Ruby

Description¶

I created a Resolv::DNS resolver with two nameservers described using their domain names. Calling #getresources on this for an A or NS (and possibly other resource types) of domain example.com returns an empty list (no results, no error). I expected it to return the corresponding resource record (IP of example.com).

Instead if the Resolv::DNS instance is created with one resolver domain name or two resolvers' IP addresses then correct results are obtained. The error occurs only when more than two nameservers [or possibly more] are specified using their domain names.

Testcases¶

I used two well-known public DNS servers to test (they have funky but legitimate TLDs):

• dns.google (8.8.4.4, 8.8.8.8)
• one.one.one.one (1.1.1.1, 1.0.0.1)

✗ Failing: two nameservers by name¶

require 'resolv' Resolv::DNS.new({nameserver: ['dns.google', 'one.one.one.one']}).getresources('example.com', Resolv::DNS::Resource::IN::A) => [] 

✓ Successful: one nameserver by name¶

(either as a single-item list or a string)

Resolv::DNS.new({nameserver: 'dns.google'}).getresources('example.com', Resolv::DNS::Resource::IN::A) => [#<Resolv::DNS::Resource::IN::A:0x000055ee95df9ad8 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=16840>] Resolv::DNS.new({nameserver: ['one.one.one.one']}).getresources('example.com', Resolv::DNS::Resource::IN::A) => [#<Resolv::DNS::Resource::IN::A:0x000055ee95517c80 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=77160>] 

✓ Successful: one nameserver by IP¶

(either as a single-item list or a string)

Resolv::DNS.new({nameserver: ['8.8.4.4']}).getresources('example.com', Resolv::DNS::Resource::IN::A) => [#<Resolv::DNS::Resource::IN::A:0x000055ee95e28d38 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=20931>] Resolv::DNS.new({nameserver: '1.1.1.1'}).getresources('example.com', Resolv::DNS::Resource::IN::A) => [#<Resolv::DNS::Resource::IN::A:0x000055ee95e41360 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=66081>] 

✓ Successful: two nameservers by IP¶

Resolv::DNS.new({nameserver: ['8.8.4.4', '1.1.1.1']}).getresources('example.com', Resolv::DNS::Resource::IN::A) => [#<Resolv::DNS::Resource::IN::A:0x000055ee95d62ea8 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=20894>] 

A little analysis¶

I didn't get to the bottom of this but this may help:

• When only one nameserver is used then a Resolv::DNS::Requester::ConnectedUDP::Sender is used; when two nameservers are used an UnconnectedUDP::Sender is used.
• The code appears to be timing out and then retried even though correct requests are made and responses received.
• In particular, if I packet trace 53/udp then the conversation is exactly as I would expect and near identical in both failing and successful cases. That is:
  • Request A record for first nameserver from local resolver
  • Receive IP of first nameserver
  • Request A record of example.com from first nameserver
  • Receive IP of example.com
    In the successful case, the conversation ends here. In the failing case, the code round-robins between the two nameservers until it finally returns [].

Test environment¶

Tested ruby 2.5.0p0 on Red Hat 6 and rubies 2.7.1p83, 2.7.2p137 on Ubuntu 20 from two different networks with identical results.