Dropbear configuration
- Follow SFTP server to provide SFTP support.
- Follow Dropbear key-based authentication to set up key-based authentication.
- Follow Secure your router's access for additional security hardening.
The SSH configuration is handled by the Dropbear subsystem of uci and the configuration file is located in /etc/config/dropbear.
Each dropbear SSH server instance uses a single section of the configuration file, and you can have multiple instances.
Sections
The dropbear configuration contains settings for the dropbear SSH server in a single section.
Dropbear
The dropbear section contains these settings. Names are case-sensitive.
| Name | Type | Required | Default | Description |
|---|---|---|---|---|
enable | boolean | no | 1 | Set to 0 to disable starting dropbear at system boot. |
verbose | boolean | no | 0 | Set to 1 to enable verbose output by the start script. |
BannerFile | string | no | (none) | Name of a file to be printed before the user has authenticated successfully. |
PasswordAuth | boolean | no | 1 | Set to 0 to disable authenticating with passwords. |
Port | integer | no | 22 | Port number to listen on. |
RootPasswordAuth | boolean | no | 1 | Set to 0 to disable authenticating as root with passwords. |
RootLogin | boolean | no | 1 | Set to 0 to disable SSH logins as root. |
GatewayPorts | boolean | no | 0 | Set to 1 to allow remote hosts to connect to forwarded ports. |
Interface | string | no | (none) | An interface name, for example lan. With this setting you can limit connections to clients that can reach the IP of this interface. It binds to addresses, not the device. It collects all IPv4, and all IPv6 - ULA, but not Link Local - on the specified interface and binds to those. It is used in dropbear's -p option by doing: “For each specified interface, collect all its IPs, up to 10 endpoints total, and add -p <ip-addr:tcp-port> for each.”1) When both are defined DirectInterface takes precedence over Interface. |
DirectInterface | string | no | (none) | A single logical interface name, for example lan. If multiple are given via a`list`, the first one is used. Runs dropbear with -l <device> i.e., it binds directly to the layer 2 network device rather than specific IPs (as in Interface).2) On layer 3, dropbear binds to 0.0.0.0 and ::. When both are defined DirectInterface takes precedence over Interface. |
keyfile | list of files | no | (none) | Path to host key file. |
rsakeyfile | file | no | (none) | Path to RSA host key file. Deprecated. See keyfile. |
SSHKeepAlive | integer | no | 300 | Ensure that traffic is transmitted at a certain interval in seconds. This is useful for working around firewalls or routers that drop connections after a certain period of inactivity. The trade-off is that a session may be closed if there is a temporary lapse of network connectivity. A setting of 0 disables keepalives. If no response is received for 3 consecutive keepalives the connection will be closed. Equivalent of OpenSSH ClientAliveInterval |
IdleTimeout | integer | no | 0 | Disconnect the session if no traffic is transmitted or received for IdleTimeout seconds even after the SSHKeepAlive keep alive pings. Equivalent of OpenSSH ClientAliveInterval multiplied on ClientAliveCountMax |
mdns | integer | no | 1 | Whether to announce the service via mDNS |
MaxAuthTries | integer | no | 3 | Amount of times you can retry writing the password when logging in before the SSH server closes the connection. |
RecvWindowSize | integer | no | 24576 | Specify the per-channel receive window buffer size. Increasing this may improve network performance at the expense of memory use. |
Default configuration
This is the default configuration:
# uci show dropbear dropbear.@dropbear[0]=dropbear dropbear.@dropbear[0].RootPasswordAuth='1' dropbear.@dropbear[0].PasswordAuth='1' dropbear.@dropbear[0].Port='22'
Extras
Multiple instances
Add a second instance of dropbear listening on port 2022.
uci add dropbear dropbear uci set dropbear.@dropbear[-1].RootPasswordAuth='1' uci set dropbear.@dropbear[-1].PasswordAuth='0' uci set dropbear.@dropbear[-1].Port="2022" uci commit dropbear service dropbear restart
If you want to connect from internet you need to allow the 2022 port on WAN interface.
OpenSSH compatibility
Dropbear does not support SFTP by itself.
Meanwhile OpenSSH client uses SFTP by default.
Use scp -O for the legacy SCP protocol if necessary.
If you do have enough space you can install the SFTP server:
opkg update opkg install openssh-sftp-server
Security considerations
- Set up public key authentication and disable password authentication if possible.
- Set up a VPN to avoid exposing SSH to the internet as a single critical vulnerability may be enough for a remote attacker to gain root access.
Problems facing with a public SSH:
- No normal group for users and no normal user.
- No facility to ban IPs with many failed login attempts.
- File system permissions are very lax on default OpenWrt.
- Preventing normal users from exploiting BusyBox to gain access to root only commands due to missing permissions for symlinks.
References
See also
- SSH articles - tunneling, SSHFS mounting etc.