SO_REUSEADDR behavior different from BSD

["Michael T Kerrisk" <mtk-lists@gmx.net>]

Gidday,

In the classical BSD sockets implementation, the 
SO_REUSEADDR socket option serves two purposes:

(a) to permit a socket to be bound to a port that currently 
    has a bound endpoint in the TIME_WAIT state, and

(b) to allow a terminated server to be restarted (and once
    again be bound to it's well-known port), even if there 
    is a child of the previous server still serving a 
    connection.

On Linux SO_REUSEADDR does serve this purpose.  However, 
other second scenario is mot permitted by SO_REUSEADDR.  
To make it clear what I mean, I’ll detail the scenario.  
Suppose we have the following:

                      Server (host Y)
                    
                      Create listening socket -- fd 4
                      bind() to INADDR_ANY:9999
                      listen()
                      accept()

Client (host X)

Create socket
connect() to Y:9999   <accept unblocks, fd 5 returned>

                      server forks --------+
                                            \ <child>
                      server terminates      closes fd 4
                                             <keeps running, with
                                             fd 5 holding the
                                             connection to client>

At this point, netstat on host Y shows the tuple 

    [local=Y-IP-addr:9999, remote=X-IP-addr:ephem-port] ESTABLISHED

                      New Server started
                      Create listening socket -- fd 4
                      set SO_REUSEADDR to 1 for fd 4

                      bind() fd 4 to port INADDR_ANY:9999
                      WHAT HAPPENS?

On FreeBSD, the bind() succeeds, and then using "netstat –an" 
on host Y shows:

    [local=Y-IP-addr:9999, remote=X-IP-addr:ephem-port] ESTABLISHED
and
    [local=*:9999, remote=*.*] CLOSED

But on Linux (2.6), the bind() fails with EADDRINUSE.

Why does Linux behave differently in this scenario?

Cheers,

Michael

-- 
Michael Kerrisk
mtk-lists@gmx.net

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