netboot: failed to parse DNS settings set by dhclient

[cuihao]

I am setting up diskless netboot for Ubuntu LiveCDs, with following kernel arguments:

```
boot=casper netboot=nfs nfsroot=192.168.89.10:/extracted/livecd toram
```

I have noticed the following strange error messages after dhcp (also see the attachment):

```
/init: /run/net-ens3.conf: line 8: 202.38.64.56: not found
/init: /run/net-ens3.conf: line 8: 202.38.64.56: not found
/init: /run/net-ens3.conf: line 8: 202.38.64.56: not found
no search or nameservers found in /run/net-ens3.conf /run/net-ens3.conf /run/net6-*.conf
/init: /run/net-ens3.conf: line 8: 202.38.64.56: not found
/init: /run/net-ens3.conf: line 8: 202.38.64.56: not found
```

I manually broke the init process, and found line 8 of /run/net-ens3.conf is:

```
IPV4DNS0=192.168.88.1 202.38.64.56 202.38.64.17
```

The problematic code is in the `netinfo_to_resolv_conf` function of `scripts/functions`. It sources `/run/net-ens3.conf` (which is obviously not in correct shell syntax due to the space inserted between multiple DNS servers) to get network settings:

```
. "$f" || { echo "WARN: failed '. \"$f\"'" 1>&2; return 1; }
```

The bug is introduced by commit a77729e9, which replaced IP-config with dhclient. While IP-config seems to write each DNS in `IPV4DNS0`, `IPV4DNS1`, etc., dhclient write all DNS in `IPV4DNS0`, which is not expected by `netinfo_to_resolv_conf`.

I can confirm 18.10 LiveCDs are affected.

[cuihao (cuihao-leo)]

Another reasonable solution is to ask isc-dhcp packagers to change the dhclient hook script `/etc/initramfs-tools/lib/etc/dhcp/dhclient-enter-hooks.d/config`.

Currently it writes all DNS in IPV4DNS0:
```
echo "IPV4DNS0=$new_domain_name_servers" >> /run/net-$interface.conf
```

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.