Tagarchief: security

iptables and dynamic DNS – part 3

This is an updated post for this updated article.

I just found back an old note about using iptables in combination with dyndns to open up access from a remote location. For instance, if you have a laptop that you take everywhere and you want to connect to your home or office. The script the other site suggested was broken, so let's write a new one.

Step 1: Create a new chain in the firewall

Create a new chain in the firewall where we can plug in the dynamic rules. On my Fedora machine, the firewall is located in /etc/sysconfig/iptables. I added the bold lines to this example.


*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
COMMIT
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
<b>:DYNAMICPARENT - [0:0]
-A INPUT -j DYNAMICPARENT</b>
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT

Step 2: Write a script

#!/bin/bash
 
HOSTNAME=myname.dyndns.org
CHECK_INTERVAL=60 #once a minute
 
IP="" #initialize $IP
while [ true ]; do
        OIP=$IP
        IP=$(dig +short $HOSTNAME | grep -iE "^[0-9]+.[0-9]+.[0-9]+.[0-9]+$"|head -n 1)
        if [ "$OIP" != "$IP" -a "$IP" != "" ]; then
                echo "Changing ip to $IP"
                /sbin/iptables -N DYNAMICNEW                    # create new rule
                /sbin/iptables -I DYNAMICNEW -s $IP -j ACCEPT   # allow new ip
                /sbin/iptables -I DYNAMICPARENT -j DYNAMICNEW   # attach new rule to its parent
 
                while [ true ]; do  # unlink old rule - if multiple exist, remove all
                        /sbin/iptables -D DYNAMICPARENT -j DYNAMICCHILD 2>/dev/null || break
                done
                /sbin/iptables -F DYNAMICCHILD #flush all old rules
                /sbin/iptables -X DYNAMICCHILD #flush all old rules
 
                /sbin/iptables -E DYNAMICNEW DYNAMICCHILD #rename new to "current"
        fi
        sleep $CHECK_INTERVAL
done

In this case, the firewall accepts all traffic from $IP, but of course you could restrict it to 1 port. Also, I focussed on IPv4, but you could easily rewrite this script to IPv6 using ip6tables. I saved the file to /usr/local/bin/dynfirewall.sh

Step 3: Run the script

I'd prefer running the script from inittab, but since Fedora doesn't work like this anymore, I put the following line in /etc/rc.d/rc.local:

/usr/local/bin/dynfirewall.sh >>/var/log/dynfirewall 2>>/var/log/dynfirewall &

Please don't forget the ampersand at the end to fork the script!!

Why is this script better than previous version?

- This script can handle cnames
- The old script used to delete old rules, before creating new ones. This one does not. Therefore, it will never leave a second where you cannot connect.

© GeekLabInfo iptables and dynamic DNS - part 3 is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info
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iptables and dynamic DNS – part 2

In 2011, I wrote this post on Dynamic DNS: https://www.geeklab.info/2011/02/iptables-and-dynamic-dns. While this is still useful, I found a newer, cooler way to do Dynamic DNS in combination with iptables. It's called libnetfilter_queue.

iptables is used to change the inner netfilter tables of the kernel. And because the kernel has no internal resolver, it is impossible for the kernel to do on-the-fly dns lookups. But by offloading this decision to userspace, it is possible. The libnetfilter_queue lib offers that functionality.

libnetfilter_queue is a userspace library providing an API to packets that have been queued by the kernel packet filter. It has bindings for Python and several other languages.

Requirements for my setup

python-NetfilterQueue - https://github.com/kti/python-netfilterqueue

libnfnetlink

libnetfilter_queue

libmnl

You may need to build the first dependency yourself. The other 3 are available in Fedora 20 by default. If you're running RHEL/CentOS, the Fedora packages can be recompiled for your setup.

iptables rule

First, you need to get iptables to enqueue specific packets to your queue.

iptables -I INPUT -p tcp --dport 631 -m state --state NEW -j NFQUEUE --queue-num 6789 -m comment --comment "Remote CUPS printer"

Queue handler

Then we write a script that handles the queue. A quick-and-dirty implementation:

#!/usr/bin/python
 
import socket
from netfilterqueue import NetfilterQueue
 
def getIP(d):
    """
    This method returns the first IP address string
    that responds as the given domain name
    """
    try:
        data = socket.gethostbyname(d)
        #ip = repr(data)
        return data
    except Exception:
        # fail gracefully!
        return False
 
def dnsfilter(pkt):
        if pkt.get_payload_len() < 0x10:
                "Don't know how to handle this too small packet"
                pkt.drop()
                return False
 
        payload=pkt.get_payload()
        srcip=".".join("{:d}".format(ord(c)) for c in payload[0x0c:0x10])
        allowedip=getIP('localhost')
        print "Debug: SRC="+srcip+" ALLOWED="+allowedip+" RESULT=",
        if srcip==allowedip:
                print "Accept"
                pkt.accept()
        else:
                print "Drop"
                pkt.drop()
 
nfqueue = NetfilterQueue()
nfqueue.bind(6789, dnsfilter)
try:
        nfqueue.run()
except KeyboardInterrupt:
        print

This is a quick-and-dirty implementation that misses basic features such as caching the result of gethostbyname. This may introduce terrible delays if used wrong.

Ubuntu/Debian

I'm running RedHat-based software on all of my machines. Above information may be useful for Ubuntu/Debian users, but it's not tested and I'm not supporting it.

Servers: RedHat Enterprise Linux/CentOS is more suitable for servers, as there's a lot of professional level support available. I think that's important, because if I say, get a car accident, I want the servers to be managable by another professional.

Desktops/Laptops: RPM packages are pretty exchangable between RedHat-based platforms. That's a good reason to run Fedora on the desktop.
© GeekLabInfo iptables and dynamic DNS - part 2 is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info
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Firewall your Exchange 2007 server

Software that is not reachable, can't be hacked. Easy as that. So if you have an cloud-based anti-spam/anti-virus filter, you can block your smtp server for badguys.

In my situation, I'm using a Windows 2008 SBS server with Exchange 2007.

  1. Start wf.msc
  2. Go to inbound rules
  3. Find MSExchangeTransportWorker and double-click it to open the properties
  4. On the tab "scope", select "These IP addresses" and add the following IPs: 'Local subnet', 127.0.0.0/8, 192.168.0.0/16, fe80::/16
  5. Also add the IPs of your anti-spam servers as well
  6. Then click OK
  7. Don't forget to check that the changes actually work by both checking an IP that can connect and one that doesn't
© GeekLabInfo Firewall your Exchange 2007 server is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info
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iptables and dynamic DNS

Please read other posts in this section as well.
They may provide better options.

I just found back an old note about using iptables in combination with dyndns to open up access from a remote location. For instance, if you have a laptop that you take everywhere and you want to connect to your home or office. The script the other site suggested was broken, so let's write a new one.

Step 1: Create a new chain in the firewall

Create a new chain in the firewall where we can plug in the dynamic rules. On my Fedora machine, the firewall is located in /etc/sysconfig/iptables. I added the bold lines to this example.


*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
COMMIT
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
<b>:DYNAMIC - [0:0]
-A INPUT -j DYNAMIC</b>
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT

Step 2: Write a script

#!/bin/bash
 
HOSTNAME=myname.dyndns.org
CHECK_INTERVAL=60 #once a minute
 
/sbin/iptables -F DYNAMIC #flush all existing rules
IP="" #initialize $IP
while [ true ]; do
    OIP=$IP
    IP=$(host $HOSTNAME | grep -iE "[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+" |cut -f4 -d' '|head -n 1)
    if [ "$OIP" != "$IP" -a "$IP" != "" ]; then
         echo "Changing ip to $IP"
         /sbin/iptables -F DYNAMIC #flush all old rules
         /sbin/iptables -I DYNAMIC -s $IP -j ACCEPT #the new rule
    fi
    sleep $CHECK_INTERVAL
done

In this case, the firewall accepts all traffic from $IP, but of course you could restrict it to 1 port. Also, I focussed on IPv4, but you could easily rewrite this script to IPv6 using ip6tables. I saved the file to /usr/local/bin/dynfirewall.sh

Step 3: Run the script

I'd prefer running the script from inittab, but since Fedora doesn't work like this anymore, I put the following line in /etc/rc.d/rc.local:

/usr/local/bin/dynfirewall.sh >>/var/log/dynfirewall 2>>/var/log/dynfirewall &

Please don't forget the ampersand at the end to fork the script!!

© GeekLabInfo iptables and dynamic DNS is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info
1 Star2 Stars3 Stars4 Stars5 Stars (2 votes, average: 5,00 out of 5)
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Recover the password of a 3COM 4250T switch

Last weekend, one of my 3com 4250T switches stopped functioning. I used a multimeter and the fuse seems okay, but it's dead as a doorknob. Since my switches are stacked to one virtual unit, I bought a second hand 4250T to replace the broken one.

As often with second hand crap, this switch contained settings from the last location. Including an unknown password.

Fortunately, the switches have a recovery mode (which can also be disabled, so below instructions may not work on another switch).

Recovery mode

I connected the switch to a RS232 port using a null-modem cable. If you know the switches IP, you can also use telnet.

Login: recover
Password: recover

*** Password Recovery Mode ***
The administrative password will be cleared if a hard reset operation is
carried out on the device within 30 seconds.

If a hard reset operation is not carried out during this period, the device
will return to the CLI login prompt

countdown = 30...29...28...

Pull the plug of the switch before the countdown reaches 0. When you boot it again, you can login using the default user "admin" and simply press enter for 'password'.

This manual may work on:
3COM SuperStack 3 4400
3COM SuperStack 3 4900
3COM SuperStack 3 4924
3COM SuperStack 3 4950
3COM SuperStack 3 4300
3COM SuperStack 3 Webcache
3COM SuperStack 3 Webcache 1000
3COM SuperStack 3 Webcache 3000
3COM SuperStack 4200 series
3COM SuperStack 3 Switch 4250T
3COM SuperStack 3 Switch 4226T
3COM Switch 4050
3COM Switch 4060
3COM 3C16115
3COM 3C16116

© GeekLabInfo Recover the password of a 3COM 4250T switch is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info
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Adobe Reader and Acrobat security update

The last year or so, Adobe Reader has had a whole lot of leaks. Trend Micro published a blog yesterday about the fact that Adobe released an out-of-band update to plug yet another security hole.

If you want to use Adobe Reader, update. But I have a better suggestion: ditch Adobe Reader all together, and install FoxIT Reader instead.

For business use, FoxIT is a great piece of software. Clean, faster, more secure, and just as easy to use. And unlike Adobe, FoxIT offers a ready-to-use MSI file for Active Directory deployment.

© GeekLabInfo Adobe Reader and Acrobat security update is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info
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