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Distributed Denial of Service (DDoS) attacks are becoming increasingly commonplace as business becomes more and more dependent on delivering services over the Internet.  One of the most common types of DDoS attacks is the well-known SYN-flood attack. It is a basic end-host resource attack designed to bring your server to its knees.  As a result, your server is unable to properly handle any new incoming connection requests.

Recently at 2014, I gave a talk focusing on how you can survive TCP SYN-flooding attacks by implementing some recently developed kernel level Netfilter/iptables defense mechanisms. In this post I will provide a more condensed version of the talk highlighting how you can use these same techniques to protect your servers running Red Hat Enterprise Linux 7 Beta.

It's important to note that while the following DDoS defense features are available in RHEL 7 Beta, they're not enabled by default.  To take advantage of them, you will first need to enable some settings including a few additional iptables rules to your existing firewall ruleset.

Why is the SYN-flood hurting the kernel?

The basic TCP scalability problem for the Linux kernel is related to how many new connections can be created per second. This directly relates to a lock per socket when in the "listen" state. For "established" state connections, it can scale very well.

The "listen" state lock is encountered not only with SYN packets, but also other initial connection state packets like SYN-ACK and ACK packets (the last three-way handshake (3WHS) packet). In the flooding attack scenario, the attacker is sending fake packets aimed at hitting the "listen" state locking problem. As such, we need a mechanism to filter out these fake initial connection attempts before the socket enters the "listen" state lock and blocks new incoming connections.

Basic conntrack filtering

With Netfilter's connection tracking system (conntrack), we can start filtering out false SYN-ACK and ACK packets before they hit the "listen" state lock. This is something, which has been possible for a long time, but is not enabled by default.

All you need is these two commands:

# iptables -A INPUT -m state --state INVALID -j DROP

# /sbin/sysctl -w net/netfilter/nf_conntrack_tcp_loose=0

The first iptables rule will catch packets that the connection tracking system has categorized as "INVALID" and not part of a known connection state.

The second sysctl setting will make the connection tracking system more strict in this categorization.  Specifically this will (among other things) help catch ACK-floods.

What about performance?

The result is a 20x increase in deflecting SYN-ACK and ACK based attacks.

Netfilter's conntrack has had a bad reputation for being slow. While this was true in the "early-days", it now offers excellent scalability for established conntracks.

The matching against existing conntrack entries is very fast and completely scalable. The conntrack system actually does lockless RCU (read-copy update) lookups for existing connections.

Essentially, this solves all other TCP-flooding packets except SYN-flooding.

Why doesn't this solve the SYN-flooding?

The conntrack system actually has a scalability problem (like the "listen" lock) when it comes to creating (or deleting) connections, which the SYN-flood will hit.

Even after fixing the conntrack lock, the SYN packets will still be sent to the socket causing the "listen" socket lock to occur. The normal mitigation technique is to send SYN-cookies and avoid creating any state until the SYN-ACK packet is seen.

Unfortunately, SYN-cookies are sent under the same "listen" state lock, so the mitigation does not solve the scalability issue. How these limitations can be worked around will be discussed later.

What's new in Red Hat Enterprise Linux 7 Beta?

During Netfilter Workshop 2013, Patrick McHardy, Martin Topholm and I got the idea of addressing this in the firewall layer in what is called "Network-Based Countermeasures".  This gave birth to the "SYNPROXY" iptables module and related changes in the Netfilter core for handling this; features now available in Red Hat Enterprise Linux 7 Beta.

The SYNPROXY module works around the two scalability issues mentioned above. First, it does parallel SYN-cookies, and second, it does not create a conntrack entry before the SYN-ACK packet is received thus avoiding the conntrack new connections lock.

The SYNPROXY module can be used on localhost as well as protecting hosts behind the firewall. Once the initial connection is established the normal conntrack system will take over and do all the needed translations (reusing part of the NAT code).

Testing on localhost connections showed a 10x performance benefit for deflecting SYN-attacks.

Configuring SYNPROXY

Configuring SYNPROXY can often be complicated without a guide. For that reason, I'm going to take you through the steps for configuring it one-by-one. (You can also use this script to simplify the setup.)

This example can be used for protecting a web-server, simply export PORT=80.

Step #1: In the "raw" table, we need to make sure connections that need protection don't create new conntrack entries for SYN packets.

# iptables -t raw -I PREROUTING -i $DEV -p tcp -m tcp --syn --dport $PORT -j CT --notrack

Step #2: Next, you need to enable more strict conntracking. This is necessary to have ACK packets (from 3WHS) marked as INVALID state.

# /sbin/sysctl -w net/netfilter/nf_conntrack_tcp_loose=0

Step #3: Now we need to catch these packets and direct them to the SYNPROXY target module. To do this, use the following rule to catch UNTRACKED SYN and INVALID packets that contain the ACK from 3WHS (and also others, but they will fall-through).

# iptables -A INPUT -i $DEV -p tcp -m tcp --dport $PORT -m state --state INVALID,UNTRACKED -j SYNPROXY --sack-perm --timestamp --wscale 7 --mss 1460

Step #4: Catch the INVALID state packets that fell-through the SYNPROXY module and drop those.  Basically, this will drop SYN-ACK based floods.

# iptables -A INPUT -m state --state INVALID -j DROP

Step #5: Remember to also enable TCP timestamps as SYN cookies utilize this TCP option field.

# sbin/sysctl -w net/ipv4/tcp_timestamps=1

Step #6: If you have a busy site, it's recommended to do some conntrack entry tuning to increase the default 64K conn limit. However, it is crucial for performance that you also remember to increase the conntrack hash size.

# echo 1000000 > /sys/module/nf_conntrack/parameters/hashsize

# /sbin/sysctl -w net/netfilter/nf_conntrack_max=2000000

Remember to set your own limits and calculate the memory usage. In this example, 2 million entries times 288 bytes = max 576.0 MB of potential memory usage. For the hash, each hash list "head" is only 8 bytes times 1 million is only 8 MB of fixed allocated memory (mind your CPU's L3 cache-size when choosing the hash size).

Considerations when using SYNPROXY

Enabling SYNPROXY does comes at a cost. The connection establishment phase is going to be slower due to the extra connection setup needed towards the end-host. When the end-host is localhost, then this extra step is obviously very fast but nonetheless adds latency.

The parameters to the SYNPROXY target module must match TCP options and settings supported by the end-host that the TCP connections are being proxied for. Detecting and setting this up is manually done per rule setting. (A helper tool "nfsynproxy" is part of iptables release 1.4.21). This unfortunately means the module cannot be easily deployed in DHCP-based firewall environments.

In the future, the plan is to auto detect end-host TCP options. This is being tracked in Red Hat BZ 1059679, and is waiting for customers requesting this feature (...does anybody get this hint?).

For more information, please check out my 2014 talk on "DDoS protection using Netfilter/iptables" or, alternatively, you can watch the video or access my slides.

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