run "ifconfig <interface> mtu <N>", while <interface> is interface name and N is the wanted MSS value + 40. Example : ifconfig eth0 mtu 1360. The MSS will be 1320 (if there are no TCP options).
The TCP MSS Adjustment feature enables the configuration of the maximum segment size (MSS) for transient packets that traverse a router, specifically TCP segments in the SYN bit set, when Point to Point Protocol over Ethernet (PPPoE) is being used in the network. PPPoE truncates the Ethernet maximum transmission unit (MTU) 1492, and if the effective MTU on the hosts (PCs) is not changed, the router in between the host and the server can terminate the TCP sessions.
use iptables --insert OUTPUT --jump TCPMSS --protocol tcp --set-mss mss to modify the MSS value.
The ip tcp adjust-mss command specifies the MSS value on the intermediate router of the SYN packets to avoid truncation.
When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size by using the MSS option field in the TCP SYN packet. The value of the MSS field is determined by the maximum transmission unit (MTU) configuration on the host. The default MSS value for a PC is 1500 bytes.