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[dns-operations] Quick DNS Report from Behind the Great Firewall of China

[dns-operations] Quick DNS report from behind the Great Firewall of China

Shane Kerr shane at time-travellers.org

Sun Jun 5 13:03:15 UTC 2016

Hello,  I'm doing some work comparing IANA root server answers with Yeti root server answers. As a baseline I decided to compare IANA servers with themselves, and discovered differences. I am doing this work in China right now, so I probably should have expected something like this, because of the Great Firewall and all of its evil.  So here are some observations about DNS and the Great Firewall of China, from my hotel room. Note that I am not encouraging anyone to bypass Chinese laws or regulations, but as a technical DNS person I find the details interesting and thought other people would too.  After a little poking around it seems that there is some DPI happening for all DNS packets that come into China via the Great Firewall. This means that, for example, the F, J, and L root servers are not impacted by the Great Firewall since they have instances in Beijing. (Packets from some root servers are *sometimes* modified, like I root, and I'm not sure why that is.) I didn't look at any TLD. I don't know if any have servers in China other than the various Chinese TLDs.  It looks like there is a blacklist. Of course I can't know the full extent, but I did notice that answers for "google.com" and "twitter.com" are modified. If you query anything on the list, then you get an A record back with some IP addresses that are not always the same, but also not completely random.  WHOIS lookup on some of the addresses shows valid networks in Germany, Azerbaijan, the UK, and so on. When I traceroute to these IP addresses though they terminate at a hop within the China Unicom network, which seems to be the provider that I'm currently connected to - so while I would consider it BGP hijacking, as Randy Bush says, "your network, your rules". 😉 My guess is that there is some disk somewhere recording all packets that try to get to those addresses and making a careful note for future auditing, but perhaps these packets are just dropped on the floor. (The NSA would save these, maybe the Chinese officials are smarter though?)  It looks like the DNS answers are being modified on the return path, not the outbound path. At least, I can query IP addresses I run for "google.com" and I see the query packets at that host ("dig @my.host google.com").  It looks like TCP is not being intercepted, so a simple approach for someone in China wishing to get clean DNS might be to use TCP to a resolver outside of China. A Linux host can usually do this by setting:     options use-vc  In the /etc/resolv.conf file.  If you want to run a resolver *inside* of China and get the answers that authority operators are actually returning, this is probably also possible, although trickier.  Right now, it seems that NS records are not being intercepted. This means that if you use QNAME minimization, a resolver in China should be able to get information on (for example) the Google name servers. UDP answers from those servers will be spoofed.  These spoofed A records are not signed (of course), so anyone performing DNSSEC validation will reject the spoofed answers.  Since TCP is not being intercepted, a resolver can:  1. Use QNAME minimization to get to the authoritative servers, then 2. Reject UDP answers because validation will break, and then 3. Use TCP to get a valid answer.  I don't know if any resolvers can work this way, but in principle it is at least *possible*.  To be clear, this won't actually let you get to any blocked server, since there are IP layer blocks in place too, but at least DNS will be correct. :) (Actually, detecting such blocks could allow someone to build a middlebox which used VPN only when needed and otherwise used normal upstream. Hm...)  Cheers,  -- Shane -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 181 bytes Desc: OpenPGP digital signature URL: <http://lists.dns-oarc.net/pipermail/dns-operations/attachments/20160605/b726a2ff/attachment.sig>

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