smallwebwaf/internal/config/config.go

// Package config reads smallwebwaf's settings. Every setting is an
// environment variable whose name starts with SWWAF_, every setting has a
// default, and this package is the one place they are read.
package config

import (
	"errors"
	"fmt"
	"log/slog"
	"math"
	"net"
	"net/http"
	"net/netip"
	"net/url"
	"path/filepath"
	"slices"
	"strconv"
	"strings"
	"time"
	"unicode/utf8"
)

// Config is smallwebwaf's settings. A timeout, size or rate limit of zero
// is off.
type Config struct {
	// ListenAddr is where smallwebwaf listens (SWWAF_LISTEN_ADDR).
	ListenAddr string
	// UpstreamURL is the app (SWWAF_UPSTREAM_URL).
	UpstreamURL *url.URL
	// Observe is true in observe mode, when SWWAF_MODE is observe rather
	// than enforce: a request that SWWAF_DENY_NETS, a ban, the country
	// lists or a rate limit would refuse is passed to the app instead, and
	// no ban is made.
	Observe bool
	// TrustedProxies are the netblocks whose X-Forwarded-For is
	// believed (SWWAF_TRUSTED_PROXIES).
	TrustedProxies []netip.Prefix
	// ClientRequestTimeout bounds reading the whole request from the
	// client (SWWAF_CLIENT_REQUEST_TIMEOUT).
	ClientRequestTimeout time.Duration
	// ClientRequestHeaderMaxBytes is the largest request line and headers
	// a client may send (SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES). It is
	// never off, and always more than 4K.
	ClientRequestHeaderMaxBytes int64
	// ClientIdleTimeout bounds how long a kept-open client connection
	// may wait for its next request (SWWAF_CLIENT_IDLE_TIMEOUT).
	ClientIdleTimeout time.Duration
	// ClientResponseTimeout bounds writing the whole response to the
	// client (SWWAF_CLIENT_RESPONSE_TIMEOUT).
	ClientResponseTimeout time.Duration
	// UpstreamRequestTimeout bounds connecting to the app and writing
	// the whole request to it (SWWAF_UPSTREAM_REQUEST_TIMEOUT).
	UpstreamRequestTimeout time.Duration
	// UpstreamResponseTimeout bounds reading the whole response from
	// the app (SWWAF_UPSTREAM_RESPONSE_TIMEOUT).
	UpstreamResponseTimeout time.Duration
	// RequestMaxBytes is the largest request body
	// (SWWAF_REQUEST_MAX_BYTES).
	RequestMaxBytes int64
	// ResponseMaxBytes is the largest response body
	// (SWWAF_RESPONSE_MAX_BYTES).
	ResponseMaxBytes int64
	// AllowNets are the netblocks whose clients skip every check
	// (SWWAF_ALLOW_NETS). RateLimitExemptNets are those whose clients the
	// rate limits neither count nor refuse (SWWAF_RATE_LIMIT_EXEMPT_NETS).
	// DenyNets are those whose clients are always refused
	// (SWWAF_DENY_NETS).
	AllowNets           []netip.Prefix
	RateLimitExemptNets []netip.Prefix
	DenyNets            []netip.Prefix
	// RateLimitPerMinute, RateLimitPerHour and RateLimitPerDay are the
	// most requests a client may make in a minute, an hour and a day
	// (SWWAF_RATE_LIMIT_PER_MINUTE, SWWAF_RATE_LIMIT_PER_HOUR and
	// SWWAF_RATE_LIMIT_PER_DAY).
	RateLimitPerMinute int64
	RateLimitPerHour   int64
	RateLimitPerDay    int64
	// RateLimitExemptPaths are the path prefixes whose requests the rate
	// limits neither count nor refuse (SWWAF_RATE_LIMIT_EXEMPT_PATHS).
	// Each starts with /.
	RateLimitExemptPaths []string
	// DeniedCountries are the countries whose clients are refused
	// (SWWAF_DENIED_COUNTRIES). ExclusivelyAllowedCountries, when not
	// empty, are the only countries whose clients are let through
	// (SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES). Both hold two-letter codes in
	// capitals, as GeoJS gives them.
	DeniedCountries             []string
	ExclusivelyAllowedCountries []string
	// BanResponse is the status a refused client is answered with, 403
	// or 429, or 0 to close the connection without an answer
	// (SWWAF_BAN_RESPONSE). It answers a banned client, a request that
	// breaks a rate limit, SWWAF_DENY_NETS and the country lists.
	BanResponse int
	// LimitBanDuration is the ban for a first broken rate limit
	// (SWWAF_LIMIT_BAN_DURATION). A limit broken again within
	// LimitBanRepeatWindow after the last ban ended bans for three times
	// as long as that ban (SWWAF_LIMIT_BAN_REPEAT_WINDOW), and a ban that
	// would be longer than MaxBanDuration is permanent instead
	// (SWWAF_MAX_BAN_DURATION). None of them can be off.
	LimitBanDuration     time.Duration
	LimitBanRepeatWindow time.Duration
	MaxBanDuration       time.Duration
	// MaxBans is the most bans held (SWWAF_MAX_BANS).
	MaxBans int
	// BanScopeV4Prefix is the length of the netblock around an IPv4
	// client that a ban covers (SWWAF_BAN_SCOPE_V4_PREFIX).
	BanScopeV4Prefix int
	// StateDir is the directory of the state files, an absolute path
	// (SWWAF_STATE_DIR). bans.json is written StateWriteDelay after a ban
	// is made (SWWAF_STATE_WRITE_DELAY), and every state file every
	// StateCounterInterval (SWWAF_STATE_COUNTER_INTERVAL). Neither can be
	// off.
	StateDir             string
	StateWriteDelay      time.Duration
	StateCounterInterval time.Duration
	// MetricsToken is the bearer token a scraper sends for the metrics
	// (SWWAF_METRICS_TOKEN), "" while it is unset and the metrics are off.
	// MetricsTopN is how many countries get series of their own in the
	// metrics (SWWAF_METRICS_TOP_N).
	MetricsToken string
	MetricsTopN  int

	// settings are the values read, as given or by default, for the
	// log line at start.
	settings []slog.Attr
}

// off is the value that switches a timeout, a size limit or a rate limit
// off.
const off = "off"

const (
	day      = 24 * time.Hour
	kibibyte = 1 << 10
	mebibyte = 1 << 20
	gibibyte = 1 << 30
	ipv4Bits = 32
	// minTokenLength is the fewest characters a token may have.
	minTokenLength = 32
	// masked is what the log shows for a token that is set.
	masked = "********"
)

var (
	errNotDuration = errors.New(
		"is not a duration such as 90s, 15m or 7d, or off")
	errNotSize = errors.New(
		"is not a size such as 512K, 100M or 5G, or off")
	errNotCount = errors.New(
		"is not a whole number of requests such as 1000, or off")
	errNotPositive = errors.New("must be more than zero, or off")
	errEmptyItem   = errors.New("has an empty item in its list")
	errNotNetblock = errors.New(
		"is not a netblock such as 10.0.0.0/8, or an address")
	errNotListenAddr = errors.New(
		"is not an address to listen on, such as :8080")
	errNotUpstreamURL = errors.New(
		"is not a URL with only a scheme, a host and an optional port, " +
			"such as http://127.0.0.1:8081")
	errNotCountry = errors.New(
		"is not a two-letter country code such as de or kp")
	errOnBothLists          = errors.New("is in SWWAF_DENIED_COUNTRIES too")
	errNotOver4K            = errors.New("is not a size of more than 4K, such as 32K")
	errNotDurationAboveZero = errors.New(
		"is not a duration above zero, such as 1h or 7d")
	errNotNumberAboveZero = errors.New(
		"is not a whole number above zero, such as 5000")
	errNotBanResponse = errors.New("is not 403, 429 or close")
	errNotV4Prefix    = errors.New(
		"is not the length of an IPv4 netblock, from 0 to 32, such as 24")
	errNotAbsolutePath = errors.New(
		"is not an absolute path, such as /var/lib/smallwebwaf")
	errShortToken    = errors.New("is shorter than 32 characters")
	errNotMode       = errors.New("is not enforce or observe")
	errNotPathPrefix = errors.New(
		"is not a path prefix starting with /, such as /assets/")
)

// FromEnvironment reads the settings with lookupEnv, normally
// os.LookupEnv. A setting that is not set takes its default. A setting
// that is set but invalid is an error that names it.
func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
	env := &environment{lookupEnv: lookupEnv}
	cfg := &Config{
		ListenAddr:           env.address("SWWAF_LISTEN_ADDR", ":8080"),
		UpstreamURL:          env.appURL("SWWAF_UPSTREAM_URL", "http://127.0.0.1:8081"),
		Observe:              env.observe("SWWAF_MODE", "enforce"),
		TrustedProxies:       env.netblocks("SWWAF_TRUSTED_PROXIES", privateRanges),
		ClientRequestTimeout: env.duration("SWWAF_CLIENT_REQUEST_TIMEOUT", "60s"),
		ClientRequestHeaderMaxBytes: env.headerSize(
			"SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES", "32K"),
		ClientIdleTimeout:       env.duration("SWWAF_CLIENT_IDLE_TIMEOUT", "120s"),
		ClientResponseTimeout:   env.duration("SWWAF_CLIENT_RESPONSE_TIMEOUT", "30m"),
		UpstreamRequestTimeout:  env.duration("SWWAF_UPSTREAM_REQUEST_TIMEOUT", "60s"),
		UpstreamResponseTimeout: env.duration("SWWAF_UPSTREAM_RESPONSE_TIMEOUT", "30m"),
		RequestMaxBytes:         env.size("SWWAF_REQUEST_MAX_BYTES", "100M"),
		ResponseMaxBytes:        env.size("SWWAF_RESPONSE_MAX_BYTES", "5G"),
		AllowNets:               env.netblocks("SWWAF_ALLOW_NETS", ""),
		RateLimitExemptNets:     env.netblocks("SWWAF_RATE_LIMIT_EXEMPT_NETS", ""),
		DenyNets:                env.netblocks("SWWAF_DENY_NETS", ""),
		RateLimitPerMinute:      env.count("SWWAF_RATE_LIMIT_PER_MINUTE", "1000"),
		RateLimitPerHour:        env.count("SWWAF_RATE_LIMIT_PER_HOUR", "10000"),
		RateLimitPerDay:         env.count("SWWAF_RATE_LIMIT_PER_DAY", "50000"),
		RateLimitExemptPaths:    env.pathPrefixes("SWWAF_RATE_LIMIT_EXEMPT_PATHS", ""),
		DeniedCountries:         env.countries("SWWAF_DENIED_COUNTRIES", ""),
		ExclusivelyAllowedCountries: env.countries(
			"SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES", ""),
		BanResponse:          env.banResponse("SWWAF_BAN_RESPONSE", "403"),
		LimitBanDuration:     env.durationNotOff("SWWAF_LIMIT_BAN_DURATION", "1h"),
		LimitBanRepeatWindow: env.durationNotOff("SWWAF_LIMIT_BAN_REPEAT_WINDOW", "24h"),
		MaxBanDuration:       env.durationNotOff("SWWAF_MAX_BAN_DURATION", "7d"),
		MaxBans:              env.numberNotOff("SWWAF_MAX_BANS", "5000"),
		BanScopeV4Prefix:     env.v4Prefix("SWWAF_BAN_SCOPE_V4_PREFIX", "32"),
		StateDir:             env.absolutePath("SWWAF_STATE_DIR", "/var/lib/smallwebwaf"),
		StateWriteDelay:      env.durationNotOff("SWWAF_STATE_WRITE_DELAY", "10s"),
		StateCounterInterval: env.durationNotOff("SWWAF_STATE_COUNTER_INTERVAL", "15m"),
		MetricsToken:         env.token("SWWAF_METRICS_TOKEN"),
		MetricsTopN:          env.numberNotOff("SWWAF_METRICS_TOP_N", "50"),
	}

	for _, country := range cfg.ExclusivelyAllowedCountries {
		if slices.Contains(cfg.DeniedCountries, country) {
			env.check("SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES",
				fmt.Errorf("%q %w", country, errOnBothLists))
		}
	}

	if env.err != nil {
		return nil, env.err
	}

	cfg.settings = env.settings

	return cfg, nil
}

// privateRanges are the private address ranges, the default trusted
// proxies.
const privateRanges = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"

// LogValue makes a Config log as each setting's name with the value it was
// given, or its default.
func (c *Config) LogValue() slog.Value {
	return slog.GroupValue(c.settings...)
}

// environment is where FromEnvironment reads the settings: it notes each
// value for the log, and keeps the first error.
type environment struct {
	lookupEnv func(string) (string, bool)
	settings  []slog.Attr
	err       error
}

// value returns a setting's value, or its default when it is not set,
// and notes it for the log.
func (e *environment) value(name, defaultValue string) string {
	value, ok := e.lookupEnv(name)
	if !ok {
		value = defaultValue
	}

	e.settings = append(e.settings, slog.String(name, value))

	return value
}

// check keeps the first error, naming the setting it is about.
func (e *environment) check(name string, err error) {
	if err != nil && e.err == nil {
		e.err = fmt.Errorf("%s: %w", name, err)
	}
}

// address reads a setting that is an address to listen on.
func (e *environment) address(name, defaultValue string) string {
	address, err := parseListenAddr(e.value(name, defaultValue))
	e.check(name, err)

	return address
}

// appURL reads a setting that is the app's URL.
func (e *environment) appURL(name, defaultValue string) *url.URL {
	upstream, err := parseUpstreamURL(e.value(name, defaultValue))
	e.check(name, err)

	return upstream
}

// observe reads the setting that is the mode, enforce or observe, and
// reports whether it is observe.
func (e *environment) observe(name, defaultValue string) bool {
	mode := e.value(name, defaultValue)
	if mode != "enforce" && mode != "observe" {
		e.check(name, fmt.Errorf("%q %w", mode, errNotMode))
	}

	return mode == "observe"
}

// netblocks reads a setting that is a list of netblocks.
func (e *environment) netblocks(name, defaultValue string) []netip.Prefix {
	netblocks, err := parseNetblocks(e.value(name, defaultValue))
	e.check(name, err)

	return netblocks
}

// duration reads a setting that is a duration.
func (e *environment) duration(name, defaultValue string) time.Duration {
	duration, err := parseDuration(e.value(name, defaultValue))
	e.check(name, err)

	return duration
}

// size reads a setting that is a number of bytes.
func (e *environment) size(name, defaultValue string) int64 {
	size, err := parseSize(e.value(name, defaultValue))
	e.check(name, err)

	return size
}

// headerSize reads the setting that is the largest request line and
// headers.
func (e *environment) headerSize(name, defaultValue string) int64 {
	size, err := parseHeaderSize(e.value(name, defaultValue))
	e.check(name, err)

	return size
}

// count reads a setting that is a number of requests.
func (e *environment) count(name, defaultValue string) int64 {
	count, err := parseCount(e.value(name, defaultValue))
	e.check(name, err)

	return count
}

// pathPrefixes reads a setting that is a list of path prefixes.
func (e *environment) pathPrefixes(name, defaultValue string) []string {
	prefixes, err := parsePathPrefixes(e.value(name, defaultValue))
	e.check(name, err)

	return prefixes
}

// countries reads a setting that is a list of countries.
func (e *environment) countries(name, defaultValue string) []string {
	countries, err := parseCountries(e.value(name, defaultValue))
	e.check(name, err)

	return countries
}

// durationNotOff reads a setting that is a duration and, unlike a
// timeout, cannot be off.
func (e *environment) durationNotOff(name, defaultValue string) time.Duration {
	duration, err := parseDurationNotOff(e.value(name, defaultValue))
	e.check(name, err)

	return duration
}

// numberNotOff reads a setting that is a whole number above zero, which
// cannot be off.
func (e *environment) numberNotOff(name, defaultValue string) int {
	number, err := parseNumberNotOff(e.value(name, defaultValue))
	e.check(name, err)

	return number
}

// banResponse reads a setting that is how a refused client is answered.
func (e *environment) banResponse(name, defaultValue string) int {
	status, err := parseBanResponse(e.value(name, defaultValue))
	e.check(name, err)

	return status
}

// v4Prefix reads a setting that is the length of an IPv4 netblock.
func (e *environment) v4Prefix(name, defaultValue string) int {
	length, err := parseV4Prefix(e.value(name, defaultValue))
	e.check(name, err)

	return length
}

// absolutePath reads a setting that is an absolute path.
func (e *environment) absolutePath(name, defaultValue string) string {
	path := e.value(name, defaultValue)
	if !filepath.IsAbs(path) {
		e.check(name, fmt.Errorf("%q %w", path, errNotAbsolutePath))
	}

	return path
}

// token reads a setting that is a bearer token. Unset, it is "", which
// switches off what it guards; set, it must be at least minTokenLength
// characters. Neither the log nor an error shows its value.
func (e *environment) token(name string) string {
	value, set := e.lookupEnv(name)
	if !set {
		e.settings = append(e.settings, slog.String(name, ""))

		return ""
	}

	e.settings = append(e.settings, slog.String(name, masked))

	if utf8.RuneCountInString(value) < minTokenLength {
		e.check(name, errShortToken)
	}

	return value
}

// parseDuration reads a duration in Go's syntax, such as 90s or 15m, a
// whole number of days such as 7d, or off.
func parseDuration(value string) (time.Duration, error) {
	if value == off {
		return 0, nil
	}

	duration, err := durationOrDays(value)
	if err != nil {
		return 0, fmt.Errorf("%q %w", value, errNotDuration)
	}

	if duration <= 0 {
		return 0, fmt.Errorf("%q %w", value, errNotPositive)
	}

	return duration, nil
}

// durationOrDays reads Go's duration syntax, or a whole number of days.
func durationOrDays(value string) (time.Duration, error) {
	days, isDays := strings.CutSuffix(value, "d")
	if !isDays {
		return time.ParseDuration(value)
	}

	n, err := strconv.ParseInt(days, 10, 64)
	if err != nil || n < 0 || n > math.MaxInt64/int64(day) {
		return 0, errNotDuration
	}

	return time.Duration(n) * day, nil
}

// parseSize reads a number of bytes with an optional K, M or G suffix, in
// powers of 1024 (1K is 1024 bytes), or off.
func parseSize(value string) (int64, error) {
	if value == off {
		return 0, nil
	}

	number, unit := splitUnit(value)

	n, err := strconv.ParseInt(number, 10, 64)
	if err != nil || n > math.MaxInt64/unit {
		return 0, fmt.Errorf("%q %w", value, errNotSize)
	}

	if n <= 0 {
		return 0, fmt.Errorf("%q %w", value, errNotPositive)
	}

	return n * unit, nil
}

// parseHeaderSize reads the largest request line and headers: a size as
// parseSize reads it, but more than 4K and never off. Go's server reads 4K
// past the limit it is given before it refuses, so proxy.New gives it this
// size less 4K, which must leave a limit.
func parseHeaderSize(value string) (int64, error) {
	size, err := parseSize(value)
	if err != nil || size <= 4*kibibyte {
		return 0, fmt.Errorf("%q %w", value, errNotOver4K)
	}

	return size, nil
}

// splitUnit splits a size into its number and the bytes its suffix
// stands for.
func splitUnit(value string) (string, int64) {
	switch {
	case strings.HasSuffix(value, "K"):
		return strings.TrimSuffix(value, "K"), kibibyte
	case strings.HasSuffix(value, "M"):
		return strings.TrimSuffix(value, "M"), mebibyte
	case strings.HasSuffix(value, "G"):
		return strings.TrimSuffix(value, "G"), gibibyte
	default:
		return value, 1
	}
}

// parseCount reads a whole number of requests, or off.
func parseCount(value string) (int64, error) {
	if value == off {
		return 0, nil
	}

	n, err := strconv.ParseInt(value, 10, 64)
	if err != nil {
		return 0, fmt.Errorf("%q %w", value, errNotCount)
	}

	if n <= 0 {
		return 0, fmt.Errorf("%q %w", value, errNotPositive)
	}

	return n, nil
}

// parseDurationNotOff reads a duration above zero, as parseDuration does,
// but not off.
func parseDurationNotOff(value string) (time.Duration, error) {
	duration, err := parseDuration(value)
	if err != nil || duration == 0 {
		return 0, fmt.Errorf("%q %w", value, errNotDurationAboveZero)
	}

	return duration, nil
}

// parseNumberNotOff reads a whole number above zero.
func parseNumberNotOff(value string) (int, error) {
	n, err := strconv.Atoi(value)
	if err != nil || n <= 0 {
		return 0, fmt.Errorf("%q %w", value, errNotNumberAboveZero)
	}

	return n, nil
}

// parseBanResponse reads how a refused client is answered: 403, 429, or
// close, which is 0.
func parseBanResponse(value string) (int, error) {
	switch value {
	case "403":
		return http.StatusForbidden, nil
	case "429":
		return http.StatusTooManyRequests, nil
	case "close":
		return 0, nil
	default:
		return 0, fmt.Errorf("%q %w", value, errNotBanResponse)
	}
}

// parseV4Prefix reads the length of an IPv4 netblock, from 0 to 32.
func parseV4Prefix(value string) (int, error) {
	n, err := strconv.Atoi(value)
	if err != nil || n < 0 || n > ipv4Bits {
		return 0, fmt.Errorf("%q %w", value, errNotV4Prefix)
	}

	return n, nil
}

// parseList splits a comma-separated list and trims the spaces around
// each item. An empty value is an empty list.
func parseList(value string) ([]string, error) {
	if strings.TrimSpace(value) == "" {
		return []string{}, nil
	}

	items := strings.Split(value, ",")
	for i, item := range items {
		items[i] = strings.TrimSpace(item)
		if items[i] == "" {
			return nil, fmt.Errorf("%q %w", value, errEmptyItem)
		}
	}

	return items, nil
}

// parseNetblocks reads a comma-separated list of netblocks.
func parseNetblocks(value string) ([]netip.Prefix, error) {
	items, err := parseList(value)
	if err != nil {
		return nil, err
	}

	netblocks := make([]netip.Prefix, 0, len(items))

	for _, item := range items {
		netblock, err := parseNetblock(item)
		if err != nil {
			return nil, err
		}

		netblocks = append(netblocks, netblock)
	}

	return netblocks, nil
}

// parseNetblock reads a netblock in CIDR form, such as 10.0.0.0/8. A bare
// address is a netblock of that address alone, a /32 or a /128.
func parseNetblock(value string) (netip.Prefix, error) {
	if strings.Contains(value, "/") {
		netblock, err := netip.ParsePrefix(value)
		if err != nil {
			return netip.Prefix{}, fmt.Errorf("%q %w", value, errNotNetblock)
		}

		return netblock.Masked(), nil
	}

	addr, err := netip.ParseAddr(value)
	if err != nil || addr.Zone() != "" {
		return netip.Prefix{}, fmt.Errorf("%q %w", value, errNotNetblock)
	}

	return netip.PrefixFrom(addr, addr.BitLen()), nil
}

// parsePathPrefixes reads a comma-separated list of path prefixes, each
// starting with /.
func parsePathPrefixes(value string) ([]string, error) {
	prefixes, err := parseList(value)
	if err != nil {
		return nil, err
	}

	for _, prefix := range prefixes {
		if !strings.HasPrefix(prefix, "/") {
			return nil, fmt.Errorf("%q %w", prefix, errNotPathPrefix)
		}
	}

	return prefixes, nil
}

// countryCodes are the two-letter codes ISO 3166-1 assigns today, and XK,
// the code in common use for Kosovo. golang.org/x/text/language cannot
// check them: it also takes withdrawn codes such as su, and reserved ones
// such as ac, as countries.
const countryCodes = `
AD AE AF AG AI AL AM AO AQ AR AS AT AU AW AX AZ
BA BB BD BE BF BG BH BI BJ BL BM BN BO BQ BR BS BT BV BW BY BZ
CA CC CD CF CG CH CI CK CL CM CN CO CR CU CV CW CX CY CZ
DE DJ DK DM DO DZ
EC EE EG EH ER ES ET
FI FJ FK FM FO FR
GA GB GD GE GF GG GH GI GL GM GN GP GQ GR GS GT GU GW GY
HK HM HN HR HT HU
ID IE IL IM IN IO IQ IR IS IT
JE JM JO JP
KE KG KH KI KM KN KP KR KW KY KZ
LA LB LC LI LK LR LS LT LU LV LY
MA MC MD ME MF MG MH MK ML MM MN MO MP MQ MR MS MT MU MV MW MX MY MZ
NA NC NE NF NG NI NL NO NP NR NU NZ
OM
PA PE PF PG PH PK PL PM PN PR PS PT PW PY
QA
RE RO RS RU RW
SA SB SC SD SE SG SH SI SJ SK SL SM SN SO SR SS ST SV SX SY SZ
TC TD TF TG TH TJ TK TL TM TN TO TR TT TV TW TZ
UA UG UM US UY UZ
VA VC VE VG VI VN VU
WF WS
XK
YE YT
ZA ZM ZW
`

// parseCountries reads a comma-separated list of country codes in either
// case, and returns them in capitals.
func parseCountries(value string) ([]string, error) {
	items, err := parseList(value)
	if err != nil {
		return nil, err
	}

	known := strings.Fields(countryCodes)
	countries := make([]string, 0, len(items))

	for _, item := range items {
		country := strings.ToUpper(item)
		if !slices.Contains(known, country) {
			return nil, fmt.Errorf("%q %w", item, errNotCountry)
		}

		countries = append(countries, country)
	}

	return countries, nil
}

// parseListenAddr checks an address to listen on: an optional host and a
// port number.
func parseListenAddr(value string) (string, error) {
	_, port, err := net.SplitHostPort(value)
	if err != nil {
		return "", fmt.Errorf("%q %w", value, errNotListenAddr)
	}

	_, err = strconv.ParseUint(port, 10, 16)
	if err != nil {
		return "", fmt.Errorf("%q %w", value, errNotListenAddr)
	}

	return value, nil
}

// parseUpstreamURL reads the app's URL: http or https, a host and an
// optional port from 1 to 65535, and nothing else, since the request's
// own path and query go to the app unchanged.
func parseUpstreamURL(value string) (*url.URL, error) {
	upstream, err := url.Parse(value)
	if err != nil {
		return nil, fmt.Errorf("%q %w", value, errNotUpstreamURL)
	}

	onlySchemeAndHost := (upstream.Scheme == "http" || upstream.Scheme == "https") &&
		upstream.Hostname() != "" && upstream.User == nil && upstream.Opaque == "" &&
		(upstream.Path == "" || upstream.Path == "/") &&
		upstream.RawQuery == "" && upstream.Fragment == ""
	if !onlySchemeAndHost {
		return nil, fmt.Errorf("%q %w", value, errNotUpstreamURL)
	}

	if upstream.Port() != "" {
		port, err := strconv.ParseUint(upstream.Port(), 10, 16)
		if err != nil || port == 0 {
			return nil, fmt.Errorf("%q %w", value, errNotUpstreamURL)
		}
	}

	return upstream, nil
}