How can dhclient be made namespace aware?

I looked into the issue myself.

What happens is that when you create a network namespace, you see /etc/resolv.conf of the host machine unless you create explicitly /etc/netns/<namespace_name>/resolv.conf, which will bind mount automatically to /etc/resolv.conf when looked up inside the network namespace. Therefore, by simply creating that path, the resolv.conf of the host won't be visibile any more on the network namespace, which will have its own resolv.conf.

The manual page of ip netns explains this:

For applications that are aware of network namespaces, the convention is to look for global network configuration files first in /etc/netns/NAME/ then in /etc/. For example, if you want a different version of /etc/resolv.conf for a network namespace used to isolate your vpn you would name it /etc/netns/myvpn/resolv.conf.

Ip netns exec automates handling of this configuration, file convention for network namespace unaware applications, by creating a mount namespace and bind mounting all of the per network namespace configure files into their traditional location in /etc.

As far as updating resolv.conf, dhclient doesn't work in network namespaces out of the box when /etc/netns/<namespace_name>/resolv.conf exists (on the other hand, when it doesn't exist, it will overwrite the resolv.conf of the host machine, since it's the only one available, but that's not really desirable). As the error in the question above shows, what happens is that dhclient prepares a temporary file with the new nameserver details in /etc/resolv.conf.dhclient-new.2740 and then tries to rename it as /etc/resolv.conf. It generates an error because /etc/resolv.conf is already bind-mounted and apparently mv isn't allowed to do this trick.

In order to make dhclient work in network namespaces, /sbin/dhclient-script should be modified.I removed this:

mv -f $new_resolv_conf /etc/resolv.conf

And replaced it with:

cat $new_resolv_conf > /etc/resolv.confrm -f $new_resolv_conf

Otherwise, dhcpcd seems to do this job correctly.