. This does not refer to the
- persistent connection or HTTP keepalive timeouts, but the
- <code>SO_LINGER</code> socket option which is enabled by
- Apache. This parameter can be adjusted by using
- <code>nettune</code> to modify parameters such as
- <code>tcp_keepstart</code> and <code>tcp_keepstop</code>.
- In later revisions, there is an explicit timer for
- connections in FIN_WAIT_2 that can be modified; contact
- HP support for details.</li>
-
- <li><a href="http://www.sgi.com/">SGI IRIX</a> can be
- patched to support a timeout. For IRIX 5.3, 6.2, and 6.3,
- use patches 1654, 1703 and 1778 respectively. If you have
- trouble locating these patches, please contact your SGI
- support channel for help.</li>
-
- <li><a href="http://www.ncr.com/">NCR's MP RAS Unix</a>
- 2.xx and 3.xx both have FIN_WAIT_2 timeouts. In 2.xx it
- is non-tunable at 600 seconds, while in 3.xx it defaults
- to 600 seconds and is calculated based on the tunable
- "max keep alive probes" (default of 8) multiplied by the
- "keep alive interval" (default 75 seconds).</li>
-
- <li><a href="http://www.sequent.com">Sequent's ptx/TCP/IP
- for DYNIX/ptx</a> has had a FIN_WAIT_2 timeout since
- around release 4.1 in mid-1994.</li>
- </ul>
-
- <p>The following systems are known to not have a
- timeout:</p>
-
- <ul>
- <li><a href="http://www.sun.com/">SunOS 4.x</a> does not
- and almost certainly never will have one because it as at
- the very end of its development cycle for Sun. If you
- have kernel source should be easy to patch.</li>
- </ul>
-
- <p>There is a <a href="http://www.apache.org/dist/httpd/contrib/patches/1.2/fin_wait_2.patch">
- patch available</a> for adding a timeout to the FIN_WAIT_2
- state; it was originally intended for BSD/OS, but should be
- adaptable to most systems using BSD networking code. You
- need kernel source code to be able to use it.</p>
-
-
-
- <h3><a name="no_lingering" id="no_lingering">Compile without using
- <code>lingering_close()</code></a></h3>
-
- <p>It is possible to compile Apache 1.2 without using the
- <code>lingering_close()</code> function. This will result
- in that section of code being similar to that which was in
- 1.1. If you do this, be aware that it can cause problems
- with PUTs, POSTs and persistent connections, especially if
- the client uses pipelining. That said, it is no worse than
- on 1.1, and we understand that keeping your server running
- is quite important.</p>
-
- <p>To compile without the <code>lingering_close()</code>
- function, add <code>-DNO_LINGCLOSE</code> to the end of the
- <code>EXTRA_CFLAGS</code> line in your
- <code>Configuration</code> file, rerun
- <code class="program"><a href="../programs/Configure.html">Configure</a></code> and rebuild the server.</p>
-
-
-
- <h3><a name="so_linger" id="so_linger">Use <code>SO_LINGER</code> as
- an alternative to <code>lingering_close()</code></a></h3>
-
- <p>On most systems, there is an option called
- <code>SO_LINGER</code> that can be set with
- <code>setsockopt(2)</code>. It does something very similar
- to <code>lingering_close()</code>, except that it is broken
- on many systems so that it causes far more problems than
- <code>lingering_close</code>. On some systems, it could
- possibly work better so it may be worth a try if you have
- no other alternatives.</p>
-
- <p>To try it, add <code>-DUSE_SO_LINGER
- -DNO_LINGCLOSE</code> to the end of the
- <code>EXTRA_CFLAGS</code> line in your
- <code>Configuration</code> file, rerun
- <code class="program"><a href="../programs/Configure.html">Configure</a></code> and rebuild the server.</p>
-
- <div class="note"><h3>NOTE</h3>Attempting to use
- <code>SO_LINGER</code> and <code>lingering_close()</code>
- at the same time is very likely to do very bad things, so
- don't.</div>
-
-
-
- <h3><a name="increase_mem" id="increase_mem">Increase the amount of memory
- used for storing connection state</a></h3>
-
- <dl>
- <dt>BSD based networking code:</dt>
-
- <dd>
- BSD stores network data, such as connection states, in
- something called an mbuf. When you get so many
- connections that the kernel does not have enough mbufs
- to put them all in, your kernel will likely crash. You
- can reduce the effects of the problem by increasing the
- number of mbufs that are available; this will not
- prevent the problem, it will just make the server go
- longer before crashing.
-
- <p>The exact way to increase them may depend on your
- OS; look for some reference to the number of "mbufs" or
- "mbuf clusters". On many systems, this can be done by
- adding the line <code>NMBCLUSTERS="n"</code>, where
- <code>n</code> is the number of mbuf clusters you want
- to your kernel config file and rebuilding your
- kernel.</p>
- </dd>
- </dl>
-
-
-
- <h3><a name="disable" id="disable">Disable KeepAlive</a></h3>
-
- <p>If you are unable to do any of the above then you
- should, as a last resort, disable KeepAlive. Edit your
- httpd.conf and change "KeepAlive On" to "KeepAlive
- Off".</p>
-
-
- </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
-<div class="section">
-<h2><a name="appendix" id="appendix">Appendix</a></h2>
-
- <p>Below is a message from Roy Fielding, one of the authors
- of HTTP/1.1.</p>
-
- <h3><a name="message" id="message">Why the lingering close
- functionality is necessary with HTTP</a></h3>
-
- <p>The need for a server to linger on a socket after a close
- is noted a couple times in the HTTP specs, but not
- explained. This explanation is based on discussions between
- myself, Henrik Frystyk, Robert S. Thau, Dave Raggett, and
- John C. Mallery in the hallways of MIT while I was at W3C.</p>
-
- <p>If a server closes the input side of the connection
- while the client is sending data (or is planning to send
- data), then the server's TCP stack will signal an RST
- (reset) back to the client. Upon receipt of the RST, the
- client will flush its own incoming TCP buffer back to the
- un-ACKed packet indicated by the RST packet argument. If
- the server has sent a message, usually an error response,
- to the client just before the close, and the client
- receives the RST packet before its application code has
- read the error message from its incoming TCP buffer and
- before the server has received the ACK sent by the client
- upon receipt of that buffer, then the RST will flush the
- error message before the client application has a chance to
- see it. The result is that the client is left thinking that
- the connection failed for no apparent reason.</p>
-
- <p>There are two conditions under which this is likely to
- occur:</p>
-
- <ol>
- <li>sending POST or PUT data without proper
- authorization</li>
-
- <li>sending multiple requests before each response
- (pipelining) and one of the middle requests resulting in
- an error or other break-the-connection result.</li>
- </ol>
-
- <p>The solution in all cases is to send the response, close
- only the write half of the connection (what shutdown is
- supposed to do), and continue reading on the socket until
- it is either closed by the client (signifying it has
- finally read the response) or a timeout occurs. That is
- what the kernel is supposed to do if SO_LINGER is set.
- Unfortunately, SO_LINGER has no effect on some systems; on
- some other systems, it does not have its own timeout and
- thus the TCP memory segments just pile-up until the next
- reboot (planned or not).</p>
-
- <p>Please note that simply removing the linger code will
- not solve the problem -- it only moves it to a different
- and much harder one to detect.</p>
-
- </div></div>
-<div class="bottomlang">
-<p><span>Available Languages: </span><a href="../en/misc/fin_wait_2.html" title="English">&nbsp;en&nbsp;</a></p>
-</div><div id="footer">
-<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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