Add ServerName argument to srvs.hNetrShareEnum

[cnotin]

I have an SMB server for which impacket smbconnection.listShares() doesn't return as many shares as Windows explorer shows me.
Impacket only returns "C$", "F$" and a few others, and "IPC$", "print$" and "prnproc$", while Windows sees many more shares which are not only technical/admin shares but also business ones.

Through a packet capture analysis I found the difference being that Windows fills the "ServerName" field in the NetShareEnumAll request which impacket doesn't (or actually fills it with a NULL byte only coming from b4cacea):
https://github.com/SecureAuthCorp/impacket/blob/d84fca225175729bdf215adca10f3b3bd5a84733/impacket/dcerpc/v5/srvs.py#L3095-L3097

We can observe that Windows fills it with "\\<ip><NULL>"
Even though the specification mentions that this field can be NULL and that the server MUST remove any preceding "\\"

This code fixes it for smbconnection.listShares() only (which is what I need at the moment...). Would you like me to add something similar to all other structures in srvs.py that have a "ServerName" field?

I tested it on the problematic server and a few others, but I can run a larger scale test :)
EDIT: I did a larger scale test and I did not notice any regression

[mohemiv]

Hello @cnotin!

I've just read the specification by your link, and I've found this:
If the Level member is 503, the server MUST return all shares in SHARE_INFO_503_I structures. Otherwise, the server MUST return the shares in which share.shi503_servername matches ServerName.

Isn't this the right way to list all the shares?

[cnotin]

Thanks @mohemiv! That's a very good idea I'll try that!

[cnotin]

I did a quick test and it wasn't very successful (the server answered with an ACCESS_DENIED error) but it didn't dig too much!

[anadrianmanrique]

Hi, I merged these changes in the context of #1721, as this PR needs to be rebased.
Thank you!