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Dropbear public-key authentication HowTo

For overview of public-key authentication read signature authentication.


Install SSH and SCP clients

Install OpenSSH for *nix, PuTTY for Windows. See the links at the end of this document.

Generate the key pair (on your PC)

If you don't have one yet, create it using ssh-keygen. If you use Windows, use puttygen.exe. puttygen.exe will provide you with output to be added later to dropbear config file you will need later; you also should save private key for puTTY configuration described later. puttygen.exe provide various key generation methods but RSA key is most compatible with LEDE ssh server (dropbear).

For low security (fast verification), just use 2048-bits key.


If you are concerned about the security, use 4096-bits instead. Since accessing to router via SSH hasn't been hackers' interest, 5120-bits key is high enough for security.

ssh-keygen -b 4096
# or 
ssh-keygen -b 5120

Append public key to authorized_keys (on your OpenWRT device)

Add the public key of your computer to the authorized_keys file on OpenWrt

On your PC:

ssh root@ "tee -a /etc/dropbear/authorized_keys" < ~/.ssh/

or if you are using Windows to generate keys and your LEDE has nano text editor installed (if not, do opkg install nano), you can paste text provided by PuTTYgen in previous steps, via clipboard in your puTTY SSH session

nano /etc/dropbear/authorized_keys

Key is appended to the /etc/dropbear/authorized_keys file on your OpenWRT device.

Connecting to OpenWrt with Public Key

If you did everything right, you can now login using your key. It will not ask you for a password.

Using the OpenSSH client

$ ssh root@

Using PuTTY on Windows

Start putty.exe and do the following:

  • Session: In “Host Name” enter the router's DNS name or IP address, e.g. for access from the LAN enter openwrt.lan or from the WAN (your registered dynamic DNS name). If you change the port for Dropbear, then also adopt the “Port” statement here. The protocol (“connection type”) is always “SSH”.
  • Connection → Data: In the box “Login details” enter the “Auto-login username” which is root.
  • Connection → SSH → Auth: In the box “Authentication Parameters” under “Private key file for Authentication” state the path to your private key file for this connection (e.g. the OpenWrt-Private-Key.ppk file you created before). Best is to click “Browse…” and select the file via the file dialog.
  • Session: Load- save or delete a stored session, enter OpenWrt-Session in Saved Sessions and click the Save button
  • (optional) Connection → SSH → Tunnels: Here you can define tunnels, which offer you the possibility to access services on your router and LAN with exposing them to the internet. The connection will be done through your SSH connection, hence tunnel. Example to access the router's WebIF: Define a “Local” tunnel with the source port 80 and the destination localhost:80; don't forget to “Add” it. This will allow you to access the router's WebIF in your browser via localhost:80. Note that the destination is always resolved on the other side of the tunnel.

TIP: To make a PuTTY shortcut with an automatically login, create one and append the saved session with an @ sign, for example call PuTTY with:

C:\> putty.exe @OpenWrt-Session

Disable password login

For more security you can disable Dropbear's password login.

root@OpenWrt:~# uci set dropbear.@dropbear[0].PasswordAuth=off
root@OpenWrt:~# uci set dropbear.@dropbear[0].RootPasswordAuth=off
root@OpenWrt:~# uci commit dropbear

See also Dropbear configuration article.


Make sure the /etc/dropbear directory is chmoded 0700 and the /etc/dropbear/authorized_keys file 0600.

root@OpenWrt:~# ls -dl /etc/dropbear/ /etc/dropbear/authorized_keys                    
drwx------    1 root     root             0 Feb 28 00:00 /etc/dropbear/                
-rw-------    1 root     root           626 Feb 28 00:00 /etc/dropbear/authorized_keys 

If mode is not the same for you, do

chmod 0700 /etc/dropbear
chmod 0600 /etc/dropbear/authorized_keys

If you think everything is OK but it still does not accept your key, check that you didn't say ssh-dsa when manually converting a multi line SSH2 key file.

docs/guide-user/security/dropbear.public-key.auth.txt · Last modified: 2018/03/31 11:04 by pipetto