Re: [PATCH] resolv/resolv.h: allow alternative resolv.conf files

[Renzo Davoli <renzo at cs dot unibo dot it>]

> Can you share more about your use case?

Sure. I am a developer of VDE, Virtual Distributed Ethernet.

One of the main features of the new implementation (VDE4) are the vde namespaces.
(user namespaces, also for users not having root access).

Simply typing vdens and a VDE-locator of a network a user gets a new namespace
connected to that network.

So for example:
  $ vdens vxvde://
  $ ip addr
  1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000
      link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
  2: vde0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
      link/ether 02:ef:ba:21:82:5a brd ff:ff:ff:ff:ff:ff
  
Users can configure their ip addresses, routing and use their favourite tools (both
		servers and clients).

As a complete example let us create a vde namespace connected to a remote slirp service:
  $ vdens cmd://'ssh vdetest.v2.cs.unibo.it vde_plug slirp://'
  $ /sbin/udhcpc -i vde0
  udhcpc (v1.22.1) started
  Sending discover...
  Sending select for 10.0.2.15...
  Lease of 10.0.2.15 obtained, lease time 86400
  $ ip link set lo up
  1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
      link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
		  inet 127.0.0.1/8 scope host o
		  valid_lft forever preferred_lft forever
		  inet6 ::1/128 scope host 
		  valid_lft forever preferred_lft forever
  2: vde0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 1000
      link/ether 7e:20:6e:12:f3:ee brd ff:ff:ff:ff:ff:ff
		  inet 10.0.2.15/24 brd 10.0.2.255 scope global vde0
		  valid_lft forever preferred_lft forever
		  inet6 fe80::7c20:6eff:fe12:f3ee/64 scope link 
		  valid_lft forever preferred_lft forever

Now a user can start any server or client... but if the nameserver defined in /etc/resolvconf (by root)
is not reachable all the resolv functions will fail. 

Why a user (not root) should not be allowed to choose her own resolver?
Why to obtain a so simple feature should I have to overlay mount /etc?

I have had the same problem in other project:
* libvdestack: provide processes their own netwoking stack(s)
https://github.com/rd235/libvdestack

* fqdndhcp: a dhcp server receiving the fully qualified domain name from the client and providing
it with the IP(v6) address retrieved fron a DNS server. In this way configuring hosts is as simple as
giving them a name. In this case it happens that sometimes specific DNS servers for the service get used, so
the daemon has to query them instead of the standard servers. I have currently solved this problem using libadns,
but all I needed was a way to use getaddrinfo using my own nameserver.
(see: https://github.com/rd235/vde_dnsutils/blob/master/resolv.c).

Any implementation/alternative approach/patch which fits the requirements (possibility to define the nameserver to use, 
no root access required) is okay to me.

Thank you.

	renzo