DNS using Name Service Switch module and Casper

[Mark Johnston]

On Wed, Jan 06, 2021 at 07:08:14PM +0300, Vasily Postnicov wrote:
> That's what I found.
> 
> At first, ping calls cap_init() in capdns_setup(). cap_init() forks a
> process, then the parent returns and the child calls casper_main_loop().
> The child and the parent both have a socket to communicate.
> casper_main_loop() calls zygote_init() and that one blocks on fork(). I do
> not know how it could be. How can fork() block?

Does you module somehow use pthread_atfork()?

> The parent process later calls cap_service_open() and that function calls
> cap_xfer_nvlist(). Because the child process is stuck somewhere in
> zygote_init() it never sends an nvlist back. So ping blocks.

Can you show output from "procstat -kk <pid>" when this hang occurs?

> All this is figured out by inserting printf()'s. LLDB refuses to run ping
> with 'error: Child exec failed'.

Presumably it needs to be run as root since ping(8) is a setuid
executable.

> вт, 5 янв. 2021 г. в 17:43, Mark Johnston <markj at freebsd.org>:
> 
> > On Tue, Jan 05, 2021 at 10:02:37AM +0300, Vasily Postnicov wrote:
> > > Hello. I wrote a simple daemon called ZeroDNS which provides
> > functionality
> > > similar to multicast DNS, namely it discovers other participating
> > machines
> > > over the LAN and stores their hostname and IPv4 address pairs.
> > >
> > > Here is a NSS module which allows the system to use information from that
> > > daemon:
> > > https://github.com/shamazmazum/nss-zero-dns
> > >
> > > You need to modify /etc/nsswitch.conf, changing the line 'hosts: files
> > dns'
> > > to 'hosts: files dns zerodns'.
> > >
> > > It all works on FreeBSD 12.2-RELEASE, but sometimes not on 13.0-CURRENT.
> > > For example, ping(8) just blocks when trying to ping a host whose name is
> > > resolvable with ZeroDNS. Turns out that programs built with casper
> > support
> > > (like ping(8) and some others) stop working with my NSS module (they just
> > > block trying to resolve the name).
> >
> > Presumably it's the casper process (i.e., cap_dns) that uses your
> > module?  If the main ping process is blocked trying to resolve a name,
> > it's waiting for the cap_dns process - where exactly is it getting
> > stuck?
> >
> > > Is there some kind of manual on how to write casper-compatible NSS
> > modules?
> >