head.daveops.net

Snippets for yer computer needs

Linux

force filesystem check on next boot

touch /forcefsck

Socket programming with /dev/tcp

exec 3<>/dev/tcp/www.google.com/80 echo -e “GET / HTTP/1.1\n\n” >&3 cat <&3

See what services are using a particular port

Run as root:

lsof -w -n -i (tcp udp):

or

netstat -luntp

See if hard drive is on its last legs

smartctl -H /dev/sda

Get reboot/shutdown history

last -x

Date utility

# Get the date from a timestamp date -d @$TIMESTAMP # Get the current time as a timestamp date +%s

Find all files with a setuid/setgid bit set

find / -perm +6000 -type f -exec ls -ld {} \; > setuid.txt &

Burn an ISO from the command prompt

cdrecord -v -data image.iso

Fixing missing shared library

Find files changed in the past day

find . -ctime -1 -type f

Disable caps lock

setxkbmap -option ctrl:nocaps

Set time on machine that doesn’t have NTP

date –set=”$(ssh user@server date)”

Inter-user communication

# Get list of logged in users who # Send message to all users wall [message] # Send message to another user’s terminal write user [ttyname] # Enable/disable terminal message mesg [n|y]

Assembly

System call table located at /usr/include/asm/unistd.h Red Hat syscall man pages installed with man-pages RPM. man 2 syscalls for a list, man 2 <syscall> for the syscall.

Put syscall in EAX, put arguments in other ExX registers, call the interrupt, result usually in EAX

ip command

ifconfig is deprecated, ip was added in Linux 2.2

# Get IP address
ip addr
# Get network interface stats
ip link
# Get network interface packet stats
ip -s link

# Enable interface
ip link set eth0 up
# Set IP address
ip address add 192.168.1.23 dev eth0

# Show routing table
ip route show

Sneaking around the open file limit

https://www.youtube.com/watch?v=_XgXCVULj0o

Open a pair of domain sockets (with socketpair) that connect to the same process. Throw the FD in one end, close the FD, then read it out of the other end. Recursively add the ring buffers…

PipeFS, SockFS, DebugFS, SecurityFS

https://www.linux.org/threads/pipefs-sockfs-debugfs-and-securityfs.9638/

setuid/setgid on executables

The kernel doesn’t execute setuid scripts, only binaries. See the UNIX FAQ for reasons why.

Kernel resources