Finally iptables works the same on every Linux distro | Hacker News Reader

Finally iptables works the same on every Linux distro

nico.schottelius.org

38 pointstelmich27 comments

For complex setups you sooner or later have to dynamically add/remove rules, based on values only known at runtime (such as an IP address obtained by DHCP). Then, a simple list of rules doesn't suffice, and some kind of programming is needed.

To solve such problems elegantly I designed my "NCD programming language" (link: https://code.google.com/p/badvpn/wiki/NCD ). The language has built-in backtracking, so in the case of iptables, the language itself makes sure any iptables rule that was added is also removed when that is necessary, in a manner not unlike exception handling in C++ etc. Link to iptables module: https://code.google.com/p/badvpn/source/browse/trunk/ncd/mod...

This looks pretty cool. Does it run the script(s) in kernel space or user space?

The interpreter is a userspace program. You can run if from a terminal if you wish and use it as a general purpose scripting language (though it's not quite that useful in that regard, yet).

Oh, and it also runs in the browser ;) http://badvpn.googlecode.com/svn/wiki/emncd.html

umm, iptables is a part of the kernel, and is exactly the same, regardless of distribution. I've used a bunch of different distros and iptables is always the same. What is this guy talking about??

He's talking about wrappers. Wrappers are dist specific. Some people like to use UFW and so on. It's their choice. But real men call iptables directly.

Agreed. The post title is wrong, it should read "Another iptables frontend, this one aims for all distribution."

And yes, real men use iptables directly. I never understood the need for wrappers, just makes it harder to debug and see whats really going on.

Note that this requires a commitment to using cdist. cdist may be great, but it's not much different from puppet or chef or bcfg2 or whatever your favorite configuration management system is.

Since iptables comes from upstream (and is closely linked to the kernel), distros running the same version of the kernel already have the same iptables quirks. (There aren't many.) The article discusses distributing an iptables config file and having it run on startup, a task which can be equally well handled by the other config management systems.

Haven't tried it for myself -- but looking at the code, it appears that the last rule needs to be changed to port 22 for SSH to work.

SSH should be immediately moved off port 22 in virtually any install.

The amount of toxic traffic hitting that port is scary.

If you think someone could get in thru the ssh port (exploiting an undisclosed vulnerability or with brute force password cracking) moving it elsewhere is not a long term solution. fail2ban is good to avoid brute force attacks, but portknocking with i.e. fwknop will be safer in more situations. If a service is not meant for the public then they shouldn't be able to even notice that is there.

fail2ban and other programs that just grep a log file are a horrible idea as most of them can't properly parse a log entry and it's possible to inject an arbitrary ip address into the log file one way or another. I'm not sure if there are any outstanding exploits for the latest version of fail2ban right now but the whole concept of searching through log files that are in a format that can change at any time, aren't delimited properly, and subject to the sysadmin customizing is a precarious one at best.

Even if fail2ban and others were perfect, all it would take is the sysadmin customizing the log format to include some random field and all the sudden it's possible for an attacker to block every IP address with one request.

Source: http://seclists.org/fulldisclosure/2007/Jun/138

A better alternative is something like pam_abl which only protects logins to a user on the system but with it ssh is essentially unbruteforceable.

Does pam_abl work when you're not using PAM for authentication--only for authorization ("account") and session management? e.g., when UsePAM is enabled, but ChallengeResponseAuthentication and PasswordAuthentication are disabled.

I believe this is the problem with using pam_tally2. Then again, if you're only allowing PubkeyAuthentication then maybe you don't care about brute force login attempts so much.

If you're worried about that, it probably shouldn't be publicly accessible in the first place.

Yes, moving ports helps, but it's not a real defense. It just lowers bandwidth costs and prevents some not-so-harmful attacks (from people who don't know whether what they've compromised has any value.)

At the very least, use public-key authentication instead of password authentication for SSH.

One nice side bonus to moving to non-standard SSH ports? Your log files are a lot less noisy.

You then also can easily detect bad behavior by watching failed connect attempts to port 22.

It's indeed a large amount of traffic. But it's not scary. None of it will get through if your SSH software is up to date, you have it configured properly, and user accounts are managed sanely. The attacks to be scared of are those that are actually targeted at you. And they will find the port SSH is running on either way.

> None of it will get through if your SSH software is up to date, you have it configured properly, and user accounts are managed sanely.

So in the majority of cases, traffic will get through? :P

Or perhaps firewall your server properly so only people who are meant to connect to port 22 can connect to it?

Oh I know, we could use iptables!

Every time somebody suggests this, you should not say it's for security purposes, but to reduce the load on a server.

And to reduce load on the person checking the logs.

Not scary if you disable password logins, implement PKI, and limit logins to specific addresses.

I get a kick out of viewing all the failed attempts in /var/log/auth

Added bonus is adding all those bad guy addresses to my blacklists.

passive port knocking should be standard.

This is interesting, but I'd rather prefer something like Ript (a Ruby DSL for Iptables) than a very specific implementation along with its assumptions.

https://github.com/bulletproofnetworks/ript

So long as you can write Ruby, this works in Chef, Puppet, or whatever.

Sounds like one of the reasons I was instructed to use Firehol by my system admin. http://en.wikipedia.org/wiki/FireHOL

Nice to run into someone else using firehol! I've been using it for years but it doesn't seem to get much love.

<plug> If you use Chef, check out AFW. https://github.com/jvehent/AFW </plug>

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