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28 .\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93
36 .Nd listen for connections on a socket
42 .Fn listen "int s" "int backlog"
44 To accept connections, a socket
47 a willingness to accept incoming connections and
48 a queue limit for incoming connections are specified with
50 and then the connections are
55 system call applies only to sockets of type
62 argument defines the maximum length the queue of
63 pending connections may grow to.
64 The real maximum queue length will be 1.5 times more than the value
70 system call on the listening socket allows the caller to change the maximum
71 queue length using a new
75 request arrives with the queue full the client may
76 receive an error with an indication of
78 or, in the case of TCP, the connection will be
81 Current queue lengths of listening sockets can be queried using
87 and the introduction of the syncache,
90 argument also determined the length of the incomplete
91 connection queue, which held TCP sockets in the process
92 of completing TCP's 3-way handshake.
93 These incomplete connections
94 are now held entirely in the syncache, which is unaffected by
98 values to help handle denial
99 of service attacks are no longer necessary.
104 .Va kern.ipc.soacceptqueue
105 specifies a hard limit on
107 if a value greater than
108 .Va kern.ipc.soacceptqueue
109 or less than zero is specified,
111 is silently forced to
112 .Va kern.ipc.soacceptqueue .
114 If the listen queue overflows, the kernel will emit a syslog message
115 using default priority LOG_DEBUG (7).
119 .Va kern.ipc.sooverprio
120 may be used to change this priority to any value in a range of 0..7
121 (LOG_EMERG..LOG_DEBUG).
125 It may be set to -1 to disable these messages.
128 .Va kern.ipc.sooverinterval
129 specifies a per-socket limit on how often the kernel will emit these messages.
130 .Sh INTERACTION WITH ACCEPT FILTERS
131 When accept filtering is used on a socket, a second queue will
132 be used to hold sockets that have connected, but have not yet
133 met their accept filtering criteria.
134 Once the criteria has been
135 met, these sockets will be moved over into the completed connection
138 If this secondary queue is full and a
139 new connection comes in, the oldest socket which has not yet met
140 its accept filter criteria will be terminated.
142 This secondary queue, like the primary listen queue, is sized
157 is not a valid descriptor.
158 .It Bq Er EDESTADDRREQ
159 The socket is not bound to a local address, and the protocol does not
160 support listening on an unbound socket.
162 The socket is already connected, or in the process of being connected.
168 The socket is not of a type that supports the operation
183 system call appeared in
185 The ability to configure the maximum
187 at run-time, and to use a negative
189 to request the maximum allowable value, was introduced in
192 .Va kern.ipc.somaxconn
194 has been replaced with
195 .Va kern.ipc.soacceptqueue
198 to prevent confusion about its actual functionality.
201 .Va kern.ipc.somaxconn
202 is still available but hidden from a
204 -a output so that existing applications and scripts continue to work.