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I have a MacBook Air running OS X Yosemite (even El Capitan) version 10.10.5 and I use ADSL connection to connect to my university internal network which works just fine but when I use the PPTP VPN to connect to the Internet I can not open any webpages no more however Telegram works and I can ping all the servers.

This problem only appears in MacBook/iPhone and Windows users don't have any problem.

And if I route my network to another computer (using this link) who share his VPN connection there is no problem with the Internet.

Ask me for more details

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2 Answers 2

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Finally I've found the answer to my question. As described in this link this problem is with MTU being to high. So the solution is:

It seems, with the ping functioning, that the problem is with your MTU being too high, as the VPN adds headers, it means packets are getting fragmented and dropped. To adjust the MTU on your mac, open

System Preferences > Network > [your connection] > Advanced > Hardware > Configure: {Manually,Automatically}

There you will see if it is configured automatically, it will be 1500, as that works fine on normal ethernet connections. You will want to adjust this down, there is a fairly simple way of understanding how large your MTU needs to be. Following this guide here, you can use ping to determine the optimum MTU. Connect your VPN and open a Terminal window:

Type: ping -c 2 -D -s 1472 www.youtube.com 1472 is the packet size in this case. You will get one of two reponses, ping: sendto: Message too long or a normal ping reply.

Since 1472 + 28 is 1500, you should get a Message too long error. Reduce the ping number down until you get a positive reply. take the functioning number, add 28 and use that as your MTU.

[Update]

As mentiond in this link you can turn automatic discovery on using this command:

  • Session only:

    sudo sysctl -w net.inet.tcp.path_mtu_discovery=0

  • Permenant:

    echo "net.inet.tcp.path_mtu_discovery=0" | sudo tee -a /etc/sysctl.conf

[Update2]

It seems sysctl is not working.

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On windows you can configure your VPN to NOT use the default (internet) route. Unfortunately on mac OS you can't configure that with GUI. You should be able with scripts, some of my collegues told me so, but I haven't done it myself.

So if your school is using a proxy or whatever to get out, you also have to configure on your Mac when you're connected with your VPN.

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  • You understood me wrong. VPN is perfect I have ping and routes are good. Jun 19, 2016 at 12:39

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