Black Ops of TCP/IP 2011 (Black Hat USA 2011)
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Black Ops of TCP/IP 2011 (Black Hat USA 2011)



Interesting network tricks

Interesting network tricks



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    Black Ops of TCP/IP 2011 (Black Hat USA 2011) Black Ops of TCP/IP 2011 (Black Hat USA 2011) Presentation Transcript

    • Black Ops Of TCP/IP 2011
      Dan Kaminsky, Chief Scientist, DKH
    • Intro
      I’m Dan Kaminsky
      I write code
      Not here to fix authentication
      Working on that
      Not here to make DNSSEC scale
      Working on that too
    • What I’m here for
      Return to form
      As a community, we’ve sort of stopped looking at network security
      Mapping networks
      Evading firewalls
      Subverting design assumptions
      This is probably the right thing – looking at attacks:
      Acquire Beachhead
      SQLi the web front end
      PDF the client backend
      Use acquired credentials to break everything else
      Netsec is only so relevant in such an environment
    • So?
      We’re going to look into it anyway. Maybe we’ll find something interesting.
    • BitCoin
      “BitCoin turns nerd forums into libertarian forums”
      It’s infected everything else in nerddom, why not this talk?
      What is it?
      Attempt at making a digital currency with no central bank
      A system with economic properties I don’t know anything about
      An overlay network upon the Internet that people think has certain properties
    • BitCoin In A Nutshell
      Built on doing three things
      TRANSFER: “I Alice, give Bob 2.1 BTC”
      Alice signs the declaration to Bob’s public key
      GOSSIP: “Heh everyone! Did you hear that Alice gave Bob 2.1 BTC?”
      Alice sends that declaration into a peer to peer network that gossips the change
      APPEND: “Everyone, the official registry of transactions should now include Alice paying Bob, Charlie paying David, and so on.”
      This is gossiped too
      Requires solving a problem so hard, it takes the world 10 minutes for someone to do it
      If it takes less than 10 minutes, it’s not hard enough
      Crypto lets you make things hard enough
      Solving the problem gives you 50 BTC (today) to Transfer
    • The Truth Of Bitcoin
      …this is not my BitCoin talk 
      Go to for a more detailed deck
      BitCoin is actually really impressive
      Entire classes of bugs are just missing
      The first five times you think you understand it, you don’t
      BitCoin has fixed almost all flaws that aren’t forced by the design
    • The Main Flaws (there are a few more)
      Does not scale
      Totally not anonymous
    • Scalability (from BitCoin’s Own Wiki)
      “Let's assume an average rate of 2000tps, so just VISA…. Shifting 60 gigabytes of data in, say, 60 seconds means an average rate of 1 gigabyte per second, or 8 gigabits per second.”
       ”A network node capable of keeping up with VISA would need roughly 50 cores + whatever is used for mining (done by separate machines/GPUs).”
      “ A 3 terabyte hard disk costs less than $200 today and will be cheaper still in future, so you'd need one such disk for every 21 days of operation (at 1gb per block).”
    • OK, so you end up with supernodes and normal nodes
      What are the characteristics of supernodes?
      They’re banks
      “Welcome to the new boss, who looks suspiciously like the old boss”
      I’m not saying banks are bad or anything
      The “peer to peer” model of BitCoin eventually goes away; as soon as the thing gets big, the entire thing switches to a banking model
      With all the elements of banking people think BitCoin is immune to, without necessarily the properties people like
      However, until then…
    • An Interesting Question
      Travis Goodspeed: “Heh Dan, any chance BitCoin can be used as a samizdat service?”
      Samizdat (Russian: самиздат; Russian pronunciation: [səmᵻˈzdat]) was a key form of dissidentactivity across the Soviet bloc in which individuals reproduced censored publications by hand and passed the documents from reader to reader.
      An old challenge
      The Internet is usually about sending data, ephemerally
      Can we use it to store data, indefinitely?
      Well, if BitCoin is eventually going to require a 3TB HD every 21 days…and is going to need to keep that data forever…
    • Len.
      Our community recently lost one of its shining lights
      If one executes:
      strings --bytes=20 ~/.bitcoin/blk0001.dat
      Strings extracts human readable text from any blob of data
      Usually used to find hardcoded interesting stuff in executables, like default passwords
      The block database of all transactions ever pushed into BitCoin, run through a filter that extracts all human readable text from the (presently) 450MB file…
    • BitLen
    • …and just because it would have made Len laugh
    • How This Works
      In BitCoin, Alice gives money to Bob by issuing a sort of challenge
      “Whoever can sign a message with the public key that hashes to the following bytes, may claim this money.”
      Well, bytes are bytes
      Instead of pushing the hash of a public key (20 bytes), we push 20 characters of a testimonial
    • Side Effect
      This does cost BTC
      About 1.0BTC in total
      There’s minimums to transferring money
      This does destroy the money
      The network thinks somewhere, there must be a public key with a hash of “Len was our friend.”
      I am OK with this.
      It is the cyber equivalent of pouring one out for your homies.
    • Can we get higher bandwidth?
      BitCoin lets you send money to a public key directly, rather than its hash
      10x increase from 20 bytes to 200 bytes
      This is not a bug
      BitCoin allows for extra data in a signature
    • Signature Expansion
      BitCoin works with small programs
      The program from the receiver is: “Put this signature and public key on a stack”
      The program from the sender is: “Take the signature and public key off the stack and make sure they’re good.”
      The receiver can put extra stuff on the stack, and yes, it still works just fine
      This is in fact a bug that is visible purely from being pedantic about the English Language
    • Illicit Signature Expansion
      Signatures can’t cover themselves
      Chicken and Egg
      So signatures also don’t cover the presence or absence of additional data within themselves
      Block appending does cover additional data
      But there is time between when transactions are first created and emitted, and when they’re included in a block append
      So it turns out anyone can add additional data to an otherwise valid transaction
    • Limited Usefulness
      If you’re just some random relay, gossiping the information, you have to compete with the real version
      Transaction fees limit you to about 1KB of embed per 0.01 BTC (14 cents)
      This does not apply to you if you generate the signature with extra data, because then you can pay fees
      This does not apply to you if you calculate the block – you can include as much as you want, up to present 2MB limit, and force everyone else to carry
      Still better than 20 bytes per 0.01 BTC 
      Yes, Travis, bitcoinfs is totally possible
    • What about Anonymity? Looking at
      Transaction Sources: These are all the same ID
      Transaction Dst’s:One of these IDs is (likely) all of the IDs on the left
    • Graphs (from Reid/Harrigan)
    • Problem: Linking pseudonyms isn’t enough
      Reid/Harrigan get lucky
      One BitCoin source publishes the IPs it gives money to
      Another user posted to a forum seeking donations to a linked ID
      They’re linking pseudonyms within BC, but they’re not linking to IP via out of band processes
      The published audit trail is noisy and deniable
      “Naturally, much of this analysis is circumstantial. We cannot say for certain whether or not these flows imply a shared agency in both incidents. There is always the possibility of drawing false inferences.”
      Is there another source of data?
    • P2P!
      There are two sources of transaction information in BitCoin
      The “blocks” that have been set in stone
      The “loose transactions” waiting to be merged into blocks
      These (effectively) always refer to a single identity
      Both are “gossiped” around the network
      Big relay race; Alice tells Bob and Charlie, Bob tells David and Eric, Charlie tells Frank and Gary
    • Subverting the Relay Race
      An attacker can just connect to every node in the public cloud at once
      “But that could take 50,000 connections!”
      Yeah, we can do that in Python now. Kernels don’t suck anymore (well as much).
      When you’re connected to every node, the first node to inform you of a transaction is the source of it
      “Done relay it because done done it”
      More or less true, and absolutely over time
      (Bonus: You can accelerate your own transactions, by relaying them to everyone yourself)
      BlitCoin – accelerated probing of BitCoin
    • Discovering Nodes
      Just scan the Internet on 8333/TCP
      Join the IRC channels!
      #bitcoin, #bitcoin00 to #bitcoin99 on LFNET
      Recursively ask every node about every other node it knows about
      “get_addr” message
      Can start from hardcoded seeds
    • Statement From Gavin Andreson, lead dev on BitCoin
      Bitcoin transactions are more private than credit card or PayPal transactions, but are less private than physical-world cash transactions. Unless you are very careful in the way you use Bitcoin (and you have the technical know-how to use it with other anonymizing technologies like Tor or i2p), you should assume that a persistent, motivated attacker will be able to associate your IP address with yourbitcoin transactions.
    • What about Tor?
      Tor indeed obfuscates IPs derived from outbound connections
      It does nothing if you’re still listening on 8333/tcp and somebody sweeps the net for you
      Bug filed in BitCoin to shut off listener when operating through Tor
    • What about unreachable nodes?
      Most are behind NAT, and only connect via outbound links
      The active inbound set is only 3000-8000 nodes
      So, you just create 3000-8000 nodes and you’re half the gossip network
      Probably only need a few hundred, since each node will collect ~7 peers and you only need one
    • Just how unreachable are they?
      Many users are behind wireless routers
      Routers implement NAT – outbound is easy, inbound is hard
      “Poor Man’s Firewall”
      Don’t mock it, it was more effective than real firewalling when it came out
      Most home routers implement UPNP – Universal Plug And Play
      UPNP allows nodes inside your network to ask the router to open up ports from the Internet
      BitCoin now supports doing this by default
      …but even if it didn’t…
    • How UPNP is supposed to work
      Internal hosts send a multicast message out via SSDP (Simple Service Discovery Protocol)
      1900/UDP M-SEARCH Multicast
      Internal UPNP nodes – media players, routers, etc – respond w/ endpoints that can be twiddled via web services requests
      1900/UDP NOTIFY Unicast
      Responses are sometimes just flooded out, in the absence of M-SEARCH
      SSDP NOTIFY messages are supposed to contain a randomized URL for UPNP messages to go to
    • Question
      UPNP is supposed to only work on internal interfaces
      “Hello Router, please let the outside world in.”
      It would be tragic if routers listened to UPNP on external interface as well..
      “Hello Router, please let the outside world (read: me) in”
    • Stats (and yes, Scanrand’s coming back)
    • More Stats
      Not all listeners on 2869 are fully open
      Would require fixed UPNP endpoints, instead of the randomized ones Microsoft uses
      Many verified listeners though
      Hundreds of thousands to millions
      Entire countries have standardized NATs that are vulnerable
    • Your Princess Is In Another Castle
      Turns out there’s a speaker talking at DEFCON about just this very subject!
      I’m a little more careful about independent rediscoveries now 
      Daniel Garcia found that there were open UPNP endpoints on the net last year
      Track 3, Friday, 17:00
      ArmijnHemelalso did some great work
      Also noted that sometimes UPNP was exposed to outside world, back in ~2007
      Still true, unfixed
    • What about outside the consumer space?
      Corporate environments
      Less about BitCoin and UPNP
      More about web services and ACLs
      Are there ways past corporate ACLs?
      Access Control Lists
      “Access to this IP is constrained to the following range”
    • Ye Olde Trick
      IP Spoofing
      Just pretend to be a source IP vaguely near the target, and you’ll probably pass ACLs
      “But BCP’s!”
      Real world, IP spoofing is not hard, as long as you’re not virtualized
      IP spoofing – the one thing the cloud isn’t very good for
    • Is IP Spoofing Still Effective?
      Sure! Let me just pull this DNS trick out of the archive…
      Generate a query for “$”
      Send query to all IPs on a network, from various IPs that network might trust
      x.1.1.1 -> x.1.100.8
      x.1.100.1 -> x.1.100.8
      Response will go back to IP you don’t control – but first, the server will try to resolve $ – from you!
      (Yes, this was another way to exploit that bug.)
      Granted, this only works for an obscure application like DNS and UDP…certainly nothing built on TCP
    • Understanding The Limits Of IP Spoofing
      Most modern protocols run over TCP, a reliable communication protocol
      1) Alice sends Bob a SYN, containing a random sequence number
      2) Bob replies with a SYN|ACK, containing both Alice’s sequence number, and his own sequence number
      3) Alice replies to Bob with an ACK, containing both sequence numbers
      Data can be sent now
      Sequence numbers become a sort of “password” for all future traffic
      If Alice spoofs her IP, she doesn’t see Bob’s sequence number, so she can’t complete step 3
    • Sequence Numbers Didn’t Used To Be Random
      Obviously if you can guess a sequence number, you can blindly inject into sessions
      So, make them random?
      Problem: Connections are identified by source port, dest port, source IP, and dest IP ->
      Sometimes, ports are recycled from one connection to the next
      What if a packet arrives from an old connection? It could look like it belongs in the new one!
      Fix this by having random sequence numbers, unless id is the same, then we go sequential in time to maximize distance
    • Example
      F(srcip, dstip, srcport, dstport,secret)+time
    • A Problem: Memory
      What if somebody just floods us with connection attempts?
      They don’t have to remember all of our “passwords”
      They don’t even need to use their own IP addresses
      We need to remember all of theirs
      This is a SYN flood, and it’s old as dirt
    • Solution: SYN Cookies
      Specified (if not invented) by Dan Bernstein in 1999
      Finally on by default in Linux in 2008
      The “password” turns into a challenge
      “If you can send this back to me, I’ll accept your data”
      Uses 3/4ths of the sequence number (24 bits) to store the hash of a secret and the four tuple, 5 bits for time, three bits for connection metadata
      5 bits is exposed to everyone publicly, 3 bits don’t matter, so there’s 24 bits of security
    • Alas
      Average of 8 million packets to bypass SYN cookies
      May be less, due to fudge factors
      Of course DJB knew this 
      “No matter what function is used, the attacker will succeed in a connection forgery after millions of random ACK packets.”
      But it’s a different reality than 1999
      Sending 8M packets is easy now, we has the bandwidth
      Forged connections have arbitrary sources
      They get through your ACLs
      They can contain arbitrary Web Services payloads
      Definitely REST, maybe SOAP
    • Are you safe if you disable SYN cookies?
      Well, not on Linux
      Linux is RFC 1948 compliant for the lower 24 bits
      Uses MD4, but still
      Upper 8 bits?
      Counter, starting at 0, increments every five minutes
      Shared between inbound and outbound connections
      So, you send a query from your actual IP once or twice to find the offset, and blindly spoof a SYN and a payload-containing ACK
      After 8M tries, you win
    • Impact on RST attacks
      Tony Watson, “Slipping In The Window”
      Noticed that only one 32 bit “password” was required for Resets (RSTs)
      Noticed that the “password” only had to be in the “window” of valid data that could sequentially be sent
      Window describes how many bytes a sender is allowed to transmit without a receiver acknowledging
      Noticed that the “window” wasn’t even limited to 16 bits; was being expanded 5-8 bits more from “Window Scaling”
      32-16-8 = 8 bits = 128 packets to kill a session on average
      New possibility: 32 - 16 – 8 – 8 = 0 bits = 1 packet will always work (assuming full sized window)
    • Beyond RST: Injection?
      RST handlers (usually) only check SEQ# (32 bits)
      ACK handlers however check both SEQ# and ACK# (64 bits)
      64 – (16 bits from Alice window) – (16 bits from server window) = 32 bits
      2B packets for 50%
      32 bits – (5 bits from Alice window scaling) – (5 bits from Bob window scaling) = 22 bits
      1M packets for 50%
      Uh oh
      22 bits – (8 bits from Alice predictable high bits) – (8 bits from Bob predictable high bits) = 6 bits
      16 packets for 50% 
    • Difficulty: Ports
      Linux randomizes the source port of a new connection by default
      You don’t worry about this when you’re doing an ACL bypass, because you control the source port and the dest port
      You do have to worry about this when injecting into other sessions though
      6 bits (from large windows and high bit disclosure) + 13 bits (port leakage) = 19 bits
      250K packets for 50% injection even with port randomization
      Note that sometimes a TCP client sets its source port (DNS, BGP)
    • Status
      This is very old code in Linux
      Predates the check in history of LinusTorvalds
      They’re figuring out the right fix that won’t cause even more problems
      There are many potential wrong fixes that are even worse
    • A Digression
      RFC1948 is an interesting construction
      Sequential and ordered with the key
      Random and unpredictable without
      Can participate with either:
      Aprivate component (the secret, mixed in with the 4-tuple), able to generate all possible sequence numbers
      A public component (a sample sequence number), transmitted over the network, successfully received and retransmitted
      Public/private cryptography with nothing but a password?
      Clearly this is impossible
      Only possible here because of intersection of network security and crypto
    • To be clear
      Passwords are a bad idea
      They’re constantly being lost and forgotten and stolen
      They are responsible for 50% of compromises
      They increasingly look like l33tspeak, and this is not helping
      But, supposing we ignore all that…and assuming that we’re stuck with them…
    • An Old Challenge [0]
      How do we use a password to log into a system without that system learning our password?
      “We hash it!”
      You’re still giving the server your plaintext password, it just isn’t storing it
      If salt (random but public prefix) is omitted, attacker can precalculate hash->password database, notice when two users use the same one
    • An Old Challenge [1]
      • “We challenge you to hash against it properly”
      “Send me the password hashed against $RANDOM”
      Digest/NTLM are more advanced versions
      Requires server to store plaintext password or password equivalent
      “We require knowledge of password to go from keypair to shared session secret”
      Requires both client and server to run fairly obscure code – good luck getting either deployed
    • So…
      Is it possible (NOT ADVISABLE, OBVIOUSLY THIS IS A BAD IDEA) to build a system where the client only remembers a password, but the server:
      Stores nothing but a normal public key
      Deploys nothing but a standard challenge to make sure the client has the matching private key, derived unilaterally from a password?
      In other words…
      Can we construct a keypair out of a password?
    • A Foreboding Question
      What vulnerability impacted all asymmetric cryptosystems, be they RSA, DSA, or ECC?
    • …ok…
      Specifically, a change to the way Debian calculated random numbers in OpenSSL
      It always calculated the same numbers 
      All asymmetric cryptosystems use entropy as follows:
      Collect: Grab random bits
      Permute: Alter those bits until they meet certain requirements. Then emit a public/private keypair
      Predictable entropy == Predictable keypairs, no matter the algorithm
    • Uh Oh
      What if we turned the Debian bug…into a feature?
      Cryptography is all about constructions
      We have hash functions, stream ciphers, block ciphers, all of which can be constructed from eachother
      Note too this is often a bad idea
      We know how to take a password and construct an everlasting stream of psuedorandom numbers from it
      “Predictable Entropy”
      We can even do so in a way that is Hard, in both CPU time and Memory
      What if you make the output of a password-seeded PRNG, the input to an asymmetric key generator?
      You’d end up with 2048 bit RSA keypairs, with a “trapdoor” in the form of a password
      This isn’t theoretical
    • Normal ssh-keygen
      # ssh-keygen -f $RANDOM -N "" | grep -i root70:94:3d:4f:c8:c1:1a:a3:88:9a:77:d7:cf:9e:44:2a root
      # ssh-keygen -f $RANDOM -N "" | grep -i root69:3e:11:4e:a5:5f:09:12:ac:e2:94:21:c4:3b:40:09 root
      # ssh-keygen -f $RANDOM -N "" | grep -i rootd6:7c:3a:c2:d5:ec:84:88:9d:da:81:2b:6f:9a:c3:9b root
    • ssh-keygen using Phidelius
      # LD_PRELOAD=./ PH_PASS="hi grandma" ssh-keygen -f $RANDOM -N ""| grep -i rootad:0d:52:2a:72:be:77:e4:b5:ca:83:bb:4f:49:ce:d2 root
      # LD_PRELOAD=./ PH_PASS="hi grandma" ssh-keygen -f $RANDOM -N "" | grep -i rootad:0d:52:2a:72:be:77:e4:b5:ca:83:bb:4f:49:ce:d2 root
    • Enter Phidelius
      Harry Potter, properly understood, is a story about the epic consequences of losing one’s password.
      Fidelius is how passwords fail in the HP universe, so…
      Phidelius hooks /dev/random, /dev/urandom, OpenSSL’s Random functions, and a few other tidbits to provide predictable entropy where it isn’t expected
      Uses a modified version of scrypt to require ~1 second processing time, and about 256MB of RAM, per crack attempt
      No GPU fun for you
      Can be seeded with a file as well
    • What Phidelius Gives You
      Generic, multi-application support for predictably generating keypairs from passwords
      ssh-keygen for SSH keys
      openssl for certificates
      Phreebird for DNSSEC keys
      Allows message signing, message encryption, client certificate authentication, etc. with nothing but a password
      Solves the “log in with a password, without the system learning your password” problem thoroughly, without you having to store anything anywhere
      With BitCoin, you could literally give money to the bearer of a word, or a photo.
    • No pain server side
      All time/memory hard requirements are limited to the client – the server just implements completely standard crypto
    • Primary Issues With Phidelius
      The obvious ones
      It uses passwords
      Passwords tend to be low entropy
      The not obvious ones
      It’s fragile
      An explicit scheme to use a password to seed an RSA key, for instance, fixes parameters like “How sure do we need to be that this number is prime?”
      As an implicit scheme, it depends on assumptions that happen to be encoded into a particular version of a particular key generator
      It’s hard to salt
      All users of the common password “password” have the same public/private keypair!
    • Salting with Phidelius
      Basic idea is that the private key is computed not just from the password, but from the public key as well
      The public key is then the carrier of the salt
      Works for protocols like SSL, fails for protocols like PGP
      Also a good channel of parameters, like “scrypt doesn’t need to use 256MB of RAM”
      Can be implemented with no magic code on server, but client needs magic code to embed metadata in public key, and to extract said magic during computation of private
    • But, to get back to TCP/IP…
      Lets talk about one last thing we can do with networks.
      We can find biased network policy, no matter how subtle
      If biased networks are affecting you, this gives you proof.
      If you are biasing your network, this is how proof will come.
    • The Topology
      Link 1
      Link 2
      Home Router
      Link 3
    • Understanding the Target
      1) “Magic box” is deployed within ISP network, in front of all links
      2) Box matches packets to policies, and applies different rules to different packets
      Can be stateless – “Do I like this packet?”
      Can be stateful – “This packet is part of a flow. Do I like this flow?”
      3) Policies can be anything and can do anything
      Limit maximum bandwidth
      Increase minimum latency
      Alter content
    • The Problem With Subtlety
      Say is 50ms slower than
      Is this because of the ISP?
      Or is this because has better hosting?
      There are many reasons why might be slower than, granting plausible deniability
    • Requirement: Normalization
      Whether the tester is accessing or, the network path should be identical (or at least uncorrelated)
      We call this normalization
      That way, any changes would be the result not of path, but of policy (presumably, and ultimately detectably, at the ISP)
    • Simple Normalization: HTTP
      Policy: “All flows associated with a HTTP request w/ Host: should be delayed by 50ms”
      Detector: Configure a single server to accept HTTP requests for,, etc.
      Then set the client to use it as a proxy server
      If traffic from the proxy server is faster for some names, than it is for others, you’ve just detected a HTTP-biased policy!
    • The Problem
      1) This is very protocol dependent
      HTTP can be made to do this at low work
      Other protocols require lots of work to implement/emulate
      2) The policy can always be specific to IP addresses
      Sniff DNS to learn which IPs to cover
      Doesn’t matter how many hundreds of test servers you have, if policies are only applied to genuine or servers
    • The Solution: N00ter
      N00ter: The Network Normalization Engine
      Start with a VPN
      Traffic is pushed from the Client to a Broker
      An IP associated with the Broker contacts Servers, who reply to the Broker
      The Broker sends traffic back to the Client
      Normally, the ISP sees nothing because traffic between Client and Broker is encrypted
      Now, instead of encrypting traffic from Broker to Client, send it back to the client
      Spoofed, as if there was no Broker
      We want the ISP to see our return traffic
      We’re trying to trigger the response, that would normally be reserved for Bing/Google, for our normalized test server
      Policy engine can’t tell, because we’re impersonating the real entities
      Traffic took the same path
      Traffic came from the same source
      Why else would we see different Quality of Service?
    • What About Forward Flow?
      The policy engine in this scenario doesn’t see traffic from Client to Server
      That’s encrypted, VPN style
      What if it just didn’t trigger the filtering policy if it didn’t see both sides of the conversation?

      Normal N00ter: Spoof the server to the client
      RotoNooter: Spoof the client to the server
      Sample A: Client talks directly to the real Google
      ISP sees SYN
      Sample B: Client talks to real Google by way of Broker, who spoofs the Client. Google replies directly.
      ISP does not see SYN
      Both samples have the same path!
      If they have different performance characteristics, it must be because of the segment of the network that no longer sees client traffic – the ISP!
    • Catch-22
      If ISP applies policy to half-flows, N00ter can differentiate the performance of the spoofed half flow of Google, versus the spoofed half flow from Bing
      If ISP applies policy only to full flows, RotoNooter can differentiate the performance of the full flow to and from the real Google, versus the half flow from the real Google
      Either way, N00ter Wins
      This is the endgame. Biased policies might as well be transparent, because they’re not going to be deniable.
    • Retaining Full Flows
      Suppose you really want the ISP to see bidirectional traffic
      Advantage: Triggers all policies. Also, opens up listeners for NATs, that might be inconvenient to get around
      Disadvantage: If the ISP sees Client->Server traffic, then the Server sees Client->Server traffic
      It may reply, interfere, complain, etc.
    • Strategy 1: Bad TCP Checksum
      Client can tunnel valid traffic to Broker, and push packets with invalid TCP checksums to Server
      Advantage: Invalid TCP checksums are ignored. Server won’t interfere. NAT almost certainly won’t check sums; Policy engine might not
      Disadvantage: Policy engine could. NAT might fix sums.
      Catch-22 with checksums
      If policy is disabled when checksums are bad, policy can be proven by having Broker provide steady stream of good sums while ISP sees the bad ones
    • Strategy 2: Low TTL
      Client can send traffic to Server with TTL that causes packets to be dropped in the middle of the Internet
      Advantage: Legitimate traffic.
      Disadvantage: Policy could note low TTL. Router may drop sessions from ICMP Time Exceeded messages. Sort of a router DoS.
      Another Catch-22: Can probably even figure out which hop the policy engine lives at, by when precisely the flow policy shifts
    • Strategy 3: The Silent Splice
      When a TCP stack receives a message not associated with an active socket, it’s supposed to RST
      But many servers have firewalls that silently ignore unassociated messages
      For Security!
      We can have the Client complete a three way handshake with a server, snipe the connection with a RST from the Broker, and then splice a connection between Broker and Server, with what Client (and ISP) think is a connection between Client and Server
      Packets from Client to Server will be ignored by server
      Packets from Server to Client are actually spoofed by Broker
      Policy Engine sees client talking to server. Policy Engine sees server talking to client. You can’t explain that.
      100% Perfect Bidirectional Flow
    • A Bit Of Warning
      If you’re passively monitoring network traffic, be aware that these techniques do mean a malicious client can make it look like they’re having a conversation with anyone
      Particularly if the server ignores unassociated traffic
      Keep complete traffic logs!
      Validate checksums
      Check TTLs
    • Where N00ter Is Now
      Emulates half flows at present
      Very very fast (written to the old LibPaketto code!)
      Supports anything that runs over IP
      If you want to know whether a network prefers XBOX360 traffic to Playstation 3 traffic, this’ll tell you.
      N00ter is extremely neutral – It Just Works
      Again, it’s just a VPN that exposes Server->Client traffic in the hopes it’ll get filtered 
    • Summary
      Networks are neat
      BitCoin isn’t anonymous
      UPNP sometimes exposes itself to the outside world
      ACLs can be bypassed using some interesting sequence number tricks and large number of packets
      Passwords can be used to seed asymmetric crypto, though they probably shouldn’t
      Subtle net neutrality hacks are doomed. Transparency or bust.
      Research hosting thanks to N2K of 3Crowd and Doxx of LyonLabs
      Anyone want to do some release engineering for me? 