Posts Tagged scripts

netcat as a logging tcp proxy

I felt I needed to write an article about netcat, so here is it !
Netcat is an incredibly usefull tool, that allows you to play with tcp connection easily from the shell.
Basically, as it name implies, it’s just cat over the network, but what its name doesn’t tell you is that it also can act as a socket listener.
So let’s play with pipes, here is one of my favourite use of netcat:

mkfifo proxypipe
cat proxypipe | nc -l -p 80 | tee -a inflow | nc localhost 81 | tee -a outflow 1>proxypipe

This command will redirect traffic from localhost:80 to localhost:81, in the inflow file you while find the incoming http request, in the outfile, you will find the http response from the server.
Similarly, you can do this:

cat proxypipe | nc -l 80 | tee -a inflow | sed 's/^Host.*/Host:' |  nc 80 | tee -a outflow >proxypipe

This will allow your browser to point to google using http://localhost .
Anyway, this is my favourite but netcat has thounds of other uses, have a look at it !
It can be usefull for file transfers (gzip|nc) , performance measurement (dd|gzip), protocol debugging (replaying requests), security testing (nc does port scan) …

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Installing redmine 0.8 on intrepid (ubuntu 8.10)

I’ve successfully insalled redmine pretty much easily but I needed to find out what packages to install with apt, which one with gem, which version …
Here is my magic receipe to install it all:

apt-get update 
apt-get install subversion mysql-server rubygems rake pwgen
# next line generates a password for the database
export PASSWORD=`pwgen -nc 8 1`
gem install -v=2.1.2 rails
cd /opt/
svn export redmine-0.8
cd redmine-0.8/
cat <<EOF >> config/database.yml
  adapter: mysql
  socket: /var/run/mysqld/mysqld.sock 
  database: redmine
  host: localhost
  username: redmine
  password: $PASSWORD
  encoding: utf8

rake db:migrate RAILS_ENV="production"
rake redmine:load_default_data RAILS_ENV="production"
apt-get remove pwgen subversion
RAILS_ENV="production" ./script/server  

And that’s it ! Redmine is running on port 3000.
I did this on an EC2 instance and it works like a charm (ami-7cfd1a15).
Maybe next article will discuss running redmine in mongrel or apache, and creating an init script for having redmine running on boot !

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subnet ping scan in shell

Today I logged in a machine I don’t want to install anything on it, but I wanted to find a machine in its network.
I came up with the little shell script that scans the subnet:


while [ $CURR -lt 255 ] ; do
  ping -c1 -t1 $SUBNET.$CURR 2>&1 >/dev/null
  if [ "$?" -eq "0" ]; then
    echo "$SUBNET.$CURR"
  let CURR=$CURR+1

This script is suboptimal but it does the stuff: It uses ping with a timeout of 1 sec, so If no machine is up, the script takes around 255 seconds to scan the subnet, it doesn’t list the machines that doesn’t reply to ping and so on … but as I said it , it does the stuff.

I tested this script in Linux and OSX.

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Biggest file in a tree

There are plenty of solutions to find biggests files in a tree on unix.
I usually used
find . -type f -exec du -sk {} \; |sort -nrk1
until I found it too slow, on a really big partition with a lot of files. It’s slow because the -exec option of find forks for each file , and the du re-fetches the inode for every files (IIRC the inode should be in the buffer cash, the really expensive part is the forking).

Now I usually use this command which is really more efficient (depending essentially on number of files
find . -type f -ls |awk '{print "$7" "$11" }' | sort -nrk1

Conclusion: Fork is expensive 🙂

As my friend nikoteen said in the comment of this post, there is a better solution:
find -ls | sort -k7
The thing is, unix people are (ohh, sorry, I am) used to use some commands with it’s usual argument for example, I often use: ls -lart, tar zcvf, netstat -atnup. And sort -nrk1 is one of that command I often use. That’s why I’m writing stupid commands with awk | sort rather than just writing a simple sort. So guys, use this command:
find -ls | sort -k7

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