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7.15. ( MTU ) - IP MASQ seems to be working fine but some sites don't work. This usually happens with WWW and FTP.

There are two possible reasons for this. The first one is VERY common and the second is very UNCOMMON.

7.15.1. Changing the MTU of a PPP link:

  • To fix the MTU issue on your PPP link, edit your /etc/ppp/options file and towards the top, add the following text on two seperate lines: "mtu 1500" and "mru 1500". Save these new changes and then restart PPP. Like above, again verify that your PPP link has the correct MTU and MTU.

  • To fix the MTU issue on a standard Ethernet link to your bridged or routed DSL, Cablemodem, etc. connection, you need to edit the correct network startup scripts for your Linux distribution. Please see the TrinityOS - Section 16 document for network optimizations.

7.15.2. Old UNIX Serial interfaces:

  • Lastly, though this isn't a common problem, some users have found the solution to the following problem. With PPP users, verify what port is your PPPd code connecting to. Is it a /dev/cua* port or a /dev/ttyS* port? It NEEDS to be a /dev/ttyS* port. The cua style is OLD and it breaks some things in very odd ways.

7.15.3. PPPoE Users:

For those users who use PPPoE (this has a maximum MTU of 1490) or for those users who choose NOT to use an MTU of 1500, not is all lost. If you reconfigure ALL of your MASQed PCs to use the SAME MTU as your external Internet link's MTU, everything should work fine. It should be noted that some PPPoE ISPs might require an MTU of 1460 for proper connectivity.

How would you do this? Follow these simple steps for your respective operating system.

The follow examples utilizes an MTU of 1490 for typical PPPoE connections for some DSL and Cablemodem users. It is recommended to use the HIGHEST values possible for all connections that are 128Kb/s and faster.

The only real reason to use smaller MTUs is to lower latency but at the cost of throughput. Please see:

http://www.ecst.csuchico.edu/~dranch/PPP/ppp-performance.html#mtu

for more details on this topic.

*** If you have had SUCCESS, FAILURE, or have procedures for OTHER operating *** systems, please email David Ranch. Thanks!

7.15.4. Linux:

------------------------------------------
1. The setting of MTU can vary from Linux distribution to distribution.  

   For Redhat: You need to edit the various "ifconfig" statements in 
               the /sbin/ifup script

   For Slackware: You need to edit the various "ifconfig" statements in 
                  the /etc/rc.d/rc1.inet

2. Here is one good, any-distribution-will-work example, edit the 
   /etc/rc.d/rc.local file and put the following at the END of the file: 

        echo "Changing the MTU of ETH0"
        /sbin/ifconfig eth0 mtu 1490

     Replace "eth0" with the interface name that is the machine's upstream 
     connection which is connected to the Internet.

3. For advanced options like "TCP Receive Windows" and such, detailed examples
   on how to edit the respective networking scripts for your specific Linux
   distro, etc., please see Chapter 16 of 
   http://www.ecst.csuchico.edu/~dranch/LINUX/index-linux.html#trinityos 
------------------------------------------

7.15.5. MS Windows 95:

------------------------------------------
1. Making ANY changes to the Registry is inheritantly risky but
   with a backup copy, you should be safe.  Proceed at your OWN RISK.

2. Goto Start-->Run-->RegEdit

3. You should make a backup copy of your Registry before continuing.  To
   do this, copy the "user.dat" and "system.dat" files from the \WINDOWS 
   directory and put them into a safe place.  It should be noted that the
   previously mentioned method of using "Regedit: Registry-->Export Registry 
   File-->Save a copy of your registry" would only do Registry MERGES and NOT 
   do a replacement.

4. Search through each of the Registry trees that end in "n" (e.g. 0007) 
   and have a Registry entry called "IPAddress", which has the IP address
   of your NIC.  Under that key, add the following:

   From http://support.microsoft.com/support/kb/articles/q158/4/74.asp

     [Hkey_Local_Machine\System\CurrentControlset\Services\Class\NetTrans\000n]

         type=DWORD
         name="MaxMTU"           (Do NOT include the quotes)
         value=1490 (Decimal)    (Do NOT include the text "(Decimal)")

         type=DWORD
         name="MaxMSS"           (Do NOT include the quotes)
         value=1450 (Decimal)    (Do NOT include the text "(Decimal>")


5. You can also change the "TCP Receive Window" which sometimes
   increases network performance SUBSTANTIALLY.  If you notice your
   throughput has DECREASED, put these items BACK to their original 
   settings and reboot.

     [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP]
        type=DWORD
        name="DefaultRcvWindow"   (Do NOT include the quotes)
        value=32768 (Decimal)     (Do NOT include the text "(Decimal>")

        type=DWORD
        name="DefaultTTL"         (Do NOT include the quotes)
        value=128 (Decimal)       (Do NOT include the text "(Decimal>")


6. Reboot to let the changes take effect.
------------------------------------------

7.15.6. MS Windows 98:

------------------------------------------
1. Making ANY changes to the Registry is inheritantly risky but
   with a backup copy, you should be safe.  Proceed at your OWN RISK.

2. Goto Start-->Run-->RegEdit

3. You should make a backup copy of your Registry before doing anything.  To
   do this, copy the "user.dat" and "system.dat" files from the \WINDOWS 
   directory and put them into a safe place.  It should be noted that the
   previously mentioned method of using "Regedit: Registry-->Export Registry 
   File-->Save a copy of your registry" would only perform Registry MERGES 
   and NOT do a replacement.

4. Search though each of the Registry trees that end in "n" (e.g. 0007) 
   and have a Registry entry called "IPAddress" which has the IP address
   of your NIC.  Under that key, add the following:

   From http://support.microsoft.com/support/kb/articles/q158/4/74.asp

     [Hkey_Local_Machine\System\CurrentControlset\Services\Class\NetTrans\000n]
         type=STRING
         name="MaxMTU"            (Do NOT include the quotes)
         value=1490 (Decimal)     (Do NOT include the text "(Decimal)")


5. You can also change the "TCP Receive Window" which sometimes
   increases network performance SUBSTANTIALLY.  If you notice your
   throughput has DECREASED, put these items BACK to their original 
   settings and reboot.

     [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP]

        type=STRING
        name="DefaultRcvWindow"    (Do NOT include the quotes)
        value=32768 (Decimal)      (Do NOT include the text "(Decimal>")

        type=STRING
        name="DefaultTTL"          (Do NOT include the quotes)
        value=128 (Decimal)        (Do NOT include the text "(Decimal>")


6. Reboot to let the changes take effect.
------------------------------------------

7.15.7. MS Windows NT 4.x

------------------------------------------
1. Making ANY changes to the Registry is inheritantly risky but
   with a backup copy, you should be safe.  Proceed at your 
   OWN RISK.

2. Goto Start-->Run-->RegEdit

3. Registry-->Export Registry File-->Save a copy of your registry
   to a reliable place

4. Create the following keys in the Registry trees, choose two
   possible Registry trees.  Multiple entries are for various 
   network devices like DialUp Networking (ppp), Ethernet NICs, 
   PPTP VPNs, etc.

   http://support.microsoft.com/support/kb/articles/Q102/9/73.asp?LN=EN-US&SD=gn&FR=0


   [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Parameters\Tcpip]
                     and
   [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<Adapter-name>\Parameters\Tcpip]

      Replace "<Adapter-Name>" with the respective name of your uplink LAN NIC 
      interface

         type=DWORD
         name="MTU"              (Do NOT include the quotes)
         value=1490 (Decimal)    (Do NOT include the text "(Decimal>")

       (Do NOT include the quotes)


 *** If you know how to also change the MSS, TCP Window Size, and the
 *** TTL parameters in NT 4.x, please email dranch@trinnet.net as I 
 *** would love to add it to the HOWTO.

5. Reboot to make the changes take effect.
------------------------------------------

7.15.8. MS Windows 2000

------------------------------------------
1. Making ANY changes to the Registry is inheritantly risky but
   with a backup copy, you should be safe.  Proceed at your 
   OWN RISK.

2. Goto Start-->Run-->RegEdit

3. Registry-->Export Registry File-->Save a copy of your registry
   to a reliable place

4. Navigate down to the key:

   [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Inter
faces\<ID for Adapter>

   Each ID Adapter has default keys for DNS, TCP/IP address, Default Gateway, 
   subnet mask, etc. Find the key one that is for your network card.

5. Create the following Entry:

      type=DWORD
      name="MTU"				(Do NOT include the quotes)
      value=1490 (Decimal)      (Do NOT include the text "(Decimal)")

http://support.microsoft.com/support/kb/articles/Q120/6/42.asp?LN=EN-US&SD=gn&FR=0


 *** If you know how to also change the MSS, TCP Window Size, and the
 *** TTL parameters in NT 2000, please email dranch@trinnet.net as I 
 *** would love to add it to the HOWTO.

5. Reboot to let the changes take effect.
------------------------------------------

As stated above, if you know how to make similar changes like these to other OSes like OS/2, MacOS, etc. please email David Ranch so it can be included in the HOWTO.