Skip to content

Document cellular hotspot MTU fix for Lima VMs #181

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
samestep wants to merge 1 commit into from
Closed

Document cellular hotspot MTU fix for Lima VMs #181

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
40 changes: 40 additions & 0 deletions README.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -255,6 +255,46 @@ And run this repo's script to generate `~/.ssh/tailnet`:
tailnet
```

### 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 the `vzNAT` interface advertises, causing a [path MTU black hole](https://en.wikipedia.org/wiki/Path_MTU_Discovery#Problems): DNS, `ping`, and TCP handshakes all work, but HTTPS connections hang right after sending the TLS ClientHello, so tools like `apt` (against HTTPS repos) and Claude Code time out while small-packet traffic looks healthy.

The `vzNAT` interface is normally named `lima0`, and it takes priority over Lima's user-mode `eth0` because of its lower route metric. Confirm that Internet traffic uses it:

```sh
ip route get 8.8.8.8
```

Then lower its MTU (use 1280 instead if 1400 still fails):

```sh
sudo ip link set dev lima0 mtu 1400
```

To make that persist across reboots, create a Netplan override, substituting the MAC address reported by `ip link show lima0`:

```sh
sudo tee /etc/netplan/99-lima-mtu.yaml >/dev/null <<'EOF'
network:
version: 2
ethernets:
lima0:
match:
macaddress: "52:55:55:25:b0:2c"
mtu: 1400
EOF
sudo chmod 600 /etc/netplan/99-lima-mtu.yaml
sudo netplan generate
sudo netplan try
```

After rebooting, verify the setting and HTTPS connectivity:

```sh
ip link show lima0
curl -fsS https://download.docker.com/ >/dev/null
```

## [Tart](tart)

This config can be used for macOS VMs created with [Tart](https://tart.run/), which comes with the host-side macOS config in this repo. First, download a macOS image:
Expand Down