rinetd: a user-mode port redirect utility
Redirects TCP connections from one IP address and port to another. rinetd
is a single-process server which handles any number of connections to
the address/port pairs specified in the file
Since rinetd runs as a single process using nonblocking I/O, it is
able to redirect a large number of connections without a severe
impact on the machine. This makes it practical to run TCP services
on machines inside an IP masquerading firewall. rinetd does not
redirect FTP, because FTP requires more than one socket.
rinetd is typically launched at boot time, using the following syntax:
The configuration file is found in the file
another file is specified using the
-c command line option.
Most entries in the configuration file are forwarding rules. The format of a forwarding rule is as follows:
bindaddress bindport connectaddress connectportFor example:
22.214.171.124 80 10.1.1.2 80
Would redirect all connections to port 80 of the "real" IP address 126.96.36.199, which could be a virtual interface, through rinetd to port 80 of the address 10.1.1.2, which would typically be a machine on the inside of a firewall which has no direct routing to the outside world.
Although responding on individual interfaces rather than on all
interfaces is one of rinetd's primary features, sometimes it is
preferable to respond on all IP addresses that belong to the server.
In this situation, the special IP address
can be used. For example:
0.0.0.0 23 10.1.1.2 23
Would redirect all connections to port 23, for all IP addresses assigned to the server. This is the default behavior for most other programs.
Service names can be specified instead of port numbers. On most systems, service names are defined in the file /etc/services.
Both IP addresses and hostnames are accepted for bindaddress and connectaddress.
Allow and Deny Rules
Configuration files can also contain allow and deny rules.
Allow rules which appear before the first forwarding rule are applied globally: if at least one global allow rule exists, and the address of a new connection does not satisfy at least one of the global allow rules, that connection is immediately rejected, regardless of any other rules.
Allow rules which appear after a specific forwarding rule apply to that forwarding rule only. If at least one allow rule exists for a particular forwarding rule, and the address of a new connection does not satisfy at least one of the allow rules for that forwarding rule, that connection is immediately rejected, regardless of any other rules.
Deny rules which appear before the first forwarding rule are applied globally: if the address of a new connection satisfies any of the global allow rules, that connection is immediately rejected, regardless of any other rules.
Deny rules which appear after a specific forwarding rule apply to that forwarding rule only. If the address of a new connection satisfies any of the deny rules for that forwarding rule, that connection is immediately rejected, regardless of any other rules.
The format of an allow rule is as follows:
Patterns can contain the following characters: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, . (period), ?, and *. The ? wildcard matches any one character. The * wildcard matches any number of characters, including zero.
This allow rule matches all IP addresses in the 206.125.69 class C domain.
Host names are NOT permitted in allow and deny rules. The performance cost of looking up IP addresses to find their corresponding names is prohibitive. Since rinetd is a single process server, all other connections would be forced to pause during the address lookup.
rinetd is able to produce a log file in either of two formats: tab-delimited and web server-style "common log format."
By default, rinetd does not produce a log file. To activate logging, add the following line to the configuration file:
By default, rinetd logs in a simple tab-delimited format containing the following information:
Date and time
Bytes received from client
Bytes sent to client
To activate web server-style "common log format" logging, add the following line to the configuration file:
Command line options
The -c command line option is used to specify an alternate configuration file.
The -h command line option produces a short help message.
The -v command line option displays the version number.
The kill -1 signal (SIGHUP) can be used to cause rinetd
to reload its configuration file without interrupting existing
connections. Under Linux(tm) the process id
is saved in the file
to facilitate the kill -HUP. An alternate
file name can be provided by using the
configuration file option.
The server redirected to is not able to identify the host the
client really came from. This cannot be corrected; however,
the log produced by rinetd provides a way to obtain this
information. Under Unix, sockets would theoretically lose data when closed
SO_LINGER turned off, but in Linux this is not the case
(kernel source comments support this belief on my part). On non-Linux Unix
platforms, alternate code which uses a different trick to work around
close() is provided, but this code is untested.
The logging is inadequate. The duration of the connection should be logged.
Copyright (c) 1997-2019 Thomas Boutell. This software is released for free use under the terms of the GNU General Public License, version 2 or higher.
Thanks are due to Bill Davidsen, Libor Pechachek, Sascha Ziemann, Joel S. Noble, the Apache Group, and many others who have contributed advice, encouragement and/or source code to this and other open software projects.