[ncat] Ability to control hostname resolution for proxy destinations

[nnposter]

In connect mode, currently all proxy destinations are directly passed onto the remote proxy server, without trying to resolve them locally. This behavior has raised some concerns.

This proposal implements a new ncat option that provides control over whether these hostnames are resolved by the remote proxy server or locally, by Ncat itself.

Usage:

ncat   ...   --proxy-dns local | remote | both | none

• local - Hostnames are resolved locally on the Ncat host. Ncat exits with error if the hostname cannot be resolved.

• remote - Hostnames are passed directly onto the remote proxy server. This is the default behavior.

• both - Hostname resolution is first attempted on the Ncat host. Unresolvable hostnames are passed onto the remote proxy server.

• none - Hostname resolution is completely disabled. Only an IPv4 or IPv6 address can be used as the proxy destination.

Note that this new option is complementing, not replacing, option --no-dns, which controls all existing hostname resolution, such as non-proxied targets, hops, source address, and the proxy server itself.

[jaymzh]

From a read-through of the code, this looks like it'll solve the issue. I'm traveling the next two weeks, so I'm not sure when I'll get a chance to actually give it a shot, but I'll try to find time this this week if I can.

[jaymzh]

probably should list the possible types here (none, local, remote, both)... and maybe what they mean...

[nnposter]

Not enough space if we want to keep it on a single line

[jaymzh]

I checked out your branch and tried to build, but the build fails on:

make: *** No rule to make target `nbase/nbase_addrset.h', needed by `targets.o'.  Stop.

[jaymzh]

Nevermind I think this is #1379 I'll give the solution there a shot.

[jaymzh]

local/remote/both all work as expected, thanks! I will note that none is not considered a valid option, despite the docs, you get:

Ncat: Invalid proxy DNS type. QUITTING.

[nnposter]

Yup; my bad. Hopefully fixed.

[nnposter]

Committed as r37586.