Add file missing from previous commit

src/rust/lqos_node_manager/src/tracker/cache/dns_cache.rs

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+//! Implements a lock-free DNS least-recently-used DNS cache.
+
+use std::net::IpAddr;
+use dashmap::DashMap;
+use dns_lookup::lookup_addr;
+use lqos_utils::unix_time::unix_now;
+use once_cell::sync::Lazy;
+
+const CACHE_SIZE: usize = 1000;
+
+struct DnsEntry {
+    hostname: String,
+    last_accessed: u64,
+}
+
+static DNS_CACHE: Lazy<DashMap<IpAddr, DnsEntry>> = Lazy::new(|| DashMap::with_capacity(CACHE_SIZE));
+
+pub fn lookup_dns(ip: IpAddr) -> String {
+    // If the cached value exists, just return it
+    if let Some(mut dns) = DNS_CACHE.get_mut(&ip) {
+        if let Ok(now) = unix_now() {
+            dns.last_accessed = now;
+        }
+        return dns.hostname.clone();
+    }
+
+    // If it doesn't, we'll be adding it.
+    if DNS_CACHE.len() >= CACHE_SIZE {
+        let mut entries : Vec<(IpAddr, u64)> = DNS_CACHE.iter().map(|v| (*v.key(), v.last_accessed)).collect();
+        entries.sort_by(|a,b| b.1.cmp(&a.1));
+        DNS_CACHE.remove(&entries[0].0);
+    }
+    let hostname = lookup_addr(&ip).unwrap_or(ip.to_string());
+    DNS_CACHE.insert(ip, DnsEntry { hostname, last_accessed: unix_now().unwrap_or(0) });
+
+
+    String::new()
+}