Monday, July 20, 2015

MACOSX MTU adjustment

Here's a quick howto adjust the interface MTU under MACOSX. Why you would adjust the interface depends on a alot of  reasons here's a few;

1> tunnel ( IPSEC/GRE/4in6/6in4/VXLAN/802.1q/QinQ/etc..)
2> other vpn methods like OpenVPN or SSLVPN
3> PPPoE over head
4> PMTUd failures 
5> reducing ip fragmentation
6> NIC tcp offload issues

1st, you want to gather the interface MTU size but this requires you to get a list of the network interfaces

Here, we will adjust the WiFi adapater;

As you can see, the interface is the set defacto 1500bytes. We will adjust the interface by using the setMTU command with a new value.

NOTE: beadvise the OS will prompt you for the admin user account similar to the below

And that's how easy it is for changing a mtu setting. Most  network interface allows you  to decrease the MTU size from the 1500bytes defacto setting, but rarely it will allow you to increase the interface MTU size.

Ken Felix
NSE ( Network Security Expert) and Route/Switching Engineer.
kfelix  -----a----t---- socpuppets ---dot---com

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