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Useful things to remember

Send email via telnet

HELO example.com
MAIL FROM: user@example.com
RCPT TO: otheruser@example.com
RCPT TO: anotheruser@example.com
DATA
From: spammer@example.com
To: people@example.com
Subject: spam alert

Buy my crappy product!!
only $19.95

Get the version of sendmail

/usr/lib/sendmail -d0 < /dev/null | grep -i version

Use Perl to edit a file in place

perl -p -i -e "s#/etc#/u/decandia/dns-parsing-test#" zonefilenamehere

Redirect STDERR and STDOUT to a file

(uname -n;uname -xerror) > file.out 2>&1

clear the sendmail queue

/usr/lib/sendmail -q -v (all messages)
/usr/lib/sendmail -qI (specific queueID)
/usr/lib/sendmail -qR (specific recipient)
/usr/lib/sendmail -qS (specific sender)
/usr/lib/sendmail -L sm-msp-queue -Ac -q1m (clientmqueue)

Use awk to sum data

awk '{SUM=$5+SUM} END {print SUM,"in kbytes"}' < datafile

Override an ActiveRecord accessor method

class Ipinterface < ActiveRecord::Base
  def macaddress=(macaddr)
    super(Ipinterface.sanitize_macaddress(macaddr))
  end

  def macaddress
    Ipinterface.sanitize_macaddress(super)
  end

  def Ipinterface.sanitize_macaddress(macaddr)
    # Take a MAC address that looks like:
    #  0:14:4f:8b:7:ba
    #   and convert it to
    #  00:14:4f:8b:07:ba
    split_mac = macaddr.split(/:/)
    adjusted_mac = Array.new
    split_mac.each do |bytes|
      adjusted_mac << bytes.rjust(2,"0")
    end
    adjusted_mac.join(':')
  end
end

Set up SSH port forwarding through an intermediary host

[user1@server:~]$ ssh -L8080:remotehost:8080 user1@jumphost

Make your xterm scrollbar work with screen

Add "termcapinfo xterm ti@:te@" to your ~/.screenrc file.

Set up a screen multiuser session

[user1@server:~]$ screen
Inside the screen session type:
Ctrl-A:multiuser on
Ctrl-A:acladd user2 [crypthash] 
[user2@server:~]$ screen -rx user1/

Copy the whole screen scroll buffer to a file

Inside the screen session type:
Ctrl-A:hardcopy -h dump_file