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GreenTunnel: anti-censorship utility designed to bypass deep packet inspection (github.com)
280 points by hieudang9 7 days ago | hide | past | web | favorite | 77 comments





This is a nice workaround for those stuck under censorship regimes such as the UK, South Korea, Turkey, India or China.

Now, Encrypted DNS (thanks to DNS over TLS/HTTPS) and HTTPS (thanks to Let's Encrypt and HSTS) are getting deployed somewhat widely.

The next step is encrypted SNI[0], and it'll get this much harder to do any meaningful DPI, for censorship or else.

[0]: https://en.wikipedia.org/wiki/Server_Name_Indication#Securit...


there are two edges to this sword.

DoH also means breaking stuff like pihole and other ad filtering. It means you trust companies like google who base their revenue off ads, or cloudflare who have censored content numerous times in the past, to serve you DNS.

its also kind of pointless if the state knows youre using it outside of a tunnel...they can just watch your next packets to see where you decided to go.


Quick thought. If software wanted to, could they not, today, bypass your DNS resolvers anyways? Choosing to use DoH on software where you control the DNS resolution seems like an unambiguous win. FWIW, the Chromium implementation of DoH upgrading only upgrades you to DoH if your configured DNS provider is known to support it via a hardcoded list.

In theory, you could have Pihole resolve using a DoH resolver and your devices resolve using Pihole and have the best of everything.

(Disclaimer: Google employee, not working on ads or Chromium or DNS.)


Also in practice. It's one of the check-boxes in the pi-hole settings.

This is a fundamental flaw of content blocking based on host name. It often happens to work, but there's no rule that says that it has to, and really no good reason why it should be guaranteed to.

Isn't there a way to use pihole as your DNS server and let it use DoH?

That way you could do DNS to pihole, do the filtering and let it use DoH to the outside world.


> cloudflare who have censored content numerous times in the past

Besides Stormfront[0], what else did they censor?

[0] https://en.wikipedia.org/wiki/Stormfront_%28website%29


I wouldn't call that censoring either. They just rejected to provide any services for them.

Indeed, "deplatforming" isn't equivalent to "censoring".

8chan

>DoH also means breaking stuff like pihole and other ad filtering.

No, it doesn't.

e.g. I run DoH behind my home's dns cache server.

>its also kind of pointless if the state knows youre using it outside of a tunnel...they can just watch your next packets to see where you decided to go.

This is where HTTPS and eSNI further help.


> e.g. I run DoH behind my home's dns cache server.

I think GP is referring to the fact that apps can now bypass network / os wide dns stub / recursive resolvers undetected with DoH.

> This is where HTTPS and eSNI further help.

I believe TLS v1.3 specifically has anti-censorship and anti-surveillance properties baked in: https://blog.cloudflare.com/rfc-8446-aka-tls-1-3/


They could have had their own resolver before, or even hard coded IPs.

Using software that doesn't respect you is the problem.


Firewalls can redirect port 53 to another IP. That prevents things from hard coding to a specific IP.

https://forum.opnsense.org/index.php?topic=9245.0


Euh, it prevents a custom resolver sure, but hard-coded IPs bypass the need for DNS completely.

pihole is a short term solution; it is the wrong long term one - it only works as these holes exist. blocking needs to be done in the browser, or your computer to be done more securely

Pi-hole helps for network devices where blocking on the device isn't possible. Examples in my household are the TV, which tries to connect to an obvious telemetry address, all my sonos devices (love em, hate em) the nest device, and the apps on all phones. I struggled to block those until the pi-hole made it easy.

Yes, it's a clever workaround. And requires no remote server.

I still prefer VPNs and Tor, but hey.


does this just become an arms race where root certificates or some sort of device management tools are forced onto citizen devices?

Kazakhstan just recently conducted an experiment with sending people an sms telling them to install a root cert. And Russia is making it mandatory to install not-yet-determined Russian software on all newly sold machines, which will quite probably soon include an FSB cert.

Why hasn't this become the modern Right to Bear Arms? The root of the second amendment was trying to ensure that one class of citizenry did not have tools at their hands to force another class of citizenry to comply. It maintained a balance. The right to encrypt and keep your data private should be a modern equivalent of the right to bear arms.

The second amendment doesn't guarantee us the right to pack heat at work. There's lots of use cases where it would be considered reasonable to block the use of certain services.

Bear in mind that as a non American, invoking a right to bear arms actively turns me off whatever you're trying to sell me. Too much damage has been done to my home country by violent groups who took up arms against the state and each other.

Just look at the past 100 years of human history and the damage that has been done because citizens had NO arms to defend their rights against a despotic government. The deaths are counted in millions.

Insurgent groups will buy 5 dollar AK-47 regardless of legality. This is about lawful citizens being able to defend their rights against unlawful governments. Or at least to raise the costs an armed group has to pay for an attempt to take power.

As another non American, I am still undecided on the issue, but I tend to be in favor of that 2nd amendment.


It is working perfectly for Turkcell Superonline, Turkey. Unfortunately, anti-censorship tools are very crucial for us these days. Thank you for your work.

Going voting is crucial.

> GET / HTTP/1.0

> Host: www.youtube.com

> We send it in 2 parts: first comes GET / HTTP/1.0 \n Host: www.you and second sends as tube.com \n .... In this example, ISP cannot find blocked word YouTube in packets and you can bypass it!

If you talk to anyone from China that this is how you bypass (HTTP) "deep packet inspection", it would sound incrediblely naive. I'm not criticizing here, thanks for developing an anti-censorship tool, but my point is, any DPI that can be bypassed in this way is simply too outdated, it's far from the state-of-art threats we are facing worldwide.

What China does today is what your ISP/government is going to do tomorrow, when they upgrade the equipment. Learning a history lesson from China, can help providing insights for developers in other countries to know where this cat-and-mouse game is heading to...

> paulryanrogers: So basically it just does two things: carefully chunking HTTP header packets and encrypted DNS? Not sure this will work for very long.

Of course it will not. I'll explain why.

---

Literally, the same technique was used in China during the early days of Great Firewall, around 2007. At that time, the "censorship stack" was simple, basically, it had...

* A brute-force IP blocking list

This is a constantly updated list of IP addresses of "unwanted" web servers, such as YouTube or Facebook. They are distributed via the BGP protocol, just like how normal routing information is distributed. Once your server enters this blacklist, nothing can be done. Not all unwanted websites enter the list due to its computational/storage costs.

* A DNS poisoning infrastructure

A list of "unwanted" domain names are maintained. These domain names are entered to the national DNS root server as records with bogus IP addresses. It was used more widely than the IP blocklist, since it has zero cost to operate, but it can only block websites in the list and it takes time for the censor to be aware of a target's existence.

* A naive keyword filtering system.

All outgoing international traffic is mirrored for inspection. A keyword inspection system attempts to match the URLs in HTTP requests against a blacklist of unwanted keywords. Rumors said the string matching was performed by hardware in ASIC/FPGA, allowing enormous throughput.

* A TCP reset attack system

Once an unwanted TCP connection is identified by the keyword inspection system, the TCP Reset attack system fires a bogus RST packet to your computer, fooled by the packet, your operating system will voluntarily work against you and terminate the connection, saving the censors' CPU time. The keyword filtering system paired with reset attack was the preferred way to carry out censorship.

That's all. The principle of operation was simple and easy to understand. So what were the options for bypassing it? There were a lot. To begin with, the blocked IP addresses were blocked, you could do nothing about it. But in the earliest day, accessing them was as simple as finding a random HTTP proxy server. Later, the inspection system was upgraded to match HTTP proxy requests. Then, you could simply play some magic tricks with your HTTP requests, like the example in the beginning, so that your request wouldn't trigger a match. Around the same time, in-browser web proxy tools became popular, they were PHP scripts running on a web server that fetched pages. However, they became useless when the keyword matching system was upgraded to match the content of the entire page, not simply the requests (remember, few sites had HTTPS). At this point, all plaintext proxy techniques and HTTP request "massaging" techniques were all officially dead.

Some naive rot13-like techniques were later implemented to some web proxies, HTTPS web proxies were also a thing, but they saw limited use.

* New: A complete keyword filtering system - Inspect all HTML pages (Was: A naive keyword filtering system)

Another target to attack was the DNS poisoning system, sometimes all you needed was a correct IP address, since not all IPs were included in the blocklist due to the costs. Initially, all one needed to do was modifying one nameserver to 8.8.8.8. However, countermeasures were quickly deployed. A simple countermeasure was rerouting 8.8.8.8 to the ISP's nameserver, continued feeding the same bogus records to you. Nevertheless, there were always alternative resolvers to use. So the system was upgraded to provide a DNS spoofing infrastructure - at the instant an outgoing DNS packet is detected, the spoofing system would immediately answer with a bogus packet. The real packet would arrive at a hundred milliseconds later, but it would be too late, your OS had already accepted the bogus result.

And ironically, even if DNSSEC was widely supported (it was not), it couldn't do anything but returning an SERVFAIL, since DNSSEC can only check whether the result was true, dropping the bogus packet and accepting the true one was outside the capabilities of a standard DNSSEC implementation.

* New: A Real-time DNS Spoofing System

Better tools were developed later, that acted like a transparent resolver between the upstream resolver and your computer, that identified the bogus results to drop them, but the use was limited. Also, at this point, the IP blocklist has been greatly expanded. Even if a correct IP could be obtained, it was still inaccessible. Around 2008 or so, a special open source project was launched by developers in China - /etc/hosts list, whenever someone found a Facebook IP address that was not in the blocklist yet, one sent patches to the project. There were also shell scripts to keep your list up-to-date.

However, a /etc/hosts list was useful but its usefulness was limited. First, it was a matter of time before a new IP address was blocked. Also, a working IP address still was restricted by the same keyword filtering system.

* New: Expanded IP Blocklist.

Some people also realized that the firewall was only able to terminate a connection by fooling the operating system. Soon, iptables rules for blocking RST packets appeared in technical blogs. By ignoring all RST packets, one essentially gained immunity at the expense of network stability, as legitimate RSTs were also ignored. Soon, the censorship responded by upgrading the reset attack system, so that RST packets were sent to both directions - even if you ignored RST, the server on the other side would still terminate it. Also, RST was now "latched-on" for a limited time, when the first RST was triggered, the target remained inaccessible in several minutes.

* New: Bidirectional TCP Reset Attack

* New: "Latched-On" Reset Attack

When HTTPS was enabled, it was impossible to perform keyword inspection in the HTML pages - at this time, censor sometimes still wished to allow partial access, only triggering the block when detected a match. This strategy cannot be applied to HTTPS, since the content was all encrypted. Some people realized some popular websites supported HTTPS but not enabled it by default, such as Wikipedia. The Great Firewall responded by implementing a HTTPS certificate matching subsystem in the keyword matching system, when a particular certificate was matched, you were greeted by a TCP RST packet (this system has been removed later when HTTPS saw widespread use).

* New: Certificate-Based HTTPS Blocking System

At this point, around 2010, the only reliable way to browse the web was using a fully-encrypted proxy, such as SSH dynamic port forwarding or a VPN, which required purchasing a VPS from a hosting provider. SSH was more popular due to its ease of use - all one needed was finding a SSH server and ran "ssh -D 1337", so that port 1337 would be a SOCKS5 proxy provided by OpenSSH. OpenVPN was reserved for heavy web users, since it's more difficult to setup, but had better performance.

From the beginning to the 2010s, anyone who was using VPN or SSH can enjoy reliable web browsing (only be disturbed from time to time due to the overloaded international bandwidth). However, the good days came to an end when the Great Firewall implemented a real-time traffic classifier, it was first applied to SSH. It observed the SSH packets in real-time and attempted to identify whether an overlay proxy traffic was carried on top of it. The blocking mechanism was enhanced as well, now it was able to dynamically inserting null route entries when it decided that the communication with a server was unwanted. The IP blocking system was also improved, now it was able to collect unwanted IP addresses at a faster rate with help of the traffic classifier. If you used SSH as a proxy, after a while the connection would be identified, with all packets dropped, repeated offenses would earn you a permanent IP block. For VPNs, the firewall implemented a real-time classifier to detect OpenVPN's TLS handshakes. When handshakes were detected, a RST packet is sent (or if you use UDP, all packets are dropped). Repeated offenses would earn you a permanent IP block as well.

New: Real-Time Traffic Classifier

New: Real-Time IP Blocking

New: Actively Updated IP Blocklist using Classifiers as Feedback

Traffic classifiers would later be expanded to cover HTTPS-in-HTTPS as well, so a naive HTTPS proxy wouldn't work, and possibly have other features, it's a mystery.

BTW, after Google exited from China, the HTTPS version was immediately blocked, and for HTTP, a ridiculous keyword blocklist was enforced and it generated huge amount of false-positive RSTs for harmless words, apparently a deliberate decision, preferring false-positive over false-negative. Eventually, all Google services had been permanently blocked. The IP block became extensive, major websites have been completely blocked, the unblocked sites were only exceptions. For most people, the arrival of widely-used HTTPS was too late and useless, since IPs were blocked. And as mentioned, SSH and VPNs were classified and blocked as well.

This was when a new generation of proxy tools started to gain popularity,


Shadowsocks being the most well-known example. From a cryptographic perspective, it was a big step backwards. Since Diffie-Hellman handshakes were subjected to traffic classifiers, these tools only used symmetric encryption with fixed keys. Their encryption protocols were ad-hoc, and not cryptographically robust. While it was a matter of fact that nobody could break a simple AES-CBC encryption, nobody would trust these tools for one's confidential data as well (for example, AEAD was unsupported for many years). But since the goal was bypassing censorship, not secrecy, they became extremely popular. It was not seen as an major issue, since the widespread use of HTTPS offered robust secrecy. DNS encryption was still essential (usually the SOCKS-5 interface was provided by these tools, SOCKS-5 can be configured to pass the original domain name to the proxy, the proxy can resolve the names inside its encrypted connection), but became less useful when used on its own, since the IP blocklist was huge by the time.

The landscape of the Internet has changed dramatically since 2013 as well. The universal adoption of HTTPS eventually rendered all keyword-based inspection useless. A few sites were considered too large to block, including Amazon AWS and GitHub. One side of the battle started becoming a mutual assured destruction game - either allowing people to exploit a large platform to publish uncensored material, or blocking the platform altogether and creating economic damages. I am confident that the MAD game will continue to play out, however, Russia's response to AWS domain fronting showed this strategy could fail if major platforms don't want to cooperate, it was a bit worrying, at least. But anyway, encrypting SNIs should be the next step.

But I digressed, back to Shadowsocks, et al, since the state was eliminated (pun intended), all one could see was encrypted raw TCP packets, there was no reliable way for the firewall to classify Shadowsocks-like tools for many years (until recently, possibly by exploiting cryptographic-related issues, but we are not sure how successful it is). But the censorship system started getting weirder and weirder - sometimes, connections break without any apparent reason at all, sometimes data rate was extremely low, sometimes a few IPs were blocked mysteriously, and so on, but life kept going on. There were several possible hypotheses, one was that the traffic classifiers were getting more and more functionalities, and occasionally they could hit something. Another was that the TCP RST was sent in a probabilistic manner to suspected endpoints to degrade reliability. The only thing that could be confirmed was the significantly increased use of QoS by the ISPs, so that all unknown protocols would be classified as "low priority", degrading the reliability of all anti-censorship tools. At this point, bad connectivity and censorship was indistinguishable.

It's safe to say, that at this point, nobody ever understands how the Great Firewall of China work anymore. This is the end of our story.

For simplicity, I skipped many less used techniques, such as Tor's domain fronting, or CDN-based circumvention, or obfsproxy4 that featured Diffie-Hellman keys indistinguishable from random strings, and possibly others. I'm well-aware of them. But it's expected that, unless everything is encrypted and all infoleak is plugged (then, we will start playing the mutual assured destruction game), all these tools are doing is an endless cat-and-mouse game.

Developers of anti-censorship tools need to consider countermeasures based on what China is currently doing. So that when the same techniques used by China are implemented by their own ISPs in the future, they are always prepared to act.


Fantastic breakdown on the recent history of censorship in China, thanks for sharing it.

You mentioned that for many of these efforts bypassing censorship trumped secrecy concerns. Is this still the case?

If I were a citizen regularly bypassing censorship of an authoritarian government, I’d be concerned for my safety if it was well documented that I regularly accessed censored material.


From what I gather, the regime doesn't really intend to arrest anybody who simply regularly accesses western websites. Some big corps also have their special VPN channels to access foreign websites so that they can do business normally. Hell, even the foreign ministry spokesperson posts regularly on Twitter. What they want is to stop this floodgate of information being opened to the common mass, that's when things could get problematic.

People are arrested for producing things that are deemed potentially destabilizing for the regime/country, but nobody as far as I know ever got arrested for accessing blocked materials.

Of course, if you are also actively producing content it would be much wiser to camouflage your identity much better, if you can. That's when the secrecy becomes a major concern.


> You mentioned that for many of these efforts bypassing censorship trumped secrecy concerns. Is this still the case?

Yes, it's still the case, but how bad is a matter of debate.

To make it specific, we can use two criteria to evaluate anti-censorship circumvention tools: (a) How cryptographically robust it is? (secrecy) and (b) How well they can avoid detection? (visibility) The situation is complicated, since they are related but independent.

First, OpenVPN has good secrecy, but high visibility, since it's handshake is obvious, and it even led to a complete block. Second, everything that exploits a bug in the DPI system will have circumvention capabilities, but bad secrecy and high visibility - ultimately, the fact that a TCP connection has been created cannot be hidden, and the fact that you are bypassing censorship will be clear - on the other hand, high visibility doesn't necessarily mean it can be blocked (fixing such a bug can be difficult). [0] Third, a protocol with cryptographic flaws (such as not providing good protection against ciphertext modification) can otherwise have low (or high) visibility, but allows attackers to compromise infosec in some ways. Finally, Tor has circumvention capabilities, excellent secrecy, but high visibility - it's anonymity depends on its large anonymity set, not hiding the fact that someone is using it (which is unpractical), and its network is completely open.

Primarily, my personal concern is whether the circumvention tools are cryptographically robust, so that my secrecy won't be compromised when I browse a HTTP website (The NSA can always wiretap at the exit node, but at least it should not be vulnerable at the entry point). I don't trust these tools, if cryptographers kept discovering implementation flaws from established protocols, why should I trust a tool with ad-hoc crypto? For example, Shadowsocks did not have any forms of forward secrecy, if someone is recording all the outgoing traffic, and later take control over my computer, using a single key allows the decryption of everything. On the other hand, some people argue that flaws may exist, but exploitable ones are rare. But still, I think it's a bad practice to lower the standard of secrecy. If I have to use them, I'll run an additional layer of TLS on top of these tools, so that my connection will always be as secure as TLS, while the outer layer provides circumvention. Fortunately, most people are protected anyway by HTTPS.

> If I were a citizen regularly bypassing censorship of an authoritarian government, I’d be concerned for my safety if it was well documented that I regularly accessed censored material.

If your goal is totally avoid detection of using any circumvention tool at all, it's going to be much harder. Many privacy tools are developed to exercise one's rights to privacy, but they are not designed to avoid detection. On the other hand, the same tools are usually promoted for citizens in oppressive regimes. This can be dangerous. For example, a full-disk encryption software that includes clickable links to its official website, with automatic update, what can possibly go wrong? If the regime is authoritarian enough, the regime can simply make a list of all users that have accessible to these servers before and hunt them down.

A huge amount of work needs to be done to fix this problem. However, if you are in China, it's not that dangerous. In the authoritarianism spectrum, there are Kazakhstan, Iran, China, Russia, and others. However, China is nowhere close to the extreme. Being economically open at large, the censorship of information in China cannot, and was never meant to prevent all forms of access. The purpose is merely to increase the costs from doing so. In fact, criticisms of the government in domestic social media are sometimes tolerated, often the censorship only kicks in when it became popular.

By installing an Internet censorship system in China, the consequences are: (a) Most people are not interested in accessing block websites, at not in a regular basis, even if methods are available. (b) Accessing information doesn't necessarily mean a change of point-of-view, especially when the opinions are completely different from one's education, personal experience, or worldview. (c) Foreign platforms cannot gain any significant influence, even if they are accessed to many. For example, the Chinese Twitter community is an interesting place (if one digs deeper below the political flamewars at the surface), you could see people coming from the entire political spectrum. There are even jokes, such as "Twitter - the future of governance in China", but they are irrelevant in the big picture. (d) IT workers are required to use Google and other blocked sites for doing one's job.

Under this background, regularly bypassing censorship in China just for web browsing is perfectly safe [1]. If you want the best invisibility, I recommend you to use the most popular VPN service used by the highest number of people, and run your own encrypted tunnel inside that. The downside is that these services are too popular to be stable, most IT workers still prefer to use a personal hosting service.

[0] Due to the increased centralization of the web, changes are expected. With SNI encryption, if all the censor can see is a connection to an unknown website on CloudFlare's server, it's less of a threat. But different opinions exist, one says the pressure of censoring everything vs. not to censor can lead to an decrease of censorship or a faster overthrow of the censorship system , but others say the censorship/anti-censorship forces are in a dynamic balance, the introduction of centralized services with SNI encryption can actually break the balance. What used to be a slow censorship progression that needed in 5 years can speed up to 2 years, creating an accelerated and more aggressive censorship, and ends up to be a net negative everyone. Whether it is the case is yet to be seen.

[1] Unless you are in regions like Xinjiang, where separatist conflicts are seen as a threat, and that the censorship has extra objectives.


Thanks for this summary. The firewall has been a lot stricter recently and it's been a real pain in the ass, even for legitimate things. I can only speculate they are using deep learning type tools now to do their blocking

> I can only speculate they are using deep learning type tools now to do their blocking

It needs careful justification before making such a statement - the censorship system has a serious constraint on computational costs - it needs to operate on the stream of the entire outgoing international traffic, and to make a decision in real-time (or for back-analysis). We are talking about many terabytes per seconds of traffic, any censorship tools that have a high computational costs cannot be deployed for such a purpose, even if it runs okay on a single PC. Also, a high false-positive rate is not acceptable, as it will create massive service disruption and practically useless.

Unlike the case for SOCKS5-over-SSH, HTTPS-over-HTTPS, or VPN handshakes, which can be detected by relatively simpler rules, most deep learning tools required excessive CPU time, so it's unlikely that complex deep learning algorithms are being used, at least not the category that costs the highest CPU time (anything with "AI").

Given these constraints, the algorithms available to the censorship system is rather limited, it seems. What types of algorithms are being used, then? Unfortunately, nobody can answer this question. This is the fundamental question people are facing today. 10 years ago, every sysadmin in China knew the censorship system works, but today, the system has became completely opaque.


Great post, thank you.

I agree with the other posters -- fascinating, detailed info. These posts should be promoted to their own HN article/discussion...

Were they doing that full page text matching in an ASIC too?! Doesn't that basically involve writing a simple parser also? Else what prevents things like usage of Google analytics/fonts etc from triggering a match and blocking?

> Else what prevents things like usage of Google analytics/fonts etc from triggering a match and blocking?

The blocking was/is complementary. Usually, domain names themselves were blocked by DNS poisoning (or IP blocking if it escalated), domains themselves (or the names of the websites) did not appear in the keyword blocklist. A link to Google Analytics or Facebook button could stuck the webpage from loading properly until a timeout, but merely mentioning or linking a domain name would not trigger a keyword match of the page itself.

The intention of keyword matching was to allow partial access while still blocking unwanted content. Usually, only the most politically unwanted keyword entered the keyword list. For example, Wikipedia could be accessed normally, but as soon as "a word that should not be named" appeared in the webpage, the connection would be reset immediately. An interesting phenomenon was, sometimes the page could partially load and stopped exactly before the forbidden word. And since the censorship system worded on mirrored traffic, sometimes a slight processing delay allowed the full page to load before the RST was received, it would be a "I'm feeling lucky moment".

Anyway, there was how the system worked before 2010. The extensive use of HTTPS rendered it useless, and it appeared that some forms of keyword filtering has already been lifted, since it's already a pointless exercise.

For quite some time after keyword matching became ineffectively, DNS poisoning remained the only form of censorship for many unwanted but not significant websites, for example, Hacker News. But recently, SNI matching was implemented.


A simple countermeasure at the ISP level: a buffer to merge 'www.you' to 'tube.com'

A far greater danger is DPI that use statistical analysis to detect possible tunnels. You want your traffic to be as close as possible to normal traffic. There is no perfect solution there. The current best is generating valid images with a hidden data payload (to download), and generating pseudo text posted on public forums or email (to upload) while limiting the download/upload ratio, by downloading random content if necessary as most people download far more than they upload.

It works best when using "known" websites like gmail (draft folder) or facebook (messenger), as all the traffic goes to a whitelisted host and look like regular usage.


>A simple countermeasure at the ISP level: a buffer to merge 'www.you' to 'tube.com'

addressed here: https://news.ycombinator.com/item?id=22656122

>A far greater danger is DPI that use statistical analysis to detect possible tunnels.

That's only an issue if there's a blanket ban on tunneling/proxies. While it's a problem in authoritarian regimes (eg. china, kazakhstan), it's not an issue in most western countries. I haven't heard of any western countries banning VPNs (yet).

>The current best is [...]

The timing information would still be suspicious. Most people aren't constantly checking their gmail/facebook multiple times a second, but normal browsing would generate packets with that frequency. It's really only undetectable if you're sending/receiving messages (eg. IM or email). A better candidate might be multiplayer game traffic. They provide a consistent stream of bits[1] to hide data in. If you're willing to set your tunnel's bandwidth to a few kilobytes a second (throttling if there's too much data, sending decoy packets if there's too little), it'd be very hard to detect any anomalies.

[1] random search: https://youtu.be/8Kvj5TZNNJ4?t=1080


> Defeating chunking would require additional memory + compute power on the DPI boxes, which I suspect ISPs don't want to bear.

It depends. ISP may be willing to spend more, if they gain more or are forced by governments to do that.

Even as is, the proposed method is still too easy to defeat, especially with IP bans: if the ISP really doesn't want to let youtube.com work, all the A and AAAA records will be blacklisted

> The timing information would still be suspicious. > It's really only undetectable if you're sending/receiving messages

Indeed, so the suggestion was to use the draft folder and FB messenger.

A better method would rotate the whitelisted websites- like using mostly gmail for 20 minutes, then facebook for 1h, etc. and of course only "on demand" so that traffic does not occur 24/7

For multiplayer game, the audio channel already provides a very simple method to stream more than a few kb per seconds.


So basically it just does two things: carefully chunking HTTP header packets and encrypted DNS?

Not sure this will work for very long.


>Not sure this will work for very long.

Maybe if it gets popular. Defeating chunking would require additional memory + compute power on the DPI boxes, which I suspect ISPs don't want to bear.


I work in the DPI field and have maintained a few DPI firewalls.

Most DPI that I know of will defeat this bypass technique, I'm not sure the author has even tested if it works.

DPI firewalls already have to support aggregating packets. It's pretty common to need more information beyond the initial packet. It's not really any more memory intensive either, you're just reading byte by byte and keeping what you need.

Heck most DPI firewalls support checking something in the outbound packets is in the inbound packets. ie - checking if a connection is performing IKE.


The DPI doesn't need to buffer packets. Searches like this are performed using a regex compiled as a DFA or similar state machine. The state maintained per flow is a few machine words at most.

You'd have better luck sending the TCP packets out-of-order. But some DPI boxes will buffer these to a small degree to catch such shenanigans.

Source: in a previous life I worked on the layer-7 inspection subsystem (among others) of a DPI box.

EDIT: Also what @cpitman said. DPI boxes will often err on the side of caution. The DPI will happily kill your goofy-but-standards-compliant flow if it can't figure out that it's safe.


Does it? I don't think you need to correlate packets, you could probably just block small packets that look like they have only part of the hostname. If they wanted to be slightly more selective, they could block small packets that have a partial hostname and have a prefix that is blocked.

In order for traffic to be open for any substantial time, the technique either has to stay hidden/unpopular or the traffic has to be hard to distinguish from normal traffic.


I believe this is more of a cat and mouse game.

Works with You Broadband, India. Thanks a lot man!

This is great if there isn't a blanket ban on VPN's, but unfortunately, it won't work in China. I've had next to zero-luck keeping my own VPN tunnels open for more than a couple of days at a time when behind the great firewall.

This isn't a VPN though, you simply run it on your computer, and it does all the magic for you.

Check out v2ray. You'll need to have your own domain, server, and a cloudflare account, but in terms of speed it is unmatched. Unfortunately many of the best tutorials are in Chinese.

Looks interesting. From https://www.v2ray.com/en/index.html it seems that it's "just" a VPN protocol / software that can tunnel over TLS. I assume the point of using your own server + Cloudflare is that it breaks IP based blocking of most VPN providers. I guess just your own server without Cloudflare would work fine for a while, but they probably have heuristics for a lot of encrypted traffic sent to a single unknown server?

The remaining question for me is about the TLS part of all this. Does China not have agreements with most external services about stripping TLS such that a lot of TLS traffic would be suspect? Or do they not mandate their citizens to use a Government provided root cert that would allow them to "securely" MITM connections? That would be how I'd do it if I were an authoritarian government.

If not, then what's their plan for the future? I could see a Firewall kind of mostly working for now on a combination of DNS, IP, and SNI filtering, but all three are going away in the near term. DNS with DNS-over-HTTP, SNI with eSNI, and IP blocking has become less plausible already through routine use of proxies like Cloudflare.


https://www.scmp.com/news/china/politics/article/3030563/big...

They want to make the networks transparent to the government, and apply machine learning for understanding the data and warnings the monitoring system will provide.

You either provide decryption keys, or your traffic will be dropped.


Yeah, that's what I figured would happen next. It's honestly very difficult to defend against an adversary that nakedly aggressive. It's like trying to browse the Internet privately on your computer at your desk at the major IT firm you work at.

Is that a SOCKS proxy, or just a HTTP proxy? The github readme does not make that clear.

Doesn't work against Virgin Media UK

Virgin is censoring things now?

All the big UK ISPs do. This is targeted at CP but who knows what else gets covered.

The Virgin Media list above has some very benign content blocked (movies, sports and even a Nintendo cheats/hacks site).

This is concerning and I'm thankful I left that terrible (for a lot of other reasons) company long ago (my current ISP doesn't seem to be blocking anything from their list).


Has been for a while. https://www.virginmedia.com/help/list-of-court-orders

Also (rightly so) anything on the Internet Watch Foundation list - https://www.iwf.org.uk/become-a-member/services-for-members/...


> rightly so

Debatable


It's a URL list of child abuse content?

That from time to time contains all kinds of false negatives and does a subpar job, see https://en.wikipedia.org/wiki/Internet_Watch_Foundation_and_... and https://fuwafuwa.moe/transparency/cocaine.ninja/2016-07-14.d... - which means that the ISPs do not "rightly so" block sites mentioned in said lists.

But even then if they did their job perfectly, not everyone agrees with this censorship policy. "ah yes, I will intercept your DNS queries and censor anything related to certain sites which I do not fit within my personal moral framework" is not something that I agree with.


ISPs in the UK have to obey court orders, and there are some court orders in place for the "big 5" ISPs around certain piracy websites.

If I try to go to thepiratebay.org I get this

--- Access to this website has been blocked under an Order of the Higher Court.

Any TalkTalk customer affected by the Court Order has a right under the Court Order to apply to vary or discharge it. Any such application must: (i) clearly indicate the identity and status of the applicant; (ii) be supported by evidence setting out and justifying the grounds of the application; and (iii) be made on 10 days notice to all of the parties to the Court Order.

For further details click here. ---

On top of that there is a voluntary scheme run by the Internet Watch Foundation to provide filtering for sites that share images of child sexual abuse. Some ISPs don't participate in this, but the vast majority do. One famous example of things that get blocked (but also reasonably quickly unblocked) are the Wikipedia page for a band. NSFW link https://en.wikipedia.org/wiki/Internet_Watch_Foundation_and_...


I believe it's illegal not to apply the Internet Watch Foundation blacklist (despite the IWF being completely unaccountable). A&A does a scheme where they'll set you up as a 1-person ISP and provide an unfiltered feed, but you have to sign something saying you're applying the blacklist yourself.

In the UK? Do you know the relevant law and could provide a link?

I briefly looked at the legal side of offering public Wi-Fi (essentially becoming an ISP) and besides debunking a ton of BS (turns out you don’t need to collect traffic data unless explicitly ordered by the government, and sign-in pages/captive portals/ToS aren’t necessary either) I never even heard of the IWF.

Curious how this law would apply to the ISPs providing connectivity to dedicated servers.


It's really good to see more tools in the privacy space, resulting in more options and fallbacks for the end-user.

I think the author should also try to market this as much as possible outside of the HN crowd, since this seems targeted at non-tech users — I could be wrong but my reasoning is that HN users who care about privacy would prefer a combination of a VPN and DoH to defend against traffic & DNS inspection, respectively.


It really depends on your threat model, but if you have a full tunnel VPN, then encrypted DNS is a lot less important.

It's working very well against South Korea, KT(ISP)

Need some more insights into proper use. I couldn't get to work in mainland China

> We send it in 2 parts: first comes GET / HTTP/1.0 \n Host: www.you and second sends as tube.com

How does this even work? "www.you" won't return a valid HTTP response and "tube.com" won't either. How can you fetch the content at "youtube.com" but splitting the domain name in half? Won't you get two completely wrong responses that don't fit together?


It's split across two network packets. It's still one request for the web server.

might add another fake packet that confuse DPI.

How does this differ from Gigsaw's Outline, Shadowsocks or V2Ray?

Any one tested if this tool work in China or not?

Why not use Tor?

Really depends on your use-case. Tor is great but easily detectable and thus blockable, and though its speed has gotten much better, it isn't as fast as other options. But again, it really depends on what your goal is.

Tor has obfuscation options, they work well.

The default endpoint list is usually blocked though as its published to the clients so you have to request an off-list endpoint.




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