[Copilot]

On macOS, RouteListener::listen() reports ordinary routes such as 10.0.0.0/8 as prefix 0, so they look like default routes. BSD truncates netmask sockaddrs to their significant bytes. When RTA_NETMASK is the last address, the remaining buffer was shorter than size_of::<sockaddr>(), so the walk skipped it and message_to_route mapped the missing entry to prefix = 0. RouteManager::list() was unaffected because its sysctl dumps carry more addresses after the netmask.

I couldn't build or test this on macOS or a BSD, so it is untested. RouteListener::listen() against the issue's RFC1918 routes should now report prefixes 8, 12, 16 and 16.

All changes are in src/unix_bsd/mod.rs:

• Sockaddr walk (message_to_route)
  • Reads sa_len first and advances by the aligned length instead of requiring a full sockaddr to remain.
  • Keeps the netmask as the raw bytes that are present (&buf[..min(sa_len, buf.len())]) rather than a &sockaddr.
  • Non-netmask addresses are still stored as &sockaddr only when a full sockaddr is available and sa_len fits in the buffer. A short non-netmask address is skipped, but the walk still advances past it so later addresses stay aligned.
  • The walk stops instead of panicking when cur_pos runs past the message.
  • The macOS ROUNDUP now uses usize, so it can't overflow u8 for sa_len ≥ 253.
• Prefix calculation
  • Copies the available bytes into a zeroed 16-byte buffer, taken from offset 4 for IPv4 and offset 8 for IPv6, then counts leading_ones.
  • This removes the transmute that read 16 bytes from a 5–8 byte region.
  • A zero-length or absent netmask still gives prefix 0, as before.
• Message type filter (deserialize_res)
  • Only RTM_ADD, RTM_DELETE, RTM_CHANGE and RTM_GET are parsed. Other types, such as RTM_IFINFO, are skipped before rtm_errno is read.
  • Previously a tun coming up produced a bogus Err(Os { code: <mtu> }). That error also discarded any RTM_ADD/RTM_DELETE batched into the same read().
  • list() uses NET_RT_DUMP, which emits RTM_GET, so it still passes the filter.

• Fixes BSD: routes with a truncated trailing netmask are reported as prefix 0 #48

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