[firewall] T314: add firewall options for MSS clamping

* clamp MSS IPv4
  set firewall options interface pppoe0 adjust-mss '1452'

* clamp MSS IPv6
  set firewall options interface pppoe0 adjust-mss6 '1452'

* disable entire rule
  set firewall options interface pppoe0 disable

Output
------

  $ sudo iptables-save -t mangle
  # Generated by iptables-save v1.4.21 on Sun Apr 21 12:56:25 2019
  *mangle
  :PREROUTING ACCEPT [1217:439885]
  :INPUT ACCEPT [290:52459]
  :FORWARD ACCEPT [920:375774]
  :OUTPUT ACCEPT [301:100053]
  :POSTROUTING ACCEPT [1221:475827]
  :VYOS_FW_OPTIONS - [0:0]
  -A FORWARD -j VYOS_FW_OPTIONS
  -A VYOS_FW_OPTIONS -o pppoe0 -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1452
  COMMIT
  Completed on Sun Apr 21 12:56:25 2019

1 files changed, 142 insertions, 0 deletions

diff --git a/src/conf_mode/firewall_options.py b/src/conf_mode/firewall_options.py
new file mode 100755
index 000000000..e2c306904
--- /dev/null
+++ b/src/conf_mode/firewall_options.py
@@ -0,0 +1,142 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2018 VyOS maintainers and contributors
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+import sys
+import os
+import copy
+
+from vyos.config import Config
+from vyos import ConfigError
+
+default_config_data = {
+    'intf_opts': [],
+    'new_chain4': False,
+    'new_chain6': False
+}
+
+def get_config():
+    opts = copy.deepcopy(default_config_data)
+    conf = Config()
+    if not conf.exists('firewall options'):
+        return None
+    else:
+        conf.set_level('firewall options')
+
+        # Parse configuration of each individual instance
+        if conf.exists('interface'):
+            for intf in conf.list_nodes('interface'):
+                conf.set_level('firewall options interface {0}'.format(intf))
+                config = {
+                    'intf': intf,
+                    'disabled': False,
+                    'mss4': '',
+                    'mss6': ''
+                }
+
+                # Check if individual option is disabled
+                if conf.exists('disable'):
+                    config['disabled'] = True
+
+                #
+                # Get MSS value IPv4
+                #
+                if conf.exists('adjust-mss'):
+                    config['mss4'] = conf.return_value('adjust-mss')
+
+                    # We need a marker that a new iptables chain needs to be generated
+                    if not opts['new_chain4']:
+                        opts['new_chain4'] = True
+
+                #
+                # Get MSS value IPv6
+                #
+                if conf.exists('adjust-mss6'):
+                    config['mss6'] = conf.return_value('adjust-mss6')
+
+                    # We need a marker that a new ip6tables chain needs to be generated
+                    if not opts['new_chain6']:
+                        opts['new_chain6'] = True
+
+                # Append interface options to global list
+                opts['intf_opts'].append(config)
+
+    return opts
+
+def verify(tcp):
+    # syntax verification is done via cli
+    return None
+
+def apply(tcp):
+    target = 'VYOS_FW_OPTIONS'
+
+    # always cleanup iptables
+    os.system('iptables --table mangle --delete FORWARD --jump {} >&/dev/null'.format(target))
+    os.system('iptables --table mangle --flush {} >&/dev/null'.format(target))
+    os.system('iptables --table mangle --delete-chain {} >&/dev/null'.format(target))
+
+    # always cleanup ip6tables
+    os.system('ip6tables --table mangle --delete FORWARD --jump {} >&/dev/null'.format(target))
+    os.system('ip6tables --table mangle --flush {} >&/dev/null'.format(target))
+    os.system('ip6tables --table mangle --delete-chain {} >&/dev/null'.format(target))
+
+    # Setup new iptables rules
+    if tcp['new_chain4']:
+        os.system('iptables --table mangle --new-chain {} >&/dev/null'.format(target))
+        os.system('iptables --table mangle --append FORWARD --jump {} >&/dev/null'.format(target))
+
+        for opts in tcp['intf_opts']:
+            intf = opts['intf']
+            mss = opts['mss4']
+
+            # Check if this rule iis disabled
+            if opts['disabled']:
+                continue
+
+            # adjust TCP MSS per interface
+            if mss:
+                os.system('iptables --table mangle --append {} --out-interface {} --protocol tcp ' \
+                          '--tcp-flags SYN,RST SYN --jump TCPMSS --set-mss {} >&/dev/null'.format(target, intf, mss))
+
+    # Setup new ip6tables rules
+    if tcp['new_chain6']:
+        os.system('ip6tables --table mangle --new-chain {} >&/dev/null'.format(target))
+        os.system('ip6tables --table mangle --append FORWARD --jump {} >&/dev/null'.format(target))
+
+        for opts in tcp['intf_opts']:
+            intf = opts['intf']
+            mss = opts['mss6']
+
+            # Check if this rule iis disabled
+            if opts['disabled']:
+                continue
+
+            # adjust TCP MSS per interface
+            if mss:
+                os.system('ip6tables --table mangle --append {} --out-interface {} --protocol tcp ' \
+                          '--tcp-flags SYN,RST SYN --jump TCPMSS --set-mss {} >&/dev/null'.format(target, intf, mss))
+
+    return None
+
+if __name__ == '__main__':
+
+    try:
+        c = get_config()
+        verify(c)
+        apply(c)
+    except ConfigError as e:
+        print(e)
+        sys.exit(1)