Archives pour la catégorie ‘Réseau’

Packet Analyzer: 15 TCPDUMP Command Examples

16/04/2019 Aucun commentaire

Packet Analyzer: efficient network traffic analysispacket analyzer

tcpdump command is also called as packet analyzer.

tcpdump command will work on most flavors of unix operating system. tcpdump allows us to save the packets that are captured, so that we can use it for future analysis. The saved file can be viewed by the same tcpdump command. We can also use open source software like wireshark to read the tcpdump pcap files.

In this tcpdump tutorial, let us discuss some practical examples on how to use the tcpdump command.

1. Capture packets from a particular ethernet interface using tcpdump -i

When you execute tcpdump command without any option, it will capture all the packets flowing through all the interfaces. -i option with tcpdump command, allows you to filter on a particular ethernet interface.

$ tcpdump -i eth1
14:59:26.608728 IP > . ack 540 win 16554
14:59:26.610602 IP >  4278 1/0/0 (73)
14:59:26.611262 IP >  26364  PTR? (45)

In this example, tcpdump captured all the packets flows in the interface eth1 and displays in the standard output.

Note: Editcap utility is used to select or remove specific packets from dump file and translate them into a given format.

2. Capture only N number of packets using tcpdump -c

When you execute tcpdump command it gives packets until you cancel the tcpdump command. Using -c option you can specify the number of packets to capture.

$ tcpdump -c 2 -i eth0
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
14:38:38.184913 IP > P 1457255642:1457255758(116) ack 1561463966 win 63652
14:38:38.690919 IP > P 116:232(116) ack 1 win 63652
2 packets captured
13 packets received by filter
0 packets dropped by kernel

The above tcpdump command captured only 2 packets from interface eth0.

Note: Mergecap and TShark: Mergecap is a packet dump combining tool, which will combine multiple dumps into a single dump file. Tshark is a powerful tool to capture network packets, which can be used to analyze the network traffic. It comes with wireshark network analyzer distribution.

3. Display Captured Packets in ASCII using tcpdump -A

The following tcpdump syntax prints the packet in ASCII.

$ tcpdump -A -i eth0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
14:34:50.913995 IP > P 1457239478:1457239594(116) ack 1561461262 win 63652
......G..7 KA....A...L.
14:34:51.423640 IP > P 116:232(116) ack 1 win 63652

Note: Ifconfig command is used to configure network interfaces

Lire la suite…

Categories: Réseau Tags:

How to Backup Linux? 15 rsync Command Examples

15/04/2019 Aucun commentaire

rsync stands for remote sync.

rsync is used to perform the backup operation in UNIX / Linux.

rsync utility is used to synchronize the files and directories from one location to another in an effective way. Backup location could be on local server or on remote server.

Important features of rsync

  • Speed: First time, rsync replicates the whole content between the source and destination directories. Next time, rsync transfers only the changed blocks or bytes to the destination location, which makes the transfer really fast.
  • Security: rsync allows encryption of data using ssh protocol during transfer.
  • Less Bandwidth: rsync uses compression and decompression of data block by block at the sending and receiving end respectively. So the bandwidth used by rsync will be always less compared to other file transfer protocols.
  • Privileges: No special privileges are required to install and execute rsync


$ rsync options source destination

Source and destination could be either local or remote. In case of remote, specify the login name, remote server name and location.

Example 1. Synchronize Two Directories in a Local Server

To sync two directories in a local computer, use the following rsync -zvr command.

$ rsync -zvr /var/opt/installation/inventory/ /root/temp
building file list ... done
sent 26385 bytes  received 1098 bytes  54966.00 bytes/sec
total size is 44867  speedup is 1.63

In the above rsync example:

  • -z is to enable compression
  • -v verbose
  • -r indicates recursive

Now let us see the timestamp on one of the files that was copied from source to destination. As you see below, rsync didn’t preserve timestamps during sync.

$ ls -l /var/opt/installation/inventory/sva.xml /root/temp/sva.xml
-r--r--r-- 1 bin  bin  949 Jun 18  2009 /var/opt/installation/inventory/sva.xml
-r--r--r-- 1 root bin  949 Sep  2  2009 /root/temp/sva.xml

Example 2. Preserve timestamps during Sync using rsync -a

rsync option -a indicates archive mode. -a option does the following,

  • Recursive mode
  • Preserves symbolic links
  • Preserves permissions
  • Preserves timestamp
  • Preserves owner and group

Now, executing the same command provided in example 1 (But with the rsync option -a) as shown below:

$ rsync -azv /var/opt/installation/inventory/ /root/temp/
building file list ... done
sent 26499 bytes  received 1104 bytes  55206.00 bytes/sec
total size is 44867  speedup is 1.63

As you see below, rsync preserved timestamps during sync.

$ ls -l /var/opt/installation/inventory/sva.xml /root/temp/sva.xml
-r--r--r-- 1 root  bin  949 Jun 18  2009 /var/opt/installation/inventory/sva.xml
-r--r--r-- 1 root  bin  949 Jun 18  2009 /root/temp/sva.xml

Lire la suite…

Categories: Réseau, Système Tags: ,

Mass-blocking IP addresses with ipset

15/04/2019 Aucun commentaire

Using ipset to block many IP addresses

I was sponsoring an upload of ipset to Debian the other day. This reminded me of ipset, a very cool program as I am going to show. It makes administering related netfilter (that is: firewall) rules easy along with a good performance. This is achieved by changing how rules match in iptables. Traditionally, an iptables rule matches a single network identity, for example a single IP address or a single network only. With ipsets you can operate on a bunch of (otherwise unrelated) addresses at once easily. If you happen to need bulk actions in your firewall, for example you want to blacklist a long list of IP addresses at once, you will love IP sets. I promise.

Drawbacks of netfilter

IP sets do exist for a longer time, however they made it into the upstream Linux kernel as of version 2.6.39. That means Debian Wheezy will support it as is; for Debian Squeeze you can either use a backported kernel or compile yourself the module shipped in the ipset-source package. In any case you additionally need the command line utilities named ipset. Thus, install that package before you can start. Having that said, Squeeze users should note the ipset syntax I am demonstrating below slightly differs from the syntax supported by the Squeeze utilities. The big picture remains the same, though.

IP utils do not conflict with iptables, but extend it in a useful way. You can combine both as you like. In fact, you still need iptables to turn IP sets into something really useful. Nonetheless you will be hitting iptables‘ limitation soon if you exceed a certain number of rules. You can combine as many conditions within a filter rule as you like, however you can only specify a single pattern for each condition. You figure, this does not scale very well if a pattern to match against does not follow a very tight definition such as a CIDR pattern.

This means you can happily filter whole network blocks such as (which translates to 255 hosts) in iptables, but there is no way to specify a particular not specially connected set of IP addresses within this range if it cannot be expressed with a CIDR prefix. For example, there is no way to block, say,, and in a single statement only. You really need to declare three rules which only differ by the IP address. Pretend, you want to prevent these three addresses from accessing your host. You would probably do something like this:

iptables -A INPUT -s -p TCP -j REJECT
iptables -A INPUT -s -p TCP -j REJECT
iptables -A INPUT -s -p TCP -j REJECT

Alternatively you could do

iptables -A INPUT -s -p TCP -j REJECT

but this would block 251 unrelated hosts, too. Not a good deal I’d say. Now, while the former alternative is annoying, what’s the problem with it? The problem is: It does not scale. Netfilter rules work like a fall-through trapdoor. This means whenever a packet enters the system, it passes through several chains and in each of these chains, netfilter checks all rules until either one rule matches or there are no rules left. In the latter case the default action applies. In netfilter terminology a chain determines when an interception to the regular packet flow occurs. In the example above the specified chain is INPUT, which applies to any incoming packet.

In the end this means every single packet which is sent to your host needs to be checked whether it matches the patterns specified in every rule of yours. And believe me, in a server setup, there are lots of packets flowing to your machine. Consider for example a single HTTP requests, which requires at very least four packets sent from a client machine to your web server. Thus, each of these packets is passing through your rules in your INPUT chain as a bare minimum, before it is eventually accepted or discarded.

This requires a substantial computing overhead for every rule you are adding to your system. This isn’t so much of a problem if you haven’t many rules (for some values of “many” as I am going to show). However, you may end up in a situation where you end up with a large rule set. For example, if you suffer from a DDoS attack, you may be tempted to block drone clients in your firewall (German; Apache2. Likewise: for Lighttpd). In such a situation you will need to add thousands of rules easily.

Being under attack, the performance of your server is poor already. However, by adding many rules to your firewall you are actually further increasing computing overhead for every request significantly. To illustrate my point, I’ve made some benchmarks. Below you find the response times of a HTTP web server while doing sequential requests for a single file of 10 KiB in size. I am explaining my measurement method in detail further below. For now, look the graph. It shows the average response time of an Apache 2 web server, divided into four parts:

  • connect time: this is the time passed by until the server completed the initial TCP handshake
  • send time: this is the time passed by which I needed to reliably send a HTTP request over the established TCP connection reliably (that means: one packet sent and waiting for acknowledged by the server)
  • first byte: this is time passed by until the server sent the first byte from the corresponding HTTP response
  • response complete: this is time passed by until the server sent all of the remaining bytes of the corresponding HTTP response (remaining HTTP header + 10 KiB of payload)


Lire la suite…

How to run iptables automatically after reboot on Debian

13/04/2019 Aucun commentaire

reboot iptablesIf you have customized iptables rules, and would like to load the customized iptables rules persistently across reboots on Debian, you can leverage if-up.d scripts that are located in /etc/network/if-up.d. On Debian, any script that is marked as executable and placed in /etc/network/if-up.d gets executed when a network interface is brought up.

In order to run iptables automatically after reboot on Debian, do the following.

First, customize iptables as you wish, and then save the current iptables rule-set using iptables-save command.

$ sudo iptables-save > /etc/firewall.conf

The above command will dump the current iptables rule set into /etc/firewall.conf file which iptables-restore command can later use to restore the same rule set.

Now create the following if-up.d script called iptables that restores the saved iptables rule set.

$ sudo vi /etc/network/if-up.d/iptables
iptables-restore < /etc/firewall.conf
$ sudo chmod  x /etc/network/if-up.d/iptables

Alternatively, you can add « iptables-restore < /etc/firewall.conf » command to /etc/rc.local, which gets executed at the end of system boot-up.

Source: Xmodulo

Sécuriser son serveur Linux

12/04/2019 Aucun commentaire

Sécuriser Linux

securiser linuxCe guide va vous apprendre à sécuriser un serveur et donc vous initier aux thématiques de la sécurité informatique. En quoi est-ce important ? Par définition, un serveur est ouvert sur le monde, un minimum de sécurité est donc intéressant afin de se prémunir des attaques les plus simplistes.

La marche à suivre sera donc la suivante :

  • présentation des failles ;

  • présentation des outils pour y pallier.

Bien entendu, je ne fais pas un cours complet de sécurité informatique, ce tutoriel est une initiation. Pour faire simple, c’est un peu comme fermer les volets la nuit.

Point de vue matériel, voilà ce qu’il vous faudra :

  • un serveur embarquant une distribution xBuntu ou Debian (pour les autres distribs, le principe sera le même, mais les commandes risquent d’être différentes) ;

  • un accès root (en ssh par exemple) et une console.

Tout le tutoriel se passe en ligne de commande afin de pouvoir être accessible à tous. Comme il faudra éditer des fichiers, veuillez vous assurer d’avoir un éditeur de fichier en ligne de commande (j’utiliserai personnellement nano qui est très simple).

C’est parti !

Filtrer le trafic via le firewall


Le firewall (pare-feu en français) est l’élément indispensable pour sécuriser son serveur. Il va en effet filtrer tout le trafic en n’autorisant que les échanges permis par l’administrateur. Sans firewall correctement réglé, tous les trafics sont plus ou moins permis (c’est-à-dire qu’un attaquant peut faire ce qu’il veut chez vous) et ce genre de faille est détectable par un simple scan de ports.

Or, le noyau Linux offre déjà un pare-feu à l’utilisateur, qu’il est possible de configurer via le logiciel iptables (normalement contenu dans /sbin/iptables). S’il n’est pas installé :

apt-get install iptables

Nous allons maintenant détailler le fonctionnement d’un firewall – relativement simple. Un firewall analyse tout le trafic et vérifie si chaque paquet échangé respecte bien ses règles (critères de filtrage). Donc, il suffit de spécifier de bonnes règles pour interdire tout trafic superflu.

Les critères peuvent être divers (filtrer les ports, les protocoles, les adresses IP, etc). De base, nous allons spécifier nos règles sur les ports. Bien entendu, il faut être le plus strict possible quant au choix des règles ; c’est pourquoi, par défaut, tout firewall se règle en premier lieu en bloquant tout, absolument tout. Ensuite, nous allons « ouvrir » (autoriser le trafic) certains ports que nous voulons utiliser (par exemple pour un serveur web, nous allons ouvrir le port 80 afin que le site web soit accessible).

Déclaration des règles

Filtrage intégral

Suit la marche à suivre pour créer les règles :

1. Créons le script :

nano /etc/init.d/firewall

Et on y écrit : #!/bin/sh

2. On efface les règles précédentes pour partir sur de bonnes bases :

iptables -t filter -F
iptables -t filter -X

3. On bloque par défaut tout le trafic (si vous êtes en ssh, bien entendu, n’exécutez pas encore le script !) :

iptables -t filter -P INPUT DROP
iptables -t filter -P FORWARD DROP
iptables -t filter -P OUTPUT DROP

4. On ne ferme pas les connexions déjà établies :

iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Nous indiquons avec les paramètres -m et --state de ne pas fermer les connexions qui sont déjà établies.

5. On autorise le loopback (on ne va pas se bloquer nous-mêmes !)

iptables -t filter -A INPUT -i lo -j ACCEPT
iptables -t filter -A OUTPUT -o lo -j ACCEPT

Note : lo signifie localhost (le serveur lui-même).

Tout est bloqué, il ne nous reste plus qu’à ouvrir les ports utilisés.

Lire la suite…