ISP Column

Encrypted DNS using DNSCrypt

Writing this post from my hotel room in Kathmandu. I found that many of the servers appear to be DNS resolvers which is unusual.

dig . ns +short
dig . ns +short

This seems unusual and is the result of basically port 53 DNS hijack. Let’s try to verify it using popular “” query.

dig @ a +short
dig @ a +short
dig @ a +short

So clearly something in middle is hijacking DNS queries and no matter whichever DNS resolver I try to use, the queries actually hit authoritative DNS via This belongs to WorldLink Communications (ISP here in Nepal) and I am just 5 hops away from it.
So what can be done about these cases? Well, one way is VPN of course but with a setup where VPN server’s IP address is hardcoded in the client and not using DNS. It works and does the task but performance can vary greatly depending on how far is the tunnel server. A better and more modern way out of it is by using encryption in DNS by using a protocol named “DNSCrypt“. DNSCrypt offers to encrypt of DNS queries from clients to the DNS resolvers. (Beyond that resolver still, follow usual non-encrypted root chain to reach authoritative DNS servers).
So how does it work?
There’s no integrated support of DNSCrypt in OS’es at this time. There are number of projects like dnscrypt-osxclient available on GitHub which enable this support.  Once configured, the client changes system’s DNS resolver to a local IP which listens for port 53 (regular/non-encrypted) requests.

cat /etc/resolv.conf |grep nameserver

The client often offers support of various open resolvers like OpenDNS, Quad9 etc.

dig @ a +short

Here it shows that DNS resolver in my case happens to be Cisco’s OpenDNS. As soon as the client gets port 53 DNS queries, it encrypts it and sends via UDP port 443 (UDP or TCP depending on provider and client configuration). The encyption is based on trusted root CA’s and associated chain as popularly used in HTTPS. This is also one of reasons why DNSCrypt is also known as DNS over HTTPS.
Here’s an example of a DNS query to resolve A record of while running tcpdumps in parallel:

sudo tcpdump -i lo0 'dst port 53' -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo0, link-type NULL (BSD loopback), capture size 262144 bytes
04:36:04.429212 IP > 31576+ A? (59)
04:36:04.532015 IP > 623+ [1au] A? (39)
2 packets captured
4 packets received by filter
0 packets dropped by kernel

This shows request went in clear text to which is configured on loopback. While in parallel if I watch for traffic towards OpenDNS public IPs, I get:

sudo tcpdump -i en0 'dst or dst' -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on en0, link-type EN10MB (Ethernet), capture size 262144 bytes
04:39:56.827824 IP > UDP, length 512
1 packet captured
63 packets received by filter
0 packets dropped by kernel

Thus all that appears here is just an encrypted packet to Cisco OpenDNS over UDP port 443.
I ran another query and saved it in pcap file. Here’s how it looks like in wireshark:

That’s all about it for now. I am going to keep encryption enabled especially when travelling from now onwards. Time to get some sleep. 🙂
Useful Links:

  1. dnscrypt-osxclient –
  2. DNSCrypt Wikipedia –
  3. DNS Over HTTPS (Google Public DNS) –
  4. DNS over TLS (Quad9) –

4 replies on “Encrypted DNS using DNSCrypt”

  1. Although it might be considered nasty thing… DNS Hijacking is done by ISPs in order to optimise delivery from their local CDNs which are sensitive to DNS, for example Akamai is such… If users do not use the Local DNS of ISP then Akamai traffic is served from somewhere abroad which is usually = bad experience and/or lower throughput and ISP cannot save money from having local content 🙂

    1. RFC 7871 takes care of it, so even if you don’t use ISP’s DNS, or run your own DNS (without forwarding), you still get geographically nearest PoP. Although geographically nearest PoP may not be efficiently routed, eitherways, DNS hijacking is not quite solution to the problem anymore, IMHO.

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