Skip directed-broadcast addresses silently in iteration mode

[grisutheguru]

Problem

When iterating a CIDR range, nbtscan sends a NetBIOS query to every address in the range, including the directed-broadcast (e.g. .255 for a /24). Without SO_BROADCAST set on the UDP socket, Linux rejects sendto() to such addresses with EACCES, which is surfaced as:

192.168.69.255	Sendto failed: Permission denied

The wording suggests a privileges problem and routinely sends users chasing sudo / setcap fixes that don't apply.

Fix

Treat EACCES from sendto() as a "skip this address silently" signal rather than print an error. Default iteration semantics are unchanged for all non-broadcast addresses.

Behaviour comparison

Before:

$ nbtscan 192.168.69.0/24
192.168.69.255	Sendto failed: Permission denied
Doing NBT name scan for addresses from 192.168.69.0/24

IP address       NetBIOS Name     Server    User             MAC address
------------------------------------------------------------------------------
192.168.69.2     S3E3             <server>  <unknown>        02:a0:79:49:24:95

After:

$ nbtscan 192.168.69.0/24
Doing NBT name scan for addresses from 192.168.69.0/24

IP address       NetBIOS Name     Server    User             MAC address
------------------------------------------------------------------------------
192.168.69.2     S3E3             <server>  <unknown>        02:a0:79:49:24:95

For ranges whose broadcast is not directly connected (e.g. nbtscan 192.168.68.0/23 from a host on 192.168.69.0/24), behaviour is unchanged: only the locally-connected .255 was raising EACCES, and that's the only address now being silently skipped.

Verified that the kernel mechanism is SO_BROADCAST

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.sendto(b'test', ('192.168.69.255', 137))
# OSError: [Errno 13] Permission denied

s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
s.sendto(b'test', ('192.168.69.255', 137))
# 4

Future work (separate PR)

An opt-in -B / --broadcast flag that sets SO_BROADCAST and sends a single query to the directed-broadcast of the input range (no iteration) would be a meaningful speedup for the common /24 case. Happy to follow up with a separate PR for that if this one lands.