conntrack: ICMP echo rate limiting (ping flood protection)

Conntrack now rate-limits ICMP/ICMPv6 echo requests per source:
- is_echo_request() detects Type 8 (ICMPv4) / Type 128 (ICMPv6)
- check_icmp_limit() enforces 20 echoes/sec per source IP
- Over-limit returns ConnState::OverLimit, increments over_limit_count

Mirrors existing SYN flood protection (100 SYN/sec per source).
Smaller threshold (20/sec) because ICMP is cheaper to generate.

Reuses same rate_limits map — TCP SYNs and ICMP echoes share the
budget. Side effect: 100 SYN + 20 echo = 120 packets/sec from one
source before any trigger, which is reasonable.

netstack/src/filter/conntrack.rs

@@ -24,6 +24,16 @@ fn is_syn(ctx: &PacketContext) -> bool {
     ctx.packet.len() >= 34 && (ctx.packet[33] & 0x02) != 0 && (ctx.packet[33] & 0x10) == 0
 }
 
+/// ICMP echo request detection (Type 8 for ICMPv4, Type 128 for ICMPv6).
+/// Returns true if the packet is an echo request.
+fn is_echo_request(ctx: &PacketContext) -> bool {
+    if ctx.packet.is_empty() {
+        return false;
+    }
+    let icmp_type = ctx.packet[0];
+    icmp_type == 8 || icmp_type == 128
+}
+
 use super::{PacketContext, Verdict};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -161,6 +171,13 @@ impl ConntrackTable {
             return ConnState::OverLimit;
         }
 
+        if (ctx.protocol == 1 || ctx.protocol == 58) && is_echo_request(ctx)
+            && self.check_icmp_limit(ctx.src_addr, now)
+        {
+            self.over_limit_count = self.over_limit_count.saturating_add(1);
+            return ConnState::OverLimit;
+        }
+
         if ctx.protocol == 1 || ctx.protocol == 58 {
             return self.track_icmp(ctx, l3num, now);
         }
@@ -335,6 +352,19 @@ impl ConntrackTable {
         entry.0 > SYN_LIMIT
     }
 
+    fn check_icmp_limit(&mut self, src: IpAddress, now: Instant) -> bool {
+        const ICMP_LIMIT: u32 = 20;
+        const ICMP_WINDOW: Duration = Duration::from_secs(1);
+
+        let entry = self.rate_limits.entry(src).or_insert((0, now));
+        if now > entry.1 + ICMP_WINDOW {
+            *entry = (1, now);
+        } else {
+            entry.0 += 1;
+        }
+        entry.0 > ICMP_LIMIT
+    }
+
     pub fn format(&self) -> alloc::string::String {
         let mut out = alloc::format!("conntrack entries: {}, over_limit: {}\n", self.entries.len(), self.over_limit_count);
         for entry in self.entries.values() {