From fd15f9d2ab6a7e5bbc07ff2e8b10c064984492ce Mon Sep 17 00:00:00 2001
From: Viacheslav Hletenko <v.gletenko@vyos.io>
Date: Thu, 18 Aug 2022 17:09:17 +0000
Subject: firewall: T4622: Add TCP MSS option

Ability to drop|accept packets based on TCP MSS size
set firewall name <tag> rule <tag> tcp mss '501-1460'
---
 interface-definitions/include/firewall/tcp-flags.xml.i | 17 +++++++++++++++++
 python/vyos/firewall.py                                |  5 +++++
 smoketest/scripts/cli/test_firewall.py                 |  8 +++++++-
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/interface-definitions/include/firewall/tcp-flags.xml.i b/interface-definitions/include/firewall/tcp-flags.xml.i
index b99896687..5a7b5a8d3 100644
--- a/interface-definitions/include/firewall/tcp-flags.xml.i
+++ b/interface-definitions/include/firewall/tcp-flags.xml.i
@@ -114,6 +114,23 @@
         </node>
       </children>
     </node>
+    <leafNode name="mss">
+      <properties>
+        <help>Maximum segment size (MSS)</help>
+        <valueHelp>
+          <format>u32:1-16384</format>
+          <description>Maximum segment size</description>
+        </valueHelp>
+        <valueHelp>
+          <format>&lt;min&gt;-&lt;max&gt;</format>
+          <description>TCP MSS range (use '-' as delimiter)</description>
+        </valueHelp>
+        <constraint>
+          <validator name="numeric" argument="--range 1-16384"/>
+          <validator name="range" argument="--min=1 --max=16384"/>
+        </constraint>
+      </properties>
+    </leafNode>
   </children>
 </node>
 <!-- include end -->
diff --git a/python/vyos/firewall.py b/python/vyos/firewall.py
index 3e2de4c3f..663c4394a 100644
--- a/python/vyos/firewall.py
+++ b/python/vyos/firewall.py
@@ -297,6 +297,11 @@ def parse_rule(rule_conf, fw_name, rule_id, ip_name):
     if tcp_flags:
         output.append(parse_tcp_flags(tcp_flags))
 
+    # TCP MSS
+    tcp_mss = dict_search_args(rule_conf, 'tcp', 'mss')
+    if tcp_mss:
+        output.append(f'tcp option maxseg size {tcp_mss}')
+
     output.append('counter')
 
     if 'set' in rule_conf:
diff --git a/smoketest/scripts/cli/test_firewall.py b/smoketest/scripts/cli/test_firewall.py
index 4de90e1ec..684a07681 100755
--- a/smoketest/scripts/cli/test_firewall.py
+++ b/smoketest/scripts/cli/test_firewall.py
@@ -177,6 +177,7 @@ class TestFirewall(VyOSUnitTestSHIM.TestCase):
         self.verify_nftables(nftables_search, 'ip filter')
 
     def test_basic_rules(self):
+        mss_range = '501-1460'
         self.cli_set(['firewall', 'name', 'smoketest', 'default-action', 'drop'])
         self.cli_set(['firewall', 'name', 'smoketest', 'enable-default-log'])
         self.cli_set(['firewall', 'name', 'smoketest', 'rule', '1', 'action', 'accept'])
@@ -203,6 +204,10 @@ class TestFirewall(VyOSUnitTestSHIM.TestCase):
         self.cli_set(['firewall', 'name', 'smoketest', 'rule', '4', 'destination', 'port', '22'])
         self.cli_set(['firewall', 'name', 'smoketest', 'rule', '4', 'recent', 'count', '10'])
         self.cli_set(['firewall', 'name', 'smoketest', 'rule', '4', 'recent', 'time', 'minute'])
+        self.cli_set(['firewall', 'name', 'smoketest', 'rule', '5', 'action', 'accept'])
+        self.cli_set(['firewall', 'name', 'smoketest', 'rule', '5', 'protocol', 'tcp'])
+        self.cli_set(['firewall', 'name', 'smoketest', 'rule', '5', 'tcp', 'flags', 'syn'])
+        self.cli_set(['firewall', 'name', 'smoketest', 'rule', '5', 'tcp', 'mss', mss_range])
 
         self.cli_set(['interfaces', 'ethernet', 'eth0', 'firewall', 'in', 'name', 'smoketest'])
 
@@ -214,7 +219,8 @@ class TestFirewall(VyOSUnitTestSHIM.TestCase):
             ['tcp flags & (syn | ack) == syn', 'tcp dport { 8888 }', 'log prefix "[smoketest-2-R]" level err', 'ip ttl > 102', 'reject'],
             ['tcp dport { 22 }', 'limit rate 5/minute', 'return'],
             ['log prefix "[smoketest-default-D]"','smoketest default-action', 'drop'],
-            ['tcp dport { 22 }', 'add @RECENT_smoketest_4 { ip saddr limit rate over 10/minute burst 10 packets }', 'drop']
+            ['tcp dport { 22 }', 'add @RECENT_smoketest_4 { ip saddr limit rate over 10/minute burst 10 packets }', 'drop'],
+            [f'tcp flags & syn == syn tcp option maxseg size {mss_range}']
         ]
 
         self.verify_nftables(nftables_search, 'ip filter')
-- 
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