NAT-to-NAT direct connections with no proxy/STUN needed | Hacker News Reader

NAT-to-NAT direct connections with no proxy/STUN needed

samy.pl

162 pointsmetabrew36 comments

Cheat sheet version: one side sends packets to a bogus IP, the other side sends ICMP TTL Exceeded packets back. Some NATs will pass the TTL Exceeded packets because that's how traceroute works; the ICMP packets are indistinguishable from "real" traceroute responses. Once both sides know each other's IP addresses, they open a tunnel by simultaneously talking outbound UDP to each other.

This is a clever trick for home NATs, but most corporate networks are doing more than NAT'ing; they're dropping everything but DNS and HTTP from a proxy.

The forest, dude, the forest.

Think about the masses behind home NAT routers and P2P. From my comments on reddit http://www.reddit.com/r/programming/comments/bjc0g/pwnat_ser...

    Server publishes it's NAT external IP and a hash of its (RSA) public key.
    [ICMP] Client MAC(client public key + server public key hash) -> Server
      (Server checks the key hash the client got is valid)
    [UDP] Server MAC(server public key + hash of client public key) -> Client
      (Client checks the public key hash matches the server's public key) ...

    The publishing first step could be just an URI like:
    pwnat://<server external ip>/pubkey_hash:<pub key hash>
    The hash could be base64 encoded in just 24 chars. Of course this
    is thinking out loud. There are protocols for this already.

Thats of course drafty and not really checked, but you can get the idea and improve on it. It looks very doable.

Also think about P2P and privacy, so far the middle men (e.g. Skype) would know who is talking to who. This is a major step.

And there's plenty of space on the spoofed ICMP reply to put anything relevant to start a conversation (randomized ports, hashes, keys, nonce).

You're right. It's a big deal. It's just not a big deal for corporate networks.

Yeah, it's not for corporate networks.

But, for example, corporate network (and security) could benefit from this, too. Virtual P2P meetings could be done this way without a third party central server or the need to install a special server software. The meeting initiator can send an email with an URI and everyone else just clicks on it running the relevant client software. Much better security-wise than most scenarios nowadays.

Cross-continent meetings are a major pain when it's over VPN or centralized.

> Much better security-wise than most scenarios nowadays.

Can you elaborate on this?

Note I'm kind of thinking out loud, and in this idea was about P2P conferencing, so it's not for local network or well connected offices. More like a very typical scenario of several people connecting from different locations (e.g. working from home plus remote offices.)

Using a VPN (centralized) adds a lot of latency and degrades the experience. Using a third party service (MS, WebEx, Lotus) often adds trusting them with your dearest corporate secrets and allows them (and the authorities on their respective countries) to spy on the conference or even simply detect your users's IPs to direct attacks to their more vulnerable home computers full of interesting data.

With some service set up like this the users can just have an "initiator" "server", who's software just checks its NAT external IP (thousands of existing free services and web pages give you this) and creates a meeting URI. They send it by some secure channel (e.g. email over vpn) to the other parties. The rest conenct to the initiator "server" and discover each other's IP. If the protocol is carefully designed the clients and server can have a shared secret on the URI to avoid Man-in-the-middle. Voilà!

This can easily be an open source product with zero server costs (unlike WebEx, Skype, MS.) Of course, this works only for clients with NAT access and others should connect in other ways (e.g. HTTPS and/or UPnP.)

My point was that even the corporate network and security teams can benefit from this in some way. It's not just to set up quake servers on college networks.

basically you are trying to hack a solution for one thing that can be solved with your wallet.

And i dont mean to pay for the most expensive option becausw that's what you isp wants.

I'm talking about hiring a smaller isp that does not goes extra lengths to block your connections

they're dropping everything but DNS and HTTP from a proxy

That won't help them either anymore:

Weaponizing dnscat with shellcode and Metasploit http://news.ycombinator.com/item?id=1204931

(a) My point isn't that enterprises can solve the (fundamentally intractable) covert channel problem. It's that this NAT tunnelling code won't work on corporate networks.

(b) DNS tunneling is also clever, but it's loud, easily detected, easily defeated, and extremely likely to get you fired.

Thank you for the followup! A quick question: after reading over "Detecting and preventing the use of DNScat" on this page:

http://tadek.pietraszek.org/projects/DNScat/ ,

it seems as if uncovering small amounts of data sent this way might be difficult. Here are the detection techniques mentioned:

  detecting unusual and malformed DNS packets (DNScat will evade this)
  detecting high number of unusual types of DNS queries (e.g. TXT) (DNScat will also evade this)
  Anomaly detection: DNS query and response length
  Anomaly detection: profiling the amount of traffic per client source IP address

Are there any other detection methods?

This is clearly only designed for home NATs. The simple fact of the matter is that most home users don't want to know how to forward ports so that they can use p2p software.

This allows one to write software so that such users can use p2p software without having a central server to discover IP addresses.

We're experimenting with this technique in our peer-to-peer software. Unfortunately we probably won't have data on what percentage of the time it works for a few weeks.

Provided everything goes okay, I'll try to post our data as a follow up.

This sounds like an interesting project, do let us know!

You should make a test version of your p2p client and ask here (and, say, on Reddit and p2p-hackers list) to help test it. You will get a few hundred sample points in a matter of hours.

Sorry for being lazy. Does it need root rights to forge ICMP?

The trick with priming NAT for receiving bogus inbound TTL-Exceeded ICMP packets is really clever. However keep in mind that this method assumes that:

(a) the client knows server's external ("Internet") IP

(b) there is exactly one server behind the NAT device

Latter is obviously quite limiting unless the "3.3.3.3" changes for every server and the client learns it together with server's external IP. This in turn means that the client still needs to communicate with some 3rd party, so this method only partly alleviates the need for the 3rd party during the tunnel setup process, but it does not completely eliminate it.

Isn't 3.3.3.3 an IP inside the General Electric Company? What happens if they ever decide to use it?

Ok, so ICMP is easy to turn off. But why couldn't this technique be done over UDP or TCP? The server could send a UDP packet to a known "fake" address and the client could spoof its source address to penetrate the nat.

Because many/most ISPs block simple outbound src addr spoofing.

pwnat works b/c ISPs don't (currently) inspect deep enough to notice spoofed addresses in the error packet contained inside ICMP.

This is an arms race b/t clever hackers and the ISP's increasingly deep packet inspection gear.

Because the ICMP message is a case where NATs are already programmed to allow arbitrarily-sourced traffic to pass. The point is that the trick discovers the client's address, so they can establish direct 2-way comms.

I know Samy well. Probably more than 10 years.

He's one of the best of the best. I'd hire him in a second if he'd let me.

This looks to be two things: a way of discovering ips and communicating them without STUN and a simple nat traversal using UDP. Th first is a little clever but not super interesting as signalling channels are a dime a dozen. The second is essentially a small subset of ICE and will fail if anyone remaps ports.

For something similar check out scottlu's FWD — http://www.scottlu.com/2008/05/fwd-tool-for-peer-to-peer-por.... It uses xmpp as the signalling channel and the ICE implementation from libjingle. This is the P2P stack we built for Google Talk several years ago. If you get this stuff right you can establish a direct connection when both parties aren't behind a full cone NAT — 90+% of the time.

This was dead-ed to allow this re-submission (thanks metabrew).

There is a great discussion @reddit http://www.reddit.com/r/programming/comments/bjc0g/pwnat_ser...

I might be showing my ignorance here, I'm not a network guy, but a vast majority of internal networks use 192.168.1.0/24 192.168.1.1/24 and so on. What happens when the source and destination networks both use the same class C addresses?

Source is talking to the external IP of destination's NAT device and vice versa. They are not talking using their internal IPs.

very clever. how hard is it for the paranoid IT guy to block ICMP time exceeded packets inbound for 3.3.3.3 but not for other hosts, to stop servers from discovering clients?

Very but as with the example he is giving, this is very useful technology for video games where hosts and clients are regularly behind NAT. UPNP has made this somewhat better but it's still no where near perfect.

If i'm not mistaken, much of the UDP punch-through technology was invented by Skype. However, skype still relies on a proxy server that every client connects to. It needs that so that the client can prepare the router to let data through from a certain client IP. These guys here found a ridiculously clever way of skipping this step which makes true P2P with both end points behind NAT possible and even easy to implement.

All of these techniques could be prevented if you knew about them.

Yet I wouldn't be surprised if the specific IP address wasn't customizable eventually when you control both sides.

On most firewalls this should be trivial. On an Cisco ASA:

access-list BLOCK_TIMEEXCEEDED deny icmp any any time-exceeded

(iirc) and then apply the acl. You should block all hosts as any could be chosen by the person. They could change 3.3.3.3 to any other IP.

NAT is not a security mechanism and does not ensure your hosts are protected. Denying tunneling of any kind is difficult as there are tunnels over most protocols. I'm not aware of any perfect prevention or detection technique, but detection could in the case of a moderate amount of data transit could possibly be done via analysis of netflow records.

And in fact they almost certainly will choose a different host. Our plan is to just buy an extra IP for one of our VPS machines and leave it unused.

Better just to block all ICMP TTL exceeded packets, breaking traceroute entirely. Normal users don't use it that much. Traceroute sucks; it's clunky, slow, and unreliable. Network people need it (and they can carve exceptions for themselves), but end-users don't.

Anyone mind posting the arguments for the server and client you're using. The server side segfaults for me.

Mine are:

Server: ./pwnat -s 2221

Client: ./pwnat -c ClientIP 3333 RemoteIP 2221

Is there a possible segfault at line 108 in udpclient.c? pport is a junk pointer at this line: sprintf(pport, "2222");

In v0.2-beta, the line reads `sprintf(pport_s, "2222");'.

how exactly is this different than netcat -u ?

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