$ unshare -r -n ls
a.txt  b.txt

$ unshare -r -n curl unix.stackexchange.com
curl: (6) Could not resolve host: unix.stackexchange.com

$ /bin/sh -c 'echo 1 > /sys/unsharethis; curl unix.stackexchange.com'

open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=4759040, ...}) = 0
mmap(NULL, 4759040, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f7ec6968000
close(3)                                = 0
unshare(CLONE_NEWUSER|CLONE_NEWNET)     = 0
open("/proc/self/setgroups", O_WRONLY)  = 3
write(3, "deny", 4)                     = 4