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 -R3142:127.0.0.1:3142 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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Ubuntu Wake on Lan WOL 
If ethtool ethX doesn't contain Wake-on: g then add pre-down /sbin/ethtool -s ethX wol g to /etc/network/interfaces

Add NETDOWN=no to /etc/default/halt

Test with /sbin/shutdown -P now and wakeonlan [-i BROADCASTADDR] MAC (apt-get install wakeonlan)

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Find duplicate filenames 
find -type f -printf "%f\n" | sort | uniq -d


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hdd performance tests 
#> hdparm -tT /dev/sda
Timing buffered disk reads: 398 MB in 3.00 seconds = 132.45 MB/sec

write:

#> dd if=/dev/zero of=./tempfile bs=1M count=1024 conv=fdatasync,notrunc
1073741824 Bytes (1,1 GB) kopiert, 12,8038 s, 83,9 MB/s

read:

#> echo 3 > /proc/sys/vm/drop_caches
#> dd if=./tempfile of=/dev/null bs=1M count=1024
1073741824 Bytes (1,1 GB) kopiert, 6,79724 s, 158 MB/s

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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:

/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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