Using TCP for DNS Queries

$ echo exit |telnet 192.168.1.5 53
Trying 192.168.1.5...

Connected to 192.168.1.5.

Escape character is '^]'.

Connection closed by foreign host.

Sweet, it looks like I can use the DNS server! Let’s make a query to be sure…

dig www.google.com @192.168.1.5




; <<>> DiG 9.11.36-RedHat-9.11.36-8.el8_8.1 <<>> www.google.com @192.168.1.5
;; global options: +cmd
;; connection timed out; no servers could be reached

Hmm, looks fishy, especially since I just used telnet to connect to port 53. Google-Fu OK, after some reading and thinking, telnet used TCP to connect to the port – DNS uses UDP. I bet that’s it. Let’s see if we can dig using TCP.

$ dig www.google.com @192.168.1.5 +tcp

; <<>> DiG 9.10.6 <<>> www.google.com @192.168.1.5 +tcp
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58292
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;www.google.com. IN A

;; ANSWER SECTION:
www.google.com. 232 IN A 64.233.177.105
www.google.com. 232 IN A 64.233.177.106
www.google.com. 232 IN A 64.233.177.99
www.google.com. 232 IN A 64.233.177.104
www.google.com. 232 IN A 64.233.177.147
www.google.com. 232 IN A 64.233.177.103

;; Query time: 19 msec
;; SERVER: 192.168.1.5#53(192.168.1.5)
;; WHEN: Thu Aug 03 09:07:05 EDT 2023
;; MSG SIZE  rcvd: 139

Well, look at that, we can use TCP for DNS. Now, how do I get NetworkManager to actually use TCP for DNS? Google-Fu Ah-ha! You can use nmcli to configure ipv4.dns-options and set “use-vc”. Let’s give it a whirl…

# cat /etc/resolv.conf
# Generated by NetworkManager
search internal.net
nameserver 192.168.1.5
# nmcli con mod ens192 ipv4.dns-options "use-vc"

# systemctl restart NetworkManager
# cat /etc/resolv.conf
# Generated by NetworkManager
search internal.net
nameserver 192.168.1.5
options use-vc

Alright, that did something. Now, let’s see if we can dig without using the “+tcp” option…hmm still times out. Does the host command work?

$ host www.google.com
www.google.com has address 64.233.177.99
www.google.com has address 64.233.177.103
www.google.com has address 64.233.177.104
www.google.com has address 64.233.177.105
www.google.com has address 64.233.177.106
www.google.com has address 64.233.177.147
www.google.com has IPv6 address 2607:f8b0:4002:c09::67
www.google.com has IPv6 address 2607:f8b0:4002:c09::69
www.google.com has IPv6 address 2607:f8b0:4002:c09::6a
www.google.com has IPv6 address 2607:f8b0:4002:c09::93

Also, this is my first post with actual html code in it, via email. I wonder if it will get formatted properly. Markdown would be much better – maybe I’ll look into that if this one works.