[resolved]system on LAN not found any more

[jlittle]

I have a tiny PC being used for streaming. I have been connecting to it for months using ssh, but today

$ ssh johnl@clare
ssh: Could not resolve hostname clare: Device or resource busy

What could have changed? I know I can just put "clare" in /etc/hosts, but I didn't need it there last week, so why now?

[Snowhog]

I'm sure you already know all this, but...

AI Overview

The error "ssh: Could not resolve hostname clare: Device or resource busy" indicates a problem with
hostname resolution (DNS issue), where the system cannot translate the name clare into an IP address. The "Device or resource busy" part suggests that a related network or name-service process might be locked or stuck.
Here are the steps to troubleshoot and resolve this issue on Kubuntu:

1. Verify Hostname Resolution using IP Address
The quickest way to confirm the issue is with the hostname, not SSH itself, is to try connecting directly with the target machine's IP address.

• Try SSH with IP: bash
  
  ssh user@<IP_Address_of_clare>
  
  
  
  (Replace <IP_Address_of_clare> with the actual IP address, e.g., 192.168.1.10).
• If this works, the problem is definitely name resolution.

2. Check the Local Hosts File
Ensure clare is correctly mapped in your local /etc/hosts file.

• Open the file in a text editor: bash
  
  sudo nano /etc/hosts
• Add an entry for clare using its IP address if it's a local network device:
  192.168.1.10 clare
  
  
  
  (Use the correct IP address for clare).
• Save and close the file.

3. Check Network Name Service Switch Configuration
The /etc/nsswitch.conf file determines the order in which your system resolves hostnames (e.g., local files, then DNS).

• Check the configuration: bash
  
  cat /etc/nsswitch.conf
• Ensure the line for hosts: includes both files and dns (and possibly mdns if using .local hostnames with Avahi). The line should look similar to this:
  hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4

4. Troubleshoot DNS Service
The "Device or resource busy" error strongly suggests that a service responsible for name resolution might be in a bad state. Restarting the system's DNS resolver service can clear the busy state.

• Restart systemd-resolved (common on modern Ubuntu/Kubuntu): bash
  
  sudo systemctl restart systemd-resolved.service
• Check its status: bash
  
  sudo systemctl status systemd-resolved.service

5. Verify Network Configuration and Firewall

• Check your DNS server settings in /etc/resolv.conf to ensure they are correct for your network. Incorrect DNS servers can cause resolution failures.
• Check if a firewall (like ufw) is blocking mDNS (port 5353) or general DNS requests, as this can lead to the "busy" state if requests time out.

6. Restart Your Machine
If software restarts don't fix the underlying "device busy" problem, a full system reboot might be necessary to reset all network interfaces and related processes.

[jlittle]

A router restart has brought back the host being recognized; weird that it had not happened before. I've been pinging a printer by name for years, it was forgotten too.

The router is really old, supplied by the ISP in 2014 I think. The ethernet ports do gigabit, or at least an internet speed test says 900/500.

[Snowhog]

The router is really old, supplied by the ISP in 2014 I think.

If you are still with that ISP, they should replace that router at no cost to you.

[jlittle]

If you are still with that ISP, they should replace that router at no cost to you.

I'll look into that, thank you.