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Articles taggués ‘netfilter’

Rate-limit Incoming Port 22 Connections

13/11/2018 Aucun commentaire

Both netfilter and pf provides rate-limit option to perform simple throttling on incoming connections on port # 22.

Iptables Example

The following example will drop incoming connections which make more than 5 connection attempts upon port 22 within 60 seconds:

#!/bin/bash
inet_if=eth1
ssh_port=22
$IPT -I INPUT -p tcp --dport ${ssh_port} -i ${inet_if} -m state --state NEW -m recent  --set
$IPT -I INPUT -p tcp --dport ${ssh_port} -i ${inet_if} -m state --state NEW -m recent  --update --seconds 60 --hitcount 5 -j DROP

Call above script from your iptables scripts. Another config option:

$IPT -A INPUT  -i ${inet_if} -p tcp --dport ${ssh_port} -m state --state NEW -m limit --limit 3/min --limit-burst 3 -j ACCEPT
$IPT -A INPUT  -i ${inet_if} -p tcp --dport ${ssh_port} -m state --state ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -o ${inet_if} -p tcp --sport ${ssh_port} -m state --state ESTABLISHED -j ACCEPT
# another one line example
# $IPT -A INPUT -i ${inet_if} -m state --state NEW,ESTABLISHED,RELATED -p tcp --dport 22 -m limit --limit 5/minute --limit-burst 5-j ACCEPT

See iptables man page for more details.

*BSD PF Example

The following will limits the maximum number of connections per source to 20 and rate limit the number of connections to 15 in a 5 second span. If anyone breaks our rules add them to our abusive_ips table and block them for making any further connections. Finally, flush keyword kills all states created by the matching rule which originate from the host which exceeds these limits.

sshd_server_ip="202.54.1.5"
table <abusive_ips> persist
block in quick from <abusive_ips>
pass in on $ext_if proto tcp to $sshd_server_ip port ssh flags S/SA keep state (max-src-conn 20, max-src-conn-rate 15/5, overload <abusive_ips> flush)

psad: Linux Detect And Block Port Scan Attacks In Real Time

09/10/2018 Comments off

Source: NixCraft

Q. How do I detect port scan attacks by analyzing Debian Linux firewall log files and block port scans in real time? How do I detect suspicious network traffic under Linux?

A. A port scanner (such as nmap) is a piece of software designed to search a network host for open ports. Cracker can use nmap to scan your network before starting attack. You can always see scan patterns by visiting /var/log/messages. But, I recommend the automated tool called psad – the port scan attack detector under Linux which is a collection of lightweight system daemons that run on Linux machines and analyze iptables log messages to detect port scans and other suspicious traffic.

psad makes use of Netfilter log messages to detect, alert, and (optionally) block port scans and other suspect traffic. For tcp scans psad analyzes tcp flags to determine the scan type (syn, fin, xmas, etc.) and corresponding command line options that could be supplied to nmap to generate such a scan. In addition, psad makes use of many tcp, udp, and icmp signatures contained within the Snort intrusion detection system.

Install psad under Debian / Ubuntu Linux

Type the following command to install psad, enter:
$ sudo apt-get update
$ sudo apt-get install psad

Lire la suite…

Nmap Reference Guide

02/10/2018 Comments off

NMAP: Host Discovery

nmap reference guideNMAP: One of the very first steps in any network reconnaissance mission is to reduce a (sometimes huge) set of IP ranges into a list of active or interesting hosts. Scanning every port of every single IP address is slow and usually unnecessary.

Of course what makes a host interesting depends greatly on the scan purposes. Network administrators may only be interested in hosts running a certain service, while security auditors may care about every single device with an IP address. An administrator may be comfortable using just an ICMP ping to locate hosts on his internal network, while an external penetration test may use a diverse set of dozens of probes in an attempt to evade firewall restrictions.

Because host discovery needs are so diverse, Nmap offers a wide variety of options for customizing the techniques used. Host discovery is sometimes called ping scan, but it goes well beyond the simple ICMP echo request packets associated with the ubiquitous ping tool. Users can skip the ping step entirely with a list scan (-sL) or by disabling ping (-Pn), or committed the network with arbitrary combinations of multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes.

The goal of these probes is to solicit responses which demonstrate that an IP address is actually active (is being used by a host or network device). On many networks, only a small percentage of IP addresses are active at any given time. This is particularly common with private address space such as 10.0.0.0/8. That network has 16 million IPs, but I have seen it used by companies with less than a thousand machines. Host discovery can find those machines in a sparsely allocated sea of IP addresses.

If no. host discovery options are given, Nmap sends an ICMP echo request, a TCP SYN packet to port 443, a TCP ACK packet to port 80, and an ICMP timestamp request. (For IPv6, the ICMP timestamp request is omitted because it is not part of ICMPv6.) These defaults are equivalent to the – PE - PS443 - PA80 - PP options. The exceptions to this are the ARP (for IPv4) and Neighbor Discovery (for IPv6) scans which are used for any targets on a local ethernet network.

For unprivileged Unix shell users, the default probes are a SYN packet to ports 80 and 443 using the connect system call. This host is often sufficient when local scanning discovery networks, but a more comprehensive set of discovery probes is recommended for security auditing. Lire la suite…

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Linux: 20 Iptables Examples For New SysAdmins

28/09/2017 Comments off

According to the official project site:

netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack. A registered callback function is then called back for every packet that traverses the respective hook within the network stack.

This Linux based firewall is controlled by the program called iptables to handles filtering for IPv4, and ip6tables handles filtering for IPv6. I strongly recommend that you first read our quick tutorial that explains how to configure a host-based firewall called Netfilter (iptables) under CentOS / RHEL / Fedora / Redhat Enterprise Linux. This post lists most simple iptables solutions required by a new Linux user to secure his or her Linux operating system from intruders.

IPTABLES Rules Example

  • Most of the actions listed in this post written with the assumption that they will be executed by the root user running the bash or any other modern shell. Do not type commands on the remote system as it will disconnect your access.
  • For demonstration purpose, I’ve used RHEL 6.x, but the following command should work with any modern Linux distro that use the netfliter.
  • It is NOT a tutorial on how to set iptables. See tutorial here. It is a quick cheat sheet to common iptables commands.

#1: Displaying the Status of Your Firewall

Type the following command as root:
# iptables -L -n -v
Sample outputs:

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Above output indicates that the firewall is not active. The following sample shows an active firewall:
# iptables -L -n -v
Sample outputs:

Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           state INVALID
  394 43586 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
   93 17292 ACCEPT     all  --  br0    *       0.0.0.0/0            0.0.0.0/0
    1   142 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  br0    br0     0.0.0.0/0            0.0.0.0/0
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           state INVALID
    0     0 TCPMSS     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp flags:0x06/0x02 TCPMSS clamp to PMTU
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
    0     0 wanin      all  --  vlan2  *       0.0.0.0/0            0.0.0.0/0
    0     0 wanout     all  --  *      vlan2   0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     all  --  br0    *       0.0.0.0/0            0.0.0.0/0

Chain OUTPUT (policy ACCEPT 425 packets, 113K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain wanin (1 references)
 pkts bytes target     prot opt in     out     source               destination

Chain wanout (1 references)
 pkts bytes target     prot opt in     out     source               destination

Where,

  • -L : List rules.
  • -v : Display detailed information. This option makes the list command show the interface name, the rule options, and the TOS masks. The packet and byte counters are also listed, with the suffix ‘K’, ‘M’ or ‘G’ for 1000, 1,000,000 and 1,000,000,000 multipliers respectively.
  • -n : Display IP address and port in numeric format. Do not use DNS to resolve names. This will speed up listing.

#1.1: To inspect firewall with line numbers, enter:

# iptables -n -L -v --line-numbers
Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  0.0.0.0/0            0.0.0.0/0           state INVALID
2    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
3    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
4    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy DROP)
num  target     prot opt source               destination
1    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
2    DROP       all  --  0.0.0.0/0            0.0.0.0/0           state INVALID
3    TCPMSS     tcp  --  0.0.0.0/0            0.0.0.0/0           tcp flags:0x06/0x02 TCPMSS clamp to PMTU
4    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
5    wanin      all  --  0.0.0.0/0            0.0.0.0/0
6    wanout     all  --  0.0.0.0/0            0.0.0.0/0
7    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0

Chain OUTPUT (policy ACCEPT)
num  target     prot opt source               destination

Chain wanin (1 references)
num  target     prot opt source               destination

Chain wanout (1 references)
num  target     prot opt source               destination

You can use line numbers to delete or insert new rules into the firewall.

#1.2: To display INPUT or OUTPUT chain rules, enter:

# iptables -L INPUT -n -v
# iptables -L OUTPUT -n -v --line-numbers

#2: Stop / Start / Restart the Firewall

If you are using CentOS / RHEL / Fedora Linux, enter:
# service iptables stop
# service iptables start
# service iptables restart

You can use the iptables command itself to stop the firewall and delete all rules:
# iptables -F
# iptables -X
# iptables -t nat -F
# iptables -t nat -X
# iptables -t mangle -F
# iptables -t mangle -X
# iptables -P INPUT ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -P FORWARD ACCEPT

Where,

  • -F : Deleting (flushing) all the rules.
  • -X : Delete chain.
  • -t table_name : Select table (called nat or mangle) and delete/flush rules.
  • -P : Set the default policy (such as DROP, REJECT, or ACCEPT).

#3: Delete Firewall Rules

To display line number along with other information for existing rules, enter:
# iptables -L INPUT -n --line-numbers
# iptables -L OUTPUT -n --line-numbers
# iptables -L OUTPUT -n --line-numbers | less
# iptables -L OUTPUT -n --line-numbers | grep 202.54.1.1

You will get the list of IP. Look at the number on the left, then use number to delete it. For example delete line number 4, enter:
# iptables -D INPUT 4
OR find source IP 202.54.1.1 and delete from rule:
# iptables -D INPUT -s 202.54.1.1 -j DROP
Where,

  • -D : Delete one or more rules from the selected chain

#4: Insert Firewall Rules

To insert one or more rules in the selected chain as the given rule number use the following syntax. First find out line numbers, enter:
# iptables -L INPUT -n –line-numbers
Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  202.54.1.1           0.0.0.0/0
2    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state NEW,ESTABLISHED 

To insert rule between 1 and 2, enter:
# iptables -I INPUT 2 -s 202.54.1.2 -j DROP
To view updated rules, enter:
# iptables -L INPUT -n --line-numbers
Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  202.54.1.1           0.0.0.0/0
2    DROP       all  --  202.54.1.2           0.0.0.0/0
3    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state NEW,ESTABLISHED

#5: Save Firewall Rules

To save firewall rules under CentOS / RHEL / Fedora Linux, enter:
# service iptables save
In this example, drop an IP and save firewall rules:
# iptables -A INPUT -s 202.5.4.1 -j DROP
# service iptables save

For all other distros use the iptables-save command:
# iptables-save > /root/my.active.firewall.rules
# cat /root/my.active.firewall.rules

#6: Restore Firewall Rules

To restore firewall rules form a file called /root/my.active.firewall.rules, enter:
# iptables-restore < /root/my.active.firewall.rules
To restore firewall rules under CentOS / RHEL / Fedora Linux, enter:
# service iptables restart

#7: Set the Default Firewall Policies

To drop all traffic:
# iptables -P INPUT DROP
# iptables -P OUTPUT DROP
# iptables -P FORWARD DROP
# iptables -L -v -n
#### you will not able to connect anywhere as all traffic is dropped ###
# ping cyberciti.biz
# wget http://www.kernel.org/pub/linux/kernel/v3.0/testing/linux-3.2-rc5.tar.bz2

#7.1: Only Block Incoming Traffic

To drop all incoming / forwarded packets, but allow outgoing traffic, enter:
# iptables -P INPUT DROP
# iptables -P FORWARD DROP
# iptables -P OUTPUT ACCEPT
# iptables -A INPUT -m state --state NEW,ESTABLISHED -j ACCEPT
# iptables -L -v -n
### *** now ping and wget should work *** ###
# ping cyberciti.biz
# wget http://www.kernel.org/pub/linux/kernel/v3.0/testing/linux-3.2-rc5.tar.bz2

#8:Drop Private Network Address On Public Interface

IP spoofing is nothing but to stop the following IPv4 address ranges for private networks on your public interfaces. Packets with non-routable source addresses should be rejected using the following syntax:
# iptables -A INPUT -i eth1 -s 192.168.0.0/24 -j DROP
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

#8.1: IPv4 Address Ranges For Private Networks (make sure you block them on public interface)

  • 10.0.0.0/8 -j (A)
  • 172.16.0.0/12 (B)
  • 192.168.0.0/16 (C)
  • 224.0.0.0/4 (MULTICAST D)
  • 240.0.0.0/5 (E)
  • 127.0.0.0/8 (LOOPBACK)

#9: Blocking an IP Address (BLOCK IP)

To block an attackers ip address called 1.2.3.4, enter:
# iptables -A INPUT -s 1.2.3.4 -j DROP
# iptables -A INPUT -s 192.168.0.0/24 -j DROP

#10: Block Incoming Port Requests (BLOCK PORT)

To block all service requests on port 80, enter:
# iptables -A INPUT -p tcp --dport 80 -j DROP
# iptables -A INPUT -i eth1 -p tcp --dport 80 -j DROP

To block port 80 only for an ip address 1.2.3.4, enter:
# iptables -A INPUT -p tcp -s 1.2.3.4 --dport 80 -j DROP
# iptables -A INPUT -i eth1 -p tcp -s 192.168.1.0/24 --dport 80 -j DROP

#11: Block Outgoing IP Address

To block outgoing traffic to a particular host or domain such as cyberciti.biz, enter:
# host -t a cyberciti.biz
Sample outputs:

cyberciti.biz has address 75.126.153.206

Note down its ip address and type the following to block all outgoing traffic to 75.126.153.206:
# iptables -A OUTPUT -d 75.126.153.206 -j DROP
You can use a subnet as follows:
# iptables -A OUTPUT -d 192.168.1.0/24 -j DROP
# iptables -A OUTPUT -o eth1 -d 192.168.1.0/24 -j DROP

#11.1: Example – Block Facebook.com Domain

First, find out all ip address of facebook.com, enter:
# host -t a www.facebook.com
Sample outputs:

www.facebook.com has address 69.171.228.40

Find CIDR for 69.171.228.40, enter:
# whois 69.171.228.40 | grep CIDR
Sample outputs:

CIDR:           69.171.224.0/19

To prevent outgoing access to www.facebook.com, enter:
# iptables -A OUTPUT -p tcp -d 69.171.224.0/19 -j DROP
You can also use domain name, enter:
# iptables -A OUTPUT -p tcp -d www.facebook.com -j DROP
# iptables -A OUTPUT -p tcp -d facebook.com -j DROP

From the iptables man page:

… specifying any name to be resolved with a remote query such as DNS (e.g., facebook.com is a really bad idea), a network IP address (with /mask), or a plain IP address …

#12: Log and Drop Packets

Type the following to log and block IP spoofing on public interface called eth1
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j LOG --log-prefix "IP_SPOOF A: "
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

By default everything is logged to /var/log/messages file.
# tail -f /var/log/messages
# grep --color 'IP SPOOF' /var/log/messages

#13: Log and Drop Packets with Limited Number of Log Entries

The -m limit module can limit the number of log entries created per time. This is used to prevent flooding your log file. To log and drop spoofing per 5 minutes, in bursts of at most 7 entries .
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -m limit --limit 5/m --limit-burst 7 -j LOG --log-prefix "IP_SPOOF A: "
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

#14: Drop or Accept Traffic From Mac Address

Use the following syntax:
# iptables -A INPUT -m mac --mac-source 00:0F:EA:91:04:08 -j DROP
## *only accept traffic for TCP port # 8080 from mac 00:0F:EA:91:04:07 * ##
# iptables -A INPUT -p tcp --destination-port 22 -m mac --mac-source 00:0F:EA:91:04:07 -j ACCEPT

#15: Block or Allow ICMP Ping Request

Type the following command to block ICMP ping requests:
# iptables -A INPUT -p icmp --icmp-type echo-request -j DROP
# iptables -A INPUT -i eth1 -p icmp --icmp-type echo-request -j DROP

Ping responses can also be limited to certain networks or hosts:
# iptables -A INPUT -s 192.168.1.0/24 -p icmp --icmp-type echo-request -j ACCEPT
The following only accepts limited type of ICMP requests:
### ** assumed that default INPUT policy set to DROP ** #############
iptables -A INPUT -p icmp --icmp-type echo-reply -j ACCEPT
iptables -A INPUT -p icmp --icmp-type destination-unreachable -j ACCEPT
iptables -A INPUT -p icmp --icmp-type time-exceeded -j ACCEPT
## ** all our server to respond to pings ** ##
iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT

#16: Open Range of Ports

Use the following syntax to open a range of ports:
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 7000:7010 -j ACCEPT 

#17: Open Range of IP Addresses

Use the following syntax to open a range of IP address:
## only accept connection to tcp port 80 (Apache) if ip is between 192.168.1.100 and 192.168.1.200 ##
iptables -A INPUT -p tcp --destination-port 80 -m iprange --src-range 192.168.1.100-192.168.1.200 -j ACCEPT

## nat example ##
iptables -t nat -A POSTROUTING -j SNAT --to-source 192.168.1.20-192.168.1.25

#18: Established Connections and Restaring The Firewall

When you restart the iptables service it will drop established connections as it unload modules from the system under RHEL / Fedora / CentOS Linux. Edit, /etc/sysconfig/iptables-config and set IPTABLES_MODULES_UNLOAD as follows:

IPTABLES_MODULES_UNLOAD = no

#19: Help Iptables Flooding My Server Screen

Use the crit log level to send messages to a log file instead of console:
iptables -A INPUT -s 1.2.3.4 -p tcp --destination-port 80 -j LOG --log-level crit

#20: Block or Open Common Ports

The following shows syntax for opening and closing common TCP and UDP ports:

Replace ACCEPT with DROP to block port:
## open port ssh tcp port 22 ##
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
 
## open cups (printing service) udp/tcp port 631 for LAN users ##
iptables -A INPUT -s 192.168.1.0/24 -p udp -m udp --dport 631 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -p tcp -m tcp --dport 631 -j ACCEPT
 
## allow time sync via NTP for lan users (open udp port 123) ##
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p udp --dport 123 -j ACCEPT
 
## open tcp port 25 (smtp) for all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 25 -j ACCEPT
 
# open dns server ports for all ##
iptables -A INPUT -m state --state NEW -p udp --dport 53 -j ACCEPT
iptables -A INPUT -m state --state NEW -p tcp --dport 53 -j ACCEPT
 
## open http/https (Apache) server port to all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
 
## open tcp port 110 (pop3) for all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 110 -j ACCEPT
 
## open tcp port 143 (imap) for all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 143 -j ACCEPT
 
## open access to Samba file server for lan users only ##
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 137 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 138 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 139 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 445 -j ACCEPT
 
## open access to proxy server for lan users only ##
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 3128 -j ACCEPT
 
## open access to mysql server for lan users only ##
iptables -I INPUT -p tcp --dport 3306 -j ACCEPT

#21: Restrict the Number of Parallel Connections To a Server Per Client IP

You can use connlimit module to put such restrictions. To allow 3 ssh connections per client host, enter:
# iptables -A INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 3 -j REJECT

Set HTTP requests to 20:
# iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 20 --connlimit-mask 24 -j DROP
Where,

  1. –connlimit-above 3 : Match if the number of existing connections is above 3.
  2. –connlimit-mask 24 : Group hosts using the prefix length. For IPv4, this must be a number between (including) 0 and 32.

#22: HowTO: Use iptables Like a Pro

For more information about iptables, please see the manual page by typing man iptables from the command line:
$ man iptables
You can see the help using the following syntax too:
# iptables -h
To see help with specific commands and targets, enter:
# iptables -j DROP -h

#22.1: Testing Your Firewall

Find out if ports are open or not, enter:
# netstat -tulpn
Find out if tcp port 80 open or not, enter:
# netstat -tulpn | grep :80
If port 80 is not open, start the Apache, enter:
# service httpd start
Make sure iptables allowing access to the port 80:
# iptables -L INPUT -v -n | grep 80
Otherwise open port 80 using the iptables for all users:
# iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
# service iptables save

Use the telnet command to see if firewall allows to connect to port 80:
$ telnet www.cyberciti.biz 80
Sample outputs:

Trying 75.126.153.206...
Connected to www.cyberciti.biz.
Escape character is '^]'.
^]

telnet> quit
Connection closed.

You can use nmap to probe your own server using the following syntax:
$ nmap -sS -p 80 www.cyberciti.biz
Sample outputs:

Starting Nmap 5.00 ( http://nmap.org ) at 2011-12-13 13:19 IST
Interesting ports on www.cyberciti.biz (75.126.153.206):
PORT   STATE SERVICE
80/tcp open  http

Nmap done: 1 IP address (1 host up) scanned in 1.00 seconds

I also recommend you install and use sniffer such as tcpdupm and ngrep to test your firewall settings.

CONCLUSION:

This post only list basic rules for new Linux users. You can create and build more complex rules. This requires good understanding of TCP/IP, Linux kernel tuning via sysctl.conf, and good knowledge of your own setup. Stay tuned for next topics:

  • Stateful packet inspection.
  • Using connection tracking helpers.
  • Network address translation.
  • Layer 2 filtering.
  • Firewall testing tools.
  • Dealing with VPNs, DNS, Web, Proxy, and other protocols.
 

Make the configuration of iptables persistent (Debian)

22/03/2017 Comments off

Objective

To make the configuration of iptables persistent on a Debian-based system

Background

The iptables and ip6tables commands can be used to instruct Linux to perform functions such as firewalling and network address translation, however the configuration that they create is non-persistent so is lost whenever the machine is rebooted. For most practical applications this is not the desired behaviour, so some means is needed to reinstate the configuration at boot time.

For security, the iptables configuration should be applied at an early stage of the bootstrap process: preferably before any network interfaces are brought up, and certainly before any network services are started or routing is enabled. If this is not done then there will be a window of vulnerability during which the machine is remotely accessible but not firewalled.

Scenario

Suppose you have a machine that you wish to protect using a firewall. You have written iptables and ip6tables rulesets, and wish to install them so that they will remain active if the machine is rebooted.

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