fix: prevent resolver from overwriting DNS cache hostname entries (#11)

When gost calls the resolver with a raw IP (e.g. from tun2socks),
LookupIP returns the IP itself. RegisterIPs then overwrites the
correct hostname entry (e.g. 160.79.104.10 -> api.anthropic.com)
with a self-referential mapping (160.79.104.10 -> 160.79.104.10).

This caused pending requests and logs to show raw IPs instead of
hostnames, worsening over time as each connection corrupted its
cache entry.

Skip cache registration when the host is already an IP address.

Author: tito

internal/greyproxy/plugins/resolver.go

@@ -40,14 +40,19 @@ func (r *Resolver) Resolve(ctx context.Context, network, host string, opts ...re
 		return nil, fmt.Errorf("resolve %s: no results", host)
 	}
 
-	// Cache all resolved IPs -> hostname
-	ipStrs := make([]string, len(ips))
-	for i, ip := range ips {
-		ipStrs[i] = ip.String()
+	// Cache all resolved IPs -> hostname, but only when host is an actual
+	// hostname (not an IP). When gost calls the resolver with a raw IP,
+	// LookupIP returns the IP itself; registering it would overwrite
+	// a correct hostname entry previously populated by the DNS handler.
+	if net.ParseIP(host) == nil {
+		ipStrs := make([]string, len(ips))
+		for i, ip := range ips {
+			ipStrs[i] = ip.String()
+		}
+		r.cache.RegisterIPs(host, ipStrs)
 	}
-	r.cache.RegisterIPs(host, ipStrs)
 
-	r.log.Debugf("resolved %s -> %v", host, ipStrs)
+	r.log.Debugf("resolved %s -> %v", host, ips)
 	return ips, nil
 }