Black Ops of DNS
Dan Kaminsky
Introduction
Who am I?Senior Security Consultant, Avaya
Enterprise Security PracticeAuthor of “Paketto Keiretsu”, a collection of
advanced TCP/IP manipulation toolsSpeaker at Black Hat Briefings
Black Ops of TCP/IP seriesGateway Cryptography w/ OpenSSH
Protocol Geek
What’s On The Plate for Today?
/* char descrip[256] = “You’ll see”; */
What is DNS
DNS: Domain Name System Mechanism for translating human-readable names
into machine routable addresses
“Like 411 for the Internet” As 411 usually but not always yields simple phone
numbers, DNS usually but not always yields IP addresses
A: Given name, find IP MX: Given name, find Mail PTR: Given IP, find name TXT: Given name, find “stuff”
“Useful” Traits of DNS(Very Very Abridged)
Hierarchical .com says where to find addresses in .doxpara.com,
and .doxpara.com says where to find addresses in foo.doxpara.com
Recursive vs. Iterative Lookups Iterative Lookup: Ask a server a question, it tells you where
to go to find out the answer Recursive Lookup: Ask a server, it goes out and finds out
the answer for you, and tells you It queries the hierarchy…which you may control
Caching Responses contain a TTL – Time To Live – within which
future requests don’t require another message to be sent
Primary Research Areas for DNS
Exploitation1999-2000 were filled with exploits against
BIND, the most common DNS serverNot terribly vulnerable now
DNS SpoofingReturning false addresses = hijack
people’s outgoing net connections
DNS Tunneling
DNS Tunneling [1]
How Client -> Server
What’s the information for BATCH-OF-ENCODED-DATA.doxpara.com?
Server -> Client The information? Why, it’s “HERES-THAT-DATA-YOU-
WERE-LOOKING-FOR”
Why? DNS is extremely permeable – it will route through
architectures where often nothing else will Captive portals for Wireless Internet “More” ;-)
Starting Simple:DNS Tunneling [0]
Who?NSTX most popular
Creates a “virtual network device” that routes IP (actually, Ethernet frames) over DNS
Linux OnlyRumors of various botnets / malware using
DNS as a covert channel
DNS Tunneling[2]:Entering Userspace
Starting “Simple” NSTX requires kernel cooperation to get at IP Lets make something that doesn’t require the
kernel, but still allows remote networking Remote Networking: “I’m on this network, but all my
traffic is routed through that network over there – preferably securely”
Normally done with VPNs (also kernel level) SSH Dynamic Forwarding allows secure remote
networking over a single TCP port (Poor Man’s VPN) So lets start with SSH over DNS
DNS Tunneling[3]:Problems
DNS is not TCP TCP moves bytestreams, DNS moves records
Blocks of data
TCP lets either side speak first, while in DNS, the server can only talk if the client asks something
TCP is 8 bit clean, while DNS can only move a limited set of characters in each direction (Base64)
This seems so familiar…
DNS Tunneling[4]:Mini-HTTP
The semantics of DNS are surprisingly similar to those of HTTPMany tools have been written with the “lets tunnel everything over HTTP” methodology because it gets through firewalls easier (see first point)Those tools that support small message sizes (like GNU httptunnel) can be quickly modified to use DNS as an alternate transport Must use separate streams for upstream vs. downstream,
since downstream reflects all data from upstream (similar to HTTP, but on a per packet basis)
But DNS has a feature HTTP servers don’t…
DNS Tunneling[5]:Recursive Redux
Recursive lookup: Ask another host to iterate through the hierarchy to find your answerWhy, it’s as if every web server was also a
web proxy...Simple Trick: Bounce your traffic off any
DNS server (like, for instance, a captive portal’s DNS for free WiFi)
But there’s better…
DNS Tunneling[6]:Set em up…
Some DNS servers are dual hosted External interface is sending names out Internal interface is bringing names in Two interfaces because one is behind the firewall
and one is in front
Subdomain Delegation It is possible to claim that “foo.doxpara.com” is
hosted at an arbitrary name server with an arbitrary IP address
…even if you yourself can’t route to that address… …someone else can…
DNS Tunneling[7]:…and knock em down
Incoming SSH to Protected Networks via Recursive DNS 1. Wrap SSHD in dns-ized httptunnel 2. Request that remote DNS daemon look up a subdomain
of a domain you control. Tell that DNS it’ll find the answer it seeks at the internal server with the SSHD-DNS.
3. SSHD-DNS receives forwarded request, responds to it. Remote DNS daemon forwards that response back to you.
4. Continue with normal SSH over DNS semantics.
Note: This is actually a bad thing
Changing the Gameplan
Most tricks require a DNS server under the requester’s control The client and the final server conspire against the
recursive server in the middle
But what if there is no DNS server under the client’s control? What can a client do with queries alone? Can two clients communicate with eachother
through a DNS server?
DNS Cache Modulation[0]
DNS stores the results of queries, along with a TTL (Time To Live) Well known Information Leakage: If someone else looked up a site
first, the TTL is different. Example: root@bsd:~# dig @129.210.8.1 mail.layerone.info; sleep 100; dig
@129.210.8.1 mail.layerone.info First Reply: mail.layerone.info. 4H IN A 66.33.213.202 Second Reply: mail.layerone.info. 3h58m19s IN A 66.33.213.202
Destructive – If nobody else made a particular query, then the probe becomes the standard bearer for max seconds.
Not well known: Two hosts can communicate with eachother through the state change introduced by a particular query
Possibly the lowest bandwidth channel available, at 1 bit per query
DNS Cache Modulation[1]
Temporal Bit Mapping The sender requests a low traffic, preferably low TTL name
from the server. The receiver requests the same name, and sees it at some
decremented TTL value. It thus knows at approximately what time the cache entry will expire.
Within some “sender window” after expiration, the sender either does (1) or does not (0) issue another request for the same name
Within some “receiver window”, the receiver checks the name after the window expiration. IF the TTL is max, then the bit was 0. IF the TTL is not max, then the bit was 1.
Capacity = 1 bit per Max TTL period (slowwww)
DNS Cache Modulation[2]
“Spatial Bit Mapping” Spread the load across multiple names, probably
each retrieved off a wildcard server Wildcard – 00000001.doxpara.com still resolves
Bits are “set” within a setting window Bits are destructively read within a reading
window, using an identical TTL strategy. Next series of bits may be sent when TTL of last
sent request by the sender expires. Capacity = 1 bit per name per Max TTL period
(slow)
DNS Cache Modulation[3]
Bidirectionality Simply use two separate channels, or timeshare one
channel It’s just a shared medium
Adaptability Anything that can be changed by one and seen by another
can be used to send data Web Counters IP ID counters in IP Stacks Whether a car was washed
“Did it increment or not? Were there sharp spikes between 6:01 and 6:02?”
“You can always send a bit” – can we send more?
History of DNS Storage
Long history of storing data in DNS Means
TXT records AXFR Zone Transfer (can be arbitrarily large, but doesn’t
work from almost any restricted network since it’s TCP) Downsides
Very inefficient – packet size (w/o AXFR) is locked below 512 bytes, and you only get a little more than half of the packet filled with a payload
DNS can get really slow, especially under load Upsides
Everyone has a DNS server, and it caches
KDNS[0]
What do we have?A Very Dynamic DNS serverA Desire to Send More Than A Bit
Fine, I’ll go host a name on a serverA challenge to send something new
What do we get?
KDNS[1]
Voice over DNS – TXT w/ Streaming AudioSpeex codec supports Voice compression at ~2kbps (best public codec) Ends up (with headers) being about 356
bytes/second We can traffic 356 bytes per second through even
extremely slow DNS servers
Power to the Caching People Use a TTL of WINDOW seconds (~20s) All listeners behind the same DNS server will split
the same “stream”
KDNS[2]
Server HOWTO <timestamp>.server.com
TTL=WINDOW TXT (or MX) = 1.0s or 0.5s of audio
latest.server.com CNAME to <window>.<timestamp>.server.com TTL=0 This may be broken by resolvers with minimum TTL
requirements (implemented to fight Dynamic DNS server loads)
KDNS[3]
Client HOWTO Retrieve latest.server.com, learn
<window>.<timestamp>.server.com Retrieve audio samples from <timestamp-
window>.server.com up through <timestamp>.server.com
Prefer to start playing at a packet that has spent some time decrementing in the cache – binary search for oldest sample within the window
Add random jitter for the start sample – this way, everyone’s at a different point in the stream, and if the “lead looker” drops, someone else will be next up to be first in line
KDNS[4]
Alternate Implementation: Proxy-Friendly HTTP Streaming Abandon DNS entirely; simply distribute 3-10s
chunks of audio over HTTP and let proxy servers share them out
Requires client cooperation, and greatly increases latency, but solves the (bandwidth) problem that proxies don’t cache streams
Proxies don’t support TTLs, though – a server side script would need to monitor that
HTTP of course supports much higher bandwidth!
Crossroads
Everything we’ve done with DNS is slow and low bandwidth? DNS servers store little data, and may return it
relatively slowly
Can’t we go any faster? Is there no DNS “solution” with capacity? Many hands make light work There are many many many DNS servers
And almost all of them cache.
DomainCast[0]
The basic concept Normal DNS operation: Talk to your own server; it
retrieves data on demand from the official server upstream
Domaincast DNS operation: Talk to servers with different blocks preloaded into their cache; each of those blocks refers to the location of other cached blocks. Upstream server has actually preloaded data (through directed queries) into all of the servers.
This is possible. This is not necessarily a good idea. Don’t try this at home.
DomainCast[1]
Sidestepping Limited Resources Capacity
20K/server = ~80 records @ 256bytes 700MB = Knoppix, a full Linux distribution 700MB / 20K = 35,000 servers required
Number of DNS servers detected in a single class A: +140,000 (More on this later)
Speed 1Kb/s * 35,000 = 35MB/s
Would require upload of ~3.5MB/s ~50% packet / formatting overhead = 17MB/s
Not going to achieve peak bandwidth (it’d shred reliability) – but not going to get 1k/s either
DomainCast[2]
Packet Formats Request: <offset>.<filename>.server.com Reply – Either TXT or MX
TXT – Base64 Representation of Fixed Size Struct (describing file size, byte offset, data, and other servers)
MX – Can also represent data as mail server addresses Requires dots every 64 characters Responses are reordered randomly – must sort by
“precedence” Some headaches with extra information being
“volunteered” MX may stress BIND servers more (triggers search tree)
DomainCast[3]
Basic Mode – All packets retrieved from master server, provide linked list pointers to same host0.file.server.com = first block, here for
second256.file.server.com = second block, here
for thirdEtc.
Domaincast[4]:Population via Brute ForceSimplest Approach: Find enough servers that recurse
DNS servers don’t move rapidly -> Scan amortizes well across many populations
Plan out who’s going to store which blocks Populate servers with data, references to planned
other servers Easy to validate caching, though not easy to fix a
broken reference Can populate out of order – 64 populating, verifying
streams
Domaincast[5][0]:Reverse Serial PropagationThe problem: To populate the first server, you need the address of the second server. But to populate the second, you need the addr of the third. What to do?
Answer: Populate backwards. The last server points nowhere. The second to last points to the last. The third to last points to the second to last. Multi-server – last server can equal “last server group”.
Server groups may be larger than packet capacity to list servers – just include a random set
Domaincast[5][1]: Reverse Serial Propagation
Can be quickly and statelessly deployed Scan networks with generic recursive probe For each incoming request seeking to service the
probe, return whatever(TTL=0) and probe with an actual block request
If a block request comes back from the recurser, populate the server
If the population packet drops, the upstream should retransmit
Move back through the file after each server group fills up
Can be much slower to populate!
Large Scale DNS Scanning[0]
Modding the scanrand codebase… Basic scanrand concept: One half spews traffic, the other
sees what comes back. Little to no communication between the two halves = fast!
Scanrand manages TCP; statelessness is a bit of a novelty to it. DNS is built on UDP, raw packet manipulation isn’t even necessary (it was useful, though).
Reflection: Stateless operation depends on sending packets out and forgetting, only to have the necessary data returned back in the response packet
TCP barely reflects back 48 bits worth of data DNS reflects back whatever was in the query = thousands of
bits if you want ‘em! Crypto signatures can be embedded in the reflection
Large Scale DNS Scanning[1]
Floodable DNS Queries implemented by miname Is anyone there, recursive or not?
Many hosts that don’t support recursion will at least provide a “NXDOMAIN”, but not all will
We need a generic query that appears to almost always work
1.0.0.127.in-addr.arpa PTR -> 127.0.0.1 = localhost Everyone is localhost
Will you recurse back to me? BIG_COOKIE.server.com
Large Scale DNS Scanning[2]
Returned ResultsDirect: You requested, they responded.Forward Lookup: You requested, they
requested back to you, you responded.Often not the server that you asked that asks
youReverse Lookup: You requested…and
someone wondered who you were asking such questions
Large Scale DNS Scanning[3]
More on Reverse Auditing Not being too secretive
root@bsd:~# host 64.81.64.164 164.64.81.64.IN-ADDR.ARPA domain name pointer
mail.dan.at.doxpara.com L0pht’s Antisniff project – locate sniffers and IDS’s by watching
reverse DNS traffic on the LAN Miname w/ Co-opted Servers: Watch sniffers and IDS’s across the
entire internet TTL=0 on PTR replies means they continually look you up when you
pass by them Hard, but not impossible, to associate reverse lookup with node
scanned to attract Temporally associated w/ scan Multiple scans, out of order, should provide required correlation Traceroutes may help too – we know who’s looking us up
Rendering The Flood
How much data could it be?[root@fire root]# ls -l dns.log -rw-r--r-- 1 root root 360215644 May 28 12:42 dns.log
How are we going to visualize this?3D space – could use the tools we used to
visualize the complexity of arbitrary data
Phentropy w/ OpenQVIS
3D Viz[0]
3D Space provides a potentially valuable perspective on extremely complex datasets
Volume tools have gotten more matureVolsuite: Free, Open Source, Full Color,
Cross Platform, FastVideo games = New Absurdly High Speed
Graphics Chipsets
3D Viz[1]
“Volumetric” – Texture Based Stacks of transparent photos Arbitrary complexity data – it all gets blurred in
Limits positional accuracy Totally static – you can’t animate the population
“Particle” – Vertex Based Dots! Dots everywhere! Arbitrary positional accuracy – floating point coordinates
can be zoomed into Limits number of particles
Very easy to make dynamic Dynamic particles can express higher dimensional data
3D Viz[2]
Dimensions Static: XYZ, Color Dynamic
Shape Direction Color Shift Trajectory Shift Brightness / “Flare”
Surprising aspect of dynamics: Motion away from source point appears to bolster memory / perception of that point
3D Viz[3]
Under Development – As yet unnamed Generic 3D PlotterCoordinates are streamed in, perhaps with
information about how to render each particle
A more generic version of the “Spinning Cube of Potential Doom”
Parallel development
Conclusion
Stuff = Cool
More Stuff Soon
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