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  1. #1
    Master Untangler
    Join Date
    Dec 2008
    Location
    Greater Omaha Area
    Posts
    259

    Default Sprint/T-Mobile Hotspot Blocking OpenVPN?

    Over the last couple three months, I've been running into more and more issues with my Sprint/T-Mobile Hotspot when trying to connect using OpenVPN. I've not had any issues using it on WiFi or some other phones but with my new S21 5G it's becoming a consistent issue. I did find some articles online about it being an issue since the APN is IPv6 but I can't add an APN with IPv4 as per the articles. Anyone encountered this and have any ideas about how to work around it?

  2. #2
    Newbie
    Join Date
    Mar 2021
    Posts
    13

    Default

    I ran into this just a week ago with my laptop connected to my Tmobile 5G hotspot n S20 Ultra 5G trying to connect to NG Firewall with Wireguard. Funny thing is, Wireguard installed on my S21 connects no problem....but not through the hotspot...never had this issue before the merger

  3. #3
    Master Untangler
    Join Date
    Dec 2008
    Location
    Greater Omaha Area
    Posts
    259

    Default

    I've tried WireGuard VPN as well with the same issues - I did spend sometime on this again this evening a managed to get it figured out.

  4. #4
    Master Untangler
    Join Date
    Dec 2008
    Location
    Greater Omaha Area
    Posts
    259

    Default

    So here's what I figured out, the Sprint/T-Mobile APN is configured for IPv6 only and all TCP traffic was being handled correct because TCP has mechanisms to lower its MSS and try again (retransmit) when the transmission fails (no ACK received). While UDP being fire and forget will never know that there is a problem continuously sending packets into oblivion. The IPv4 UDP packets with the IPv6 packet overhead turned out to be too large hence getting split and then dropped. To resolve this I needed to figure out what MTU to use, for that one can use ping -f <WAN IP Address> -l 1500 decrementing the size by 10 until the ping is successful. Adjusted the MTU within Untangle for that WAN Interface and bingo, OpenVPN as well as WireGuard VPN was working again.
    Last edited by chmcwill; 08-30-2021 at 07:50 PM.
    jcoehoorn likes this.

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