Honour forwarded headers only from trusted proxies (closes #259)

A new ClientIP middleware sets request.RemoteAddr to the client's
address before the request is logged. X-Forwarded-For and X-Real-IP
count only when the peer is in TRUSTED_PROXIES, which defaults to the
RFC 1918 ranges. The login rate limit, the session's IP address and
hostname, and the request log all use that address.

Model: opus-5-5

internal/config/config_test.go

@@ -1,11 +1,14 @@
 package config_test
 
 import (
+	"net/netip"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
 
+	"github.com/spf13/viper"
 	"sneak.berlin/go/neoirc/internal/config"
 	"sneak.berlin/go/neoirc/internal/globals"
 	"sneak.berlin/go/neoirc/internal/logger"
@@ -140,8 +143,7 @@ func settingsOf(cfg *config.Config) settings {
 }
 
 func TestBadSettingStopsStartup(t *testing.T) {
-	// For each setting, the first value does not parse and the others
-	// parse but are out of range.
+	// Each value is invalid for its setting.
 	tests := []struct {
 		key    string
 		values []string
@@ -158,6 +160,10 @@ func TestBadSettingStopsStartup(t *testing.T) {
 		{key: "MESSAGE_MAX_AGE", values: []string{"30d", "0", "-1h"}},
 		{key: "QUEUE_MAX_AGE", values: []string{"720", "0s", "-24h"}},
 		{key: "SESSION_IDLE_TIMEOUT", values: []string{"a month", "0", "-1m"}},
+		{key: "TRUSTED_PROXIES", values: []string{
+			"proxy.example.com", "10.0.0.0/33", "192.0.2.1:8080",
+			"::ffff:10.0.0.0/104", "fe80::1%eth0",
+		}},
 	}
 
 	for _, test := range tests {
@@ -206,3 +212,124 @@ func TestIRCListenAddrEmptyTurnsListenerOff(t *testing.T) {
 		t.Errorf("IRCListenAddr = %q, want empty", cfg.IRCListenAddr)
 	}
 }
+
+func TestTrustedProxies(t *testing.T) {
+	// Each case lists addresses that must be in the trusted set and
+	// addresses that must not.
+	tests := []struct {
+		name      string
+		set       bool
+		value     string
+		trusted   []string
+		untrusted []string
+	}{
+		{
+			name:  "unset trusts the RFC 1918 ranges",
+			set:   false,
+			value: "",
+			trusted: []string{
+				"10.1.2.3", "172.16.0.1", "172.31.255.254", "192.168.1.1",
+			},
+			untrusted: []string{
+				"127.0.0.1", "172.32.0.1", "192.0.2.1", "2001:db8::1",
+			},
+		},
+		{
+			name:      "a value replaces the default",
+			set:       true,
+			value:     "198.51.100.0/24, 192.0.2.7,2001:db8::/32",
+			trusted:   []string{"198.51.100.200", "192.0.2.7", "2001:db8::5"},
+			untrusted: []string{"10.4.5.6", "192.168.4.4", "192.0.2.8"},
+		},
+		{
+			name:      "a mapped IPv6 address is trusted as IPv4",
+			set:       true,
+			value:     "::ffff:192.0.2.7",
+			trusted:   []string{"192.0.2.7"},
+			untrusted: []string{"10.7.8.9"},
+		},
+		{
+			name:      "the empty string trusts nothing",
+			set:       true,
+			value:     "",
+			trusted:   nil,
+			untrusted: []string{"10.0.0.1", "192.168.0.1", "::1"},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			// t.Setenv puts back whatever value was there after the
+			// test, including when Unsetenv removes it.
+			t.Setenv("TRUSTED_PROXIES", test.value)
+
+			if !test.set {
+				err := os.Unsetenv("TRUSTED_PROXIES")
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			proxies := newConfig(t).TrustedProxies
+
+			for _, addr := range test.trusted {
+				if !proxies.Contains(netip.MustParseAddr(addr)) {
+					t.Errorf("%s is not trusted", addr)
+				}
+			}
+
+			for _, addr := range test.untrusted {
+				if proxies.Contains(netip.MustParseAddr(addr)) {
+					t.Errorf("%s is trusted", addr)
+				}
+			}
+		})
+	}
+}
+
+func TestTrustedProxiesNotAStringInYAMLStopsStartup(t *testing.T) {
+	// YAML reads the first value as a list and the second as a map.
+	values := []string{"[10.0.0.1]", "{proxy: 10.0.0.1}"}
+
+	for _, value := range values {
+		t.Run(value, func(t *testing.T) {
+			// t.Setenv puts back whatever value was there after the
+			// test, including when Unsetenv removes it.
+			t.Setenv("TRUSTED_PROXIES", "")
+
+			err := os.Unsetenv("TRUSTED_PROXIES")
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// loadConfig names the app neoirc-test, so config.New reads
+			// $HOME/.config/neoirc-test/neoirc-test.yaml.
+			home := t.TempDir()
+			t.Setenv("HOME", home)
+
+			dir := filepath.Join(home, ".config", "neoirc-test")
+
+			err = os.MkdirAll(dir, 0o700)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			err = os.WriteFile(
+				filepath.Join(dir, "neoirc-test.yaml"),
+				[]byte("TRUSTED_PROXIES: "+value+"\n"), 0o600,
+			)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// Viper keeps the values it read from this file after the
+			// file is gone. Reset clears them for later tests.
+			t.Cleanup(viper.Reset)
+
+			_, err = loadConfig(t)
+			if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
+				t.Errorf("error = %v, want one naming TRUSTED_PROXIES", err)
+			}
+		})
+	}
+}