> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mobster.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Could not connect to lockdownd: Invalid HostID (-21)

> Your iPhone no longer recognises this Mac's pairing. Unplug it, plug it back in and tap Trust.

You see this: libimobiledevice prints `ERROR: Could not connect to lockdownd: Invalid HostID (-21)` when this Mac's pairing record for the phone is missing or no longer matches.

## Why it happens

The phone forgot this Mac, or this Mac holds an old pairing.

## Fix

1. Unplug the iPhone and plug it back in.
2. Unlock it and tap **Trust**, then enter your passcode.
3. Choose **Check again** in Mobster for Mac, or run `mobster doctor`.

## Check it worked

`idevice_id -l` prints the phone's UDID, and `mobster doctor` prints no `not trusted` line for it.

Checked 26 Oct 2026 against Mobster 0.4.0. Back to [Troubleshooting](https://docs.mobster.dev/troubleshooting.md#iphone).


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