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brian
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on 26 Jan 98
PR:

If SCO's going to break their links, I'm not going to go searching for where they
moved it to.

Cold and rainy and dark.
ml" -->

<H1 A… Show more
PR:

If SCO's going to break their links, I'm not going to go searching for where they

moved it to.

Cold and rainy and dark.

ml" -->

<H1 ALIGN="CENTER">Connections in the FIN_WAIT_2 state and Apache</H1>

<OL>

<LI><H2>What is the FIN_WAIT_2 state?</H2>

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

sent 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>

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>

<LI><H2>But why does it happen?</H2>

There are numerous reasons for it happening, some of them may not

yet be fully clear.  What is known follows.<P>

<H3>Buggy clients and persistent connections</H3>

Several clients have a bug which pops up when dealing with

<A HREF="../keepalive.html">persistent connections</A> (aka keepalives).

When the connection is idle and the server closes the connection

(based on the <A HREF="../mod/core.html#keepalivetimeout">

KeepAliveTimeout</A>), 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>The user exits the client, which on some (most?) clients

           causes the OS to fully shutdown the connection.

       <LI>The FIN_WAIT_2 times out, on servers that have a timeout

           for this state.

</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>

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>Mozilla/2.02 (X11; I; FreeBSD 2.1.5-RELEASE i386)

       <LI>Mozilla/3.01Gold (X11; I; SunOS 5.5 sun4m)

       <LI>MSIE 3.01 on the Macintosh

       <LI>MSIE 3.01 on Windows 95

</UL><P>

This does not appear to be a problem on:

<UL>

       <LI>Mozilla/3.01 (Win95; I)

</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 /export/home/cvs/CVSROOT/cvsedit

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