/etc/ssh/sshd_config:

...
Match user USER
  PasswordAuthentication no
  AllowTcpForwarding yes
  X11Forwarding no
  PermitTunnel no
  GatewayPorts no
  AllowAgentForwarding no
  ChrootDirectory /home/USER

But if the client needs a login shell this failed.

Well you could tell the client to not use a login shell:

ssh -N -L2222:IP:22 USER@SERVER

putty: SSH / Protocol Option enable "Don't start a shell or command at all"

or use an own loginshell where the user only can press return to disconnect:

sudo useradd USER -d /home/USER -s /bin/bash
sudo mkdir /home/USER
sudo chown USER:USER /home/USER
sudo su USER
cd
touch .hushlogin    (is used to not display motd's)
mkdir .ssh
chmod 0700 .ssh 
cd .ssh 
ssh-keygen -t rsa -b 4096 
mv id_rsa.pub authorized_keys
exit
sudo passwd -d USER
sudo chown root:root /home/USER          (for sshd chroot)
sudo cp own_loginshell /home/USER/

move /home/USER/.ssh/id_rsa out to your test account and test with "ssh -i id_rsa USER@IP"

I had some troubles with a chrooted environment:

/etc/passwd USER:x:ID:ID::/home/USER:/own_loginshell

.hushlogin is NOT working and motd with last login is shown!
pam.d/sshd is running all scripts in /etc/update-motd.d.

Solution move .hushlogin to /home/USER/home/USER

To hide motd and have no delays at login we could add an exception in pam.d/sshd for our user USER

session [default=2 success=ignore] pam_succeed_if.so quiet user != USER

before

session optional pam_motd.so motd=/run/motd.dynamic
session optional pam_motd.so noupdate

/etc/passwd USER:x:ID:ID::/:/own_loginshell

.hushlogin is working but before sshd is doing a chroot it checks the key against
authorized_keys based on our homedir (is / for chroot) wee need to

add "AuthorizedKeysFile /home/USER/.ssh/authorized_keys" to sshd_config

Compiling

For 64Bit there is something different to 32Bit and with my studies i compiled with

gcc -s -Os -nostdlib -ffreestanding own_loginshell.c -o own_loginshell

#> chroot /ROOTDIR ./own_loginshell
chroot: failed to run command ‘./own_loginshell’: No such file or directory

With

#> strace chroot /ROOTDIR ./own_loginshell

you only see

execve("./own_loginshell", ["./own_loginshell"], 0x7fff17d94fe8 /* 24 vars */) = -1 ENOENT (No such file or directory)

But with the help of "readelf -l own_loginshell" you see

[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]

You have to copy /lib64/ld-linux-x86-64.so.2 to CHROOT/lib64/

Or compile with "-static"

own_loginshell.c:

/*
   simple program to print to stdout and read from stdin without libc for x86-64

   taken from https://hero.handmade.network/forums/code-discussion/t/861-compiling_without_libc_on_linux

   gcc -s -Os -nostdlib -ffreestanding -static own_loginshell.c -o own_loginshell
*/

#include <stddef.h>
#include <syscall.h>

static void exit(int code)
{
    __asm__ __volatile__(
        "syscall"
        :
        : "a"(__NR_exit)
        : "cc", "rcx", "r11", "memory");
    __builtin_unreachable(); // syscall above never returns
}

// returns negative value for error (for example, if error is EINVAL, then -EINVAL is returned)
static int write(int fd, const void *buf, size_t size)
{
    long result;
    __asm__ __volatile__(
        "syscall"
        : "=a"(result)
        : "0"(__NR_write), "D"(fd), "S"(buf), "d"(size)
        : "cc", "rcx", "r11", "memory");
    return result;
}

static int read(int fd, char *buf, size_t size)
{
    long result;
    __asm__ __volatile__(
        "syscall"
        : "=a"(result)
        : "0"(__NR_read), "D"(fd), "S"(buf), "d"(size)
        : "cc", "rcx", "r11", "memory");
    return result;
}
    
void _start()
{   
    char text[] = "press enter to close connection";

    // for this example let's ignore result of write
    // but you should really handle it
    // 1 is stdout file handle
    write(1, text, sizeof(text) - 1);
    read(0, text, 1);
    
    exit(0);
}

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---

Install oathtool.

sudo apt-get install oathtool libpam-oath

Generate a secret.

export HEX_SECRET=$(head -10 /dev/urandom | md5sum | cut -b 1-30)

Generate the TOTP details, 6 digits long.

oathtool --verbose --totp $HEX_SECRET

Enter the base32 secret in Android FreeOTP.

Create and populate the /etc/security/users.oath file.

sudo bash -c "echo HOTP/T30 $USER - $HEX_SECRET >> /etc/security/users.oath"
sudo chmod 0600 /etc/security/users.oath

Forget the secret!

unset HEX_SECRET

prefix /etc/pam.d/sshd with

auth sufficient pam_oath.so usersfile=/etc/security/users.oath window=10 digits=6

Allow this in sshd and restart.

sudo sed -Ei -e 's/(ChallengeResponseAuthentication) no/\1 yes/' /etc/ssh/sshd_config
sudo service ssh restart

Test with

ssh localhost

You should see:
One-time password (OATH) for `USER':

To avoid otp for some users prefix /etc/pam.d/sshd with

auth [success=1 default=ignore] pam_succeed_if.so user in user1:user2

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---

I need a ssh login for restricted portforward but without a shell for the user.

This got realized with /etc/ssh/sshd_config:

...
Subsystem sftp internal-sftp

Match user USER
  PasswordAuthentication yes
  AllowAgentForwarding no
  X11Forwarding no
  ForceCommand internal-sftp
  PermitOpen localhost:22
  ChrootDirectory /home/USER

"PermitOpen"restricts portforward to localhost:22 (remote port forward is not restricted)

But if the client needs a login shell this failes.

Well you could tell the client to not use a login shell:

ssh -N -L2222:IP:22 USER@SERVER

putty: SSH / Protocol Option enable "Don't start a shell or command at all"

or use an own loginshell where the user can only press return to disconnect:

/etc/ssh/sshd_config:

...
Subsystem sftp internal-sftp

Match user USER
  PasswordAuthentication yes
  AllowAgentForwarding no
  X11Forwarding no
  PermitOpen 127.0.0.1:2222
  ChrootDirectory /home/USER

sudo touch /home/USER/.hushlogin
sudo cp own_loginshell /home/USER/
/etc/passwd:USER:x:ID:ID::/:/own_loginshell

own_loginshell.c:

/*
   simple program to print to stdout and read from stdin without libc

   taken from http://crazychenz.com/archives/107
   (http://stackoverflow.com/questions/2548486/compiling-without-libc)

   modified 2011 by Peter Holik (peter@holik.at)

   gcc -nostdlib -nostartfiles -fno-builtin own_loginshell.c -o own_loginshell
*/

/* Types - I've defined these just to match the kernel's macros, typedefs, and structs */
typedef unsigned int size_t;

/* Syscalls */
exit(int error_code) {
        /* The asm call is a GCC thing that allows us to put assembly
         * inline with our C code. This particular use is the extended version,
         * which provides a very clean and easy way to map variables in
         * our code with registers in the assembly code.
         */
        asm("int  $0x80"
            : // no output registers
            : "a" (1), "b" (error_code)
        );
}

size_t read(unsigned int fd, char * buf, size_t count) {
        size_t ret;
        /* In this call, we have a return value, which know will be
         * of type size_t, so we put the value of %eax into ret.
         */
        asm("int  $0x80"
            : "=a" (ret)
            : "a" (3), "b" (fd), "c" (buf), "d" (count)
        );
        return ret;
}

size_t write(unsigned int fd, const char * buf, size_t count) {
        size_t ret;
        asm("int  $0x80"
            : "=a" (ret)
            : "a" (4), "b" (fd), "c" (buf), "d" (count)
        );
        return ret;
}

/* Notice that there is no main in this code, that is because
 * main is not _really_ required. All that is _really_ required
 * is the entry point for Linux to execute. I'd suggest
 * always using a main() for compatibility reasons.
 */
void _start() {
        char *buf = "press enter to close connection";

        write(1, buf, 31);
        read(0, buf, 1);
        exit(0);
}

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---

useradd -s /bin/false -m SFTPUSER
mkdir /home/SFTPUSER/.ssh
ssh-keygen -t rsa -b 2048 -N '' -f /home/SFTPUSER/.ssh/id_rsa
chown -R SFTPUSER:SFTPUSER /home/SFTPUSER/.ssh
chmod 600 /home/SFTPUSER/.ssh/id_rsa

/etc/ssh/sshd_config:

Subsystem       sftp    internal-sftp

Match user SFTPUSER
    PasswordAuthentication no
    AllowTcpForwarding no
    X11Forwarding no
    ForceCommand internal-sftp
    ChrootDirectory /chroot

chown root:root /chroot
mkdir /chroot/SFTPUSERDIR
chown SFTPUSER:SFTPUSER /chroot/SFTPUSERDIR

echo "put FILENAME" | \
sftp -oIdentityFile=/home/SFTPUSER/.ssh/id_rsa \
     -oTCPKeepAlive=no -oServerAliveInterval=15 \
     SFTPUSER@localhost:SFTPUSERDIR

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---

I want to rsync to a remote host to a given directory.

local-host:

ssh-keygen -t rsa

keyfilename: ~/.ssh/rsync
ssh-copy-id -i .ssh/rsync rsyncuser@remote-host

rsync files with ssh:

rsync -vaHxr --delete \
      -e "ssh -i ~/.ssh/rsync -c arcfour -o Compression=no -x" \
      LOCALDIR rsyncuser@remote-host:

remote-host

/home/rsyncuser/.ssh/authorized_keys:

from="192.168.0.2,",command="/home/rsyncuser/validate-rsync.sh",
 no-pty,no-agent-forwarding,no-port-forwarding
 ssh-dss 012345678...

limit access with from (optional).
On sucessfully ssh login command is executed.
Read More...

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---

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