FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
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Article Id 191381
DescriptionWhen browsing the Internet, some sites are not viewable.
  • All FortiGate units
  • FortiOS 3.0
Steps or Commands

Problem: Certain web sites are not viewable. The Fortigate is configured to use PPPoE to connect to the ISP.

Solution:Use the "tcp-mss-sender" option in the firewall policy configuration.


HTTP Client----(internal)FGT(pppoe)----dsl----ISP----Internet----Web Server
----Ethernet MTU 1500----PPPoE MTU 1492………..Ethernet MTU 1500

The reason for this is that a PPPoE frame takes an extra eight bytes off the standard Ethernet MTU of 1500. When the server sends the large packet with DF bit set to 1, the ADSL provider's router either does not send an 'ICMP fragmentation needed' packet or the packet gets dropped along the path to the web server. In either case, the web server never knows a fragmentation is required to reach the client.

After you configure 'set tcp-mss-sender' on the firewall policy setting, this command changes the incoming packets and sends the packets with a new TCP MSS (maximum sending size) value out the downstream (external) interface. By default the MSS is MTU minus 40 byes (TCP and IP headers). When the HTTP client initiates a TCP connection, the following example changes the MSS value from 1460 to 1452 when leaving the PPPoE interface and eventually reaches the web server. The web server will also choose the smaller MSS, and therefore no fragmentation is needed. The client can now view web pages properly.

config firewall policy
   edit 1
     set srcintf "internal"
     set dstintf "external"
     set srcaddr "all"
     set dstaddr ""
     set action accept
     set schedule "always"
     set service "ANY"
     set tcp-mss-sender 1452
     set nat enable

Alternatively, you can also edit the option on the internal interface(s) of the FortiGate unit rather than individual firewall policies.

For example:

config system interface
  edit <port_name>
    set tcp-mss 1452

The <port_name> can be replaced by any internal-facing port.