So here I do want the kernel to fragment it given the fact that DF=0. Guess because route MTU is not honored for forwarded traffic ( fixed recently in kernel), packets are not fragmented on the VPN gateway node before VTI interface. The gateway node reassemble it and send it into VTI interface. Problem is I cannot ping into the oppsite domain. I have been able to establish a working tunnel, at least according to the routers they both state 'IPsec SA Established'. Office 2 Server running SBS2003 premium into a Netgear FVS318G router. Because when this node is acting as a vpn gateway, another node may send fragmented packets to the gateway node based on path MTU. Office 1 Server running SBS2008 premium Into a Netgear SXR5308 Router/Firewall. However, when I test to a host across the VPN, I can only send packets of 1410 bytes (MTU 1438). I want the kernel to fragment the packet when DF=0.
Set mtu for vpn tunnel windows#
To manually test the maximum MTU size on a path, you can use the ping command on a Windows computer.
Following pings are working because packets are fragmented by kernel based on the path MTU (1438)Because options such as tunnel key (RFC 2890) are not supported, the GRE+IP IP header will always be 24 bytes.
Set mtu for vpn tunnel plus#
Because -M dont, DF is not set for the packet. For GRE over IPsec, the IP MTU of the GRE tunnel interface should be set below the egress interface MTU by at least the overhead of IPsec encryption and the 24-byte GRE+IP header (20-byte IP header plus 4-byte GRE header). Note that the first packet triggers a ICMP frag needed. Even though there is a different value between the outputs, the firewall shows a correct value in both cases. tunnel linux:a1 id: 1 type: IPSec tunnel mtu: 1448. Doesn't kernel ipsec stack support that?Įth0: 1500 ping 10.240.3.2 -s 1411 -M dont I know PMTUD, but I do want it to fragment the packets (DF=0).
Instead, kernel is returning a ICMP packet frag needed.
I find that even DF bit is not set in the packet, it's not fragmented. A VTI interface is created to route traffic through the ipsec tunnel. I used strongswan to connect to a google cloud VPN gateway.