Игорь Олемской — практические заметки по системному администрированию Linux CentOS

Today, on my 28th birthday, I'm finally delivering on a promise to my readers which I made about two months ago. I've written a guide on how to host a web application redundantly in a cloud environment. While it's still a bit of a rough draft, it should be a good starting point for those who haven't worked in virtualized environments before. Also, it may show some of the more experienced systems administrators a new way to do things.

The guide: Redundant Cloud Hosting Guide

As always, if you find anything in the guide that needs improvement, I'm all ears.

A simple guide to redundant cloud hosting is a post from: Major Hayden's Racker Hacker blog.

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Все нижеописанное относится к GNU/Linux 2.6.x. ДДОС совершенно тупой, разномастный: syn/tcp/udp/icmp flood тупо на все открытые порты, мегабит на 60. UDP срали вообще куда попало. Но основная атака конечно на HTTP. По этому, тушим сервисы и пишем….

Немного sysctl…

Немного iptables….

А теперь запускаем nginx/httpd и прочую лабуду. Надеюсь заработает. У меня заработало, как будто ничего и не происходило

After I wrote a recent post on best practices for iptables, I noticed that I forgot to mention comments for iptables rules. They can be extremely handy if you have some obscure rules for odd situations.

To make an iptables rule with a comment, simply add on the following arguments to the rule:

-m comment --comment "limit ssh access"

Depending on your distribution, you may need to load the ipt_comment or xt_comment modules into your running kernel first.

A full iptables rule to limit ssh access would look something like this:

iptables -A INPUT -j DROP -p tcp --dport 22 -m comment --comment "limit ssh access"

Adding comments to iptables rules is a post from: Major Hayden's Racker Hacker blog.

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Regardless of the type of hosting you're using — dedicated or cloud — it's important to take network interface security seriously. Most often, threats from the internet are the only ones mentioned. However, if you share a private network with other customers, you have just as much risk on that interface.

Many cloud providers allow you access to a private network environment where you can exchange data with other instances or other services offered by the provider. The convenience of this access comes with a price: other instances can access your instance on the private network just as easily as they could on the public interface.

Here are some security tips for your private interfaces:

Disable the private interface
This one is pretty simple. If you have only one instance or server, and you don't need to communicate privately with any other instances, just disable the interface. Remember to configure your networking scripts to leave the interface disabled after reboots.

Use packet filtering
The actual mechanism will vary based on your operating system, but filtering packets is the one of the simplest ways to secure your private interface. You can take some different approaches with them, but I find the easiest method is to allow access from your other instances and reject all other traffic.

For additional security, you can limit access based on ports as well as source IP addresses. This could prevent an attacker from having easy access to your other instances if they're able to break into one of them.

Configure your daemons to listen on the appropriate interfaces
If there are services that don't need to be listening on the private network, don't allow them to listen on your private interface. For example, MySQL might need to listen on the private interface so the web server can talk to it, but apache won't need to listen on the private interface. This reduces the profile of your instance on the private network and makes it a less likely target for attack.

Use hosts.allow and hosts.deny
Many new systems administrators forget about how handy tcpwrappers can be for limiting access. If your firewall is down in error, host.allow and hosts.deny could be an extra layer of protection. It's important to ensure that the daemons you are attempting to control are build with tcpwrappers support. Daemons like sshd support it, but apache and MySQL do not.

Encrypt all traffic on the private network
Just because it's called a «private» network doesn't mean that your traffic can traverse the network privately. You should always err on the side of caution and encrypt all traffic traversing the private network. You can use ssh tunnels, stunnel, or the built-in SSL features found in most daemons.

This also brings up an important point: you should know how your provider's private network works. Are there safeguards to prevent sniffing? Could someone else possibly ARP spoof your instance's private IP addresses? Is your private network's subnet shared among many customers?

With all of that said, it's also very important to have proper change control policies so that administrators working after you are fully aware of the security measures in place and why they are important. This will ensure that all of the administrators on your instances will understand the security of the system and they should be able to make sensible adjustments later for future functionality.

©2010 Racker Hacker. All Rights Reserved.

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Для того, чтобы в контейнере OpenVZ под CentOS заработала блокировка iptables вида (разрешается не больше 4 соединений для порта 22 в течение 180 секунд):

-A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -m recent --update --seconds 180 --hitcount 4 --name DEFAULT --rsource -j DROP
-A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -m recent --set --name DEFAULT --rsource

Необходимо в файле /etc/vz/vz.conf разрешить следующие iptables-модули:

IPTABLES="iptable_filter ipt_multiport ip_conntrack ipt_REJECT"

По-умолчанию ip_conntrack отсутствует в этом списке, iptables при добавлении приведенных выше правил не ругается, но и ничего не работает.

IP Sets – расширение пакетного фильтра ядра iptables, которое позволяет комфортно работать с большими наборами адресов/сетей. При написании правил iptables при этом мы оперируем именованными списками.

Это, пожалуй, единственный на сегодняшний день доступный способ писать правила для большого количества хостов/сетей, не перечисляя каждый из них в отдельном правиле. Если ваш сайт, работающий под linux, подвергается DDOS, вы наверняка уже знаете про IP Sets. Если нет — http://ipset.netfilter.org/.

Утилита ipset живет в одноименном пакете, а все необходимые патчи ядра входят в состав пакета netfilter-extensions-source, который собирается и устанавливается при помощи утилиты module-assistant (которая для удобства имеет более короткое имя: m-a).

Примерный сценарий установки ip sets выглядит так:

$ sudo apt-get install ipset
$ sudo apt-get install netfilter-extensions-source
$ uname -a
Linux host.domain.tld 2.6.26-1-686 #1 SMP Fri Mar 13 18:08:45 UTC 2009 i686 GNU/Linux
# версия пакета linux-headers должна совпадать с версией ядра, для которого собирается netfilter-extensions
$ sudo apt-get install linux-headers-2.6.26-1-686
$ sudo m-a build netfilter-extensions
$ sudo m-a update && m-a a-i netfilter-extensions

Версия пакета заголовков ядра должна точно соответствовать версии активного ядра, для которого собирается ipset.