diff --git a/sources/tech/20211204 How to use dig.md b/sources/tech/20211204 How to use dig.md new file mode 100644 index 0000000000..21744938c5 --- /dev/null +++ b/sources/tech/20211204 How to use dig.md @@ -0,0 +1,279 @@ +[#]: subject: "How to use dig" +[#]: via: "https://jvns.ca/blog/2021/12/04/how-to-use-dig/" +[#]: author: "Julia Evans https://jvns.ca/" +[#]: collector: "lujun9972" +[#]: translator: " " +[#]: reviewer: " " +[#]: publisher: " " +[#]: url: " " + +How to use dig +====== + +Hello! I talked to a couple of friends recently who mentioned they wished they knew how to use `dig` to make DNS queries, so here’s a quick blog post about it. + +When I first started using `dig` I found it a bit intimidating – there are so many options! I’m going to leave out most of dig’s options in this post and just talk about the ones I actually use. + +Also I learned recently that you can set up a `.digrc` configuration file to make its output easier to read and it makes it SO MUCH nicer to use. + +I also drew a [zine page about dig][1] a few years ago, but I wanted to write this post to include a bit more information. + +### 2 types of dig arguments: query and formatting + +There are 2 main types of arguments you can pass to `dig`: + + 1. arguments that tell dig **what DNS query to make** + 2. arguments that tell dig how to **format the response** + + + +First, let’s go through the query options. + +### the main query options + +The 3 things you usually want to control about a DNS query are: + + 1. the **name** (like `jvns.ca`). The default is a query for the empty name (`.`). + 2. the **DNS query type** (like `A` or `CNAME`). The default is `A`. + 3. the **server** to send the query to (like `8.8.8.8`). The default is what’s in `/etc/resolv.conf`. + + + +The format for these is: + +``` + + dig @server name type + +``` + +Here are a couple of examples: + + * `dig @8.8.8.8 jvns.ca` queries Google’s public DNS server (`8.8.8.8`) for `jvns.ca`. + * `dig ns jvns.ca` makes an query with type `NS` for `jvns.ca` + + + +### `-x`: make a reverse DNS query + +One other query option I use occasionally is `-x`, to make a reverse DNS query. Here’s what the output looks like. + +``` + + $ dig -x 172.217.13.174 + 174.13.217.172.in-addr.arpa. 72888 IN PTR yul03s04-in-f14.1e100.net. + +``` + +`-x` isn’t magic – `dig -x 172.217.13.174` just makes a `PTR` query for `174.13.217.172.in-addr.arpa.`. Here’s how to make exact the same reverse DNS query without using `-x`. + +``` + + $ dig ptr 174.13.217.172.in-addr.arpa. + 174.13.217.172.in-addr.arpa. 72888 IN PTR yul03s04-in-f14.1e100.net. + +``` + +I always use `-x` though because it’s less typing. + +### options for formatting the response + +Now, let’s talk about arguments you can use to format the response. + +I’ve found that the way `dig` formats DNS responses by default is pretty overwhelming to beginners. Here’s what the output looks like: + +``` + + ; <<>> DiG 9.16.20 <<>> -r jvns.ca + ;; global options: +cmd + ;; Got answer: + ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28629 + ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1 + + ;; OPT PSEUDOSECTION: + ; EDNS: version: 0, flags:; udp: 4096 + ; COOKIE: d87fc3022c0604d60100000061ab74857110b908b274494d (good) + ;; QUESTION SECTION: + ;jvns.ca. IN A + + ;; ANSWER SECTION: + jvns.ca. 276 IN A 172.64.80.1 + + ;; Query time: 9 msec + ;; SERVER: 192.168.1.1#53(192.168.1.1) + ;; WHEN: Sat Dec 04 09:00:37 EST 2021 + ;; MSG SIZE rcvd: 80 + +``` + +If you’re not used to reading this, it might take you a while to sift through it and find the IP address you’re looking for. And most of the time, you’re only interested in one line of this response (`jvns.ca. 180 IN A 172.64.80.1`). + +Here are my 2 favourite ways to make dig’s output more manageable. + +**way 1: +noall +answer** + +This tells dig to just print what’s in the “Answer” section of the DNS response. Here’s an example of querying for the `NS` records for `google.com`. + +``` + + $ dig +noall +answer ns google.com + google.com. 158564 IN NS ns4.google.com. + google.com. 158564 IN NS ns1.google.com. + google.com. 158564 IN NS ns2.google.com. + google.com. 158564 IN NS ns3.google.com. + +``` + +The format here is: + +``` + + NAME TTL TYPE CONTENT + google.com 158564 IN NS ns3.google.com. + +``` + +By the way: if you’ve ever wondered what `IN` means, it’s the “query class” and stands for “internet”. It’s basically just a relic from the 80s and 90s when there were other networks competing with the internet like “chaosnet”. + +**way 2: +short** + +This is like `dig +noall +answer`, but even shorter – it just shows the content of each record. For example: + +``` + + $ dig +short ns google.com + ns2.google.com. + ns1.google.com. + ns4.google.com. + ns3.google.com. + +``` + +### you can put formatting options in `digrc` + +If you don’t like dig’s default format (I don’t!), you can tell it to use a different format by default by creating a `.digrc` file in your home directory. + +I really like the `+noall +answer` format, so I put `+noall +answer` in my `~/.digrc`. Here’s what it looks like for me when I run `dig jvns.ca` using that configuration file. + +``` + + $ dig jvns.ca + jvns.ca. 255 IN A 172.64.80.1 + +``` + +So much easier to read! + +And if I want to go back to the long format with all of the output (which I do sometimes, usually because I want to look at the records in the Authority section of the response), I can get a long answer again by running: + +``` + + $ dig +all jvns.ca + +``` + +### dig +trace + +The last dig option that I use is `+trace`. `dig +trace` mimics what a DNS resolver does when it looks up a domain – it starts at the root nameservers, and then queries the next level of nameservers (like `.com`), and so on until it reaches the authoritative nameserver for the domain. So it’ll make about 30 DNS queries. (I checked using tcpdump, it seems to make 2 queries to get A/AAAA records for each of the root nameservers so that’s already 26 queries. I’m not really sure _why_ it does this because it should already have those IPs hardcoded, but it does.) + +I find this mostly useful for understanding how DNS works though, I don’t think that I’ve used it to solve a problem. + +### why dig? + +Even though there are simpler tools to make DNS queries (like `dog` and `host`), I find myself sticking with dig. + +What I like about dig is actually the same thing I **don’t** like about dig – it shows a lot of detail! + +I know that if I run `dig +all`, it’ll show me all of the sections of the DNS response. For example, let’s query one of the root nameservers for `jvns.ca`. The response has 3 sections I might care about – Answer, Authority, and Additional. + +``` + + $ dig @h.root-servers.net. jvns.ca +all + ;; Got answer: + ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18229 + ;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 4, ADDITIONAL: 9 + ;; WARNING: recursion requested but not available + + ;; OPT PSEUDOSECTION: + ; EDNS: version: 0, flags:; udp: 1232 + ;; QUESTION SECTION: + ;jvns.ca. IN A + + ;; AUTHORITY SECTION: + ca. 172800 IN NS c.ca-servers.ca. + ca. 172800 IN NS j.ca-servers.ca. + ca. 172800 IN NS x.ca-servers.ca. + ca. 172800 IN NS any.ca-servers.ca. + + ;; ADDITIONAL SECTION: + c.ca-servers.ca. 172800 IN A 185.159.196.2 + j.ca-servers.ca. 172800 IN A 198.182.167.1 + x.ca-servers.ca. 172800 IN A 199.253.250.68 + any.ca-servers.ca. 172800 IN A 199.4.144.2 + c.ca-servers.ca. 172800 IN AAAA 2620:10a:8053::2 + j.ca-servers.ca. 172800 IN AAAA 2001:500:83::1 + x.ca-servers.ca. 172800 IN AAAA 2620:10a:80ba::68 + any.ca-servers.ca. 172800 IN AAAA 2001:500:a7::2 + + ;; Query time: 103 msec + ;; SERVER: 198.97.190.53#53(198.97.190.53) + ;; WHEN: Sat Dec 04 11:23:32 EST 2021 + ;; MSG SIZE rcvd: 289 + +``` + +`dog` also shows the records in the “additional” section , but it’s not super explicit about which is which (I guess the `+` means it’s in the additional section?). It doesn’t seem to show the records in the “Authority” section. + +``` + + $ dog @h.root-servers.net. jvns.ca + NS ca. 2d0h00m00s A "c.ca-servers.ca." + NS ca. 2d0h00m00s A "j.ca-servers.ca." + NS ca. 2d0h00m00s A "x.ca-servers.ca." + NS ca. 2d0h00m00s A "any.ca-servers.ca." + A c.ca-servers.ca. 2d0h00m00s + 185.159.196.2 + A j.ca-servers.ca. 2d0h00m00s + 198.182.167.1 + A x.ca-servers.ca. 2d0h00m00s + 199.253.250.68 + A any.ca-servers.ca. 2d0h00m00s + 199.4.144.2 + AAAA c.ca-servers.ca. 2d0h00m00s + 2620:10a:8053::2 + AAAA j.ca-servers.ca. 2d0h00m00s + 2001:500:83::1 + AAAA x.ca-servers.ca. 2d0h00m00s + 2620:10a:80ba::68 + AAAA any.ca-servers.ca. 2d0h00m00s + 2001:500:a7::2 + +``` + +And `host` seems to only show the records in the “answer” section (in this case no records) + +``` + + $ host jvns.ca h.root-servers.net + Using domain server: + Name: h.root-servers.net + Address: 198.97.190.53#53 + Aliases: + +``` + +Anyway, I think that these simpler DNS tools are great (I even made my own [simple web DNS tool][2]) and you should absolutely use them if you find them easier but that’s why I stick with dig. `drill`’s output format seems very similar to `dig`’s though, and maybe `drill` is better! I haven’t really tried it. + +### that’s all! + +I only learned about `.digrc` recently and I love using it so much, so I hope it helps some of you spend less time sorting though dig output! + +Someone on Twitter pointed out that it would be nice if there were a way to tell dig to show a short version of the response which also included the response’s status (like `NOERROR`, `NXDOMAIN`, `SERVFAIL`, etc), and I agree! I couldn’t find an option in the man page that does that though. + +-------------------------------------------------------------------------------- + +via: https://jvns.ca/blog/2021/12/04/how-to-use-dig/ + +作者：[Julia Evans][a] +选题：[lujun9972][b] +译者：[译者ID](https://github.com/译者ID) +校对：[校对者ID](https://github.com/校对者ID) + +本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出 + +[a]: https://jvns.ca/ +[b]: https://github.com/lujun9972 +[1]: https://wizardzines.com/comics/dig/ +[2]: https://dns-lookup.jvns.ca/