resolv.conf loses search directive with domain during pxe boot

[Impact]

 * netcfg currently ends up overwriting DNS search domains provided by correctly configured DHCP servers in the installer environment.

 * Any preseed command that relies on hostname resolution via a search domain fail at runtime.

 * The change in this bug allows netcfg to detect that the DHCP-server provided a domain.

[Test Case]

 * TBP

[Regression Potential]

 * TBP

[Other Info]

 * Anything else you think is useful to include
 * Anticipate questions from users, SRU, +1 maintenance, security teams and the Technical Board
 * and address these questions in advance

[Original Report]

When performing a pxe boot using the netboot image from the precise-server-amd64 iso, resolv.conf is modified and the search directive containing the domain provided by dhcp is removed. I can see that the entry is picked up during the boot of the image initially. The file is then overwritten removing the search directive. This seems to correspond with a reverse dns lookup being used to obtain a hostname as shown in the log section provided. Running dhclient after this restores the entry in resolv.conf.

In monitorng resolv.conf, I found that /etc/resolv.conf is populated with the following information initially during the pxe boot, which is the result that is needed:

search provision.example.com
nameserver 10.1.0.2

---------------------------

It is then modified to the following:

nameserver 10.1.0.2

------------------------
Apr 5 14:03:33 netcfg[2088]: DEBUG: No RA received; attempting IPv4 autoconfig
Apr 5 14:03:33 netcfg[2088]: WARNING **: Started DHCP client; PID is 2147
Apr 5 14:03:34 dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 1
Apr 5 14:03:34 dhclient: DHCPREQUEST of 10.1.0.112 on eth0 to 255.255.255.255 port 67
Apr 5 14:03:34 dhclient: DHCPOFFER of 10.1.0.112 from 10.1.0.2
Apr 5 14:03:34 dhclient: DHCPACK of 10.1.0.112 from 10.1.0.2
Apr 5 14:03:34 dhclient: bound to 10.1.0.112 -- renewal in 7192 seconds.
Apr 5 14:03:36 kernel: [ 21.621493] eth0: no IPv6 routers present
Apr 5 14:03:36 netcfg[2088]: DEBUG: Reading domain name returned via DHCP
Apr 5 14:03:36 netcfg[2088]: DEBUG: DHCP domain name is '**************'
Apr 5 14:03:36 netcfg[2088]: DEBUG: Reading NTP servers from DHCP info
Apr 5 14:03:36 netcfg[2088]: DEBUG: Read NTP server 10.1.0.2
Apr 5 14:03:36 netcfg[2088]: DEBUG: Reading nameservers from /etc/resolv.conf
Apr 5 14:03:36 netcfg[2088]: DEBUG: Read nameserver 10.1.0.2
Apr 5 14:03:36 netcfg[2088]: DEBUG: State is now 1
Apr 5 14:03:36 netcfg[2088]: DEBUG: State is now 2
Apr 5 14:03:36 netcfg[2088]: DEBUG: State is now 5
Apr 5 14:03:36 netcfg[2088]: DEBUG: Using DNS to try and obtain default hostname
Apr 5 14:03:36 netcfg[2088]: DEBUG: Getting default hostname from rDNS lookup of autoconfigured address 10.1.0.112
Apr 5 14:03:36 netcfg[2088]: DEBUG: getnameinfo() returned -3: errno 0 (Success)
Apr 5 14:03:36 netcfg[2088]: DEBUG: Getting default hostname from rDNS lookup of autoconfigured address fe80::250:56ff:feb8:5e
Apr 5 14:03:36 netcfg[2088]: DEBUG: getnameinfo() returned -2: errno 0 (Success)

-------------------

pxelinux.cfg entry:

LABEL default
  MENU DEFAULT
  MENU LABEL Ubuntu 12.04 (64-bit) Installer
  IPAPPEND 2
  KERNEL ubuntu/12.04/amd64/linux
  APPEND interface=auto lang=en_US locale=en_US console-setup/layoutcode=us priority=critical debian-installer/allow_unauthenticated=true initrd=ubuntu/12.04/amd64/initrd.gz url=http://kickstart/cgi-bin/example.cgi ramdisk_size=10882 root=/dev/ram0 devfs=mount,dall rw --