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.
After preparing to receive connections, print the local ip address and port number to standard output.
Fall back to IPv4 sockets. This is necessary for terminally broken systems like OpenBSD which will not let IPv6 sockets connect to V4-mapped IPv6 addresses. Please note that this also applies to DNS lookups, so you will have to use an DNS resolver with an IPv6 address to accept IPv6 connections. Use DNSCACHEIP to set the DNS resolver IP dynamically.
Force IPv6 mode in UCSPI environment variables, even for IPv4 connections. This will set $PROTO to TCP6 and put IPv4-mapped IPv6 addresses in TCPLOCALIP and TCPREMOTEIP.
- -b backlog
Allow up to backlog simultaneous SYN_RECEIVEDs. Default: 20. On some systems, backlog is silently limited to 5. See listen(2) for more details.
- -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 rules.cdb.
This feature can be used to reduce latency in protocols where the client waits for a greeting from the server.
- -c limit
Do not handle more than limit simultaneous connections. If there are limit simultaneous copies of program running, defer acceptance of a new connection until one copy finishes. limit must be a positive integer. Default: 40.
- -d (Default.)
Delay sending data for a fraction of a second whenb ever the remote host is responding slowly, to make better use of the network.
Never delay sending data; enable TCP_NODELAY. This is appropriate for interactive connections.
- -I ifname
Bind to the network interface ifname ("eth0" on Linux, for example). This is only defined and needed for IPv6 link-local addresses.
- -g gid
Switch group ID to
after preparing to receive connections. gid must be a positive integer.
Leave IP options alone. If the client is sending packets along an IPv4/IPv6 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.
Quiet. Do not print any messages.
- -Q (Default.)
Print error messages.
- -u uid
Switch user ID to uid after preparing to receive connections. uid must be a positive integer.
Verbose. Print all available messages.
- -x rules.cdb
Follow the rules compiled into rules.cdb by tcprules. These rules may specify setting environment variables or rejecting connections from bad sources.
tcpserver does not read rules.cdb into memory; you can rerun tcprules to change tcpserver's behavior on the fly.
Continue processing, even if rules.cdb is not readable.
- -h (Default.)
Look up the remote host name and set TCPREMOTEHOST.
Do not look up the remote host name.
- -l localname
Do not look up the local host name; use localname for TCPLOCALHOST.
Paranoid. After looking up the remote host name, look up the IP addresses for that name, and make sure one of them matches TCPREMOTEIP. If none of them do, unset TCPREMOTEHOST.
- -P (Default.)
Attempt to obtain TCPREMOTEINFO from the remote host.
- -R (Default.)
Do not attempt to obtain TCPREMOTEINFO from the remote host.
- -t timeout
Give up on the TCPREMOTEINFO connection attempt after timeout seconds. Default: 26.
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 127.0.0.1, 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
::ffff:a.b.c.d for the IPv4 address a.b.c.d
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.
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 local information: TCPLOCALIP, TCP6LOCALIP, TCPLOCALPORT, TCP6LOCALPORT, TCPLOCALHOST, TCP6LOCALHOST.
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.
argv0(1), fixcr(1), recordio(1), tcpclient(1), tcprules(1), listen(2), tcp-environ(5)