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

README.md

@@ -1339,8 +1339,9 @@ Every session has a hostmask in IRC's form, `nick!username@hostname`:
   from, or that IP address when the lookup gives no name. The lookup waits up to
   three seconds.
 
-Over the HTTP API, the server takes the client's IP address from the first entry
-of the `X-Forwarded-For` header, then `X-Real-IP`, then the TCP connection (see
+Over the HTTP API, the client's IP address is the address of the TCP connection.
+When that connection comes from a trusted proxy, the server reads the address
+from `X-Forwarded-For` or `X-Real-IP` instead (see
 [Reverse proxy](#reverse-proxy)).
 
 Every user can see a hostmask: in `RPL_WHOISUSER` (`311`), `RPL_WHOREPLY`
@@ -1450,10 +1451,9 @@ over the limit gets `429` `too many login attempts, try again later` with
 attempt is more than 15 minutes old. Session creation has no rate limit; the
 hashcash stamp is its cost.
 
-The IP address comes from the first entry of `X-Forwarded-For`, else from
-`X-Real-IP`, else from the TCP connection. Without a reverse proxy that
-overwrites those headers, a client can set them itself and so pick any address
-(see [Reverse proxy](#reverse-proxy)).
+The IP address is the client's address as described in
+[Reverse proxy](#reverse-proxy). A client outside the trusted proxies cannot
+change it with `X-Forwarded-For` or `X-Real-IP`.
 
 ### Writing a client
 
@@ -1714,10 +1714,14 @@ Every setting is an environment variable. The server also reads a `.env` file in
 its working directory, and a YAML file `neoirc.yaml` in `/etc/neoirc/` or
 `$HOME/.config/neoirc/` with the same names as keys. A variable from the
 environment wins over one in `.env`, and both win over `neoirc.yaml`. An
-environment variable set to the empty string counts as unset, except
-`IRC_LISTEN_ADDR`, where the empty string turns the IRC listener off. A
-`neoirc.yaml` that is not valid YAML stops the server at startup. A setting that
-does not parse or is out of range also stops the server at startup, and the
+environment variable set to the empty string counts as unset, with two
+exceptions:
+
+- For `IRC_LISTEN_ADDR`, the empty string turns the IRC listener off.
+- For `TRUSTED_PROXIES`, the empty string trusts no proxy.
+
+A `neoirc.yaml` that is not valid YAML stops the server at startup. A setting
+that does not parse or is out of range also stops the server at startup, and the
 error names the setting.
 
 - `PORT` (default `8080`): the HTTP port. `0` lets the system pick a free port.
@@ -1747,6 +1751,11 @@ error names the setting.
   address. A fraction such as `0.5` is allowed. It must be above `0`.
 - `LOGIN_RATE_BURST` (default `5`): how many attempts an address may make at
   once before the rate applies. It must be at least `1`.
+- `TRUSTED_PROXIES` (default `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`): the
+  reverse proxies whose `X-Forwarded-For` and `X-Real-IP` headers the server
+  believes. It is a comma-separated list of networks, such as `10.0.0.0/8`, and
+  single addresses, such as `192.0.2.1`. A list replaces the default entirely.
+  The empty string trusts nothing. See [Reverse proxy](#reverse-proxy).
 - `SESSION_IDLE_TIMEOUT` (default `720h`, 30 days): a duration with a unit, `h`
   for hours, `m` for minutes or `s` for seconds, such as `720h` or `36h30m`. A
   client with no request for this long is removed, and a session left with no
@@ -1804,14 +1813,34 @@ Run the server behind a reverse proxy that terminates TLS. The server itself
 speaks plain HTTP, and the auth cookie is `Secure`, so browsers send it only
 over HTTPS. The proxy must:
 
-- Set `X-Forwarded-For` to the client's address, replacing any value the client
-  sent. The server reads that header first and trusts it from anyone; it reads
-  `X-Real-IP` only when `X-Forwarded-For` is absent. Without a proxy that
-  overwrites the header, a client can choose its own address, which sets its
-  hostname and lets it dodge the login rate limit.
+- Connect from an address in `TRUSTED_PROXIES`. The default trusts the private
+  ranges `10.0.0.0/8`, `172.16.0.0/12` and `192.168.0.0/16`. The setting must
+  hold the address the server sees the proxy connect from. For a proxy on the
+  same host:
+    - With the server outside Docker, that address is `127.0.0.1` or `::1`. The
+      default does not trust them, so set `TRUSTED_PROXIES=127.0.0.1,::1`.
+    - Through a published Docker port, that address is the Docker bridge
+      gateway, such as `172.17.0.1`. The default trusts it.
+- Add the client's address to `X-Forwarded-For`.
 - Wait at least 60 seconds for a response. A long poll can take 30 seconds, and
   the server's own write timeout is 60 seconds.
 
+The server finds the client's address in these steps:
+
+- A connection from outside `TRUSTED_PROXIES` is the client. The server ignores
+  its `X-Forwarded-For` and `X-Real-IP` headers.
+- For a connection from a trusted proxy, the server reads `X-Forwarded-For` from
+  right to left. It skips trusted addresses and takes the first untrusted one.
+  When every entry is trusted, it takes the leftmost.
+- Without `X-Forwarded-For`, the server takes `X-Real-IP`.
+- When an entry it reads is not an IP address, the server uses the proxy's
+  address.
+
+The client's address sets the session's hostname and counts toward the login
+rate limit. Any connection from a trusted address can choose its client address
+through `X-Forwarded-For`. If clients connect directly from private addresses,
+set `TRUSTED_PROXIES` to the proxy's address alone.
+
 Caddy:
 
 ```
@@ -1861,9 +1890,21 @@ The IRC listener has no TLS.
 
 The server logs with Go's `slog`, one line per event, to standard output. The
 lines are JSON, or plain text when standard output is a terminal. Every HTTP
-request is logged with its method, URL, protocol, status, latency, remote
-address, user agent, referrer and request ID. `DEBUG=true` adds debug-level
-lines.
+request is logged with:
+
+- start time
+- method
+- URL
+- protocol
+- status
+- latency
+- client address
+- user agent
+- referrer
+- request ID
+
+The client address is the one found as described in
+[Reverse proxy](#reverse-proxy). `DEBUG=true` adds debug-level lines.
 
 ### The database
 
@@ -1946,8 +1987,8 @@ and their sessions to end. Then it closes the database.
   gets a message.
 - `internal/hashcash/`: stamp checking and minting.
 - `internal/ratelimit/`: the per-address login rate limiter.
-- `internal/middleware/`: request logging, cross-origin requests, the
-  `Content-Security-Policy` header and metrics.
+- `internal/middleware/`: finding the client's address, request logging,
+  cross-origin requests, the `Content-Security-Policy` header and metrics.
 - `internal/healthcheck/` and `internal/stats/`: the health check and its
   counters.
 - `internal/logger/` and `internal/globals/`: logging setup and build

go.mod

@@ -15,6 +15,7 @@ require (
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/viper v1.21.0
 	go.uber.org/fx v1.24.0
+	go4.org/netipx v0.0.0-20260823151212-3075585bcbeb
 	golang.org/x/crypto v0.48.0
 	golang.org/x/time v0.6.0
 	modernc.org/sqlite v1.45.0

go.sum

@@ -111,6 +111,8 @@ go.yaml.in/yaml/v2 v2.4.2 h1:DzmwEr2rDGHl7lsFgAHxmNz/1NlQ7xLIrlN2h5d1eGI=
 go.yaml.in/yaml/v2 v2.4.2/go.mod h1:081UH+NErpNdqlCXm3TtEran0rJZGxAYx9hb/ELlsPU=
 go.yaml.in/yaml/v3 v3.0.4 h1:tfq32ie2Jv2UxXFdLJdh3jXuOzWiL1fo0bu/FbuKpbc=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+go4.org/netipx v0.0.0-20260823151212-3075585bcbeb h1:XBM4hvfwGAttkkiTIFfeigdfcL1xIfdKXqFdgiHGtDs=
+go4.org/netipx v0.0.0-20260823151212-3075585bcbeb/go.mod h1:PLyyIXexvUFg3Owu6p/WfdlivPbZJsZdgWZlrGope/Y=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
 golang.org/x/crypto v0.48.0 h1:/VRzVqiRSggnhY7gNRxPauEQ5Drw9haKdM0jqfcCFts=

internal/config/config.go

@@ -6,12 +6,15 @@ import (
 	"fmt"
 	"log/slog"
 	"math"
+	"net/netip"
 	"os"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/spf13/viper"
 	"go.uber.org/fx"
+	"go4.org/netipx"
 	"sneak.berlin/go/neoirc/internal/globals"
 	"sneak.berlin/go/neoirc/internal/logger"
 
@@ -32,8 +35,16 @@ Type /help for available commands.`
 const DefaultDBURL = "file:///var/lib/neoirc/state.db" +
 	"?_pragma=journal_mode(WAL)&_pragma=busy_timeout(5000)"
 
+// defaultTrustedProxies is TRUSTED_PROXIES when it is not set: the
+// private address ranges of RFC 1918.
+const defaultTrustedProxies = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
+
 var errOutOfRange = errors.New("setting out of range")
 
+var errNeverMatches = errors.New("entry can never match a connection")
+
+var errNotString = errors.New("not a string")
+
 // Params defines the dependencies for creating a Config.
 type Params struct {
 	fx.In
@@ -64,13 +75,14 @@ type Config struct {
 	LoginRateLimit     float64
 	LoginRateBurst     int
 	IRCListenAddr      string
+	TrustedProxies     *netipx.IPSet
 	params             *Params
 	log                *slog.Logger
 }
 
 // New creates a new Config by reading from files and environment variables.
-// It returns an error when a number, true/false or duration setting does
-// not parse or is out of range.
+// It returns an error when a number, true/false, duration or address
+// setting does not parse or is out of range.
 func New(
 	_ fx.Lifecycle, params Params,
 ) (*Config, error) {
@@ -103,6 +115,7 @@ func New(
 	viper.SetDefault("LOGIN_RATE_LIMIT", "1")
 	viper.SetDefault("LOGIN_RATE_BURST", "5")
 	viper.SetDefault("IRC_LISTEN_ADDR", ":6667")
+	viper.SetDefault("TRUSTED_PROXIES", defaultTrustedProxies)
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -151,10 +164,10 @@ func New(
 	return cfg, nil
 }
 
-// parseSettings reads the number and true/false settings into cfg and
-// checks the duration settings. Each number and duration must be above
-// zero. PORT and NEOIRC_HASHCASH_BITS may also be zero: port 0 lets the
-// system pick a port, and 0 bits turns proof of work off. The error
+// parseSettings reads the number, true/false and address settings into
+// cfg and checks the duration settings. Each number and duration must be
+// above zero. PORT and NEOIRC_HASHCASH_BITS may also be zero: port 0 lets
+// the system pick a port, and 0 bits turns proof of work off. The error
 // names each setting that does not parse or is out of range.
 func parseSettings(cfg *Config) error {
 	return errors.Join(
@@ -168,9 +181,89 @@ func parseSettings(cfg *Config) error {
 		checkDuration("MESSAGE_MAX_AGE", cfg.MessageMaxAge),
 		checkDuration("QUEUE_MAX_AGE", cfg.QueueMaxAge),
 		checkDuration("SESSION_IDLE_TIMEOUT", cfg.SessionIdleTimeout),
+		parseTrustedProxies(&cfg.TrustedProxies),
 	)
 }
 
+// parseTrustedProxies sets *value to the addresses in TRUSTED_PROXIES, a
+// comma-separated list of networks such as 10.0.0.0/8 and single
+// addresses such as 192.0.2.1. The empty string trusts nothing. Viper
+// counts an empty environment variable as unset, so the environment is
+// read directly as well.
+func parseTrustedProxies(value **netipx.IPSet) error {
+	setting := viper.Get("TRUSTED_PROXIES")
+
+	fromEnv, set := os.LookupEnv("TRUSTED_PROXIES")
+	if set {
+		setting = fromEnv
+	}
+
+	// A YAML list or map in the config file is not a string, and
+	// viper.GetString would turn it into the empty string.
+	raw, isString := setting.(string)
+	if !isString {
+		return fmt.Errorf("cannot parse TRUSTED_PROXIES: %v is %w",
+			setting, errNotString)
+	}
+
+	var builder netipx.IPSetBuilder
+
+	if strings.TrimSpace(raw) != "" {
+		for entry := range strings.SplitSeq(raw, ",") {
+			err := addTrustedProxy(&builder, strings.TrimSpace(entry))
+			if err != nil {
+				return fmt.Errorf("cannot parse TRUSTED_PROXIES: %w", err)
+			}
+		}
+	}
+
+	ipSet, err := builder.IPSet()
+	if err != nil {
+		return fmt.Errorf("cannot parse TRUSTED_PROXIES: %w", err)
+	}
+
+	*value = ipSet
+
+	return nil
+}
+
+// addTrustedProxy adds entry, a network with a slash or a single
+// address, to builder. An IPv4 address written in IPv6 form, such as
+// ::ffff:192.0.2.1, is added as IPv4. A network in that form and an
+// address with a zone are errors, since no connection could match them.
+func addTrustedProxy(builder *netipx.IPSetBuilder, entry string) error {
+	if strings.Contains(entry, "/") {
+		prefix, err := netip.ParsePrefix(entry)
+		if err != nil {
+			return err //nolint:wrapcheck // the caller names the setting
+		}
+
+		// A connection from an IPv4 address is checked in IPv4 form.
+		if prefix.Addr().Is4In6() {
+			return fmt.Errorf("%w: %s is an IPv4 network in IPv6 form",
+				errNeverMatches, entry)
+		}
+
+		builder.AddPrefix(prefix)
+
+		return nil
+	}
+
+	addr, err := netip.ParseAddr(entry)
+	if err != nil {
+		return err //nolint:wrapcheck // the caller names the setting
+	}
+
+	// IPSet.Contains is false for every address with a zone.
+	if addr.Zone() != "" {
+		return fmt.Errorf("%w: %s has a zone", errNeverMatches, entry)
+	}
+
+	builder.Add(addr.Unmap())
+
+	return nil
+}
+
 // parseInt sets *value to the setting key read as a whole number. It
 // returns an error when the value does not parse or is below minimum.
 func parseInt(key string, minimum int, value *int) error {

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)
+			}
+		})
+	}
+}

internal/handlers/api.go

@@ -42,25 +42,9 @@ func (hdlr *Handlers) maxBodySize() int64 {
 	return defaultMaxBodySize
 }
 
-// clientIP extracts the connecting client's IP address
-// from the request, checking X-Forwarded-For and
-// X-Real-IP headers before falling back to RemoteAddr.
+// clientIP returns the IP address in request.RemoteAddr, which the
+// ClientIP middleware has set to the client's address.
 func clientIP(request *http.Request) string {
-	if forwarded := request.Header.Get("X-Forwarded-For"); forwarded != "" {
-		// X-Forwarded-For can contain a comma-separated list;
-		// the first entry is the original client.
-		parts := strings.SplitN(forwarded, ",", 2) //nolint:mnd
-		ip := strings.TrimSpace(parts[0])
-
-		if ip != "" {
-			return ip
-		}
-	}
-
-	if realIP := request.Header.Get("X-Real-IP"); realIP != "" {
-		return strings.TrimSpace(realIP)
-	}
-
 	host, _, err := net.SplitHostPort(request.RemoteAddr)
 	if err != nil {
 		return request.RemoteAddr

internal/handlers/api_test.go

@@ -4281,6 +4281,117 @@ func TestLoginRateLimitExceeded(t *testing.T) {
 	}
 }
 
+func TestLoginRateLimitIgnoresForwardedHeaders(t *testing.T) {
+	t.Parallel()
+
+	// The test client connects from 127.0.0.1, which is outside the
+	// default trusted proxies, so the server must ignore these headers.
+	tests := []struct {
+		name   string
+		header string
+	}{
+		{name: "X-Forwarded-For", header: "X-Forwarded-For"},
+		{name: "X-Real-IP", header: "X-Real-IP"},
+	}
+
+	loginBody, err := json.Marshal(map[string]string{
+		nickKey: "unknownnick", passwordKey: "wrongpassword",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			tserver := newTestServer(t)
+
+			// The default burst is 5 attempts, so the sixth gets 429
+			// although each claims a new address.
+			var status int
+
+			for attempt := range 6 {
+				request, rErr := http.NewRequestWithContext(
+					t.Context(), http.MethodPost,
+					tserver.url("/api/v1/login"),
+					bytes.NewReader(loginBody),
+				)
+				if rErr != nil {
+					t.Fatal(rErr)
+				}
+
+				request.Header.Set("Content-Type", "application/json")
+				request.Header.Set(
+					test.header, fmt.Sprintf("203.0.113.%d", attempt+1),
+				)
+
+				resp, dErr := http.DefaultClient.Do(request)
+				if dErr != nil {
+					t.Fatal(dErr)
+				}
+
+				status = resp.StatusCode
+				_ = resp.Body.Close()
+			}
+
+			if status != http.StatusTooManyRequests {
+				t.Fatalf("sixth attempt got %d, want 429", status)
+			}
+		})
+	}
+}
+
+func TestLoginRateLimitPerClientBehindTrustedProxy(t *testing.T) {
+	// The test client connects from 127.0.0.1. Trusted as a proxy, it
+	// passes each client's address in X-Forwarded-For. t.Setenv rules
+	// out t.Parallel.
+	t.Setenv("TRUSTED_PROXIES", "127.0.0.1")
+
+	tserver := newTestServer(t)
+
+	// An empty JSON object is refused after the rate limit counts the
+	// attempt, so no attempt checks a password or looks up a hostname.
+	loginFrom := func(forwardedFor string) int {
+		t.Helper()
+
+		request, err := http.NewRequestWithContext(
+			t.Context(), http.MethodPost,
+			tserver.url("/api/v1/login"), strings.NewReader("{}"),
+		)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		request.Header.Set("Content-Type", "application/json")
+		request.Header.Set("X-Forwarded-For", forwardedFor)
+
+		resp, err := http.DefaultClient.Do(request)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		_ = resp.Body.Close()
+
+		return resp.StatusCode
+	}
+
+	// The default burst is 5 attempts per address.
+	for range 5 {
+		loginFrom("203.0.113.1")
+	}
+
+	status := loginFrom("203.0.113.1")
+	if status != http.StatusTooManyRequests {
+		t.Errorf("sixth attempt from 203.0.113.1 got %d, want 429", status)
+	}
+
+	status = loginFrom("203.0.113.2")
+	if status == http.StatusTooManyRequests {
+		t.Error("first attempt from 203.0.113.2 got 429")
+	}
+}
+
 func TestLoginRateLimitAllowsNormalUse(t *testing.T) {
 	t.Parallel()
 

internal/middleware/middleware.go

@@ -5,6 +5,9 @@ import (
 	"log/slog"
 	"net"
 	"net/http"
+	"net/netip"
+	"slices"
+	"strings"
 	"time"
 
 	basicauth "github.com/99designs/basicauth-go"
@@ -15,6 +18,7 @@ import (
 	"github.com/slok/go-http-metrics/middleware/std"
 	"github.com/spf13/viper"
 	"go.uber.org/fx"
+	"go4.org/netipx"
 	"sneak.berlin/go/neoirc/internal/config"
 	"sneak.berlin/go/neoirc/internal/globals"
 	"sneak.berlin/go/neoirc/internal/logger"
@@ -62,6 +66,77 @@ func ipFromHostPort(hostPort string) string {
 	return host
 }
 
+// ClientIP returns middleware that replaces request.RemoteAddr with the
+// client's address and keeps the port of the TCP connection. The
+// X-Forwarded-For and X-Real-IP headers count only when the connection
+// comes from an address in TRUSTED_PROXIES.
+func (mware *Middleware) ClientIP() func(http.Handler) http.Handler {
+	trusted := mware.params.Config.TrustedProxies
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(
+				writer http.ResponseWriter,
+				request *http.Request,
+			) {
+				peer, err := netip.ParseAddrPort(request.RemoteAddr)
+				if err == nil {
+					client := clientAddr(
+						peer.Addr().Unmap(), request.Header, trusted,
+					)
+					request.RemoteAddr = netip.AddrPortFrom(
+						client, peer.Port(),
+					).String()
+				}
+
+				next.ServeHTTP(writer, request)
+			})
+	}
+}
+
+// clientAddr returns the client's address for a request from peer. A
+// peer outside trusted is the client, whatever its headers say. A
+// trusted peer is a proxy. Its X-Forwarded-For list is read from the
+// right, past the trusted addresses, to the first untrusted one; when
+// all are trusted, the client is the leftmost. Without X-Forwarded-For,
+// the client is X-Real-IP. An entry that is not an IP address gives
+// peer.
+func clientAddr(
+	peer netip.Addr, header http.Header, trusted *netipx.IPSet,
+) netip.Addr {
+	if !trusted.Contains(peer) {
+		return peer
+	}
+
+	forwarded := strings.Join(header.Values("X-Forwarded-For"), ",")
+	if forwarded == "" {
+		realIP, err := netip.ParseAddr(
+			strings.TrimSpace(header.Get("X-Real-IP")),
+		)
+		if err != nil {
+			return peer
+		}
+
+		return realIP.Unmap()
+	}
+
+	client := peer
+
+	for _, entry := range slices.Backward(strings.Split(forwarded, ",")) {
+		addr, err := netip.ParseAddr(strings.TrimSpace(entry))
+		if err != nil {
+			return peer
+		}
+
+		client = addr.Unmap()
+		if !trusted.Contains(client) {
+			break
+		}
+	}
+
+	return client
+}
+
 type loggingResponseWriter struct {
 	http.ResponseWriter
 

internal/middleware/middleware_test.go

@@ -0,0 +1,189 @@
+package middleware_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"testing"
+	"time"
+
+	"go4.org/netipx"
+	"sneak.berlin/go/neoirc/internal/config"
+	"sneak.berlin/go/neoirc/internal/globals"
+	"sneak.berlin/go/neoirc/internal/logger"
+	"sneak.berlin/go/neoirc/internal/middleware"
+)
+
+// The tests trust 10.0.0.0/8. Clients use the documentation ranges
+// 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24.
+const (
+	trustedNetwork = "10.0.0.0/8"
+	proxyPeer      = "10.0.0.1:4000"
+	directPeer     = "192.0.2.1:4000"
+	client         = "203.0.113.7"
+	clientWithPort = "203.0.113.7:4000"
+	realIPClient   = "203.0.113.8"
+)
+
+type clientIPTest struct {
+	name       string
+	remoteAddr string
+	forwarded  []string
+	realIP     string
+	want       string
+}
+
+func clientIPTests() []clientIPTest {
+	return []clientIPTest{
+		{
+			name:       "untrusted peer ignores forged headers",
+			remoteAddr: directPeer,
+			forwarded:  []string{client},
+			realIP:     realIPClient,
+			want:       directPeer,
+		},
+		{name: "trusted peer without headers", remoteAddr: proxyPeer, want: proxyPeer},
+		{
+			name:       "trusted peer gives rightmost untrusted entry",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{"198.51.100.9, 203.0.113.7, 10.0.0.2"},
+			want:       clientWithPort,
+		},
+		{
+			name:       "entries split over several headers",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{"198.51.100.9", client, "10.0.0.2"},
+			want:       clientWithPort,
+		},
+		{
+			name:       "all-trusted chain gives leftmost entry",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{"10.0.0.3, 10.0.0.2"},
+			want:       "10.0.0.3:4000",
+		},
+		{
+			name:       "X-Real-IP without X-Forwarded-For",
+			remoteAddr: proxyPeer,
+			realIP:     realIPClient,
+			want:       "203.0.113.8:4000",
+		},
+		{
+			name:       "X-Forwarded-For wins over X-Real-IP",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{client},
+			realIP:     realIPClient,
+			want:       clientWithPort,
+		},
+		{
+			name:       "unparseable X-Forwarded-For entry gives peer",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{"203.0.113.7, not-an-ip, 10.0.0.2"},
+			want:       proxyPeer,
+		},
+		{
+			name:       "unparseable entry past the client is not read",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{"not-an-ip, 203.0.113.7"},
+			want:       clientWithPort,
+		},
+		{
+			name:       "unparseable X-Real-IP gives peer",
+			remoteAddr: proxyPeer,
+			realIP:     "not-an-ip",
+			want:       proxyPeer,
+		},
+		{
+			name:       "mapped IPv6 peer is unmapped and trusted",
+			remoteAddr: "[::ffff:10.0.0.1]:4000",
+			forwarded:  []string{client},
+			want:       clientWithPort,
+		},
+		{
+			name:       "mapped IPv6 peer outside the set",
+			remoteAddr: "[::ffff:192.0.2.1]:4000",
+			forwarded:  []string{client},
+			want:       directPeer,
+		},
+		{
+			name:       "IPv6 client",
+			remoteAddr: proxyPeer,
+			forwarded:  []string{"2001:db8::1"},
+			want:       "[2001:db8::1]:4000",
+		},
+	}
+}
+
+func TestClientIP(t *testing.T) {
+	t.Parallel()
+
+	mware := newMiddleware(t, trustedNetwork)
+
+	for _, test := range clientIPTests() {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			var got string
+
+			handler := mware.ClientIP()(http.HandlerFunc(func(
+				_ http.ResponseWriter, request *http.Request,
+			) {
+				got = request.RemoteAddr
+			}))
+
+			request := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/", nil,
+			)
+			request.RemoteAddr = test.remoteAddr
+
+			for _, value := range test.forwarded {
+				request.Header.Add("X-Forwarded-For", value)
+			}
+
+			if test.realIP != "" {
+				request.Header.Set("X-Real-IP", test.realIP)
+			}
+
+			handler.ServeHTTP(httptest.NewRecorder(), request)
+
+			if got != test.want {
+				t.Errorf("RemoteAddr = %q, want %q", got, test.want)
+			}
+		})
+	}
+}
+
+// newMiddleware returns a Middleware that trusts the network trusted.
+func newMiddleware(t *testing.T, trusted string) *middleware.Middleware {
+	t.Helper()
+
+	var builder netipx.IPSetBuilder
+
+	builder.AddPrefix(netip.MustParsePrefix(trusted))
+
+	trustedSet, err := builder.IPSet()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	globs := &globals.Globals{
+		Appname:   "neoirc-test",
+		Version:   "test",
+		StartTime: time.Now(),
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: globs})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mware, err := middleware.New(nil, middleware.Params{
+		Logger:  log,
+		Globals: globs,
+		Config:  &config.Config{TrustedProxies: trustedSet},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return mware
+}

internal/server/routes.go

@@ -23,6 +23,7 @@ func (srv *Server) SetupRoutes() {
 
 	srv.router.Use(middleware.Recoverer)
 	srv.router.Use(middleware.RequestID)
+	srv.router.Use(srv.mw.ClientIP())
 	srv.router.Use(srv.mw.Logging())
 
 	if viper.GetString("METRICS_USERNAME") != "" {