Useful commands

IPTables - Change Port of outgoing and incoming packets

Question: Is it possible to configure IPTables in a way that all outgoing packets to a certain IP and Port are altered with a different port and do the same for incoming packets?

I have to work with a server routinely and the Hoster decided that SSH access will only be possible on port 222 instead of the default 22.

This always causes a headache when ssh, scp or rsyncing. You always have to remember to add the port parameter

Answer: Yes it should certainly be possible to setup iptable rules to NAT outgoing traffic. You really should only need to create a rule that deals with the output traffic. You shouldn't need a rule to do anything to the returning packets. The state-ful nature of netfilter will deal with this for you.

You would probably need to use a rule like one of these.

# if you want to redirect requests from the local machine
iptables -t nat -A OUTPUT--destination remote.host.ip \
         -p tcp  --dport 22 -j DNAT --to-destination remote.host.ip:222

# if you want to redirect requests on a device inline
iptables -t nat -A PREROUTING --destination remote.host.ip \
         -p tcp  --dport 22 -j DNAT --to-destination remote.host.ip:222

Another simple solution would be to simply setup an SSH configuration file for the server and specify the port in your config.

# list of all names, you might commonly use for this host.
Host foo foo.example.org foo.example 
    # real hostname
    Hostname real.example.org 

    Port 222

Some Useful commands in linux.

To view all lines in single line of any text file

sed -e '/^$/d' file |  tr '\n' ' '

To Grep pattern with in a single command: ( will find shankar word and print only shankar)

grep -o 'shankar' file

To Grep 3rd field from matching pattern line  

awk '/Unix/{print $3}' file

To list all files starting with a and ending with cd. 

ls a?cd

There are few standard text processing tools which are used very often on the Unix-like system.

• No regular expression is used:
• cat(1) concatenates files and outputs the whole content.
• tac(1) concatenates files and outputs in reverse.
• cut(1) selects parts of lines and outputs.
• head(1) outputs the first part of files.
• tail(1) outputs the last part of files.
• sort(1) sorts lines of text files.
• uniq(1) removes duplicate lines from a sorted file.
• tr(1) translates or deletes characters.
• diff(1) compares files line by line

Basic regular expression (BRE) is used:

• grep(1) matches text with patterns.
• ed(1) is a primitive line editor.
• sed(1) is a stream editor.
• vim(1) is a screen editor.
• emacs(1) is a screen editor. (somewhat extended BRE)

Extended regular expression (ERE) is used:

• egrep(1) matches text with patterns.
• awk(1) does simple text processing.
• tcl(3tcl) can do every conceivable text processing: re_syntax(3). Often used   with tk(3tk).
• perl(1) can do every conceivable text processing. perlre(1).
• pcregrep(1) from the pcregrep package matches text with Perl Compatible Regular Expressions (PCRE) pattern.
• python(1) with the re module can do every conceivable text processing. See   "/usr/share/doc/python/html/index.html".
  See the ksh or sh-posix man page for more info on these. In bash you can use the help command.
  alias :-
  to list aliases
  let :-
  to do maths. Eg:- let x=2*4-5 ; echo $x
  set :-
  to list the current variables and their values.
  typeset -f :-
  to list functions
  typeset -i :-
  to list integer variables
  which :-
  to find where a program is (in some shells you use whence instead)

  Unix accessories

  See the corresponding man page for more info on these. Note that many of the commands have dozens of options that might save you needing to write code.
  wc:-
  This counts words lines and characters. To count the number of files in a directory, try ls | wc -l
  grep:-
  `grep Kwan /etc/passwd' prints all lines in /etc/passwd that include `Kwan'. The return value indicates whether any such lines have been found.
  sed:-
  a stream editor. Doing ls | sed s/a/o/g will produce a listing where all the 'a' characters become 'o'. Numerous tricks and tutorials are available from the Handy one-liners for SED file.
  basename:-
  Strips the directory and (optionally) suffix from a filename.
  tr:-
  Translates one set of characters to another set. For example, try

    echo "date" | tr d l

  The following does case conversion

    echo LaTeX | tr '[:upper:]' '[:lower:]'

  You can also use it as follows, ``capturing" the output in a variable

    old="LaTeX"
    new=$(echo $old | tr '[:upper:]' '[:lower:]')

  test:-
  This program can be used to check the existence of a file and its permissions, or to compare numbers and strings. Note that ` if [ -f data3 ]' is equivalent to ` if test -f data3'; both check for a file's existence, but the [...] construction is faster - it's a builtin. Note that you need spaces around the square brackets. Some examples,

  if [ $LOGNAME = tpl ] # Did the user log in as tpl? 
  if [ $num -eq 6 ]     # Is the value of num 6?
  if [ $(hostname) = tw000 ] # Does hostname produce `tw000'?
                             # i.e. is that the machine name?

  sort:-
  `ls -l | sort -nr +4' lists files sorted according to what's in column 4 of ls -l's output (their size, usually).
  cut:-
  Extracts characters or fields from lines. cut -f1 -d':' < /etc/passwd prints all the uid's in the passwd file. The following code shows a way to extract parts of a filename.

  filename=/tmp/var/program.cc
  
  b=$(basename $filename)
  prefix=$(echo $b | cut -d. -f 1)
  suffix=$(echo $b | cut -d. -f 2)

  find:-
  finds files in and below directories according to their name, age, size, etc.

  Exercises

  Suppose in a directory that you wanted to change all the filenames ending in .f77 so that they instead ended in .f90. How would you go about it?If you're new to Unix you may need to find out how to change filenames. Typing apropos filename will list the programs and routines whose summaries mention filename. Alas, the most useful command isn't mentioned there. Typing apropos rename lists mv which is what you need.
  Maybe next you might try ``mv *.f77 *.f90''. Alas, this won't work - the shell will replace *.f77 by a list of filenames and the resulting command will fail. You need to use a loop of some sort. You also need a way to remove a suffix (look up basename) and how to add a new suffix. It's worth experimenting with a single name first. Do filename=test.f77 and see if you can produce a test.f90 string from $filename . One solution is

  for filename in *.f77
  do
    b=$(basename $filename .f77)
    mv $filename $b.f90
  done

  Here are some more exercises with useful man pages suggested. None of the solutions to these should be much more than a dozen lines long. Answers to some of these are in the next section.
  1. Change the args script supplied earlier so that if no argument is provided, ``They are'' isn't printed, and if exactly 1 argument is provided, ``... 1 argument'' rather than ``... 1 arguments'' is printed (use if)
  2. Read in two numbers from the keyboard and print their sum (see the read, echo and let commands in the shell manual page).
  3. Write a shell script that given a person's uid, tells you how many times that person is logged on. (who, grep, wc)
  4. Write a shell script called lsdirs which lists just the directories in the current directory (test).
  5. Write a shell script called see taking a filename name as argument which uses ls if   the file's a directory and more if the file's otherwise (test)
  6. Write a shell script that asks the user to type a word in, then tells the user how long that word is. (read, wc)
  7. In many versions of unix there is a -i argument for cp so that you will be prompted for confirmation if you are about to overwrite a file. Write a script called cpi which will prompt if necessary without using the -i argument. (test)
  8. Write a shell script that takes a uid as an argument and prints out that person's name, home directory, shell and group number. Print out the name of the group corresponding to the group number, and other groups that person may belong to. (groups, awk, cut. Also look at/etc/passwd and /etc/groups).
  9. Sort /etc/passwd using the uid (first field) as the key. (sort)
  10. Suppose that you want to write the same letter to many people except that you want each letter addressed to the senders personally. This mailmerge facility can be created using a shell script. Put the names of the recipients (one per line) in a file called names, create a texfile called template which has NAME wherever you want the person's name to appear and write a script (using sed) to produce a temporary file called letter from the template file.

Eval :-

Consider the following

word1=one
word2=two
number=1

The variable word$number has the value one but how can the value be accessed? If you try echo $word$number or even echo ${word$number} you won't get the right answer. You want $number processed by the shell, then the resulting command processed. One way to do this is to use eval echo \$word$number - the \ symbol delays the interpretation of the first $ character until eval does a second pass.

Alternative notations :-

((a=a<<2+3)) is the equivalent of let a="a<<2+3" - note that this variant saves on quotemarks.

  [[ -f /etc/passwd ]]
     echo the passwd file exists

is another way of doing

  if [ -f /etc/passwd ]
  then
     echo the passwd file exists
  fi

or you could use the logical && operator, which like its C cousin only evaluates the second operand if the first is true.

  [ -f /etc/passwd ] && echo the passwd file exists

Bash features:-

C-like for loops are possible.

for((i=1; $i<3; i=i+1))
do
  echo $i
done

Regular expressions (wildcard characters etc) can be used in some comparisons. In the following, ?[aeiou]? is a regular expression that matches a character followed by one of a, e, i , o ,u, followed by a character

for word in four six sly
do
  if [[ $word == ?[aeiou]? ]]
  then
    echo $word is 3 letters long with a central vowel
  else
    echo $word is not 3 letters long with a central vowel
  fi
done