FR: When sharing a device into a new tailnet default to original IP address

[patrickwolf]

What are you trying to do?

When I share a device with another tailnet the device receives a brand new IP address on other tailnet
This article outlines this in detail: https://tailscale.com/blog/choose-your-ip

How should we solve this?

Very easy. When adding a shared device to a tailnet check if the original IP is in use in this tailnet.
If its in use pick a new one
If its not in use keep the original IP

This way a large amount of cases devices will have the same IP on multiple networks and it does allow to reference devices with that IP in a large number of cases.

And for the networks where there is an overlap those admins can make manual decisions.

So it defaults for easy and memorable and has options when its more complex

What is the impact of not solving this?

This makes sense and works but there is an issue. When I write a Network document its not impossible to write down IP addresses as they are different on each shared device. ie there is no general convention anymore what the IP address is

Anything else?

Thanks for the great product and for considering this change

### Tasks

[bradfitz]

When adding a shared device to a tailnet check if the original IP is in use in this tailnet.
If its in use pick a new one
If its not in use keep the original IP

Hm, I thought that was already our behavior. Did you see otherwise?

When I write a Network document its not impossible to write down IP addresses as they are different on each shared device. ie there is no general convention anymore what the IP address is

Two options not mentioned here for that:

• hostnames are stable (they just resolve differently depending on persepective)
• Tailscale IPv6 address are globally unique across tailnets, if you need to use IPs

[bartoszhernas]

I faced another problem, not sure if connected here or it's a new bug I should report?

1. User A shared a device with IP address 1.1.1.1
2. User B accepts this device and change the IP address to 1.1.1.1

Everyone is happy until...

1. User A had to reauthenticate the device, so he removed old device, registered new one, and set 1.1.1.1 as ip address
2. User B accepts the new device, but the IP address is different. He tries to select 1.1.1.1 as it's IP only to get error "Address already in use". He looks everywhere but no where is this address used anymore.

[patrickwolf]

I faced another problem, not sure if connected here or it's a new bug I should report?

yes its clearly another issue and might get more attention in its own ticket as my ticket is about default values

[patrickwolf]

Hm, I thought that was already our behavior. Did you see otherwise?

Yes Tailnet provides a random IP address instead of defaulting to the IP address set in the "master" network which is sharing the device.

Tailscale IPv6 address are globally unique across tailnets, if you need to use IPs

whoever came up with IPv6 just didn't make it very sexy for humans so I imagine IP4 will stay important for a while

So please can we implement that Tailnet defaults to using the same IP address as the host network that is sharing? There is no harm in it as its just a default and it does create more consistency. Thanks!

[bartoszhernas]

I faced another problem, not sure if connected here or it's a new bug I should report?

yes its clearly another issue and might get more attention in its own ticket as my ticket is about default values

OK, I've made a new issue #15532

[patrickwolf]

When adding a shared device to a tailnet check if the original IP is in use in this tailnet.
If its in use pick a new one
If its not in use keep the original IP

@bradfitz looks like my request might have gotten forgotten? i did put though into it and its such a simple solution without any side effects as its just adjusting the default IP from a random one to a more useful one... can you please escalate it so it gets at least considered? thank you