Termux Remote Access

https://wiki.termux.com/wiki/Remote_Access

Termux is capable of accessing remote devices by using some common tools. It is also possible to turn a device running Termux into remote controlled server.

Security warning: this FTP server only supports anonymous login, so there is no authentication and anyone can connect to your device and do nasty things. Use it only in your own network or use SFTP instead.

Termux provides a lightweight FTP server so you can manage your files via remote connection. This server is part of the included busybox package, so you don't have to install anything.

To start FTP server you have to run ftpd via tcpsvd:

$ tcpsvd -vE 0.0.0.0 1024 ftpd /

Command shown above will start FTP server on port 1024 from the root (/) directory.

By default, ftpd is started in read-only mode, i.e. you can only download files or perform directory listing. If you want to upload files, then ftpd should be started this way:

$ tcpsvd -vE 0.0.0.0 1024 ftpd -w /

Then you can connect to Termux from another device. For example, to connect to Termux with a command line FTP client (from PC):

$ ftp 192.168.1.3 1024

You shouldn't start FTP on ports lower than 1024. Trying will give the following error message:

tcpsvd: bind: Permission denied

SSH provides a secure way for accessing remote hosts and replaces tools such as telnet, rlogin, rsh, ftp. Termux provides SSH via two packages: dropbear and openssh. If you never used these tools before, it is recommended to install openssh as it is more common.

Using the SSH client

You can obtain an SSH client by installing either openssh or dropbear.

Usage example

To login to a remote machine where the ssh daemon is running at the standard port (22):

ssh user@hostname_or_ip

Same as above, but if the ssh daemon running on different port, e.g. 8022:

ssh -p 8022 user@hostname_or_ip

Using public key authentication with ssh running on the standard port and a private key stored in the file id_rsa:

ssh -i id_rsa user@hostname_or_ip

Note, that if id_rsa will be stored in ~/.ssh directory, you can omit specifying it in the command. But if you have multiple keys, it is necessary to pick a specific key with -i {path\_to_privkey}.

SSH Agent

Important note: this does not work for Dropbear.

If you wish to use an SSH agent to avoid entering passwords, the Termux openssh package provides a wrapper script named ssha (note the a at the end) for ssh, which:

This means that the agent will prompt for a key password at first run, but remember the authorization for subsequent runs.

Using the SSH server

OpenSSH

OpenSSH (also known as OpenBSD Secure Shell) is a suite of secure networking utilities based on the Secure Shell (SSH) protocol, which provides a secure channel over an unsecured network in a client–server architecture.

Starting and stopping OpenSSH server

Since Termux does not use initialization system, services are started manually from command line.

To start OpenSSH server, you need to execute this command:

sshd

If you need to stop sshd, just kill it's process:

pkill sshd

SSH daemon does logging to syslog, you can view it by running logcat -s 'syslog:*'. You can do that either from Termux or ADB.

Setting up password authentication

Password authentication is enabled by default. This will allow you to get started with it much easier. Before proceeding, make sure that you understand that password authentication is less secure than a pubkey-based one.

  1. Ensure that everything is up to date and package openssh is installed:
pkg upgrade
pkg install openssh
  1. Password authentication is enabled by default in configuration file. But you can still review it ($PREFIX/etc/ssh/sshd_config), it should be like this:
PrintMotd no
PasswordAuthentication yes
PubkeyAcceptedKeyTypes +ssh-dss
Subsystem sftp /data/data/com.termux/files/usr/libexec/sftp-server
  1. Set new password. Execute command passwd. While program allows minimal password length is 1 character, the recommended password length is more than 8-10 characters. Passwords are not printed to console.
$ passwd
New password:
Retype new password:
New password was successfully set.

Setting up public key authentication

Public key authentication is the recommended way for logging in using SSH. To use this type of authentication, you need to have a public/private key pair. For successful login, the public key must exist in the authorized keys list on remote machine while private key should be kept safe on your local host.

In the following example it will be assumed that you want to establish public key authentication between your PC (host) and your Android device running Termux (remote). It also will be assumed that you running Linux distribution on your PC.

  1. If you do not have keys, you can generate them. In this example we will generate RSA key. On PC, execute this command:
ssh-keygen -t rsa -b 2048 -f id_rsa

The command shown above generates private RSA key with 2048 bit key length and saves it to file id_rsa. In the same directory you can find a file id_rsa.pub – it is a public key.

Important note: 2048 bit is the minimal key length that is considered safe. You can use higher values, but do not use higher than 4096 as remote server may not support big keys.

  1. Copy key to the remote machine (Termux). Password authentication has to be enabled in order to install pubkey on remote machine. Now do:

ssh-copy-id -p 8022 -i id_rsa IP_ADDRESS

Do not forget to replace IP_ADDRESS with the actual LAN IP address of your device. It can be determined by using command ifconfig.

If everything was okay, you will see a message like this one:

Number of key(s) added: 1

Now try logging into the machine, with:

ssh -p 8022 192.168.1.4

and check to make sure that only the key(s) you wanted were added.

  1. From this point password authentication can be disabled. Edit file $PREFIX/etc/ssh/sshd_config and replace line beginning with "PasswordAuthentication" with
PasswordAuthentication no

Then execute command pkill sshd; sshd in order to restart server with updated configuration file.

Dropbear

Dropbear is a software package written by Matt Johnston that provides a Secure Shell-compatible server and client. It is designed as a replacement for standard OpenSSH for environments with low memory and processor resources, such as embedded systems.

Important note: Dropbear does not provide SFTP server.

Starting and stopping Dropbear server

Same as for OpenSSH, you will need to execute it's binary manually. Also, unlike OpenSSH, Dropbear does not use a configuration file but only command line arguments.

Server is running in background, both password and public key authentication available. To achieve this, just type in console:

dropbear

If you need only public key authentication, do this instead:

dropbear -s

Also, server can be started in foreground. For this purpose use a parameter -F:

dropbear -F

Server started in foreground can be stopped by just Ctrl-C key combination. If it is in the background, then you can use a pkill:

pkill dropbear

Setting up password authentication

Same as for OpenSSH, password authentication is enabled by default.

Everything you have to do, is:

  1. Make sure that everything is up to date and dropbear is installed:
pkg upgrade
pkg install dropbear

  1. Set password by executing command passwd.

  2. Start dropbear server. You can execute either just dropbear to start it in background or dropbear -F to start it in the foreground.

Setting up public key authentication

Same as for OpenSSH, you can put your keys by using ssh-copy-id. But if you consider to setup a public key authentication from Termux to something else, it is worth to mention some important differences between OpenSSH and Dropbear.

  1. Dropbear uses a different command for generating keys. Example of generating RSA key (2048 bit):

dropbearkey -t rsa -f id_rsa -s 2048

  1. Public key should be obtained manually. To do this, you have to use dropbearkey again, but in different way:

dropbearkey -f id_rsa -y

  1. Dropbear and OpenSSH uses a different key formats. To use a Dropbear's key in OpenSSH, you will have to convert it:

dropbearconvert dropbear openssh ./id_rsa ./id_rsa_openssh

This procedure can be done vice versa to obtain a key in Dropbear's format:

dropbearconvert openssh dropbear ./id_rsa_openssh ./id_rsa_dropbear

Using the SFTP

Package OpenSSH provides a tool for accessing remote hosts over SFTP. This will allow you to work with files in same way as via FTP but with better security.

Connecting to Termux (sshd listening on port 8022):

$ sftp -P 8022 192.168.1.20

Connecting to somewhere else (sshd listening on standard port):

$ sftp sftp.example.com

However, to use command line SFTP client you should know some basic commands:

This is not a complete list of SFTP commands. To view all available commands, consider to view man page (man sftp) or view short help in interactive SFTP session by issuing command help.

Mosh is a remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes.

Mosh usage example

Important note: Mosh should be installed on both client and server side.

Connecting to remote host (sshd listening on standard port):

mosh user@ssh.example.com

Connecting to Termux (sshd listening on port 8022):

mosh --ssh="ssh -p 8022" 192.168.1.25

Rsync is a tool for synchronizing files with remote hosts or local directories (or drives). For better experience of using rsync, make sure that package openssh (or dropbear) is installed.

Rsync usage example

Sync your photos with PC:

$ rsync -av /sdcard/DCIM/ user@192.168.1.20:~/Pictures/Android/

Get photos from remote Android device:

$ rsync -av -e 'ssh -p 8022' 192.168.1.3:/sdcard/DCIM/ /sdcard/DCIM/

Sync local directories (e.g. from external sdcard to Termux home):

$ rsync -av /storage/0123-4567/myfiles ~/files

You may want to see man page (man rsync) to learn more about it's usage.

See Also

Accessing Termux from the Internet

Connecting to Termux with SSH over USB

tags: #networking

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