Articles taggués ‘firewall’

SIP Server IPTABLES Sample firewall Rules !

26/11/2021 Aucun commentaire

SIP Server protection

IPtables rules

iptables -I INPUT -p udp -m udp –dport 5060 -m string –string "REGISTER sip:" –algo bm -m recent –set –name VOIP –rsource
iptables -I INPUT -p udp -m udp –dport 5060 -m string –string "REGISTER sip:" –algo bm -m recent –update –seconds 60 –hitcount 12 –rttl –name VOIP –rsource -j DROP
iptables -I INPUT -p udp -m udp –dport 5060 -m string –string "INVITE sip:" –algo bm -m recent –set –name VOIPINV –rsource
iptables -I INPUT -p udp -m udp –dport 5060 -m string –string "INVITE sip:" –algo bm -m recent –update –seconds 60 –hitcount 12 –rttl –name VOIPINV –rsource -j DROP
iptables -I INPUT -p udp -m hashlimit –hashlimit 6/sec –hashlimit-mode srcip,dstport –hashlimit-name tunnel_limit -m udp –dport 5060 -j ACCEPT
iptables -I INPUT -p udp -m udp –dport 5060 -j DROP

# RTP – the media stream
# (related to the port range in /etc/asterisk/rtp.conf)
iptables -A INPUT -p udp -m udp –dport 10000:20000 -j ACCEPT

# MGCP – if you use media gateway control protocol in your configuration
iptables -A INPUT -p udp -m udp –dport 2727 -j ACCEPT

Sample script

# Clear any existing firewall stuff before we start
/sbin/iptables –flush
# As the default policies, drop all incoming traffic but allow all
# outgoing traffic. This will allow us to make outgoing connections
# from any port, but will only allow incoming connections on the ports
# specified below.
# Allow connections from my machines
/sbin/iptables -A INPUT -p tcp -i $EXIF -m state –state NEW -s -j ACCEPT
/sbin/iptables –policy INPUT DROP
/sbin/iptables –policy OUTPUT ACCEPT
# Allow all incoming traffic if it is coming from the local loopback device
/sbin/iptables -A INPUT -i lo -j ACCEPT
# Accept all incoming traffic associated with an established connection, or a "related" connection
/sbin/iptables -A INPUT -i $EXIF -m state –state ESTABLISHED,RELATED -j ACCEPT
# Check new packets are SYN packets for syn-flood protection
/sbin/iptables -A INPUT -p tcp ! –syn -m state –state NEW -j DROP
# Drop fragmented packets
/sbin/iptables -A INPUT -f -j DROP
# Drop malformed XMAS packets
/sbin/iptables -A INPUT -p tcp –tcp-flags ALL ALL -j DROP
# Drop null packets
/sbin/iptables -A INPUT -p tcp –tcp-flags ALL NONE -j DROP
# Allow connections to port (4501) – ssh. You can add other ports you need in here
/sbin/iptables -A INPUT -p tcp -i $EXIF –dport 4501 -m state –state NEW -j ACCEPT
# Allow connections to port (4500) – Webmin . You can add other ports you need in here
/sbin/iptables -A INPUT -p tcp -i $EXIF –dport 4500 -m state –state NEW -j ACCEPT
# Allow connections to port (80&443) – www. You can add other ports you need in here
/sbin/iptables -A INPUT -p tcp -i $EXIF –dport 80 -m state –state NEW -j ACCEPT
/sbin/iptables -A INPUT -p tcp -i $EXIF –dport 443 -m state –state NEW -j ACCEPT
# Allow connections from my machines
/sbin/iptables -A INPUT -p tcp -i $EXIF -m state –state NEW -s -j ACCEPT
# Allow SIP connections
/sbin/iptables -A INPUT -p udp -i $EXIF –dport 5060 -m udp -j ACCEPT
/sbin/iptables -A INPUT -p tcp -i $EXIF –dport 5060 -m tcp -j ACCEPT
/sbin/iptables -A INPUT -p udp -i $EXIF –dport 10000:20000 -m udp -j ACCEPT
# Allow icmp input so that people can ping us
/sbin/iptables -A INPUT -p icmp –icmp-type 8 -m state –state NEW -j ACCEPT
# Log then drop any packets that are not allowed. You will probably want to turn off the logging
#/sbin/iptables -A INPUT -j LOG
/sbin/iptables -A INPUT -j REJECT

Source: Ahmad Sabry ElGendi

Categories: Réseau, Sécurité Tags: ,

Neat tricks with iptables

21/11/2021 Aucun commentaire

tricks iptablesNeat tricks with iptables: The past few months have seen me digging deep into the world of TCP/IP and firewalls. It has been a fascinating journey into packet queueing and TCP headers, three-way handshakes and ICMP broadcasts.

The result of this research has been the ongoing creation of a firewall to protect my laptop against open networks, and my Internet server from port scanning and DoS attacks. I’m pretty certain I haven’t even scratched the surface yet, but I have found some settings to protect against the most common attacks. Below I’ll summarize the major pieces of my new firewall, and the logic behind it.

Address spoofing: win with iptables

The easiest way to fool a server is to construct a packet that whose source address is faked, or spoofed. This is surprisingly easy to do. To craft packets, I use a very powerful network analysis tool called Scapy. Scapy will allow you to create packets on the fly, transmit them, and scan your network for any response.

For example, let’s say I’m on my local network (which I am right now, as I write this), connected via wireless as192.168.15.113. I’m going to interact with the router, which is at For the purposes of analysis, I’ve also setup a virtual machine running on, so I can see what happens when I spoof the packet.

So, let’s say I spoof an ICMP echo-request packet, sent to .1 (router) from .113 (me) but spoofed as if it had come from .114 (virtual machine). In Scapy this is quite easy to do. I run two scapy session in two terminal windows. In the first I type:

>>> send(IP(src="", dst="")/ICMP())
Sent 1 packets.

Although my machine is at .113, I’m telling scapy to set the source address for the ICMP echo-request packet to.114, which is the host I want to attack. I’m sending this “ping” to the router, which should now send its response back to .114 instead of me.

In my other terminal window, I run scapy again, this time in promiscuous mode as a packet sniffer. Promiscuous mode means that it will capture all packets seen on the network, not just those destined for my own machine. Here’s what I see:

>>> sniff(filter="icmp")
0000 Ether / IP / ICMP > echo-request 0
0001 Ether / IP / ICMP > echo-reply 0

I ran the sniffer, then did the ping, then stopped the sniffer by pressing Control-C. I can see that two ICMP packets were seen during the sniff. By showing the contents of these packets, I can see both the packet that I transmitted, and the response – which came back to .114!

That’s a spoof. How can it be used to attack someone? Read on in the next section, since what we just did forms the basis for a smurf attack.

Some packet spoofs, however, are more obvious. For example, a packet coming from the Internet bound for a private IP address or certain broadcast addresses, such as address beginning with 192.168 or 224. These are never valid, so it’s a good idea to drop such packets immediately upon receipt. Here are the iptables rules to do this:

# Reject packets from RFC1918 class networks (i.e., spoofed)
iptables -A INPUT -s     -j DROP
iptables -A INPUT -s -j DROP
iptables -A INPUT -s  -j DROP
iptables -A INPUT -s    -j DROP

iptables -A INPUT -s      -j DROP
iptables -A INPUT -d      -j DROP
iptables -A INPUT -s      -j DROP
iptables -A INPUT -d      -j DROP
iptables -A INPUT -s        -j DROP
iptables -A INPUT -d        -j DROP
iptables -A INPUT -d -j DROP
iptables -A INPUT -d  -j DROP

Here’s the same thing, now for ipfw users:

# Verify the reverse path to help avoid spoofed packets.  This means any
# packet coming from a particular interface must have an address matching the
# netmask for that interface.
ipfw add 100 deny all from any to any not verrevpath in

# Deny all inbound traffic from RFC1918 address spaces (spoof!)
ipfw add 110 deny all from to any in
ipfw add 120 deny all from to any in
ipfw add 130 deny all from to any in
ipfw add 140 deny all from to any in

ipfw add 150 deny all from to any in
ipfw add 160 deny all from any to in
ipfw add 170 deny all from to any in
ipfw add 180 deny all from any to in
ipfw add 190 deny all from to any in
ipfw add 200 deny all from any to in
ipfw add 210 deny all from any to in
ipfw add 220 deny all from any to in

Lire la suite…

Categories: Réseau, Sécurité Tags: ,

iptables revisited: a not so ordinary ‘firewall’

20/11/2021 Aucun commentaire

iptables revisited

Source: Per Linde, Martynas Pumputis and Guillermo Rodr ́ıguez iptables revisited: a not so ordinary ‘firewall’

iptables revisited: Abstract

At the present time, security on the internet, and networks in general have evolved, and become an issue that should not be disregarded. It is well known that many experts recommend Linux as the main operating system for the machines that have to be in charge of security (also for a desktop computer). Linux included a basic firewall tool called ipchains in the series of its kernel until 2.4 version, though after that version it switched to iptables. iptables is known for its efficiency and functionality, but the enormous functionality means a more complex tool to be configured. This paper will overview some mechanisms to do advanced configuration of iptables based on two main scenarios. The different configurations presented will try to prove the remarkable power of iptables as an independent firewall and also as a tool that can work in conjuntion with other tools usually incorporated when included this one.


iptables is a firewall developed by the Netfilter Project 1 . Presently, this firewall is becoming more and more popular (both among end users and network administrators). The popularity of this firewall it closely related to Linux operating system, because iptables works with Linux kernels 2.4 and 2.6 and almost every major Linux distribution comes with pre-installed iptables firewall. This firewall is also known as a stateful packet filter. It is a main difference between iptables and ipchains (an ancestor of iptables that was used with Linux kernel versions up to 2.4). The firewall supports not only a packet filtering, but it is also able to log, forward packets and it could be used together with such tools as psad, snort, etc. In this paper we will look a bit deeper into more advance iptables configurations, but first of all, we would like to introduce a reader to a basic usage of iptables. The core of the firewall consists of four parts [7]: tables, chains, matches and targets. A system administrator is able to define an iptables policy, i.e. tables of chains, which describe how a kernel should react against different groups of packets. Rules are used to create a chain – collection of rules that is applied to every packet. There are five predefined chains:

  • INPUT: used for incoming packets before routing.
  • OUTPUT: used for packets coming into the box itself.
  • FORWARD: used for packets being routed through the box.
  • PREROUTING: used for locally-generated packets before routing.
  • POSTROUTING: used for packets as they about to leave iptables.

Every rule should have a set of matches, that helps to filter packets (e.g. -d matches a destination IP, and it also has to have a target – an action which should be performed when a rule matches on a packet (e.g. ACCEPT, DROP, LOG…). All the examples presented in this paper (excluding the one used at section 6) will be based on the scenario presented in the figure 1. In the following scenario, WebServer#1 ( assigned to its ‘eth0’ interface), WebServer#2 ( assigned to its ‘eth0’ interface) and PC ( assigned to its ‘eth0’ interface) belong to a local network and every packet that comes from the Internet/LAN is filtered by the firewall (‘eth1’ interface for LAN’s traffic, ‘eth0’ for Internet’s traffic). PC ( assigned to ‘eth0’ interface) is reachable directly through the Internet. Capture du 2016-04-19 17:53:32 The listing 1 shows a couple of rules, the first one appends a rule to the end of the INPUT chain and it specifies that every packet from the source with IP address will beDROPed and the second rule logs all outgoing connections from eth1 interface.

iptables -A INPUT -s -j DROP
iptables -A OUTPUT -o eth1 -j LOG

Lire la suite…

How to receive a million packets per second

20/11/2021 Aucun commentaire

receive million packetsLast week during a casual conversation I overheard a colleague saying: “The Linux network stack is slow! You can’t expect it to do more than 50 thousand packets per second per core!”

That got me thinking. While I agree that 50kpps per core is probably the limit for any practical application, what is the Linux networking stack capable of? Let’s rephrase that to make it more fun:

On Linux, how hard is it to write a program that receives 1 million UDP packets per second?

Hopefully, answering this question will be a good lesson about the design of a modern networking stack.

First, let us assume:

  • Measuring packets per second (pps) is much more interesting than measuring bytes per second (Bps). You can achieve high Bps by better pipelining and sending longer packets. Improving pps is much harder.
  • Since we’re interested in pps, our experiments will use short UDP messages. To be precise: 32 bytes of UDP payload. That means 74 bytes on the Ethernet layer.
  • For the experiments we will use two physical servers: “receiver” and “sender”.
  • They both have two six core 2GHz Xeon processors. With hyperthreading (HT) enabled that counts to 24 processors on each box. The boxes have a multi-queue 10G network card by Solarflare, with 11 receive queues configured. More on that later.
  • The source code of the test programs is available here: udpsender, udpreceiver.


Let’s use port 4321 for our UDP packets. Before we start we must ensure the traffic won’t be interfered with by the iptables:

receiver$ iptables -I INPUT 1 -p udp --dport 4321 -j ACCEPT  
receiver$ iptables -t raw -I PREROUTING 1 -p udp --dport 4321 -j NOTRACK  

A couple of explicitly defined IP addresses will later become handy:

receiver$ for i in `seq 1 20`; do   
              ip addr add 192.168.254.$i/24 dev eth2; 
sender$ ip addr add dev eth3  

1. The naive approach

To start let’s do the simplest experiment. How many packets will be delivered for a naive send and receive?

The sender pseudo code:

fd = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)  
fd.bind(("", 65400)) # select source port to reduce nondeterminism  
fd.connect(("", 4321))  
while True:  
    fd.sendmmsg(["x00" * 32] * 1024)

While we could have used the usual send syscall, it wouldn’t be efficient. Context switches to the kernel have a cost and it is be better to avoid it. Fortunately a handy syscall was recently added to Linux: sendmmsg. It allows us to send many packets in one go. Let’s do 1,024 packets at once.

The receiver pseudo code:

fd = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)  
fd.bind(("", 4321))  
while True:  
    packets = [None] * 1024
    fd.recvmmsg(packets, MSG_WAITFORONE)

Similarly, recvmmsg is a more efficient version of the common recv syscall.

Let’s try it out:

sender$ ./udpsender  
receiver$ ./udpreceiver1  
  0.352M pps  10.730MiB /  90.010Mb
  0.284M pps   8.655MiB /  72.603Mb
  0.262M pps   7.991MiB /  67.033Mb
  0.199M pps   6.081MiB /  51.013Mb
  0.195M pps   5.956MiB /  49.966Mb
  0.199M pps   6.060MiB /  50.836Mb
  0.200M pps   6.097MiB /  51.147Mb
  0.197M pps   6.021MiB /  50.509Mb

With the naive approach we can do between 197k and 350k pps. Not too bad. Unfortunately there is quite a bit of variability. It is caused by the kernel shuffling our programs between cores. Pinning the processes to CPUs will help:

sender$ taskset -c 1 ./udpsender  
receiver$ taskset -c 1 ./udpreceiver1  
  0.362M pps  11.058MiB /  92.760Mb
  0.374M pps  11.411MiB /  95.723Mb
  0.369M pps  11.252MiB /  94.389Mb
  0.370M pps  11.289MiB /  94.696Mb
  0.365M pps  11.152MiB /  93.552Mb
  0.360M pps  10.971MiB /  92.033Mb

Now, the kernel scheduler keeps the processes on the defined CPUs. This improves processor cache locality and makes the numbers more consistent, just what we wanted.

Lire la suite…

Reduce firewall configuration complexity using iptables with chains

19/11/2021 Aucun commentaire

firewall configuration: Introduction

Reduce firewall configuration complexity: Setting up a firewall on your *nix box, being it a workstation, laptop, or server, is always a good idea. In most cases, you can do with some simple firewall rules, f.e. on your laptop, block all incoming requests (except the established connections, i.e. the replies on the outgoing requests you made), or on a simple webserver (allow port 80 only).

But if you need more complex rules, f.e. a server that hosts a website available for the entire internet, but with an ssh and samba service that should only be available for the local subnet, or even some specific IP addresses, it becomes a bit more complex.
And if you want to filter the outgoing traffic as well, your iptables rules get a mess after a while, and when you want to change anything, chances of a mistake or forgetting something are high, which may result in locking yourself out of your box (at least for remote access), or leaving something open that shouldn’t.

To make your rules more manageable, you can make use of chains in your iptables rules. I got some inspiration in an article that uses chains to make iptables more efficient (faster). My goal was to get easier to read and configure iptables rules, but it will result in faster handling of packets as well.


  • A web service should be available from all networks (i.e. internet) on port 80 (http) and 443 (https)
  • The server can be managed remotely using ssh (port 20) and webmin (port 10000), but only from a limited set of IP addresses (admin PC’s).
  • The server hosts a samba service (several TCP and UDP ports), that should only be available from a limited set of IP addresses (admin + webmaster PC’s).
  • Outgoing connections will be filtered, but some services should be allowed (dns, dhcp, smtp, ntp) and some external websites should be available to get updates.




When using this option, you can filter for established connections. If you define it in both the INPUT and OUTPUT rules, you only have to define in the INPUT rules which NEW incoming requests should be allowed, and in the OUTPUT rules which NEW outgoing request are allowed. The established connections will be allowed and should not be redefined (making the configuration a lot more readable and maintainable). An example allowing only an ssh service without using the ESTABLISHED state would be :

# iptables -A INPUT -p tcp --dport ssh -j ACCEPT
# iptables -A INPUT -j REJECT
# iptables -A OUTPUT -p tcp --sport ssh -j ACCEPT
# iptables -A OUTPUT -j REJECT

Basically, every incoming/outgoing connection is dropped, except if the incoming packet has port 22 (ssh) as destination, or if the outgoing packet was sent from port 22 (which is the reply of the ssh server).

When using ESTABLISHED state, this will be :

# iptables -A INPUT -p tcp --dport ssh -j ACCEPT
# iptables -A INPUT -j REJECT
# iptables -A OUTPUT -m state --state ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -j REJECT

Now, every incoming/outgoing connection is dropped, except if the incoming packet has port 22 (ssh) as destination, or if the packet belongs to an established connection. Because incoming connections to port 22 are allowed, the firewall will remember a packet coming in, creating a ‘connection’ for the host/port the packet originates from when the ssh server replies to it. So when the reply of the ssh server is sent out, it matches an ‘established’ connection and will be allowed out.

In this example, the benefit of using the connection state is not clear, but when more allowed incoming services are added, they only have to be added on the INPUT chain, but not on the OUTPUT chain, because they are covered by the ESTABLISHED rule.
In the first example (without the ESTABLISHED rule), every allowed incoming connection should be repeated in the OUTPUT chain, matching the packets sent for the outgoing connection, which results in an equal amount of rules on both chains.
If you want to do filtering in both directions (allowing incoming request for listening services and outgoing request for remote services), this can become very messy, and almost unmaintainable without making mistakes.

Introducing chains

When two services (on different ports) should be available to a limited but identical list of IP addresses.
Without using chains, for every combination of port and IP a rule should be created :

# iptables -A INPUT -p tcp -m tcp -s --dport 22 -j ACCEPT
# iptables -A INPUT -p tcp -m tcp -s --dport 22 -j ACCEPT
# iptables -A INPUT -p tcp -m tcp -s --dport 22 -j ACCEPT

# iptables -A INPUT -p tcp -m tcp -s --dport 10000 -j ACCEPT
# iptables -A INPUT -p tcp -m tcp -s --dport 10000 -j ACCEPT
# iptables -A INPUT -p tcp -m tcp -s --dport 10000 -j ACCEPT

Resulting in a lot of rules, and when an IP address has to be changed/added/removed, this has to be done for every corresponding rule.

When using chains, this can be much easier. Imagine, that you first check if the packet matches the destination port, and if it does, jump to a new chain, where a list of IP addresses is checked. :

// create new chain admin_IP
# iptables -N admin_IP

// add rules to chain admin_IP
# iptables -A admin_IP -s -j ACCEPT
# iptables -A admin_IP -s -j ACCEPT
# iptables -A admin_IP -s -j ACCEPT
// drop all packets that are not matched by previous rules
# iptables -A admin_IP -j DROP

// filter ports in INPUT chain
# iptables -A INPUT -p tcp -m tcp --dport 22 -j admin_IP
# iptables -A INPUT -p tcp -m tcp --dport 10000 -j admin_IP

As you can see, there is are several benefits of putting the IP addresses in a separate chain :

  • the list of IP addresses in the separate chain can be reused for both ports, so they have to be defined only once.
  • adding/changing/removing an IP address is much easier
  • there is a better overview of the firewall rules.

Lire la suite…

Categories: Réseau, Sécurité Tags: ,