hostap with a wifi usb stick on demand 
After plug in of a wifi usb stick linux act's as a hostap.

apt-get install isc-dhcp-server hostapd

changes in /etc/hostapd/hostapd.conf
wpa_pairwise=TKIP CCMP

iface wlan0 inet static
hostapd /etc/hostapd/hostapd.conf
up iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
up /etc/init.d/isc-dhcp-server restart
down iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE
down killall hostapd

first check vendor and product id with lsusb:

ACTION=="add", SUBSYSTEMS=="usb", ATTRS{idVendor}=="148f", ATTRS{idProduct}=="3070", \
RUN+="/sbin/ifup wlan0"
ACTION=="remove", SUBSYSTEM=="net", KERNEL=="wlan0", RUN+="/sbin/ifdown wlan0"

On booting this does not work for me so i started the hostap by

lsusb | grep -q "148f:3070" && /sbin/ifup wlan0

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transcode a mjpeg ip cam on demand with a cgi script using ffmpeg 
To minimize bandwidth for video streaming i have to transcode mjpeg to h264.
To do this on demand ffserver is no option for me.
Therefore a small cgi script on the webserver with ffmpeg did the trick:


echo -e "Content-type: video/avi\n"

#ffmpeg -an -analyzeduration 0 -f mjpeg -r 8 -i http://IP_CAM:PORT \
# -c:v libx264 -preset ultrafast -r 8 -threads 2 -b:v 150k -f avi - 2>/dev/null &

avconv -an -analyzeduration 0 -f mjpeg -r 8 -i http://IP_CAM:PORT \
-c:v libx264 -pre ultrafast -r 8 -threads 2 -b:v 150k -f avi - 2>/dev/null &
trap "kill $pid" SIGTERM SIGPIPE

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port forward with iptables 
iptables -I FORWARD -i INTERFACE -p tcp --dport PORT -j ACCEPT
iptables -A PREROUTING -t nat -i INTERFACE -p tcp --dport PORT -j DNAT --to-destination LOCAL_IP

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apt-get behind a server port redirected with ssh to a proxy 
There is an apt proxy (Apt-Cacher NG), a workstation, a server and a server behind that server.
workstation> ssh -R3142:proxy:3142 server

server> ssh -R3142: server_behind

server_behind> cat /etc/apt/apt.conf.d/01proxy
Acquire::http { Proxy "http://localhost:3142"; }

server_behind> apt-get update; apt-get upgrade

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sftp and port forward only login 
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:

Subsystem sftp internal-sftp

Match user USER
PasswordAuthentication yes
AllowAgentForwarding no
X11Forwarding no
ChrootDirectory /home/USER

sudo touch /home/USER/.hushlogin
sudo cp own_loginshell /home/USER/

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

taken from

modified 2011 by Peter Holik (

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);

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