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286 lines
10 KiB
Markdown
pinewall is translating
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Anatomy of a Linux DNS Lookup – Part I
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============================================================
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Since I [work][3] [a][4] [lot][5] [with][6] [clustered][7] [VMs][8], I’ve ended up spending a lot of time trying to figure out how [DNS lookups][9] work. I applied ‘fixes’ to my problems from StackOverflow without really understanding why they work (or don’t work) for some time.
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Eventually I got fed up with this and decided to figure out how it all hangs together. I couldn’t find a complete guide for this anywhere online, and talking to colleagues they didn’t know of any (or really what happens in detail)
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So I’m writing the guide myself.
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_If you’re looking for Part II, click [here][1]_
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Turns out there’s quite a bit in the phrase ‘Linux does a DNS lookup’…
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* * *
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_“How hard can it be?”_
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* * *
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These posts are intended to break down how a program decides how it gets an IP address on a Linux host, and the components that can get involved. Without understanding how these pieces fit together, debugging and fixing problems with (for example) `dnsmasq`, `vagrant landrush`, or `resolvconf` can be utterly bewildering.
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It’s also a valuable illustration of how something so simple can get so very complex over time. I’ve looked at over a dozen different technologies and their archaeologies so far while trying to grok what’s going on.
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I even wrote some [automation code][10] to allow me to experiment in a VM. Contributions/corrections are welcome.
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Note that this is not a post on ‘how DNS works’. This is about everything up to the call to the actual DNS server that’s configured on a linux host (assuming it even calls a DNS server – as you’ll see, it need not), and how it might find out which one to go to, or how it gets the IP some other way.
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* * *
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### 1) There is no such thing as a ‘DNS Lookup’ call
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* * *
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_This is NOT how it works_
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* * *
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The first thing to grasp is that there is no single method of getting a DNS lookup done on Linux. It’s not a core system call with a clean interface.
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There is, however, a standard C library call called which many programs use: `[getaddrinfo][2]`. But not all applications use this!
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Let’s just take two simple standard programs: `ping` and `host`:
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```
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root@linuxdns1:~# ping -c1 bbc.co.uk | head -1
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PING bbc.co.uk (151.101.192.81) 56(84) bytes of data.
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```
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```
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root@linuxdns1:~# host bbc.co.uk | head -1
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bbc.co.uk has address 151.101.192.81
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```
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They both get the same result, so they must be doing the same thing, right?
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Wrong.
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Here’s the files that `ping` looks at on my host that are relevant to DNS:
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```
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root@linuxdns1:~# strace -e trace=open -f ping -c1 google.com
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open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
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open("/lib/x86_64-linux-gnu/libcap.so.2", O_RDONLY|O_CLOEXEC) = 3
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open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
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open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 4
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open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 4
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open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 4
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open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
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open("/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 4
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open("/etc/host.conf", O_RDONLY|O_CLOEXEC) = 4
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open("/etc/hosts", O_RDONLY|O_CLOEXEC) = 4
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open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
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open("/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 4
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open("/lib/x86_64-linux-gnu/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 4
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PING google.com (216.58.204.46) 56(84) bytes of data.
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open("/etc/hosts", O_RDONLY|O_CLOEXEC) = 4
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64 bytes from lhr25s12-in-f14.1e100.net (216.58.204.46): icmp_seq=1 ttl=63 time=13.0 ms
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[...]
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```
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and the same for `host`:
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```
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$ strace -e trace=open -f host google.com
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[...]
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[pid 9869] open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/libdst.cat", O_RDONLY) = -1 ENOENT (No such file or directory)
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[pid 9869] open("/usr/share/locale/en/libdst.cat", O_RDONLY) = -1 ENOENT (No such file or directory)
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[pid 9869] open("/usr/share/locale/en/LC_MESSAGES/libdst.cat", O_RDONLY) = -1 ENOENT (No such file or directory)
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[pid 9869] open("/usr/lib/ssl/openssl.cnf", O_RDONLY) = 6
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[pid 9869] open("/usr/lib/x86_64-linux-gnu/openssl-1.0.0/engines/libgost.so", O_RDONLY|O_CLOEXEC) = 6[pid 9869] open("/etc/resolv.conf", O_RDONLY) = 6
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google.com has address 216.58.204.46
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[...]
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```
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You can see that while my `ping` looks at `nsswitch.conf`, `host` does not. And they both look at `/etc/resolv.conf`.
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We’re going to take these two `.conf` files in turn.
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* * *
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### 2) NSSwitch, and `/etc/nsswitch.conf`
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We’ve established that applications can do what they like when they decide which DNS server to go to. Many apps (like `ping`) above can refer (depending on the implementation (*)) to NSSwitch via its config file `/etc/nsswitch.conf`.
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###### (*) There’s a surprising degree of variation in
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ping implementations. That’s a rabbit-hole I
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_didn’t_ want to get lost in.
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NSSwitch is not just for DNS lookups. It’s also used for passwords and user lookup information (for example).
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NSSwitch was originally created as part of the Solaris OS to allow applications to not have to hard-code which file or service they look these things up on, but defer them to this other configurable centralised place they didn’t have to worry about.
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Here’s my `nsswitch.conf`:
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```
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passwd: compat
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group: compat
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shadow: compat
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gshadow: files
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hosts: files dns myhostname
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networks: files
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protocols: db files
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services: db files
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ethers: db files
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rpc: db files
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netgroup: nis
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```
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The ‘hosts’ line is the one we’re interested in. We’ve shown that `ping` cares about `nsswitch.conf` so let’s fiddle with it and see how we can mess with `ping`.
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* ### Set `nsswitch.conf` to only look at ‘files’
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If you set the `hosts` line in `nsswitch.conf` to be ‘just’ `files`:
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`hosts: files`
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Then a `ping` to google.com will now fail:
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```
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$ ping -c1 google.com
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ping: unknown host google.com
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```
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but `localhost` still works:
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```
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$ ping -c1 localhost
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PING localhost (127.0.0.1) 56(84) bytes of data.
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64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.039 ms
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```
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and using `host` still works fine:
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```
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$ host google.com
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google.com has address 216.58.206.110
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```
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since, as we saw, it doesn’t care about `nsswitch.conf`
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* ### Set `nsswitch.conf` to only look at ‘dns’
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If you set the `hosts` line in `nsswitch.conf` to be ‘just’ dns:
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`hosts: dns`
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Then a `ping` to google.com will now succeed again:
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```
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$ ping -c1 google.com
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PING google.com (216.58.198.174) 56(84) bytes of data.
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64 bytes from lhr25s10-in-f174.1e100.net (216.58.198.174): icmp_seq=1 ttl=63 time=8.01 ms
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```
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But `localhost` is not found this time:
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```
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$ ping -c1 localhost
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ping: unknown host localhost
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```
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Here’s a diagram of what’s going on with NSSwitch by default wrt `hosts` lookup:
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* * *
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_My default ‘`hosts:`‘ configuration in `nsswitch.conf`_
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* * *
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### 3) `/etc/resolv.conf`
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We’ve seen now that `host` and `ping` both look at this `/etc/resolv.conf` file.
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Here’s what my `/etc/resolv.conf` looks like:
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```
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# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
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# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
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nameserver 10.0.2.3
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```
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Ignore the first two lines – we’ll come back to those (they are significant, but you’re not ready for that ball of wool yet).
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The `nameserver` lines specify the DNS servers to look up the host for.
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If you hash out that line:
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```
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#nameserver 10.0.2.3
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```
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and run:
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```
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$ ping -c1 google.com
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ping: unknown host google.com
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```
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it fails, because there’s no nameserver to go to (*).
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###### * Another rabbit hole: `host` appears to fall back to
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127.0.0.1:53 if there’s no nameserver specified.
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This file takes other options too. For example, if you add this line to the `resolv.conf` file:
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```
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search com
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```
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and then `ping google` (sic)
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```
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$ ping google
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PING google.com (216.58.204.14) 56(84) bytes of data.
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```
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it will try the `.com` domain automatically for you.
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### End of Part I
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That’s the end of Part I. The next part will start by looking at how that resolv.conf gets created and updated.
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Here’s what you covered above:
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* There’s no ‘DNS lookup’ call in the OS
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* Different programs figure out the IP of an address in different ways
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* For example, `ping` uses `nsswitch`, which in turn uses (or can use) `/etc/hosts`, `/etc/resolv.conf` and its own hostname to get the result
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* `/etc/resolv.conf` helps decide:
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* which addresses get called
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* which DNS server to look up
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If you thought that was complicated, buckle up…
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--------------------------------------------------------------------------------
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via: https://zwischenzugs.com/2018/06/08/anatomy-of-a-linux-dns-lookup-part-i/
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作者：[dmatech][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://twitter.com/dmatech2
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[1]:https://zwischenzugs.com/2018/06/18/anatomy-of-a-linux-dns-lookup-part-ii/
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[2]:http://man7.org/linux/man-pages/man3/getaddrinfo.3.html
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[3]:https://zwischenzugs.com/2017/10/31/a-complete-chef-infrastructure-on-your-laptop/
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[4]:https://zwischenzugs.com/2017/03/04/a-complete-openshift-cluster-on-vagrant-step-by-step/
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[5]:https://zwischenzugs.com/2017/03/04/migrating-an-openshift-etcd-cluster/
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[6]:https://zwischenzugs.com/2017/03/04/1-minute-multi-node-vm-setup/
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[7]:https://zwischenzugs.com/2017/03/18/clustered-vm-testing-how-to/
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[8]:https://zwischenzugs.com/2017/10/27/ten-things-i-wish-id-known-before-using-vagrant/
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[9]:https://zwischenzugs.com/2017/10/21/openshift-3-6-dns-in-pictures/
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[10]:https://github.com/ianmiell/shutit-linux-dns/blob/master/linux_dns.py
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