Snippets of Information

(to be sorted later)

Ethernet MAC Addresses

I conclude, that software generated MAC addresses in the Xen context need to start with

00:16:3e:xx:xx:xx

printf '00:16:3e:%02x:%02x:%02x\n' $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256))
ip link set $IFACE address $HWADDR

or¹⁾

x2-xx-xx-xx-xx-xx
x6-xx-xx-xx-xx-xx
xa-xx-xx-xx-xx-xx
xe-xx-xx-xx-xx-xx

printf '%02x:%02x:%02x:%02x:%02x:%02x\n' $(( (RANDOM%256 | 0x02) & 0xfe )) $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256))
ip link set $IFACE address $HWADDR

SSH Host Key Fingerprints

ssh

ssh-keygen -lf <pub key file>

The part to be entered at the ssh prompt when connecting to unknown hosts is after the number, i.e. starting with the alg. specifier and need not contain the comment.

less in screen

screen less ipfire

screen, less, ipfire

~/.screenrc

# workaround for the glitch, that "less" inside "screen" 5 on 
# "ipfire" 2.29 shows blinking text instead of reverse video:
setenv LESS_TERMCAP_so ^[[7m
setenv LESS_TERMCAP_se ^[[27m
:
:

Credits: ChatGPT

Using $(...) in Nagios'/Icingas command_line

nagios icinga bash

nagios, icinga, bash

$ needs to be escaped by a second $$

/etc/icinga/commands.cfg

   command_line /usr/local/bin/send_something.sh "$$(printf '%s\n%s' '$HOSTNAME' '$HOSTOUTPUT')"

Credits: Nagios Object Definitions at old-www-mathrice.pages.math.cnrs.fr (Well actually that was the first relevant Startpage hit after ChatGPT failed completely)
Reference: Command Definition - The primary source of the above

Notifications via ntfy at every login

ntfy pam

ntfy, pam

/etc/pam.d/common-session

:
session default=ignore pam_exec.so quiet /bin/sh -c [ test 'open_session' = "$PAM_TYPE" && /usr/bin/wget -q -O /dev/null --post-data "$PAM_USER hat sich gerade von $PAM_RHOST über $PAM_TTY eingeloggt" --header "X-Title: Login" --header "X-Priority: 4" --header "X-Tags: unlock" https://ntfy.sh/my_ntfy_topic]

$PAM_TTY will be ssh when appropriate

References:

man 5 pam.conf
man 8 pam_exec

What's happening on port X?

lsof network

lsof, network

lsof -iUDP:5353
lsof -iTCP:22

Refactor rcs Versioning respecting comments

shell bash rcs

shell, bash, rcs

for i in `seq 1 17`
do
  rlog -r1.2.1.$i /etc/RCS/fstab,v > /tmp/work/log/1.2.1.$i
  co -u -M -r1.2.1.$i /etc/RCS/fstab,v
  log=$( ( egrep -m1 '^date: .*author:.*state:' ; sed '$d' | fgrep -v '*** empty log message ***' ) < /tmp/work/log/1.2.1.$i | sed '1d' )
  rcs -l1 /etc/RCS/fstab,v
  ci -d -m"$log" /etc/RCS/fstab,v
done

diff  <(rlog -r1 /etc/RCS/fstab,v )  <(rlog -r1.2.1 /etc/RCS/fstab,v )

for i in `seq 17 -1 1`
do
  j=$(( i+2 ))
  rcsdiff -r1.2.1.$i -r1.$j /etc/RCS/fstab,v \
  && rcs -o1.2.1.$i /etc/RCS/fstab,v
done

Make bash remember all whitespace including newlines when typing in multiline commands

bash

shopt -s lithist

Make zsh split variable contents at whitespace

zsh

setopt shwordsplit

Make less display things coloured

shell less diff

shell, less, diff

apt install --no-install-recommends source-highlight colordiff ccze
export LESSOPEN="| /usr/share/source-highlight/src-hilite-lesspipe.sh %s"
export LESS="-R"
less  index.php
rcsdiff index.php | colordiff | less -SRF
rlog index.php | ccze -A | less -SRF

Getting the serial number of my Laptop

~ # dmidecode -s system-serial-number

Remove disconnected but still visible USB disks

linux USB udev

linux, USB, udev

udevadm trigger
# ? # udevadm trigger --action=add 
echo 1 > /sys/class/block/sd[x]/device/delete

PDF

Split

(unverified suggestions by ChatGPT for future reference)

qpdf input.pdf --split-pages output-%d.pdf

pdfseparate input.pdf page-%d.pdf # poppler

gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH \
 -dFirstPage=1 -dLastPage=1 \
 -sOutputFile=page1.pdf input.pdf

pdftk input.pdf burst output page-%02d.pdf

Compress

Split PDF into pages by PDF Arranger

-

mkdir orig gs
mv -iv *.pdf orig
cd orig
for file in *.pdf
do
  echo "$file"
  gs -sDEVICE=pdfwrite        \
     -dCompatibilityLevel=1.4 \
     -dPDFSETTINGS=/ebook     \
     -dNOPAUSE                \
     -dQUIET                  \
     -dBATCH                  \
     -sOutputFile="../gs/$file" "$file"
done
cd ..

Merge pages by PDF Arranger

Credits: ChatGPT (for the ghostscript part)

ChatGPT:

Yes. In fact, you don't need to split the PDF into individual pages at all.

pdfwrite operates on the entire document, processing every page in sequence. The compression, image downsampling, font handling, and object optimization are all applied across the whole PDF.

Combine / Merge