Mac OSX – Testing Packet Fragmentation Over VPN

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KB ID 0001204 Dtd 15/06/16

Problem

Many moons ago I wrote a post about a problem where I had no RDP over a VPN connection, and all the hoops I jumped though to troubleshoot and fix the problem.

Today I had a similar problem, I was connected to a client via Cisco AnyConnect, and I had hair-pinned that traffic, from the client site, over an IPSEC VPN to their servers in the Data Center. Pings were successful, but not RDP. 

To be honest this affects various types of TCP traffic, but it only ever seems to trip me up with RDP! In the past I used to ping and set the traffic to ‘not fragment’ and manually set the packet size, then I increased/decreased the packet size until I found the optimal MTU size like this;

Windows- Packet Fragementation

But these are Windows options, how can I do the same thing on my Mac?

 

Solution

Well things on the Mac are even easier! It will even find the value for you, (in a manner of sorts). You set the minimum packet size, and the maximum packet size, and ask it to increment each ping packet by one. Below I’ve narrowed the scope to fit it on one screenshot;

MAC-OSX - Packet Fragementation

Syntax;

ping -g {lowest-value} -G {highest-value} -h 1 -D {IP-address}

As you can see above, I started at 1370 and the last ‘working’ ping was sequence 8, so the last working value was 1378

 

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Author: PeteLong

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