tcpserver - accept incoming TCP connections


tcpserver [ -146jpPhHrRoOdDqQv ] [ -c limit ] [ -x rules.cdb ] [ -B banner] [ -g gid ] [ -u uid ] [ -b backlog ] [ -l localname ] [ -t timeout ] [ -I ifname ] host port program [ arg ... ]


tcpserver waits for connections from TCP clients. For each connection, it runs program with the given arguments, with descriptor 0 reading from the network and descriptor 1 writing to the network.

The server's address is given by host and port. host can be 0, allowing connections from any host; or a particular IP address, allowing connections only to that address; or a host name, allowing connections to the first IP address for that host. port may be a numeric port number or a port name. If port is 0, tcpserver will choose a free port.

tcpserver sets up several environment variables, as described in tcp-environ(5).

tcpserver exits when it receives SIGTERM.


Data-Gathering Options


tcpserver can specifically bound to the local available IPv4 -4 or IPv6 -6 addresses (or both; omitting this option) on the host while starting the service.

Setting up tcpserver with host 0 will bind to any available IP address on any interface, including the loopback addresses ::1 and, however excluding IPv6 link-local unicast (LLU) addresses for the given port.

In order to bind tcpserver to a LLU IPv6 address (ie. fe80::a:b:c) additionally the interface name -i ifname has to be provided.

Note: The kernel might use temporary changing SLAAC addresses.

IPv4-mapped IPv6 addresses

Setting up tcpserver with simultaneous bindings to IPv4 and IPv6 addresses, the Unix kernel emplopys the mapping

for the IPv4 address

Setting up tcpserver in IPv4 mode only -4 or in compatibility mode, IPv4 addresses are considered in their native format without the IPv6 prefix. Thus, usual IPv4 addresses can be entered in rules.cdb even in common IPv4/IPv6 bindings.

Loopback addresses

IPv4 and IPv6 have a significant different understanding of loopback addresses:


Instead of using the respective IP addresses, the alias


should be used for the binding and a potential filter-rule instead:


Environment variables read

tcpserver reads the environment variable DNSCACHEIP which can be used to specify the recursive DNS Resolver.

Environment Variables set

tcpserver provides the following environment variables:

PROTO this is either TCP or TCP6.

The interface name for IPv6 connections: TCP6INTERFACE.


The IP address of connection: TCPREMOTEIP and TCP6REMOTEIP which is the IPv4-mapped IPv6 address in case of IPv4 connections.

The remote port number: TCPREMOTEPORT, TCP6REMOTEPORT.

The hostname (FQDN) of the peer, if available: TCPREMOTEHOST, TCP6REMOTEHOST.

In case of a successful IDENT lookup: TCPREMOTEINFO.


Invoking tcpserver with the option -v will provide a logging of the session including (if possible) the hostnames in case the option -h is given, as well as the used IP addresses and ports of the peers. The displayed IP addresses are shown either als generic compactified IPv6 addresses, or as IPv6-mapped IPv4 addresses unless tcpserver is solely bound to an IPv4 address.

See also

argv0(1), fixcr(1), recordio(1), tcpclient(1), tcprules(1), listen(2), tcp-environ(5)