forward http requests to an other host 
iptables -t nat -A PREROUTING -p tcp -d THISIP --dport 80 -j DNAT --to-destination OTHERIP:80
iptables -t nat -A POSTROUTING -p tcp -d OTHERIP --dport 80 -j MASQUERADE
iptables -I FORWARD -p tcp -d OTHERIP --dport 80 -j ACCEPT


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Multiple em28xx USB devices not automatically detected by the module 
I got multiple USB devices working with em28xx module.

Usually a module option gives the em28xx module the order of the devices if not automatically detected
(ex. options em28xx card=80,92,9,9).

But the order at boot is every time different (order for module option not usb detection).

The only working solution for me is to manual loading, scan the order from kernel logs, unload and load with the scanned order:

/etc/modprobe.d/em28xx
blacklist em28xx
blacklist em28xx_v4l
blacklist em28xx_dvb
blacklist em28xx_rc


/etc/systemd/system/em28xx.service
[Unit]
Description=em28xx card sequence detection
After=local-fs.target
Before=motion.service vdr.service

[Service]
ExecStart=/usr/local/bin/em28xx_detect.sh
Type=oneshot

[Install]
WantedBy=multi-user.target


/usr/local/bin/em28xx_detect.sh
#!/bin/sh

modprobe em28xx
sleep 1
modprobe -r em28xx_rc
modprobe -r em28xx_v4l
modprobe -r em28xx_dvb
modprobe -r em28xx

IFS="
"
for line in $(dmesg|grep em28xx|grep 'New device'|tail -4)
do
[ -z "$cards" ] || cards="${cards},"
case "$line" in
*eb1a:2821*) cards="${cards}9" ;;
*2013:024c*) cards="${cards}80" ;;
*2013:0258*) cards="${cards}92" ;;
esac
done

modprobe em28xx card=$cards disable_ir=1 disable_usb_speed_check=1

sleep 10


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portforward only key based chroot ssh access without a shell for the user under 64 Bit 
/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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ssh otp 
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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convert to mp4 
ffmpeg -i input.mp4 -pix_fmt yuv420p -vcodec libx264 -acodec aac output.mp4


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