Linux implementation of 'get_mac_address_by_ip'

getaddrif() does not return the information indexed by adapter
so we have to build our own index.  Tested on my Linux vm - seems
to work!

src/linux.rs

@@ -1,5 +1,10 @@
 #![allow(dead_code)]
 
+use std::{
+    collections::HashMap,
+    net::{IpAddr, Ipv4Addr, Ipv6Addr},
+};
+
 use crate::MacAddressError;
 use nix::ifaddrs::*;
 
@@ -30,6 +35,54 @@ pub fn get_mac(name: Option<&str>) -> Result<Option<[u8; 6]>, MacAddressError> {
     Ok(None)
 }
 
+/// Uses the `getifaddrs` call to retrieve a list of network interfaces on the
+/// host device and returns the MAC address that matching the adapter with
+/// the given IP
+///
+/// Because nix returns all of the IP's and MAC's in a combined list, we need
+/// to map the IP to an inteface name and track the MAC addresses by interface name
+/// and see if there's a match
+pub fn get_mac_address_by_ip(ip: &IpAddr) -> Result<Option<[u8; 6]>, MacAddressError> {
+    let ifiter = getifaddrs()?;
+
+    let mut ip_on_inferface: Option<String> = None;
+    let mut mac_to_interface: HashMap<String, Option<[u8; 6]>> = HashMap::new();
+    for interface in ifiter {
+        if let Some(iface_address) = interface.address {
+            // is this a mac address?
+            if let Some(link) = iface_address.as_link_addr() {
+                mac_to_interface.insert(interface.interface_name.clone(), link.addr());
+                // did we just find what we're looking for?
+                if let Some(intf_name) = &ip_on_inferface {
+                    if *intf_name == interface.interface_name {
+                        return Ok(link.addr());
+                    }
+                }
+            }
+            if let Some(adapter_ip) = if let Some(sin4) = iface_address.as_sockaddr_in() {
+                // v4 addr?
+                Some(IpAddr::from(Ipv4Addr::from(sin4.ip())))
+            } else if let Some(sin6) = iface_address.as_sockaddr_in6() {
+                // v6 addr?
+                Some(IpAddr::from(Ipv6Addr::from(sin6.ip())))
+            } else {
+                // something else, ignore
+                None
+            } {
+                // found an IP for this adapter - if it's the one we're looking for, save it
+                if adapter_ip == *ip {
+                    ip_on_inferface = Some(interface.interface_name.clone());
+                    if let Some(mac) = mac_to_interface.get(&interface.interface_name) {
+                        return Ok(mac.clone());
+                    }
+                }
+            }
+        }
+    }
+
+    Ok(None)
+}
+
 pub fn get_ifname(mac: &[u8; 6]) -> Result<Option<String>, MacAddressError> {
     let ifiter = getifaddrs()?;