Create DNS query with Netcat or /dev/udp/

You have used tcpdump. But you should then have used wireshark the same way you did later: you would have seen that you made the same mistake as in the link you provided: you included the extra IPv4+UDP headers: you must only provide the UDP payload to the netcat command, since it sends an IPv4 UDP packet already.

Here's the data in hex without any superfluous addresses:

IPv4 header:

4500 003c b0b4 4000 4011 73c3 0a00 020f 0a2a 0001

UDP header (the 0028 is the length of the whole UDP part: 8+20 => payload of length 0x20 = 32 bytes):

bdd4 0035 0028 4f24

UDP payload:

cfc9 0100 0001 0000 0000 0000 0a64 7563 6b64 7563 6b67 6f03 636f 6d00 0001 0001

xxd can use the -p parameter and not write but more importantly not need any address with -r (which avoids having to rewrite and now renumber those addresses):

-p | -ps | -postscript | -plain
Output in postscript continuous hexdump style. Also known as plain hexdump style.

When reversed (-r) it will also ignore any white spaces wherever they are.

Here's a test done (I just put back your own DNS server IP instead of the test one which worked fine):

$ echo cfc9 0100 0001 0000 0000 0000 0a64 7563 6b64 7563 6b67 6f03 636f 6d00 0001 0001 |
    xxd -p -r | nc -q1 -w2 -n -u 10.42.0.1 53 | xxd
00000000: cfc9 8180 0001 0001 0004 0000 0a64 7563  .............duc
00000010: 6b64 7563 6b67 6f03 636f 6d00 0001 0001  kduckgo.com.....
00000020: c00c 0001 0001 0000 005e 0004 2872 b19c  .........^..(r..
00000030: c00c 0002 0001 0002 91bb 0014 0464 6e73  .............dns
00000040: 3303 7030 3505 6e73 6f6e 6503 6e65 7400  3.p05.nsone.net.
00000050: c00c 0002 0001 0002 91bb 0007 0464 6e73  .............dns
00000060: 31c0 41c0 0c00 0200 0100 0291 bb00 0704  1.A.............
00000070: 646e 7332 c041 c00c 0002 0001 0002 91bb  dns2.A..........
00000080: 0007 0464 6e73 34c0 41                   ...dns4.A

Replacing netcat with socat: replace nc -q1 -w2 -n -u 10.42.0.1 53 with socat udp4:10.42.0.1:53 -. It deals better with EOF conditions (so no need to wait two seconds).

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If you want to use bash's built-in /dev/udp instead of netcat or socat then it becomes a bit more complicated to handle a full-duplex socket with two half-duplex commands (one only writes, the other only reads). For this non-interactive case, no need to fork the first output command (else, for more complex cases, the ; before dd can be replaced with & to have output and input working in parallel). It still requires dd in order to grab only one packet and finish, thus avoiding xxd blocking:

( echo cfc9 0100 0001 0000 0000 0000 0a64 7563 6b64 7563 6b67 6f03 636f 6d00 0001 0001 | xxd -p -r >&3; dd bs=9000 count=1 <&3 2>/dev/null | xxd ) 3>/dev/udp/10.42.0.1/53