Posted by gmendoza on September 20, 2008 under Tech Tips | 
Introduced first in Ubuntu 8.04, UFW is Ubuntu’s “uncomplicated firewall”, a remarkably easy to use tool for creating simple iptables firewall rules. The goal behind UFW is to make it easy for administrators and even third party packages to work with firewall rules in a clean and consistent manner. When UFW is enabled, the default set of rules work very well for the average server or desktop platform, as it blocks all non-essential inbound network access without hobbling certain types of useful protocols and return traffic.
In the following example, we will set up a very simple firewall adequate for almost anyone.
First, let’s check the status of UFW, and the currently installed iptables rule set. The following displays that UFW is disabled and that there are no rules for iptables INPUT chain.
Check firewall status
sudo ufw status
Firewall not loaded
sudo iptables -L INPUT -n | column -t
Chain INPUT (policy DROP)
target prot opt source destination
Enable UFW
Now, let’s enable UFW and examine the change to iptables’ INPUT chain.
sudo ufw enable
Firewall started and enabled on system startup
sudo iptables -L INPUT -n | column -t
Chain INPUT (policy DROP)
target prot opt source destination
ufw-before-input all -- 0.0.0.0/0 0.0.0.0/0
ufw-after-input all -- 0.0.0.0/0 0.0.0.0/0
The default policy was changed to drop all traffic, and two new chains are referenced. For a much better understanding of what the default rules are, take a look at the files “/etc/ufw/before.rules” and “/etc/ufw/after.rules“.
Connection Tracking
For your convenience, UFW also enables some very useful connection tracking rules, which intelligently inspect outbound application traffic and dynamically allows the return traffic for you. By default, TCP, UDP, FTP and IRC connection tracking modules are loaded, but others may be added to the IPT_MODULES variable in the file “/etc/default/ufw“.
For example, I sometimes need to use TFTP for sending and receiving firmware to and from routers. So I typically add “nf_conntrack_tftp” to the variable IPT_MODULES.
IPT_MODULES="nf_conntrack_ftp nf_nat_ftp nf_conntrack_irc nf_nat_irc nf_conntrack_tftp"
Remember to reload UFW so that the conntrack module is loaded.
sudo /etc/init.d/ufw restart
Allowing inbound services
If your system runs server applications such as DNS, SSH, TFTP and web, then you can add them to your firewall rules using these very simple commands. If you don’t run servers on your machine, this step can be skipped.
sudo ufw allow 53
sudo ufw allow 22/tcp
sudo ufw allow 69/udp
sudo ufw allow 80/tcp
Notice that the first command I used did not specify UDP or TCP. When omitted, UFW adds both protocols. DNS uses TCP for larger DNS exchanges like zone transfers and huge replies, so you’ll probably want both.
UFW displays the results very nicely.
sudo ufw status
Firewall loaded
To Action From
-- ------ ----
53:tcp ALLOW Anywhere
53:udp ALLOW Anywhere
22:tcp ALLOW Anywhere
69:udp ALLOW Anywhere
80:tcp ALLOW Anywhere
SYN cookies and more
UFW can be used to load kernel options, too. These are defined in “/etc/ufw/sysctl.conf“. For example, I wanted to enable SYN cookies which was added to thwart certain TCP DoS attacks. Modify the following line to 1 in order to enable the feature.
net/ipv4/tcp_syncookies=1
Logging can suck
Okay, if you’re on a busy network and don’t want to fill up your syslog, you might want to disable UFW’s logging.
sudo ufw logging off
And really that’s all there is to it. Be sure to check out the man page for some more examples and features you may be interested in.
Posted by gmendoza on October 8, 2007 under Tech Tips |
Performing GnuPG functions from Vim is actually pretty helpful if you work heavily with both applications on a regular basis. I was recently looking for a simple way to both word wrap and clearsign various text files within Vim, and found just what I was looking for.
1. (optional) Set the word wrap of text in Vim to a maximum text width of 70 characters. This can be done manually, or by simply adding the following text to your “~/.vimrc” file:
:set textwidth=70
2. As for the ability to clearsign, encrypt, decrypt and verify the text from within Vim, you can create command mode aliases as shortcuts for longer commands in Vim. Simply add the following to your “~/.vimrc” file:
:cmap cs %!gpg --clearsign
:cmap es %!gpg -seat
:cmap ee %!gpg -eat
:cmap de %!gpg -d
Once you save the changes to your .vimrc file, open any text file with vim, enter command mode, and type any of the shortcuts mentioned in step two; “cs” to clearsign, “es” to encrypt and sign, “ee” to encrypt with no signature, and “de” to decrypt or verify. The shortcut will display the command about to be issued, to which you can hit can enter to execute it. You will be prompted for recipients, and/or the private key passphrase depending on the function you choose.
If you are familiar with GnuPG syntax, you can change or add any of the above commands to your liking. For instance, for those of you with multiple PGP keys, you can add the “-u”option to specify which one you would like to use.
To wrap an existing unwrapped text file, simply higlight the entire message by placing the Vim cursor at the top of the file, press <shift>+V, followed by <shift>+G. This highlights all text as you will notice. While everything is highlighted, simply press “gq”. This will wrap everything according to your “textwidth” variable.
Feel free to test it out, and provide as much feedback as you like. Have fun.
Special Note:
When using the “textwidth” variable, you may find that it is useful to toggle the paste function. If you are pasting text that has a larger text width than that of which you have specified in Vim (in this case 70 characters), then your paste will automatically be word wrapped to 70.
You may not want this behavior, so the two opposing options you can set manually are:
:set paste
:set nopaste
Better yet, you can map a quick function key to toggle it on or off by adding the following to your .vimrc file:
:set pastetoggle=<F10>
To test, while in insert mode of Vim, press the F10 key, and you will notice that the mode will be clearly identified with:
"-- INSERT (paste) --"
This will allow you to paste text in it’s unwrapped form.
Posted by gmendoza on September 2, 2007 under Tech Tips |
DenyHosts is a project that adds a protective layer to an SSH server by automatically blocking malicious hosts that use brute force or dictionary attacks. If you have SSH services enabled and accessible from the internet, you will likely have thousands of failed login attempts from several sources within a very short period of time. DenyHosts monitors all login attempts, and based on a customizable rule-set can block hosts from making further connections if an attack pattern is matched.
Using tcp_wrappers, the DenyHosts service elegantly manages entries in the /etc/hosts.deny file, adding and removing hosts when thresholds are crossed. e.g. Three failed logins with unknown user accounts; Three failed logins with root account; Five failed logins with known user accounts; Unblock host after a set period of time; etc. You can also specify whether DenyHosts blocks access to SSH or ALL services, thereby mitigating any other attack vectors the offender might try next.
A most valuable feature that makes DenyHosts even more attractive is the optional centralized reporting system. The service can be configured to report all abusive hosts to the DenyHosts collection server, and automatically import a list of IP addresses that others have reported. This network of intelligence gathering and incident response helps to thwart a large number of attacks before they happen, because the attackers (most of which are automated bots) are blocked before they have a chance to move on to other protected servers.Other useful features include email notification when hosts are blocked, and counter resets after successful authentication to prevent accidental blacklisting caused by fat fingered admins. :-)
For those of you using Ubuntu 7.04 (Feisty Fawn) and above, it is available in the Universe repository:
sudo apt-get install denyhosts
Edit and customize /etc/denyhosts.conf for your desired options, and restart the service:
sudo /etc/init.d/denyhosts restart
Ubuntu 6.06.1 LTS will need a manual installation, as it is not included in the repositories.
Be sure to check out the project at http://denyhosts.sourceforge.net.
