root/fwknop/tags/fwknop-0.9.7-pre1/fwknop_serv

#!/usr/bin/perl -w
#
############################################################################
#
# File: fwknop_serv
#
# Purpose: To provide a minimal TCP server over which the fwknop client can
#          connect to send the SPA packet.  This breaks the traditional SPA
#          model of only using a single packet to transmit desired access
#          modifications, but if you want to send SPA packets over the Tor
#          network then this server is necessary.  A circuit through the
#          Tor network is built up over successive TCP connections, and
#          there is no way to send packets through Tor without an
#          established circuit.
#
############################################################################
#
# $Id$
#

use IO::Socket;
use POSIX;
use strict;

my $config_file = '/etc/fwknop/fwknop.conf';
my %config = ();

my @required_vars = qw(
    TCPSERV_PORT
    TCPSERV_PID_FILE
);

&setup();

my $server = IO::Socket::INET->new(
    LocalPort => $config{'TCPSERV_PORT'},
    Type   => SOCK_STREAM,
    Reuse  => 1,
    Listen => 5
) or die $!;

&drop_privs();

### trivial loop; we just want the local TCP stack to accept connections;
### fwknopd gets it data from pcap anyway
while (my $client = $server->accept()) {

    $client->recv(my $request, 1500, 0);
}

close $server;

exit 0;

#================================= end main ===============================

sub drop_privs() {

    my ($login, $pass, $uid, $gid) = getpwnam('nobody');
    unless ($uid and $gid) {
        warn "[-] Could not get UID and GID of user nobody";
    }

    ### drop privileges
    if ($uid and $gid) {
        POSIX::setuid($uid);
        POSIX::setgid($gid);
    }

    return;
}

sub setup() {

    ### import config
    &import_config();

    ### make sure all the vars we need are actually in the config file.
    &required_vars();

    ### validate config
    &validate_config();

    my $pid = fork();
    exit 0 if $pid;
    die "[*] $0: Couldn't fork: $!" unless defined $pid;
    POSIX::setsid() or die "[*] $0: Can't start a new session: $!";

    ### write our pid out to disk
    &writepid();

    return;
}

sub validate_config() {
    unless ($config{'TCPSERV_PORT'} > 0
            and $config{'TCPSERV_PORT'} < 65535) {
        die "[*] TCPSERV_PORT must be between 1 and 65535";
    }
    return;
}

sub required_vars() {
    for my $var (@required_vars) {
        unless (defined $config{$var}) {
            die "[*] Variable $var is not defined in $config_file";
        }
    }
    return;
}

sub uniquepid() {
    if (-e $config{'TCPSERV_PID_FILE'}) {
        my $caller = $0;
        open PIDFILE, "< $config{'TCPSERV_PID_FILE'}";
        my $pid = <PIDFILE>;
        close PIDFILE;
        chomp $pid;
        if (kill 0, $pid) {  # fwknop_serv is already running
            die "[*] fwknop_serv (pid: $pid) is already running!  Exiting.\n";
        }
    }
    return;
}

sub writepid() {
    open P, "> $config{'TCPSERV_PID_FILE'}" or die "[*] Could not open ",
        "$config{'TCPSERV_PID_FILE'}: $!";
    print P $$, "\n";
    close P;
    chmod 0600, $config{'TCPSERV_PID_FILE'};
    return;
}

sub import_config() {
    open C, "< $config_file" or die "[*] Could not open ",
        "config file $config_file: $!";
    while (<C>) {
        next if /^\s*#/;
        for my $var (@required_vars) {
            if (/^\s*$var\s+(\S+);/) {
                $config{$var} = $1;
            }
        }
    }
    close C;
    return;
}