netdiag: parse and display MIB-II error/drop counters

IfaceStats struct gains rx_errors, tx_errors, rx_dropped, tx_dropped.
Stats output now shows error/drop line when any of these are non-zero:
    eth0    rx: ...B tx: ...B
            errors: rx=N tx=N  drops: rx=N tx=N

Mirrors Linux ifconfig output format.
Suppresses the extra line when all counters are 0 (clean state).

netdiag/src/main.rs

@@ -31,6 +31,10 @@ struct IfaceStats {
     rx_packets: u64,
     tx_bytes: u64,
     tx_packets: u64,
+    rx_errors: u64,
+    tx_errors: u64,
+    rx_dropped: u64,
+    tx_dropped: u64,
 }
 
 fn read_iface_stats(dev: &str) -> Option<IfaceStats> {
@@ -38,12 +42,19 @@ fn read_iface_stats(dev: &str) -> Option<IfaceStats> {
     if raw.is_empty() {
         return None;
     }
-    let mut s = IfaceStats { rx_bytes: 0, rx_packets: 0, tx_bytes: 0, tx_packets: 0 };
+    let mut s = IfaceStats {
+        rx_bytes: 0, rx_packets: 0, tx_bytes: 0, tx_packets: 0,
+        rx_errors: 0, tx_errors: 0, rx_dropped: 0, tx_dropped: 0,
+    };
     for line in raw.lines() {
-        if line.starts_with("rx_bytes=") { s.rx_bytes = parse_counter(line); }
-        else if line.starts_with("rx_packets=") { s.rx_packets = parse_counter(line); }
-        else if line.starts_with("tx_bytes=") { s.tx_bytes = parse_counter(line); }
-        else if line.starts_with("tx_packets=") { s.tx_packets = parse_counter(line); }
+        if let Some(v) = line.strip_prefix("rx_bytes=") { s.rx_bytes = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("rx_packets=") { s.rx_packets = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("tx_bytes=") { s.tx_bytes = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("tx_packets=") { s.tx_packets = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("rx_errors=") { s.rx_errors = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("tx_errors=") { s.tx_errors = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("rx_dropped=") { s.rx_dropped = v.trim().parse().unwrap_or(0); }
+        else if let Some(v) = line.strip_prefix("tx_dropped=") { s.tx_dropped = v.trim().parse().unwrap_or(0); }
     }
     Some(s)
 }
@@ -164,6 +175,12 @@ fn show_stats() {
                 "    {:<6}  rx: {:>10} B ({:>6} pkts)  tx: {:>10} B ({:>6} pkts)",
                 dev, s.rx_bytes, s.rx_packets, s.tx_bytes, s.tx_packets
             );
+            if s.rx_errors != 0 || s.tx_errors != 0 || s.rx_dropped != 0 || s.tx_dropped != 0 {
+                println!(
+                    "    {:>7}  errors: rx={} tx={}  drops: rx={} tx={}",
+                    "", s.rx_errors, s.tx_errors, s.rx_dropped, s.tx_dropped
+                );
+            }
         }
     }