Analysis of the GFW's Unconditional Port 443 Block on August 20, 2025
(gfw.report)217 points by kotri 4 days ago
217 points by kotri 4 days ago
Thanks for sharing this. I researched this for my A level project a few years ago, and this is a really neat cross reference. I didn't mention V2Ray as much.
How is traffic controlled inside PRC? Is GFW a central hub for all traffic between all hosts? Or between residential ASNs and commercial ones only? In the UK and Iran a lot of censorship was implemented by leaning on ISPs at IP level (eg BT Cleanfeed) and with DNS blocks but I haven’t kept up to date with how networks might handle residential hosting. Maybe internal traffic is just all banned?
> How is traffic controlled inside PRC?
Unknown. I haven't seen any injected fake DNS or reset packets so far to domestic hosts. But there are rumors that Google's servers in Beijing (AS24424) was once black holed.
> Is GFW a central hub for all traffic between all hosts?
It's supposed to has centralized management system, but not a single hub.
> Or between residential ASNs and commercial ones only?
Yes, the injecting devices are deployed in IXPs, the AS borders. See <Internet censorship in China: Where does the filtering occur?>.
> In the UK and Iran a lot of censorship was implemented by leaning on ISPs at IP level (eg BT Cleanfeed) and with DNS blocks but I haven’t kept up to date with how networks might handle residential hosting.
I believe Iran has more centralized system like China controlled by Tehran.
> Maybe internal traffic is just all banned?
No, internal HTTPS traffic is not banned in that hour.
It's in operators but managed by the regional government.
So what's blocked differs by region
Terrible, this is Internet curfew. It's not uncommon to imagine they'd shutdown Internet across border during any war (like against Taiwan).
> Terrible, this is Internet curfew.
If you think this is bad...
You can't even have a blog in China without authorization. It doesn't matter if you pay "AWS" for a machine. It won't open port 80 or 443 until you get an ICP recordal. Which you can only do if you are in China, and get the approval. It should also be displayed in the site, like a license plate. The reason "AWS" is in quotes is because it isn't AWS, they got kicked out. In Beijing, it is actually Sinnet, in Nginxia it's NWCD
You can only point to IPs in China from DNS servers in China - if you try to use, say, Route53 in the US and add an A record there, you'll get a nasty email (fail to comply, and your ports get blocked again, possibly for good).
In a nutshell, they not only can shutdown cross border traffic (and that can happen randomly if the Great Firewall gets annoyed at your packets, and it also gets overloaded during China business hours), but they can easily shutdown any website they want.
I laughed when I saw "Nginxia", thinking it was a portmanteau of, well, nginx and wuxia, a Chinese fiction genre. Reality is much less funny when I looked up NWCD, and you likely just made a typo of Ningxia.
"Xia" would map to a single character (code point) in Chinese. For instance, in simplified Chinese, it could be 下 (xia, meaning down), 侠 (martial arts - like the xia in wuxia), or any number of other homophones. Since the characters are already combinatorial, I'm not sure a Chinese speaker would think of this as a portmanteau.
> You can only point to IPs in China from DNS servers in China - if you try to use, say, Route53 in the US and add an A record there, you'll get a nasty email (fail to comply, and your ports get blocked again, possibly for good).
Wait what? So I can DoS any Web site in China by creating a rogue DNS record that points to its IP address, even under a completely unrelated domain? How would they even find those records?
I guess they would find it the moment someone in China using a Chinese resolver tries to resolve your rogue record, since that would recurse to one of the root mirrors in China, which presumably feeds this mechanism.
Seems like a very minor speed bump in your plan, though: presumably something like https://www.chinafirewalltest.com would achieve that, or send a few emails for folks to click.
I wonder if this is actually tied to Chinese domains and Chinese run registrars? That way it would be easy to flag the usage of foreign nameservers and there's no DoS risk.
> It should also be displayed in the site, like a license plate.
https://de.wikipedia.org/wiki/Impressumspflicht (Mandatory real name & address, not only for business, but private persons with web presence, too.
Same for Domain/DNS(which applies to everything in the European Union))
Not all Western companies comply with Beijing, like Route53, a name I've never heard of; Cloudflare seems to be most popular in China.
But yeah, they can shutdown anything unless proxy server is widely used. as <Nearly 90% of Iranians now use a VPN to bypass internet censorship>.
In fact, it’s a common tactic to do something unusual, in a recurrent way, so people aren’t alerted when it happens for real. (When the Mossad stole 7 boats from a French port (that they had fully paid), they prepared a few months in advance by having the pilots start the engines every night at 23:00, pretending they needed it against the cold temperatures. When they day came, they started the engines and left, no-one saw it coming).
It could also be a test to look for surprising things that break, in case they want to do this permanently at some later point.
It was five boats [1], an good story nonetheless. Think whatever you want about Mossad, it can not be denied that these guys have balls.
Could you bring something like a starlink mini for backup i wonder? Id imagine this would be very worrying being stuck there as a foreigner in such a situation.
> Starlink connects you to the internet via a ground station in the country where you are registered
Not true anymore.
> and the antenna will also only operate in an approved zone (depending on your country and account type). You cannot use it in China.
This is still correct.
No it does not. Against a huge state adversary like China it does not matter. They have satellites looking down so they can quickly locate any starlink users. And then ...
The only thing that could bypass is GPS + laser links (meaning physically aiming a laser both on the ground AND on a satellite). You cannot detect that without being in the direct path of the laser (though of course you can still see the equipment aiming the laser, so it doesn't just need to work it needs to be properly disguised). That requires coherent beams (not easy, but well studied), aimed to within 2 wavelengths of distance at 160km (so your direction needs to be accurate to 2 billionths of a degree, obviously you'll need stabilization), at a moving target, using camouflaged equipment.
This is not truly beyond current technology, but you can be pretty confident even the military doesn't have this yet.
That's what's so great about LoRA. Decentralized txt msgs, ultra cheap radios people run at home or wherever. $10-35USD ON AMAZON. Least txts get through.
The most depressing is that what happens in China, will eventually happen in the west too. I'm sure certain US, UK, and EU bureaucrats are already crafting campaigns about how this ability will 'save the children' and that it should be implemented immediately (politicians and certain other selected people will be exempt of course).
There's nothing inevitable about this. Civil society needs to organize, coordinate, and spend money on PR about this.
Right now liberal people mostly sit back and wait for things to get better, it's not enough. (Also going and walking up and down is not really effective.)
It's inevitable because we've seen time and again that all it takes to get the public opinion behind this kind of thing is to talk about how it is needed to catch pedophiles and terrorists.
And if you talk back? Why, you must be a pedophile or a terrorist, otherwise why would you have anything to hide?
It's gotten bad enough that people here on HN - Hacker News! - non-ironically make more or less this argument.
It is inevitable, because the means by which civil society can organize, coordinate, and spend money on PR about this, are all firmly in the control of a very few people. These same people are generally on the side of more centralized control, because they are the ones who will wield it.
> Right now liberal people mostly sit back and wait for things to get better
First they came for the socialists, and I did not speak out because I was not a socialist.
Then they came for the trade unionists, and I did not speak out because I was not a trade unionist.
Then they came for the Jews, and I did not speak out because I was not a Jew.
Then they came for me and there was no one left to speak for me.
Think of how many people who have remote jobs with American companies couldn't connect to their meetings while they "work from home" while secretly being in China!
Normally they have to fight VPN issues anyway, but having a sovereign state inject your packets is certainly a fun new one.
Anyone operating in/around China who needs a real VPN has a service they pay for and use that isn't mainstream that isn't blocked (using V2ray or similar). There's a reason why Shadowrocket is the number 1 app on the app store. I'm sure there are a lot of cases of people using e.g., off-the-shelf VPN apps and have trouble, but power users in China are always running a VPN, usually to Japan, that doesn't have this problem.
How do you propose users in China will magically get around a nation state injecting packets?
That's literally what VPNs are for.
If you aren't aware: a Virtual Private Network creates a fully encrypted link between you and a remote node. So long as your encryption keys are secure, there's no way for anyone (even a global superpower) to listen to or intrude on that connection. There is no possible way to break into this connection, even with the entire planet's computing resources.
From the outside, all you can see is a stream of encrypted data between two nodes. You cannot tell where the traffic goes once it exits the VPN server or what it contains.
The only way to compromise a VPN connection is the most straightforward and pedestrian: compromise the VPN host and directly spy on their clients with their own hardware.
The GFW certainly can and has detected such encrypted streams and blocked them for being un-inspectable. With a VPN you can perfectly hide what you're doing and you can perfectly prevent intrusion. You cannot prevent someone noticing you're using a VPN. China can simply blanket ban connections that look like VPN traffic. But they cannot tell what you're doing with that VPN.
I suspect those connections worked fine.
It’s good to know the boss.
I definitely appreciate that a percentage of so called "employees" are actually just full fledged Chinese nationals, living permanently in China, paid a salary to pretend to be an American who had their identity stolen.
But there absolutely is also a non-negligible number of Chinese and Indian nationals, who have some type of visa status in the US (especially a green card) who spend many months in their original countries making $200,000 or more per year while living like royalty in their home countries :)
How common can this really be? And what kind of companies? I’m finding it really hard to imagine this to be widespread.
I live in a popular Digital Nomad friendly country, and myself included, work with Europe/American companies roughly matching their time zones.
Now, the people I work with know that I'm not really located in the same time zone, but I know people who don't bother to mention it. I rarely get phone calls, but I have a roaming connection active for banking/OTP/etc. Plenty of cheap cafes with great WiFi (500mbps+ almost everywhere), and several times cheaper too.
Microsoft was caught doing it for the US federal government, so presumably Chinese software engineers are working on other Microsoft products too.
I'll just say Microsoft is not the only company doing that, and there are also Chinese-owned SAASes which American companies pay for.
Sadly much more common than it should be. The durations vary widely, but with the price of airline tickets and the nature of corporate software engineering jobs, it's extremely easy to self-justify a month abroad. The US government allows 6 months officially for green card holders.
If it wasn't literally 10x cheaper to live abroad than it is to live in Seattle/San Jose, it wouldn't be as prevalent. And not to mention, the quality of life is often better at the 10x cheaper price as well.
I can give you as much proof as you would like!
Lookup the North Korean version of this with the laptop farms
Example: https://www.justice.gov/opa/pr/justice-department-announces-...
China spans 9.6M km. It has some of the biggest and most modern megacities (Beijing, Shanghai, Chongqing, Shenzhen to name a few) and features ancient historical wonders like the Great Wall, Forbidden City and Terracotta Warriors.
The nature spans salt lakes and rainbow mountains akin to South America, to the Northern Lights in Mohe down to karst formations of Guilin shared with Vietnam's Halong Bay.
The cuisine is diverse and dishes popular in places like Xi'an reveal lasting influences dating back to the Silk Road.
If you can't find "somewhere really nice" amongst the myriad people and locations you haven't tried.
Have you ever been to China?
Because they have some of the most beautiful scenery and buildings I've seen and I've been to dozens of countries.
Personally I wouldn't go there for remote work, because the internet interference is a pain but a holiday definitely.
You say that because you don't hold a Chinese or Indian passport. Now think of those who do, who have family obligations, food preferences, local bank accounts.
How would one get around this if they found themselves in such a situation?
Well for starters recreate the situation and test out different approaches. Thanks to the detailed analysis that can be attempted.
If I understand right, a good next step would would be with eBPF or some type of proxy ignore the forged RST+ACK at the beginning.
Then it would come testing to see if sending a bunch of ACK packets, perhaps with sequence numbers that would when reconstructed could complete the handshake. Trying to send them alongside the SYN+ACK or even before if it can be predicted. Maybe try sending some packets with sequence id 0 as well to see what happens.
> ignore the forged RST+ACK
See <Ignoring the Great Firewall of China> in 2006. That won't work if RST/ACK was injected to both sides.
> Then it would come testing to see if sending a bunch of ACK packets, perhaps with sequence numbers that would when reconstructed could complete the handshake. Trying to send them alongside the SYN+ACK or even before if it can be predicted. Maybe try sending some packets with sequence id 0 as well to see what happens.
This is an interesting approach already being utilized, namely TCB desync. But currently most people tend to buy VPN/proxy services rather than studying this.
> Imagine what people would say about Cloudflare if they had an hour long outage
That Cloudflare had an outage. Not America.
I mean... it got blocked by their censorship infrastructure, does it really matter if it only got misconfigured?
Yeah, dont want their citizens to voice anti-CCP thoughts
> A Brief, Incomplete, and Mostly Subjective History of Chinese Internet censorship and its countermeasures
https://danglingpointer.fun/posts/GFWHistory
Posted 6 days ago (https://news.ycombinator.com/item?id=44898892)