CheckAllDNSScheduledTask - undefined method 'unpack' for nil (NoMethodError) len = readable_socks[0].read(2).unpack('n')[0]

Start at CheckAllDNSScheduledTask and inspect the project's resolv dependency, then review Resolv::DNS::Requester::TCP#recv_reply and the Resolv::DNS#fetch_resource fallback described in the issue. Update resolv from 0.6.2 to 0.7.1 and verify the scheduled DNS checks no longer surface the nil.unpack error.

Describe the bug

Sometimes, when CheckAllDNSScheduledTask is running, we see the following error:

undefined method 'unpack' for nil (NoMethodError) len = readable_socks[0].read(2).unpack('n')[0]

To Reproduce

This is hard, I guess have a lot of domains that need to do a DNS check?

Explanation and suggested solution from Claude:

The failing line lives in Resolv::DNS::Requester::TCP#recv_reply (resolv 0.6.2, line 931):

def recv_reply(readable_socks)
  len = readable_socks[0].read(2).unpack('n')[0]   # <-- nil.unpack here
  reply = @socks[0].read(len)
  return reply, nil
end

DNS-over-TCP frames every message with a 2-byte big-endian length prefix, which is what read(2).unpack('n') is decoding. recv_reply is only called after the requester's request loop has done an IO.select/wait_readable and been told the socket is readable. The catch: a socket whose peer has closed the connection also reports as readable, and IO#read(2) returns nil at EOF rather than blocking. So when the nameserver (or a firewall/middlebox) closes the TCP connection without sending a complete reply, read(2) is nil and nil.unpack('n') raises your NoMethodError.

Why it only happens sometimes, and via the DNS task. Resolv only uses TCP as a fallback. Looking at Resolv::DNS#fetch_resource, it queries over UDP first and only retries over TCP when the UDP reply comes back with the truncation bit set (reply.tc == 1). This stdlib path doesn't negotiate a large EDNS0 buffer, so truncation — and thus the TCP fallback — kicks in for domains with big record sets: lots of TXT (heavy SPF/DKIM/verification records), many MX or NS, DNSSEC, etc. CheckAllDNSScheduledTask walks TXT/MX/CNAME/NS across every domain and track-domain, so it's the task most likely to hit a truncated response, fall back to TCP, and then meet a nameserver that drops the TCP connection. That's your "sometimes."

Why 0.6.2 turns it into a crash. The request loop does guard the socket read — but only for Errno::ECONNREFUSED and Errno::ECONNRESET, which it converts to a timeout. A clean FIN (graceful close → read returns nil) isn't an exception, so it slips past that rescue and surfaces as the raw NoMethodError.

The fix. Upstream hardened exactly this. The first released version containing the fix is resolv 0.7.1, where recv_reply now does:

len_data = readable_socks[0].read(2)
raise EOFError if len_data.nil? || len_data.bytesize != 2
len = len_data.unpack('n')[0]
reply = @socks[0].read(len)
raise EOFError if reply.nil? || reply.bytesize != len

So the practical fix is to update the gem to 0.7.1.