Install Iptables rules to set TCPMSS for 'lo' interface (#3452)

* Install Iptables rules to set TCPMSS for lo interface
* Moved implementation to hostcfgd to maintain at one place

files/image_config/hostcfgd/hostcfgd

@@ -8,6 +8,7 @@ import subprocess
 import syslog
 import copy
 import jinja2
+import ipaddr as ipaddress
 from swsssdk import ConfigDBConnector
 
 # FILE
@@ -49,6 +50,82 @@ def obfuscate(data):
     else:
         return data
 
+class Iptables(object):
+    def __init__(self):
+        '''
+        Default MSS to 1460 - (MTU 1500 - 40 (TCP/IP Overhead))
+        For IPv6, it would be 1440 - (MTU 1500 - 60 octects)
+        '''
+        self.tcpmss = 1460
+        self.tcp6mss = 1440
+
+    def is_ip_prefix_in_key(self, key):
+        '''
+        Function to check if IP address is present in the key. If it
+        is present, then the key would be a tuple or else, it shall be
+        be string
+        '''
+        return (isinstance(key, tuple))
+
+    def load(self, lpbk_table):
+        for row in lpbk_table:
+            self.iptables_handler(row, lpbk_table[row])
+
+    def command(self, chain, ip, ver, op):
+        cmd = 'iptables' if ver == '4' else 'ip6tables'
+        cmd += ' -t mangle --{} {} -p tcp --tcp-flags SYN SYN'.format(op, chain)
+        cmd += ' -d' if chain == 'PREROUTING' else ' -s'
+        mss = self.tcpmss if ver == '4' else self.tcp6mss
+        cmd += ' {} -j TCPMSS --set-mss {}'.format(ip, mss)
+
+        return cmd
+
+    def iptables_handler(self, key, data, add=True):
+        if not self.is_ip_prefix_in_key(key):
+            return
+
+        iface, ip = key
+        ip_str = ip.split("/")[0]
+        ip_addr = ipaddress.IPAddress(ip_str)
+        if isinstance(ip_addr, ipaddress.IPv6Address):
+            ver = '6'
+        else:
+            ver = '4'
+
+        self.mangle_handler(ip_str, ver, add)
+
+    def mangle_handler(self, ip, ver, add):
+        if not add:
+            op = 'delete'
+        else:
+            op = 'check'
+
+        iptables_cmds = []
+        chains = ['PREROUTING', 'POSTROUTING']
+        for chain in chains:
+            cmd = self.command(chain, ip, ver, op)
+            if not add:
+                iptables_cmds.append(cmd)
+            else:
+                '''
+                For add case, first check if rule exists. Iptables just appends to the chain
+                as a new rule even if it is the same as an existing one. Check this and
+                do nothing if rule exists
+                '''
+                ret = subprocess.call(cmd, shell=True)
+                if ret == 0:
+                    syslog.syslog(syslog.LOG_INFO, "{} rule exists in {}".format(ip, chain))
+                else:
+                    # Modify command from Check to Append
+                    iptables_cmds.append(cmd.replace("check", "append"))
+
+        for cmd in iptables_cmds:
+            syslog.syslog(syslog.LOG_INFO, "Running cmd - {}".format(cmd))
+            try:
+                subprocess.check_call(cmd, shell=True)
+            except subprocess.CalledProcessError as err:
+                syslog.syslog(syslog.LOG_ERR, "{} - failed: return code - {}, output:\n{}"
+                              .format(err.cmd, err.returncode, err.output))
 
 class AaaCfg(object):
     def __init__(self):
@@ -159,6 +236,9 @@ class HostConfigDaemon:
         self.aaacfg = AaaCfg()
         self.aaacfg.load(aaa, tacacs_global, tacacs_server)
         self.hostname_cache=""
+        lpbk_table = self.config_db.get_table('LOOPBACK_INTERFACE')
+        self.iptables = Iptables()
+        self.iptables.load(lpbk_table)
 
     def aaa_handler(self, key, data):
         self.aaacfg.aaa_update(key, data)
@@ -215,11 +295,23 @@ class HostConfigDaemon:
 
         self.hostname_cache = hostname
 
+    def lpbk_handler(self, key, data):
+        key = ConfigDBConnector.deserialize_key(key)
+        #Check if delete operation by fetch existing keys
+        keys = self.config_db.get_keys('LOOPBACK_INTERFACE')
+        if key in keys:
+            add = True
+        else:
+            add = False
+
+        self.iptables.iptables_handler(key, data, add)
+
     def start(self):
         self.config_db.subscribe('AAA', lambda table, key, data: self.aaa_handler(key, data))
         self.config_db.subscribe('TACPLUS_SERVER', lambda table, key, data: self.tacacs_server_handler(key, data))
         self.config_db.subscribe('TACPLUS', lambda table, key, data: self.tacacs_global_handler(key, data))
         self.config_db.subscribe('DEVICE_METADATA', lambda table, key, data: self.hostname_handler(key, data))
+        self.config_db.subscribe('LOOPBACK_INTERFACE', lambda table, key, data: self.lpbk_handler(key, data))
         self.config_db.listen()