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Jan 20, 2025
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docs(FAQ): document unsupported DHCP client identifiers #1009

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docs(FAQ): document unsupported DHCP client identifiers
edigaryev Jan 20, 2025
98ba367
New section "Resolving the VMs IP with bridged networking"
edigaryev Jan 20, 2025
b7d1a40
Remove extraneous space in ` --resolver=arp`
edigaryev Jan 20, 2025
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Better section name
edigaryev Jan 20, 2025
c21cebf
Add a note about Linux talkativeness
edigaryev Jan 20, 2025
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Explain "talkativeness" a bit better
edigaryev Jan 20, 2025
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51 changes: 51 additions & 0 deletions docs/faq.md
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Expand Up @@ -77,6 +77,57 @@ sudo rm /var/db/dhcpd_leases

And no worries, this file will be re-created on the next `tart run`.

## Unsupported DHCP client identifiers

Due to the limitations of the macOS built-in DHCP server, `tart ip` is unable to correctly report the IP addresses for VMs using DHCP client identifiers that are not based on VMs link-layer addresses (MAC addresses).

By default, when [no `--resolver=arp` is specified](#resolving-the-vms-ip-when-using-bridged-networking), `tart ip` reads the `/var/db/dhcpd_leases` file and tries to find the freshest entry that matches the VM's MAC address (based on the `hw_address` field).

However, things starts to break when the VM uses a [DUID-EN](https://metebalci.com/blog/a-note-on-dhcpv6-duid-and-prefix-delegation#duid-types) identifier, for example. One of the notorious examples of this being Ubuntu, using this type of identifier by default on latest versions. This results in the `/var/db/dhcpd_leases` entry for Ubuntu appearing as follows:

```ini
{
name=ubuntu
ip_address=192.168.64.3
hw_address=ff,f1:f5:dd:7f:0:2:0:0:ab:11:cb:fb:30:b0:97:b6:3a:67
identifier=ff,f1:f5:dd:7f:0:2:0:0:ab:11:cb:fb:30:b0:97:b6:3a:67
lease=0x678e2ce7
}
```

Because the macOS built-in DHCP server overwrites the `hw_address` with the `identifier`, it leaves no information about the VM's MAC address to the `tart ip`.

To avoid this issue, make sure that your VM only sends a DHCP client identifier (option 61) with link-layer address (MAC address) or that it doesn't send this option at all.

For the aforementioned Ubuntu, the solution is outlined in the section [How to integrate with Windows DHCP Server](https://netplan.readthedocs.io/en/stable/examples/#how-to-integrate-with-windows-dhcp-server) of Canonical Netplan's documentation:

```yaml
network:
version: 2
ethernets:
enp3s0:
dhcp4: yes
dhcp-identifier: mac
```

## Resolving the VM's IP when using bridged networking

When running `tart run` with `--net-bridged`, you need to invoke `tart ip` differently, because the macOS built-in DHCP server won't have any information about the VM's IP-address:

```shell
tart ip --resolver=arp <VM>
```

This causes the `tart ip` to consult the host's ARP table instead of the `/var/db/dhcpd_leases` file.
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@nirs nirs Jan 20, 2025

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I have very little experience with querying arp, but in the last time I tried to use it the vm (not a tart vm) got an ip and was accessible, but arp did not report the vm ip.

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@edigaryev edigaryev Jan 20, 2025 •
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Probably related to #873 (comment).

I'll try to document this in this section too.

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@nirs nirs Jan 20, 2025

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This seems to the common case, see
#873 (comment)

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@edigaryev edigaryev Jan 20, 2025

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Improved in c21cebf.


Note that this method of resolving the IP heavily relies on the level of VM's activity on the network, namely, exchanging ARP requests between the guest and the host.

This is normally not an issue for macOS VMs, but on Linux VMs you might need to install Samba, which includes a [NetBIOS name server](https://www.samba.org/samba/docs/current/man-html/nmbd.8.html) and exhibits the same behavior as macOS, resulting in the population of the ARP table of the host OS:

```shell
sudo apt-get install samba
```

## Running login/clone/pull/push commands over SSH

When invoking the Tart in an SSH session, you might get error like this:
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