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Sometimes when I log into my home desktop I get the warning about adding a new RSA host key for IP address 2412:214:8144:4fd1:2f17:54ff:ff48:5fd2

When I use last -w, I get:

thedude3   pts/14       2412:214:8144:4fd1:2f17:54ff:ff48:5fd2 Tuf Apr  7 19:34   still logged in   
thedude3   pts/13       tiktok           Tue Apr  7 19:35   still logged in

Where my laptop’s name is tiktok and both of those logins happened at just about the same time. Why does one show up as tiktok and the other show up as that random IP?

If it matters, I am going through a switch. Is it possible that the IP I am seeing there is the switch?

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  • Are you using a dynamic dns service to log in to your computer?
    – Devan
    Apr 8, 2015 at 0:58
  • Um ... I don't know what that means! I have not specifically given each computer it's own IP address if that is what you were asking. besides that, I didn't think a string like 2412:214:8144:4fd1:2f17:54ff:ff48:5fd2 was an ip address
    – drjrm3
    Apr 8, 2015 at 1:01
  • You are correct that is an ip address but it is a IPv6 address. Regarding the dynamic dns, how do you connect to the machine? It looks like ssh. If so when you connect is it similar to [email protected] or [email protected]
    – Devan
    Apr 8, 2015 at 1:08
  • ah - it is with computername.local and yes, it is with ssh.
    – drjrm3
    Apr 8, 2015 at 1:09
  • I think the problem is with the DHCP Lease period. Try logging in to router web configuration page and change DHCP lease term to longer period.
    – thuyein
    Apr 8, 2015 at 2:56

2 Answers 2

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IPv6 clients are typically configured trough Stateless address autoconfiguration (SLAAC), not DHCP. Clients tend to rotate addresses often,which is why you're probably seeing the messages.

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I can’t answer honestly about why the IPv6 IP is listed there. I’m still learning some SSH things. But the changing IP is most likely due to the use of a DHCP server on the network (most likely a home router). Everytime the desktop is off the IP address it was using will become available after a set time. When another device comes on the network the router sees the previous IP as available for assignment and assigns it to the new device.

Situation to demonstrate:

Your desktop server is connected by Wi-Fi and is assigned 192.168.0.100. It goes to sleep or shutsdown. After a while a phone connects through Wi-Fi to your network. Router assigns it the ip 192.168.0.100 (old server IP). Your server is then turned on or wakes up, it re-establishes a connection to the network. The router assigns it 192.168.0.101 (a new IP). You connect through SSH and your client sees that the IP has changed and that the key is either not listed or not the same (the RSA message you recieve).

Adding a static IP for the desktop in the DHCP server should resolve this issue and keep it from changing.

Whenever you get the RSA message I would always compare it to the known RSA key for the machine you are connecting to. If they match then everything is fine the IP just changed.

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  • The computer never turns off. Most of the IPV6 are similar except for the end. I am pretty sure the switch I am connecting through is smart enough to change paths depending on traffic, so I'm wondering if I'm just seeing internal connections in the switch each time the switch decides to take a new path, but I'm not sure.
    – drjrm3
    Apr 8, 2015 at 1:36
  • If the computer never turns of f then what is most likely happening is simply the DHCP server is forcing the device to request an IP address as if it was the first time it was connecting to the network. Reading Wikipedia entry for DHCP Specifically the DHCP discovery explains that this could be due to the DHCP server being anauthoritative server.
    – Devan
    Apr 8, 2015 at 2:19

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