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Document Tart cellular hotspot MTU fix #195

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Expand Up @@ -185,6 +185,40 @@ Start up the VM, optionally using the [`--nested`](https://tart.run/faq/#nested-
tart run --no-graphics --nested ubuntu
```

### Cellular hotspot MTU

When the Mac is connected to the Internet through a cellular hotspot, the end-to-end path may not support the 1500-byte MTU that Tart's NAT interface advertises, causing a [path MTU black hole](https://en.wikipedia.org/wiki/Path_MTU_Discovery#Problems). DNS, `ping`, and TCP handshakes may still work, while HTTPS connections hang after sending the TLS ClientHello, so tools such as Git and Claude Code time out.

Lower the Tart network interface's MTU (use 1280 if 1400 still fails):

```sh
sudo ip link set dev enp0s1 mtu 1400
```

To make this persist across reboots, create a Netplan override, substituting the interface name and MAC address reported by `ip route get 8.8.8.8` and `ip link show enp0s1`:

```sh
sudo tee /etc/netplan/99-tart-mtu.yaml >/dev/null <<'EOF'
network:
version: 2
ethernets:
tart:
match:
macaddress: "9e:a0:61:3b:12:50"
mtu: 1400
EOF
sudo chmod 600 /etc/netplan/99-tart-mtu.yaml
sudo netplan generate
sudo netplan apply
```

Verify the setting and HTTPS connectivity:

```sh
ip link show enp0s1
git ls-remote https://github.com/git/git.git >/dev/null
```

Leave that running and, in a different terminal, give the VM your public SSH key give the VM your public SSH key so you don't need to type the password each time you connect:

```sh
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