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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93 29.\" $FreeBSD$ 30.\" 31.Dd April 14, 2020 32.Dt LISTEN 2 33.Os 34.Sh NAME 35.Nm listen 36.Nd listen for connections on a socket 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/socket.h 41.Ft int 42.Fn listen "int s" "int backlog" 43.Sh DESCRIPTION 44To accept connections, a socket 45is first created with 46.Xr socket 2 , 47a willingness to accept incoming connections and 48a queue limit for incoming connections are specified with 49.Fn listen , 50and then the connections are 51accepted with 52.Xr accept 2 . 53The 54.Fn listen 55system call applies only to sockets of type 56.Dv SOCK_STREAM 57or 58.Dv SOCK_SEQPACKET . 59.Pp 60The 61.Fa backlog 62argument defines the maximum length the queue of 63pending connections may grow to. 64The real maximum queue length will be 1.5 times more than the value 65specified in the 66.Fa backlog 67argument. 68A subsequent 69.Fn listen 70system call on the listening socket allows the caller to change the maximum 71queue length using a new 72.Fa backlog 73argument. 74If a connection 75request arrives with the queue full the client may 76receive an error with an indication of 77.Er ECONNREFUSED , 78or, in the case of TCP, the connection will be 79silently dropped. 80.Pp 81Current queue lengths of listening sockets can be queried using 82.Xr netstat 1 83command. 84.Pp 85Note that before 86.Fx 4.5 87and the introduction of the syncache, 88the 89.Fa backlog 90argument also determined the length of the incomplete 91connection queue, which held TCP sockets in the process 92of completing TCP's 3-way handshake. 93These incomplete connections 94are now held entirely in the syncache, which is unaffected by 95queue lengths. 96Inflated 97.Fa backlog 98values to help handle denial 99of service attacks are no longer necessary. 100.Pp 101The 102.Xr sysctl 3 103MIB variable 104.Va kern.ipc.soacceptqueue 105specifies a hard limit on 106.Fa backlog ; 107if a value greater than 108.Va kern.ipc.soacceptqueue 109or less than zero is specified, 110.Fa backlog 111is silently forced to 112.Va kern.ipc.soacceptqueue . 113.Pp 114If the listen queue overflows, the kernel will emit a LOG_DEBUG syslog message. 115The 116.Xr sysctl 3 117MIB variable 118.Va kern.ipc.sooverinterval 119specifies a per-socket limit on how often the kernel will emit these messages. 120.Sh INTERACTION WITH ACCEPT FILTERS 121When accept filtering is used on a socket, a second queue will 122be used to hold sockets that have connected, but have not yet 123met their accept filtering criteria. 124Once the criteria has been 125met, these sockets will be moved over into the completed connection 126queue to be 127.Xr accept 2 Ns ed . 128If this secondary queue is full and a 129new connection comes in, the oldest socket which has not yet met 130its accept filter criteria will be terminated. 131.Pp 132This secondary queue, like the primary listen queue, is sized 133according to the 134.Fa backlog 135argument. 136.Sh RETURN VALUES 137.Rv -std listen 138.Sh ERRORS 139The 140.Fn listen 141system call 142will fail if: 143.Bl -tag -width Er 144.It Bq Er EBADF 145The argument 146.Fa s 147is not a valid descriptor. 148.It Bq Er EDESTADDRREQ 149The socket is not bound to a local address, and the protocol does not 150support listening on an unbound socket. 151.It Bq Er EINVAL 152The socket is already connected, or in the process of being connected. 153.It Bq Er ENOTSOCK 154The argument 155.Fa s 156is not a socket. 157.It Bq Er EOPNOTSUPP 158The socket is not of a type that supports the operation 159.Fn listen . 160.El 161.Sh SEE ALSO 162.Xr netstat 1 , 163.Xr accept 2 , 164.Xr connect 2 , 165.Xr socket 2 , 166.Xr sysctl 3 , 167.Xr sysctl 8 , 168.Xr accept_filter 9 169.Sh HISTORY 170The 171.Fn listen 172system call appeared in 173.Bx 4.2 . 174The ability to configure the maximum 175.Fa backlog 176at run-time, and to use a negative 177.Fa backlog 178to request the maximum allowable value, was introduced in 179.Fx 2.2 . 180The 181.Va kern.ipc.somaxconn 182.Xr sysctl 3 183has been replaced with 184.Va kern.ipc.soacceptqueue 185in 186.Fx 10.0 187to prevent confusion about its actual functionality. 188The original 189.Xr sysctl 3 190.Va kern.ipc.somaxconn 191is still available but hidden from a 192.Xr sysctl 3 193-a output so that existing applications and scripts continue to work. 194