The `tcprules` program

tcprules compiles rules for tcpserver.

Interface

     tcprules cdb tmp

tcpserver optionally follows rules to decide whether a TCP connection is acceptable. For example, the rule

     18.23.0.32:deny

prohibits connections from IP address 18.23.0.32.

tcprules reads rules from its standard input and writes them into cdb in a binary format suited for quick access by tcpserver.

tcprules can be used while tcpserver is running. It ensures that cdb is updated atomically. It does this by first writing the rules to tmp and then moving tmp on top of cdb. If tmp already exists, it is destroyed. The directories containing cdb and tmp must be writable to tcprules; they must also be on the same filesystem.

If there is a problem with the input or with tmp, tcprules complains and leaves cdb alone.

The binary cdb format is portable across machines.

Rule format

A rule is one line. A file containing rules may also contain comments: lines beginning with # are ignored.

Each rule contains an address, a colon, and a list of instructions, with no extra spaces. When tcpserver receives a connection from that address, it follows the instructions.

Addresses

tcpserver looks for rules with various addresses:

$TCPREMOTEINFO@$TCPREMOTEIP, if $TCPREMOTEINFO is set;
$TCPREMOTEINFO@=$TCPREMOTEHOST, if $TCPREMOTEINFO is set and $TCPREMOTEHOST is set;
$TCPREMOTEIP;
=$TCPREMOTEHOST, if $TCPREMOTEHOST is set;
shorter and shorter prefixes of $TCPREMOTEIP ending with a dot;
shorter and shorter suffixes of $TCPREMOTEHOST starting with a dot, preceded by =, if $TCPREMOTEHOST is set;
=, if $TCPREMOTEHOST is set; and finally
the empty string.

tcpserver uses the first rule it finds. You should use the -p option to tcpserver if you rely on $TCPREMOTEHOST here.

For example, here are some rules:

     joe@127.0.0.1:first
     18.23.0.32:second
     :third
     127.:fourth

If $TCPREMOTEIP is 10.119.75.38, tcpserver will follow the third instructions.

If $TCPREMOTEIP is 18.23.0.32, tcpserver will follow the second instructions.

If $TCPREMOTEIP is 127.0.0.1 and $TCPREMOTEINFO is bill, tcpserver will follow the fourth instructions.

If $TCPREMOTEIP is 127.0.0.1 and $TCPREMOTEINFO is joe, tcpserver will follow the first instructions.

You can use tcprulescheck to see how tcpserver will interpret rules in cdb.

Address ranges

tcprules treats 1.2.3.37-53:ins as an abbreviation for the rules 1.2.3.37:ins, 1.2.3.38:ins, and so on up through 1.2.3.53:ins. Similarly, 10.2-3.:ins is an abbreviation for 10.2.:ins and 10.3.:ins.

Instructions

The instructions in a rule must begin with either allow or deny. deny tells tcpserver to drop the connection without running anything. For example, the rule

     :deny

tells tcpserver to drop all connections that aren't handled by more specific rules.

The instructions may continue with some environment variables, in the form ,var="x". tcpserver adds an environment variable $var with value x. For example,

     10.0.:allow,RELAYCLIENT="@fix.me"

adds an environment variable $RELAYCLIENT with value @fix.me. The quotes may be replaced by any repeated character:

     10.0.:allow,RELAYCLIENT=/@fix.me/

Any number of variables may be listed: