How to debug network issues in Unix — “dig”

Photo by Sai Kiran Anagani on Unsplash

In my day-by-day job, I started to use lots of BASH commands to debug network issues, and I did never not many of them. This is why I decided to write here all those commands and how do I use usually.

This will be a series. Here the table of contents:

1. dig (this first story)
2. ping
3. whois
4. openssl
5. nslookup
6. traceroute and mtr
7. iptables
8. network configuration and statistics
9. tcpdump

The first one I used frequently is dig. As the man page stated,

dig (domain information groper) is a flexible tool for interrogating DNS name servers.

It performs DNS lookups and displays the answers that are returned from the name server(s) that were queried.

Most DNS administrators use dig to troubleshoot DNS problems because of its flexibility, ease of use and clarity of output.

Basically, you would use dig to check if there is some issue on resolving DNS records for the URL you are calling.

This is an example of dig usage.

$ dig www.nodered.org
; <<>> DiG 9.10.6 <<>> www.nodered.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16648
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.nodered.org.       IN  A
;; ANSWER SECTION:
www.nodered.org.    300 IN  A   104.27.163.46
www.nodered.org.    300 IN  A   104.27.162.46
;; Query time: 81 msec
;; SERVER: 9.0.138.50#53(9.0.138.50)
;; WHEN: Fri Aug 09 17:34:44 CEST 2019
;; MSG SIZE  rcvd: 76

You could also query a specific DNS server using dig @sever URL syntax, or see all the calls performed using dig +trace URL syntax.

$ dig +trace www.nodered.org
; <<>> DiG 9.10.6 <<>> +trace www.nodered.org
;; global options: +cmd
.           483576  IN  NS  d.root-servers.net.
.           483576  IN  NS  j.root-servers.net.
.           483576  IN  NS  i.root-servers.net.
.           483576  IN  NS  l.root-servers.net.
.           483576  IN  NS  m.root-servers.net.
.           483576  IN  NS  c.root-servers.net.
.           483576  IN  NS  e.root-servers.net.
.           483576  IN  NS  k.root-servers.net.
.           483576  IN  NS  b.root-servers.net.
.           483576  IN  NS  f.root-servers.net.
.           483576  IN  NS  g.root-servers.net.
.           483576  IN  NS  h.root-servers.net.
.           483576  IN  NS  a.root-servers.net.
;; Received 239 bytes from 9.0.138.50#53(9.0.138.50) in 44 ms
org.            172800  IN  NS  d0.org.afilias-nst.org.
org.            172800  IN  NS  a0.org.afilias-nst.info.
org.            172800  IN  NS  c0.org.afilias-nst.info.
org.            172800  IN  NS  a2.org.afilias-nst.info.
org.            172800  IN  NS  b0.org.afilias-nst.org.
org.            172800  IN  NS  b2.org.afilias-nst.org.
org.            86400   IN  DS  9795 7 2 3922B31B6F3A4EA92B19EB7B52120F031FD8E05FF0B03BAFCF9F891B FE7FF8E5
org.            86400   IN  DS  9795 7 1 364DFAB3DAF254CAB477B5675B10766DDAA24982
org.            86400   IN  RRSIG   DS 8 1 86400 20190825050000 20190812040000 59944 . je1SWNWwQ1qR6emHZzzRj7t8LAmlxQEEIvHMNd6+O5gNt91Y7pH2X5MG EbPW2mNqDc6FBrGIhl+s6JIVTTtUyzmN9CSun15Cx8+SDHU4mtXKo3WR PXw/8+kAZ954+YzKfJ5EdHH2RBJCJtggK1eRVtLiIJTX+NKquD1/H8v/ 1ucsDI/z7qvAccnk3zK1yYTsZLuVQ0LDCL/ZrMzBmrj+fxLAYzSRGAe5 CjWzaIBUxzpzEtdpOhAsc4uHEonHY5iMb00Vro7dRTEqVW1mgPUw+aVI f29jsZneCo+7+yzI0aNDKKI+Yyi8nENxDMkogNW9BiN6VNcCbZszDX4W c30Aag==
;; Received 817 bytes from 2001:503:ba3e::2:30#53(a.root-servers.net) in 166 ms
nodered.org.        86400   IN  NS  matt.ns.cloudflare.com.
nodered.org.        86400   IN  NS  rafe.ns.cloudflare.com.
h9p7u7tr2u91d0v0ljs9l1gidnp90u3h.org. 86400 IN NSEC3 1 1 1 D399EAAB H9PARR669T6U8O1GSG9E1LMITK4DEM0T  NS SOA RRSIG DNSKEY NSEC3PARAM
h9p7u7tr2u91d0v0ljs9l1gidnp90u3h.org. 86400 IN RRSIG NSEC3 7 2 86400 20190902154211 20190812144211 44078 org. ggDUmLgHGFcctRfWnh/oIYXq/XcoXh753avQl2nFFJb10WePtNDlSQBz Ca7rIt+JDbKCco2EwDe8hLnSVMvUBfBigHcfudxzWXfMvdd3cpIONQoP mgYNq2tClQ/0yKahU+tpmtSzo+OakAstgZx9akqWJ4jRDrCGfxlN4Zya /2A=
l5kla3ec7t89nhvdgs2ap0gv4mhcp7tg.org. 86400 IN NSEC3 1 1 1 D399EAAB L5L0QRECKPN957E40C0KDPESHVU3QU07  NS DS RRSIG
l5kla3ec7t89nhvdgs2ap0gv4mhcp7tg.org. 86400 IN RRSIG NSEC3 7 2 86400 20190830152905 20190809142905 44078 org. egEr+azrmYo20+kszyiDC01cgq77JdDi8/5YKnKbjLjJzHtwSaSdM28g /+/DD12LGILgYu9X4Ufzk7v1X2MM41/WjUbwMKpOZRLvtg5GaVPNUoD9 m/Z+H23xwT6zy/9DCE3xCsIS3zrB2PXcD531+O1JsujNiLKbGuHq0bxq Ouk=
;; Received 592 bytes from 199.249.120.1#53(b2.org.afilias-nst.org) in 32 ms
www.nodered.org.    300 IN  A   104.27.163.46
www.nodered.org.    300 IN  A   104.27.162.46
;; Received 76 bytes from 173.245.59.131#53(matt.ns.cloudflare.com) in 32 ms

Originally published at https://gabriele-decapoa.github.io.