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[Sam Varshavchik]

Making the rounds of various technical mailing lists yesterday, with a  
subject that's typically a variation of "Just for yucks, and giggles" is a  
link to a commit to systemd's git, adding DNS caching to systemd; in one,  
huge 857 line glop. Here's its entire commit message: "resolved: add DNS  
cache".

And, it's beauty to read. Made me teary-eyed to know that systemd will cache  
my DNS queries. Not sure why systemd, all of a sudden, needs to make DNS  
queries. But if it does, it's going to cache them now! Such a sight to  
behold makes my heart skip a beat: now, not just bind, or pretty much every  
DNS server that automatically caches DNS for you – but so will systemd!

But, seriously folks, systemd's DNS caching achieves absolutely 100%  
nothing. Really. Surprised? Well, you'll be shocked to know that the DNS  
server that systemd queries for that DNS RR, such as the stock "bind",  
already automatically caches all recursive DNS replies!! systemd's own  
caching produces absolutely zero useful results. On the oft chance that it  
sends a query for a non-cached RR, the local DNS server will cache the  
response, before returning it to systemd, and then use the cached response  
for all future queries. That's what DNS servers do: provide caching for  
local clients. It's inherent in DNS's design: DNS was explicitly designed to  
use aggressive caching, it's an internet-wide, distributed, locally cached  
database.

Isn't modern technology amazing?

I'm willing to consider the possibility that I missed something obvious,  
some obvious value-added result from caching DNS RRs directly by systemd,  
and I'll stick around for someone to enlighten me; or if Occam's razor  
applies, and the author of that commit had no idea that bind already  
automatically caches DNS responses, and, more importantly, that its caching  
algorithms are a result of painful lessons learned from various DNS cache  
poisoning attacks, that have circulated around the intertubes, for the last  
couple of years.

The only possible use case for this kind of caching approach would be if:

A) You do not have a local DNS server nearby; and you have non-negligible  
latencies to whatever DNS server you use.

B) Your queries tend to be for domains that your DNS servers are not  
authoritative for, so they'll benefit from local caching.

So, can someone explain to me how likely this is going to be the case in a  
typical deployment scenario that systemd is targeted for; in an average  
corporate environment where systemd's alleged benefits will supposedly shine?

I would guess that a typical systemd deployment, in a corporate/business  
environment, will certainly have multiple, low-latency DNS servers nearby,  
won't that be the case? And, if so, then this is just another potentially  
exploitable security hole in systemd, nothing more.

P.S. After I wrote the above, poking around, Google dumped this onto my  
screen:

http://seclists.org/oss-sec/2014/q4/592

Mental note to myself: go back and check the timestamp of the systemd git  
commit – before, or after, this was disclosed…

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