feat: configure /etc/hosts

[yundantianchang]

Local hosts may have effects on hosts in network namespace.

[janedoe-lab]

@yundantianchang , how hard it would be to modify it to alternatively accept local hosts file in case more than just localhost needs to be defined in /etc/hosts?

[nlzy]

Hi,

The configuration to /etc/resolv.conf and /etc/nsswitch.conf are necessary for DNS redirection. Especially on some systems that used systemd-resolved. Otherwise, DNS requests will not be sent to the 'fake gateway', and will not be handled by nsproxy.

But I don't know why /etc/hosts should be modified. Seems like doing some extra things but out-of-topic.

[yundantianchang]

@nlzy, in "/etc/nsswitch.conf", there is a line "hosts: files dns". This means that dns queries will first use the "/etc/hosts" file and then "/etc/resolv.conf". However, if there are certain lines in "/etc/hosts" for the host to block certain domains, the application in nsproxy will use the same "/etc/hosts" file and will also be blocked. With this patch, nsproxy and the host are isolated, and the host's "/etc/hosts" file does not have any impact on applications in nsproxy.

[yundantianchang]

@janedoe-lab, for the current codes, may need to edit the code directly.