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DDoS Attacks Via DNS Recursion
JehCt writes "Associated Press is running a story about how the recursion feature of open DNS servers can be used to launch massive distributed denial of service (DDoS) attacks: 'First detected late last year, the new attacks direct such massive amounts of spurious data against victim computers that even flagship technology companies could not cope.' A thread at WebmasterWorld explains, 'To make a long story short, having a DNS server that allows recursion for the Internet is like running an open SMTP relay.'"
That's why you run djbdns -- by default it's closed to recursive queries.
Don't piss off The Angry Economist
I am quite a fan of djbns, but the key here is to separate authoritative and
recursive, which is something that DJB has been preaching for a while.
Consequently djbdns won't do this, but it is quite possible to make bind not
do this also. (In fact Bind now has come round and reccomended this.)
It seems to me like a no-brainer, why is splitting the two such a problem?
SDNS wouldn't hurt either, but that will take a lot more doing.
Put this line in your zone definition:
recursion no;
Problem solved.
Name servers are specialized computers that help direct Internet traffic to its destinations. The attacker then sent falsified requests to the compromised directory computer, which unleashed overwhelming floods of amplified data aimed wherever the attacker wanted.
Suggestion:
-Verify requests
-Verify directory computers have not been comprimised
-Disallow amplified data
-Build a new secure system for handling traffic
He who knows best knows how little he knows. - Thomas Jefferson
That's self-referential, not recursive. One does not immediately imply the other. GNU, on the other hand, is recursive.
Javascript + Nintendo DSi = DSiCade
No compromise needed. You just send requests to the DNS server spoofing yourself as the victim's IP. (UDP is much easier to spoof, and can be sent out very quickly.) The replies, which are some 30 times larger than the requests, get sent to the spoofed IP (victim). It is a classic form of amplification attack.
Another problem:
(Quoting a post on the other site)"they can send a 70 byte packet to your DNS server, and your DNS server will send a 500+ byte packet to the victim. With EDNS0, that can be 4,000+ bytes.
So with a dialup account, it would be possible to saturate a T1.
There's plenty of ways for them to mess with you without any 'compromised' machines on your network.
No, most of his software is copyrighted. The only djb software which is in the public domain is software that he has explicitly given to the public domain. The term for the rest of his software is "license-free". You don't need a license to use it. Just download it! Copyright law lets you do anything you want with a copyrighted work, except redistribute it. You can publish patches, as we've done with netqmail.
Don't piss off The Angry Economist
His license forbids distributing binaries unless they are made from his sources. You want to add any of the many well known patches? Great, you distribute his source and your patches, you do not distribute patched sources and you do not distribute binaries.
No way is DJB software public domain.
In fact, I bet a dollar you don't even know what public domain is.
Infuriate left and right
Correct. Here is the CERT writeup from 2000.
Intron: the portion of DNA which expresses nothing useful.
For enterprise systems a split-split DNS design is the best. There are three components to this design:
ADVERTISER
RESOLVER
INTERNAL
The advertiser sits outside, Internet-facing, and is only responsible for resolving outside queries for your own domains. It does not do recursion or dynamic updates, and has a secured cache.
The resolver and internal sit inside, are intranet-facing, and handle internal requests for outside domains, and internal requests for internal domains respectively.
There are lots of articles on-line which show how to set this up.
I am not interested in articles about life extension advancements.
That's a self-referential paradox, not a recursive statement. The grandparent is an example of a recursive statement.
Javascript + Nintendo DSi = DSiCade
Lets say that your local LAN and WLAN networks are 192.168.0/24 and 192.168.1/24, respectively. Make the following additions to your /etc/bind/named.conf.options (or equivalent):
-- -pjk Perry Kundert perry@kundert.ca http://kundert.2y.net
The problem is not only with the amplitude increase, but with the multiple responses. The most common amplification ratio is 2.5:1 (a 47 byte packet with a 117 byte return). Compound that with the fact that it is trivial to find servers that replay this back 8-12 times, and you've got a real problem. 8 * 2.5 gives you a total of a 20x amplification of the packets you send out, which is fairly significant. Also, servers that replay these responses up to 24 times are not uncommon. This type of thing has been around for years, but it is only now coming into the spotlight as it becomes a more common method of attack.
This is old news. If you're running an open DNS server, you're very likely participating in someonelse's DDoS attack and have been for the last couple years. We bought a company last year and part of my job was to assimilate their DNS systems that were reportedly flaking out constantly. I can't speak to the people running the servers before me, but the diagnosis was easy. Once we turned off recursion and convinced the network not to let spoofed UDP packets enter the network, the attacks stopped instantly.
http://www.dnsreport.com/tools/dnsreport.ch?domain =slashdot.org
FAIL Open DNS servers ERROR: One or more of your nameservers reports that it is an open DNS server. This usually means that anyone in the world can query it for domains it is not authoritative for (it is possible that the DNS server advertises that it does recursive lookups when it does not, but that shouldn't happen). This can cause an excessive load on your DNS server. Also, it is strongly discouraged to have a DNS server be both authoritative for your domain and be recursive (even if it is not open), due to the potential for cache poisoning (with no recursion, there is no cache, and it is impossible to poison it). Also, the bad guys could use your DNS server as part of an attack, by forging their IP address. Problem record(s) are:
Server 66.35.250.12 reports that it will do recursive lookups. [test]
Server 12.152.184.136 reports that it will do recursive lookups. [test]
Server 12.152.184.135 reports that it will do recursive lookups. [test]
See this page for info on closing open DNS servers.
Back in the bind 4 days, when I did serious DNS, my company wanted a few servers visible in their domain(s) for external dns host resolution.
For people behind the firewall, they wanted a far more extensive list of hosts that were not to be seen for queries outside the firewall.
I did this by using scp to transfer the zone files from the external to the internal DNS server; the internal server would then "cat" the additional hosts to the zone and HUP the named.
AFAIK modern BIND uses "zones" so you can accomplish the above on one server, if you want. I've never used it, but I can see a number of situations where I'd need my above solution even with this feature.
What BIND needs is not a "recursion no;" option, but instead a "recursion eth0;" or "recursion 1.2.3.*;" so recursive queries must originate from a trusted network.
Remember also that not everyone in the world uses BIND - people with ActiveDirectory or NDS name servers might be screwed until a vendor patch.
view "internal" {
match-clients {
10.0.0.0/8;
};
recursion yes;
zone "example.com" {
yadda yadda yadda;
};
};
view "external" {
match-clients {
any;
};
recursion no;
zone "example.com" {
blah blah blah;
};
};
<xml><I><am><so><damn>Web 2.0</damn></so></am></I></xml>
This isn't just a simple DDoS because DNS servers point many other resources to the attack target. This makes this a Distributed Reflective Denial of Service Attack, or DRDoS. I published an article on this topic in 2600 Hacker Quarterly magazine in 2004. I was a network\security student when I wrote it so it might not teach you ubergeeks anything new.
http://hyppy.zapto.org/DRDoS-Spyrochaete.html
There already is a fix in BIND (at least in the 9.2.4 release shipped with RHEL 4 & all like distros). Just add this to your "options" section of your bind.conf:
allow-recursion { localhost; mygroup; 10.10.10.1; 10.2.3.0/24; };
This would allow the localhost, the machines on the mygroup ACL, one computer at 10.10.10.1 and all the hosts in 10.2.3.0/24 access to recursive queries.
If you don't need to provide recursive lookups at all, you can just use this:
recursion no;
[End of diatribe. We now return you to your regularly scheduled programming...] - Larry Wall in Configure from the perl
In Bind9 you don't have to return cached data, so though it happens by default you can turn it off ("additional-from-cache"):
view "internal" {
match-clients { internals; guests; };
recursion yes;
zone "." {
type hint;
file "bootstrap/cache";
};
zone "example.com"{
type master;
file "example-int.com";
};
};
view "external" {
match-clients { any; };
recursion no;
additional-from-auth no;
additional-from-cache no;
zone "example.com"{
type master;
file "example-ext.com";
allow-query { any; };
};
};
---------
I believe that should prevent bind from being too useful from the outside.