zlacker

It's bit curious that traditionally UNIX systems did not run local DNS resolver daemons and instead the resolv.conf (and nsswitch.conf) persisted for so long. In addition to potentially simplifying configuration, having a daemon would allow system-wide dns caching, something I'd imagine would have been especially valuable back in the days of slow networks. Unix has daemons for everything else so that's why it feels odd that name resolution got baked into libc

>>zokier+(OP)
Some of the early unix systems did have a local daemon. The much-maligned SunOS NIS/YP services as one example.

>>zokier+(OP)
maybe for most cases nscd was enough. not exactly a dns-cache but a hostname cache on one layer up.

>>zokier+(OP)
> traditionally

because in the tradition there was a (forwarding) dns server somewhere in the local network to do caching for everybody.

nowadays most decent linux distributions have a very good caching dns resolver (systemd-resolved) so that's not an issue anymore.

>>zokier+(OP)
Unix predates DNS; the nsswitch.conf tells the c libraries how to convert names to IP addresses. This behavior is actually dependent on which libc you're using...

To resolve names, you can ask /etc/hosts for the name / IP conversion; you can also ask DNS, or ldap or NIS; probably there are many I've forgotten about.

solaris: https://docs.oracle.com/cd/E19683-01/806-4077/6jd6blbbe/inde...

glibc: https://man7.org/linux/man-pages/man5/nsswitch.conf.5.html

musl appears to not have an nsswitch.conf or a way to configure name to number resolution behavior?

>>zokier+(OP)
The model in which the DNS was developed (back in the mid-80s) was CPU/memory was a more expensive resource than sending/receiving a small datagram to a centralized location within a campus over a local LAN (external connectivity off campus was also considered expensive but necessary).

The fact that this model is still largely assumed is due to inertia.

>>zokier+(OP)
> In addition to potentially simplifying configuration, having a daemon would allow system-wide dns caching, something I'd imagine would have been especially valuable back in the days of slow networks. Unix has daemons for everything else so that's why it feels odd that name resolution got baked into libc

Having a daemon would add complexity, take up RAM and CPU, and be unnecessary in general. There really weren't that many daemons running in the olden times.

DNS resolution is expected to be fast, since it's (supposed to be) UDP-based. It's also expected that there is a caching DNS resolver somewhere near the client machines to reduce latency and spread load (in the old days the ISP would provide them, then later as "home routers" became a thing, the "home router" provided them too).

Finally, as networks were fairly slow, you just didn't do a ton of network connections, so you shouldn't be doing a ton of DNS lookups. But even if you did, the DNS lookup was way faster than your TCP connection, and the application could cache results easily (I believe Windows did cache them in its local resolver, and nscd did on Linux/Unix)

If you really did need DNS caching (or anything else DNS related), you would just run Bind and configure it to your needs. Configuring Bind was one of the black arts of UNIX so that was avoided whenever possible :)

>>zokier+(OP)
I don't see why system wide dns caching would be of use?

How many different programs in the same process space hit so many common external services individual caching of names is not sufficient?

Article lists a bunch of fun with systemd running junk in containers that seem counterproductive to me. A lot of systemd stuff seems to be stuff useful on a laptop that ends up where it's really not wanted.

Local dns caching seems like a solution looking for a problem to me. I disable it whereever I can. I have local(ish) dns caches on the network. But not inside lxc containers or Linux hosts.

>>bandie+z7
I used to work for a huge email sender (constant contact). Our mail servers needed to perform an absurd number of lookups while receiving and validating mail. When I was there, we used dnscache, running locally, on all our mail servers. But even with a local dnscache, the overhead of making DNS requests and handling their responses was high enough that adding nscd made a noticeable improvement in CPU usage.

>>sleepy+1u
i guess this shows that looking up getent hostname database cache is faster than looking up local dns cache because the former is simpler in data structure?

>>bandie+iv
I didn't dig into it too deeply at the time, but I think part of it was that you don't need to open and write to a socket, so that's avoiding some system calls (socket(), bind(), sendto(), close()). IIRC we had nscd set up so clients directly read from shared memory that nscd kept updated, rather than getting requests over a socket.

There's also probably some savings around not having to convert between the structures used by gethostbyname and DNS questions&answers.