firewall.py

import subprocess, re
import helpers
from helpers import *


def ipt_chain_exists(name):
    argv = ['iptables', '-t', 'nat', '-nL']
    p = subprocess.Popen(argv, stdout = subprocess.PIPE)
    for line in p.stdout:
        if line.startswith('Chain %s ' % name):
            return True
    rv = p.wait()
    if rv:
        raise Fatal('%r returned %d' % (argv, rv))


def ipt(*args):
    argv = ['iptables', '-t', 'nat'] + list(args)
    debug1('>> %s\n' % ' '.join(argv))
    rv = subprocess.call(argv)
    if rv:
        raise Fatal('%r returned %d' % (argv, rv))


# We name the chain based on the transproxy port number so that it's possible
# to run multiple copies of sshuttle at the same time.  Of course, the
# multiple copies shouldn't have overlapping subnets, or only the most-
# recently-started one will win (because we use "-I OUTPUT 1" instead of
# "-A OUTPUT").
def do_iptables(port, subnets):
    chain = 'sshuttle-%s' % port

    # basic cleanup/setup of chains
    if ipt_chain_exists(chain):
        ipt('-D', 'OUTPUT', '-j', chain)
        ipt('-D', 'PREROUTING', '-j', chain)
        ipt('-F', chain)
        ipt('-X', chain)

    if subnets:
        ipt('-N', chain)
        ipt('-F', chain)
        ipt('-I', 'OUTPUT', '1', '-j', chain)
        ipt('-I', 'PREROUTING', '1', '-j', chain)

        # create new subnet entries
        for snet,swidth in subnets:
            ipt('-A', chain, '-j', 'REDIRECT',
                '--dest', '%s/%s' % (snet,swidth),
                '-p', 'tcp',
                '--to-ports', str(port),
                '-m', 'ttl', '!', '--ttl', '42'  # to prevent infinite loops
                )


def ipfw_rule_exists(n):
    argv = ['ipfw', 'list']
    p = subprocess.Popen(argv, stdout = subprocess.PIPE)
    for line in p.stdout:
        if line.startswith('%05d ' % n):
            if line[5:].find('ipttl 42') < 0:
                raise Fatal('non-sshuttle ipfw rule #%d already exists!' % n)
            return True
    rv = p.wait()
    if rv:
        raise Fatal('%r returned %d' % (argv, rv))


def sysctl_get(name):
    argv = ['sysctl', '-n', name]
    p = subprocess.Popen(argv, stdout = subprocess.PIPE)
    line = p.stdout.readline()
    rv = p.wait()
    if rv:
        raise Fatal('%r returned %d' % (argv, rv))
    if not line:
        raise Fatal('%r returned no data' % (argv,))
    assert(line[-1] == '\n')
    return line[:-1]


def _sysctl_set(name, val):
    argv = ['sysctl', '-w', '%s=%s' % (name, val)]
    debug1('>> %s\n' % ' '.join(argv))
    rv = subprocess.call(argv, stdout = open('/dev/null', 'w'))


_oldctls = []
def sysctl_set(name, val):
    oldval = sysctl_get(name)
    if str(val) != str(oldval):
        _oldctls.append((name, oldval))
        return _sysctl_set(name, val)
    

def ipfw(*args):
    argv = ['ipfw', '-q'] + list(args)
    debug1('>> %s\n' % ' '.join(argv))
    rv = subprocess.call(argv)
    if rv:
        raise Fatal('%r returned %d' % (argv, rv))


def do_ipfw(port, subnets):
    sport = str(port)

    # cleanup any existing rules
    if ipfw_rule_exists(port):
        ipfw('del', sport)

    while _oldctls:
        (name,oldval) = _oldctls.pop()
        _sysctl_set(name, oldval)

    if subnets:
        sysctl_set('net.inet.ip.fw.enable', 1)
        sysctl_set('net.inet.ip.forwarding', 1)
        
        # create new subnet entries
        for snet,swidth in subnets:
            ipfw('add', sport, 'fwd', '127.0.0.1,%d' % port,
                 'log', 'tcp',
                 'from', 'any', 'to', '%s/%s' % (snet,swidth),
                 'not', 'ipttl', '42')


def program_exists(name):
    paths = (os.getenv('PATH') or os.defpath).split(os.pathsep)
    for p in paths:
        fn = '%s/%s' % (p, name)
        if os.path.exists(fn):
            return not os.path.isdir(fn) and os.access(fn, os.X_OK)


# This is some voodoo for setting up the kernel's transparent
# proxying stuff.  If subnets is empty, we just delete our sshuttle rules;
# otherwise we delete it, then make them from scratch.
#
# This code is supposed to clean up after itself by deleting its rules on
# exit.  In case that fails, it's not the end of the world; future runs will
# supercede it in the transproxy list, at least, so the leftover rules
# are hopefully harmless.
def main(port, subnets):
    assert(port > 0)
    assert(port <= 65535)

    if os.getuid() != 0:
        raise Fatal('you must be root (or enable su/sudo) to set the firewall')

    if program_exists('ipfw'):
        do_it = do_ipfw
    elif program_exists('iptables'):
        do_it = do_iptables
    else:
        raise Fatal("can't find either ipfw or iptables; check your PATH")

    # because of limitations of the 'su' command, the *real* stdin/stdout
    # are both attached to stdout initially.  Clone stdout into stdin so we
    # can read from it.
    os.dup2(1, 0)

    debug1('firewall manager ready.\n')
    sys.stdout.write('READY\n')
    sys.stdout.flush()

    # ctrl-c shouldn't be passed along to me.  When the main sshuttle dies,
    # I'll die automatically.
    os.setsid()

    # we wait until we get some input before creating the rules.  That way,
    # sshuttle can launch us as early as possible (and get sudo password
    # authentication as early in the startup process as possible).
    line = sys.stdin.readline(128)
    if not line:
        return  # parent died; nothing to do
    if line != 'GO\n':
        raise Fatal('firewall: expected GO but got %r' % line)
    try:
        if line:
            debug1('firewall manager: starting transproxy.\n')
            do_it(port, subnets)
            sys.stdout.write('STARTED\n')
        
        try:
            sys.stdout.flush()
            
            # Now we wait until EOF or any other kind of exception.  We need
            # to stay running so that we don't need a *second* password
            # authentication at shutdown time - that cleanup is important!
            while sys.stdin.readline(128):
                pass
        except IOError:
            # the parent process died for some reason; he's surely been loud
            # enough, so no reason to report another error
            return

    finally:
        try:
            debug1('firewall manager: undoing changes.\n')
        except:
            pass
        do_it(port, [])