checker-sip/checker/sip_probe.go

package checker

import (
	"bufio"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"io"
	"net"
	"strconv"
	"strings"
)

// sipResponse is the minimal parsed form of a SIP response line + headers
// we need to power the rule and the report.
type sipResponse struct {
	StatusCode   int
	StatusPhrase string
	Server       string
	UserAgent    string // some stacks use this instead of Server
	Contact      string
	Allow        []string
}

// buildOptionsRequest returns a ready-to-send SIP OPTIONS message for
// the given target / transport pair.
//
// The message is deliberately minimal and RFC 3261 §11-conforming: just
// enough for any SIP stack to recognise it as an OPTIONS ping.
func buildOptionsRequest(target string, port uint16, transport Transport, localAddr string, userAgent string) string {
	tUpper := "UDP"
	switch transport {
	case TransportTCP:
		tUpper = "TCP"
	case TransportTLS:
		tUpper = "TLS"
	}

	branch := "z9hG4bK-" + randHex(8)
	tag := randHex(6)
	// Use an RFC 2606 reserved TLD so the host part of Call-ID never
	// resolves to a real domain we don't control.
	callID := randHex(12) + "@checker-sip.invalid"

	sipScheme := "sip"
	if transport == TransportTLS {
		sipScheme = "sips"
	}

	var requestURI string
	if transport == TransportTLS {
		requestURI = fmt.Sprintf("%s:%s:%d", sipScheme, target, port)
	} else {
		requestURI = fmt.Sprintf("%s:%s:%d;transport=%s", sipScheme, target, port, strings.ToLower(tUpper))
	}

	// Via uses the remote transport name; local address is a best-effort
	// hint that servers echo back via ;rport. We don't actually listen
	// on it, this is a one-shot probe.
	lines := []string{
		"OPTIONS " + requestURI + " SIP/2.0",
		"Via: SIP/2.0/" + tUpper + " " + localAddr + ";branch=" + branch + ";rport",
		"Max-Forwards: 70",
		"From: \"happyDomain\" <sip:check@checker-sip.invalid>;tag=" + tag,
		"To: <" + sipScheme + ":ping@" + target + ">",
		"Call-ID: " + callID,
		"CSeq: 1 OPTIONS",
		"User-Agent: " + userAgent,
		"Accept: application/sdp",
		"Content-Length: 0",
	}
	return strings.Join(lines, "\r\n") + "\r\n\r\n"
}

func randHex(n int) string {
	b := make([]byte, n)
	_, _ = rand.Read(b)
	return hex.EncodeToString(b)
}

// parseSIPResponse reads a SIP response from r and extracts the fields
// we care about. It tolerates bodies (reads Content-Length bytes) and
// truncates defensively so a chatty server can't OOM us.
func parseSIPResponse(r io.Reader) (*sipResponse, error) {
	br := bufio.NewReaderSize(io.LimitReader(r, 16*1024), 8*1024)

	statusLine, err := br.ReadString('\n')
	if err != nil {
		return nil, fmt.Errorf("read status line: %w", err)
	}
	statusLine = strings.TrimRight(statusLine, "\r\n")
	if !strings.HasPrefix(statusLine, "SIP/2.0 ") {
		return nil, fmt.Errorf("not a SIP response: %q", trunc(statusLine, 80))
	}
	_, rest, _ := strings.Cut(statusLine, " ")
	parts := strings.SplitN(rest, " ", 2)
	code, convErr := strconv.Atoi(strings.TrimSpace(parts[0]))
	if convErr != nil {
		return nil, fmt.Errorf("non-numeric status code %q", parts[0])
	}
	phrase := ""
	if len(parts) == 2 {
		phrase = strings.TrimSpace(parts[1])
	}

	resp := &sipResponse{StatusCode: code, StatusPhrase: phrase}

	for {
		line, err := br.ReadString('\n')
		if err != nil && err != io.EOF {
			return resp, fmt.Errorf("read header: %w", err)
		}
		line = strings.TrimRight(line, "\r\n")
		if line == "" {
			break
		}
		// Fold continuation lines per RFC 3261 §7.3.1: a header line
		// starting with whitespace continues the previous one. We don't
		// need perfect fidelity, so just skip continuations.
		if line[0] == ' ' || line[0] == '\t' {
			continue
		}
		colon := strings.IndexByte(line, ':')
		if colon < 0 {
			continue
		}
		name := strings.ToLower(strings.TrimSpace(line[:colon]))
		value := strings.TrimSpace(line[colon+1:])

		switch name {
		case "server", "s":
			resp.Server = value
		case "user-agent":
			resp.UserAgent = value
		case "contact", "m":
			if resp.Contact == "" {
				resp.Contact = value
			}
		case "allow":
			// SIP allows multiple Allow headers *or* comma-separated;
			// handle both.
			for m := range strings.SplitSeq(value, ",") {
				m = strings.TrimSpace(strings.ToUpper(m))
				if m != "" {
					resp.Allow = append(resp.Allow, m)
				}
			}
		}
		if err == io.EOF {
			break
		}
	}

	return resp, nil
}

func trunc(s string, n int) string {
	if len(s) <= n {
		return s
	}
	return s[:n] + "…"
}

// localAddrFor returns a best-effort "host:port" describing the local
// side of conn, or "0.0.0.0:0" if conn is nil (UDP probe before dial).
func localAddrFor(conn net.Conn) string {
	if conn == nil {
		return "0.0.0.0:0"
	}
	return conn.LocalAddr().String()
}