Date: Fri, 8 Jan 2021 10:45:41 -0500 From: Mark Johnston <markj@freebsd.org> To: Vasily Postnicov <shamaz.mazum@gmail.com> Cc: freebsd-net@freebsd.org Subject: Re: DNS using Name Service Switch module and Casper Message-ID: <X/h%2BJRmXmrOfmXBM@raichu> In-Reply-To: <CADnZ6BmUJxVZx155j8opJKNsHJBE5mWz9D=MBE0Y_xu-kgOBfQ@mail.gmail.com> References: <CADnZ6Bke=9%2B_pMc6rkbheNUWS-H6_X14%2Bf%2BWz5cfUCD=BTwk=g@mail.gmail.com> <X/R7Ahz8sz5v%2BoFa@raichu> <CADnZ6BmUJxVZx155j8opJKNsHJBE5mWz9D=MBE0Y_xu-kgOBfQ@mail.gmail.com>
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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@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? > >
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