checker-dav/internal/dav/discover.go

package dav

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"strings"
)

// Discover resolves the DAV context URL for domain following RFC 6764:
// /.well-known/{caldav,carddav} first (cheap and works for the common case),
// then SRV/TXT. An explicit override shortcuts everything.
//
// The returned Observation.Discovery is fully populated with whatever was
// learned along the way, even if every step fails — the report leans on the
// captured evidence to tell the user which leg of the discovery broke.
func Discover(ctx context.Context, client *http.Client, kind Kind, domain, explicitURL string) DiscoveryResult {
	res := DiscoveryResult{}

	if explicitURL != "" {
		res.ContextURL = explicitURL
		res.Source = "explicit"
		return res
	}

	// 1. /.well-known — this is the #1 misconfig hotspot, so we always probe
	//    it even if SRV below might have worked, to surface the mistake.
	wellKnown := "https://" + domain + kind.WellKnownPath()
	res.WellKnownURL = wellKnown
	ctxURL, chain, code, err := followWellKnown(ctx, client, wellKnown)
	res.WellKnownCode = code
	res.WellKnownChain = chain
	if err != nil {
		res.WellKnownError = err.Error()
	} else if ctxURL != "" {
		res.ContextURL = ctxURL
		res.Source = "well-known"
	}

	// 2. SRV + TXT fallback (also informational even when well-known worked).
	discoverSRV(ctx, kind, domain, &res)

	if res.ContextURL == "" && len(res.SecureSRV) > 0 {
		target := res.SecureSRV[0]
		path := res.TXTPath
		if path == "" {
			path = "/"
		}
		res.ContextURL = srvURL(target, path, true)
		res.Source = "srv-txt"
	}

	if res.ContextURL == "" && res.Error == "" {
		res.Error = "could not resolve a context URL via /.well-known or SRV"
	}
	return res
}

// followWellKnown issues a GET against path and follows up to 5 redirects
// manually so we can capture the redirect chain. The well-known endpoint
// SHOULD return a 3xx (RFC 6764 §5); returning 200 is a common misconfig we
// want to flag, and 404 means the site-owner forgot to set it up.
func followWellKnown(ctx context.Context, client *http.Client, u string) (finalURL string, chain []string, firstCode int, err error) {
	chain = make([]string, 0, 5)
	cur := u
	for i := 0; i < 5; i++ {
		req, reqErr := http.NewRequestWithContext(ctx, http.MethodGet, cur, nil)
		if reqErr != nil {
			return "", chain, firstCode, reqErr
		}
		// Use a no-redirect client snapshot so we observe each hop.
		c := *client
		c.CheckRedirect = func(_ *http.Request, _ []*http.Request) error { return http.ErrUseLastResponse }
		resp, doErr := c.Do(req)
		if doErr != nil {
			return "", chain, firstCode, doErr
		}
		resp.Body.Close()
		chain = append(chain, fmt.Sprintf("%d %s", resp.StatusCode, cur))
		// We track the *first* response code because the rule cares about
		// whether /.well-known itself redirected. A chain like 301→200 is
		// correct; a chain starting with 200 is the misconfig we flag.
		if i == 0 {
			firstCode = resp.StatusCode
		}

		if resp.StatusCode >= 300 && resp.StatusCode < 400 {
			loc := resp.Header.Get("Location")
			if loc == "" {
				return "", chain, firstCode, errors.New("redirect with empty Location header")
			}
			next, parseErr := resolveLocation(cur, loc)
			if parseErr != nil {
				return "", chain, firstCode, parseErr
			}
			cur = next
			continue
		}

		if resp.StatusCode == http.StatusOK {
			return cur, chain, firstCode, nil
		}

		return "", chain, firstCode, fmt.Errorf("unexpected status %d", resp.StatusCode)
	}
	return "", chain, firstCode, errors.New("too many redirects")
}

func resolveLocation(base, loc string) (string, error) {
	baseURL, err := url.Parse(base)
	if err != nil {
		return "", err
	}
	locURL, err := url.Parse(loc)
	if err != nil {
		return "", err
	}
	return baseURL.ResolveReference(locURL).String(), nil
}

func discoverSRV(ctx context.Context, kind Kind, domain string, res *DiscoveryResult) {
	resolver := net.DefaultResolver

	type srvResult struct {
		records []SRVRecord
		err     error
	}
	secureCh := make(chan srvResult, 1)
	plainCh := make(chan srvResult, 1)
	go func() {
		r, err := lookupSRV(ctx, resolver, kind.ServiceName(true), "tcp", domain)
		secureCh <- srvResult{r, err}
	}()
	go func() {
		r, err := lookupSRV(ctx, resolver, kind.ServiceName(false), "tcp", domain)
		plainCh <- srvResult{r, err}
	}()

	secureRes := <-secureCh
	if secureRes.err != nil && !isNoSuchHost(secureRes.err) {
		res.SRVError = secureRes.err.Error()
	}
	res.SecureSRV = secureRes.records

	plainRes := <-plainCh
	if plainRes.err != nil && !isNoSuchHost(plainRes.err) && res.SRVError == "" {
		res.SRVError = plainRes.err.Error()
	}
	res.PlaintextSRV = plainRes.records

	// Pull the TXT path hint from whichever SRV target we plan to use.
	var txtName string
	if len(res.SecureSRV) > 0 {
		txtName = kind.ServiceName(true) + "._tcp." + trimTrailingDot(res.SecureSRV[0].Target)
	} else if len(res.PlaintextSRV) > 0 {
		txtName = kind.ServiceName(false) + "._tcp." + trimTrailingDot(res.PlaintextSRV[0].Target)
	}
	if txtName != "" {
		txts, err := resolver.LookupTXT(ctx, txtName)
		if err != nil && !isNoSuchHost(err) {
			res.TXTError = err.Error()
		}
		for _, t := range txts {
			if strings.HasPrefix(t, "path=") {
				res.TXTPath = strings.TrimPrefix(t, "path=")
				break
			}
		}
	}
}

func lookupSRV(ctx context.Context, r *net.Resolver, service, proto, name string) ([]SRVRecord, error) {
	_, addrs, err := r.LookupSRV(ctx, strings.TrimPrefix(service, "_"), proto, name)
	if err != nil {
		return nil, err
	}
	out := make([]SRVRecord, 0, len(addrs))
	for _, a := range addrs {
		out = append(out, SRVRecord{
			Target:   trimTrailingDot(a.Target),
			Port:     a.Port,
			Priority: a.Priority,
			Weight:   a.Weight,
		})
	}
	return out, nil
}

func srvURL(r SRVRecord, path string, secure bool) string {
	scheme := "https"
	defaultPort := uint16(443)
	if !secure {
		scheme = "http"
		defaultPort = 80
	}
	host := r.Target
	if r.Port != defaultPort {
		host = fmt.Sprintf("%s:%d", r.Target, r.Port)
	}
	if !strings.HasPrefix(path, "/") {
		path = "/" + path
	}
	return scheme + "://" + host + path
}

func trimTrailingDot(s string) string {
	return strings.TrimSuffix(s, ".")
}

func isNoSuchHost(err error) bool {
	var dnsErr *net.DNSError
	if errors.As(err, &dnsErr) {
		return dnsErr.IsNotFound
	}
	return false
}