---
name: "tcpserver"
section: "1"
platform: "linux"
package: "ucspi-tcp"
version: "1:0.88-13"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/tcpserver"
---

# tcpserver(1)

accepts incoming TCP connections.

## NAME

tcpserver - accepts incoming TCP connections.

## SYNOPSIS

**tcpserver** \[ *opts* \] *host* *port* *prog*

## DESCRIPTION

*opts* is a series of getopt-style options. *host* is one argument. *port* is one argument. *prog* consists of one or more arguments.

**tcpserver** waits for connections from TCP clients. For each connection, it runs *prog*, with descriptor 0 reading from the network and descriptor 1 writing to the network. It also sets up several environment variables.

The server's address is given by *host* and *port*. *port* may be a name from /etc/services or a number; if it is 0, **tcpserver** will choose a free TCP port. *host* may be 0, allowing connections to any local IP address; or a dotted-decimal IP address, allowing connections only to that address; or a host name, allowing connections to the first IP address for that host. Host names are fed through qualification using dns\_ip4\_qualify.

**tcpserver** exits when it receives SIGTERM.

## OPTIONS

**General options:**

- **-q**

  Quiet. Do not print error messages.

- **-Q**

  (Default.) Print error messages.

- **-v**

  Verbose. Print error messages and status messages.

**Connection options:**

- **-c *n***

  Do not handle more than *n* simultaneous connections. If there are *n* simultaneous copies of prog running, defer acceptance of a new connection until one copy finishes. *n* must be a positive integer. Default: 40.

- **-x *cdb***

  Follow the rules compiled into *cdb* by [**tcprules**(1)](/tcprules/1.md). These rules may specify setting environment variables or rejecting connections from bad sources. You can rerun [**tcprules**(1)](/tcprules/1.md) to change the rules while **tcpserver** is running.

- **-X**

  With **-x *cdb,*** allow connections even if *cdb* does not exist. Normally **tcpserver** will drop the connection if *cdb* does not exist.

- **-B *banner***

  Write *banner* to the network immediately after each connection is made. **tcpserver** writes *banner* before looking up $TCPREMOTEHOST, before looking up $TCPREMOTEINFO, and before checking *cdb*. This feature can be used to reduce latency in protocols where the client waits for a greeting from the server.

- **-g *gid***

  Switch group ID to *gid* after preparing to receive connections. *gid* must be a positive integer.

- **-u *uid***

  Switch user ID to *uid* after preparing to receive connections. *uid* must be a positive integer.

- **-U**

  Same as **-g** $GID **-u** $UID. Typically $GID and $UID are set by [**envuidgid**(8)](/envuidgid/8.md).

- **-1**

  After preparing to receive connections, print the local port number to standard output.

- **-b *n***

  Allow a backlog of approximately *n* TCP SYNs. On some systems, *n* is silently limited to 5. On systems supporting SYN cookies, the backlog is irrelevant.

- **-o**

  Leave IP options alone. If the client is sending packets along an IP source route, send packets back along the same route.

- **-O**

  (Default.) Kill IP options. A client can still use source routing to connect and to send data, but packets will be sent back along the default route.

- **-d**

  Delay sending data for a fraction of a second whenever the remote host is responding slowly. This is currently the default, but it may not be in the future; if you want it, set it explicitly.

- **-D**

  Never delay sending data; enable TCP\_NODELAY.

**Data-gathering options:**

- **-h**

  (Default.) Look up the remote host name in DNS to set the environment variable $TCPREMOTEHOST.

- **-H**

  Do not look up the remote host name in DNS; remove the environment variable $TCPREMOTEHOST. To avoid loops, you must use this option for servers on TCP port 53.

- **-p**

  Paranoid. After looking up the remote host name in DNS, look up the IP addresses in DNS for that host name, and remove the environment variable $TCPREMOTEHOST if none of the addresses match the client's IP address.

- **-P**

  (Default.) Not paranoid.

- **-l *localname***

  Do not look up the local host name in DNS; use *localname* for the environment variable $TCPLOCALHOST. A common choice for *localname* is 0. To avoid loops, you must use this option for servers on TCP port 53.

- **-r**

  (Default.) Attempt to obtain $TCPREMOTEINFO from the remote host.

- **-R**

  Do not attempt to obtain $TCPREMOTEINFO from the remote host. To avoid loops, you must use this option for servers on TCP ports 53 and 113.

- **-t *n***

  Give up on the $TCPREMOTEINFO connection attempt after *n* seconds. Default: 26.

## SEE ALSO

[tcprules(1)](/tcprules/1.md), [tcprulescheck(1)](/tcprulescheck/1.md), [argv0(1)](/argv0/1.md), [fixcrio(1)](/fixcrio/1.md), [recordio(1)](/recordio/1.md), [rblsmtpd(1)](/rblsmtpd/1.md), [tcpclient(1)](/tcpclient/1.md), who@(1), date@(1), finger@(1), http@(1), [tcpcat(1)](/tcpcat/1.md), [mconnect(1)](/mconnect/1.md), [tcp-environ(5)](/tcp-environ/5.md)

http://cr.yp.to/ucspi-tcp.html