1package main

2

3import (

4 "bufio"

5 "context"

6 "crypto/tls"

7 "errors"

8 "fmt"

9 "io"

10 "log"

11 "net"

12 "net/mail"

13 "os"

14 "path/filepath"

15 "slices"

16 "strings"

17 "time"

18

19 "github.com/mjl-/sconf"

20

21 "github.com/mjl-/mox/dns"

22 "github.com/mjl-/mox/mox-"

23 "github.com/mjl-/mox/sasl"

24 "github.com/mjl-/mox/smtp"

25 "github.com/mjl-/mox/smtpclient"

26)

27

28var submitconf struct {

29 LocalHostname string `sconf-doc:"Hosts don't always have an FQDN, set it explicitly, for EHLO."`

30 Host string `sconf-doc:"Host to dial for delivery, e.g. mail.<domain>."`

31 Port int `sconf-doc:"Port to dial for delivery, e.g. 465 for submissions, 587 for submission, or perhaps 25 for smtp."`

32 TLS bool `sconf-doc:"Connect with TLS. Usually for connections to port 465."`

33 STARTTLS bool `sconf-doc:"After starting in plain text, use STARTTLS to enable TLS. For port 587 and 25."`

34 Username string `sconf-doc:"For SMTP authentication."`

35 Password string `sconf-doc:"For password-based SMTP authentication, e.g. SCRAM-SHA-256-PLUS, CRAM-MD5, PLAIN."`

36 AuthMethod string `sconf-doc:"If set, only attempt this authentication mechanism. E.g. SCRAM-SHA-256-PLUS, SCRAM-SHA-256, SCRAM-SHA-1-PLUS, SCRAM-SHA-1, CRAM-MD5, PLAIN. If not set, any mutually supported algorithm can be used, in order listed, from most to least secure. It is recommended to specify the strongest authentication mechanism known to be implemented by the server, to prevent mechanism downgrade attacks."`

37 From string `sconf-doc:"Address for MAIL FROM in SMTP and From-header in message."`

38 DefaultDestination string `sconf:"optional" sconf-doc:"Used when specified address does not contain an @ and may be a local user (eg root)."`

39 RequireTLS RequireTLSOption `sconf:"optional" sconf-doc:"If yes, submission server must implement SMTP REQUIRETLS extension, and connection to submission server must use verified TLS. If no, a TLS-Required header with value no is added to the message, allowing fallback to unverified TLS or plain text delivery despite recpient domain policies. By default, the submission server will follow the policies of the recipient domain (MTA-STS and/or DANE), and apply unverified opportunistic TLS with STARTTLS."`

40}

41

42type RequireTLSOption string

43

44const (

45 RequireTLSDefault RequireTLSOption = ""

46 RequireTLSYes RequireTLSOption = "yes"

47 RequireTLSNo RequireTLSOption = "no"

48)

49

50func cmdConfigDescribeSendmail(c *cmd) {

51 c.params = ">/etc/moxsubmit.conf"

52 c.help = `Describe configuration for mox when invoked as sendmail.`

53 if len(c.Parse()) != 0 {

54 c.Usage()

55 }

56

57 err := sconf.Describe(os.Stdout, submitconf)

58 xcheckf(err, "describe config")

59}

60

61func cmdSendmail(c *cmd) {

62 c.params = "[-Fname] [ignoredflags] [-t] [<message]"

63 c.help = `Sendmail is a drop-in replacement for /usr/sbin/sendmail to deliver emails sent by unix processes like cron.

64

65If invoked as "sendmail", it will act as sendmail for sending messages. Its

66intention is to let processes like cron send emails. Messages are submitted to

67an actual mail server over SMTP. The destination mail server and credentials are

68configured in /etc/moxsubmit.conf, see mox config describe-sendmail. The From

69message header is rewritten to the configured address. When the addressee

70appears to be a local user, because without @, the message is sent to the

71configured default address.

72

73If submitting an email fails, it is added to a directory moxsubmit.failures in

74the user's home directory.

75

76Most flags are ignored to fake compatibility with other sendmail

77implementations. A single recipient or the -t flag with a To-header is required.

78With the -t flag, Cc and Bcc headers are not handled specially, so Bcc is not

79removed and the addresses do not receive the email.

80

81/etc/moxsubmit.conf should be group-readable and not readable by others and this

82binary should be setgid that group:

83

84 groupadd moxsubmit

85 install -m 2755 -o root -g moxsubmit mox /usr/sbin/sendmail

86 touch /etc/moxsubmit.conf

87 chown root:moxsubmit /etc/moxsubmit.conf

88 chmod 640 /etc/moxsubmit.conf

89 # edit /etc/moxsubmit.conf

90`

91

92 // We are faking that we parse flags, this is non-standard, we want to be lax and ignore most flags.

93 args := c.flagArgs

94 c.flagArgs = []string{}

95 c.Parse() // We still have to call Parse for the usage gathering.

96

97 // Typical cron usage of sendmail:

98 // anacron: https://salsa.debian.org/debian/anacron/-/blob/c939c8c80fc9419c11a5e6be5cbe84f03ad332fd/runjob.c#L183

99 // cron: https://github.com/vixie/cron/blob/fea7a6c5421f88f034be8eef66a84d8b65b5fbe0/config.h#L41

100

101 var from string

102 var tflag bool // If set, we need to take the recipient(s) from the message headers. We only do one recipient, in To.

103 o := 0

104 for i, s := range args {

105 if s == "--" {

106 o = i + 1

107 break

108 }

109 if !strings.HasPrefix(s, "-") {

110 o = i

111 break

112 }

113 s = s[1:]

114 if strings.HasPrefix(s, "F") {

115 from = s[1:]

116 log.Printf("ignoring -F %q", from) // todo

117 } else if s == "t" {

118 tflag = true

119 }

120 o = i + 1

121 // Ignore options otherwise.

122 // todo: we may want to parse more flags. some invocations may not be about sending a message. for now, we'll assume sendmail is only invoked to send a message.

123 }

124 args = args[o:]

125

126 // todo: perhaps allow configuration of config file through environment variable? have to keep in mind that mox with setgid moxsubmit would be reading the file.

127 const confPath = "/etc/moxsubmit.conf"

128 err := sconf.ParseFile(confPath, &submitconf)

129 xcheckf(err, "parsing config")

130

131 var recipient string

132 if len(args) == 1 && !tflag {

133 recipient = args[0]

134 if !strings.Contains(recipient, "@") {

135 if submitconf.DefaultDestination == "" {

136 log.Fatalf("recipient %q has no @ and no default destination configured", recipient)

137 }

138 recipient = submitconf.DefaultDestination

139 } else {

140 _, err := smtp.ParseAddress(args[0])

141 xcheckf(err, "parsing recipient address")

142 }

143 } else if !tflag || len(args) != 0 {

144 log.Fatalln("need either exactly 1 recipient, or -t")

145 }

146

147 // Read message and build message we are going to send. We replace \n

148 // with \r\n, and we replace the From header.

149 // todo: should we also wrap lines that are too long? perhaps only if this is just text, no multipart?

150 var sb strings.Builder

151 r := bufio.NewReader(os.Stdin)

152 header := true // Whether we are in the header.

153 fmt.Fprintf(&sb, "From: <%s>\r\n", submitconf.From)

154 var haveTo bool

155 for {

156 line, err := r.ReadString('\n')

157 if err != nil && err != io.EOF {

158 xcheckf(err, "reading message")

159 }

160 if line != "" {

161 if !strings.HasSuffix(line, "\n") {

162 line += "\n"

163 }

164 if !strings.HasSuffix(line, "\r\n") {

165 line = line[:len(line)-1] + "\r\n"

166 }

167 if header && line == "\r\n" {

168 // Bare \r\n marks end of header.

169 if !haveTo {

170 line = fmt.Sprintf("To: <%s>\r\n", recipient) + line

171 }

172 if submitconf.RequireTLS == RequireTLSNo {

173 line = "TLS-Required: No\r\n" + line

174 }

175 header = false

176 } else if header {

177 t := strings.SplitN(line, ":", 2)

178 if len(t) != 2 {

179 log.Fatalf("invalid message, missing colon in header")

180 }

181 k := strings.ToLower(t[0])

182 if k == "from" {

183 // We already added a From header.

184 if err == io.EOF {

185 break

186 }

187 continue

188 } else if tflag && k == "to" {

189 if recipient != "" {

190 log.Fatalf("only single To header allowed")

191 }

192 s := strings.TrimSpace(t[1])

193 if !strings.Contains(s, "@") {

194 if submitconf.DefaultDestination == "" {

195 log.Fatalf("recipient %q has no @ and no default destination is configured", s)

196 }

197 recipient = submitconf.DefaultDestination

198 } else {

199 addrs, err := mail.ParseAddressList(s)

200 xcheckf(err, "parsing To address list")

201 if len(addrs) != 1 {

202 log.Fatalf("only single address allowed in To header")

203 }

204 recipient = addrs[0].Address

205 }

206 }

207 if k == "to" {

208 haveTo = true

209 }

210 }

211 sb.WriteString(line)

212 }

213 if err == io.EOF {

214 break

215 }

216 }

217 if header && submitconf.RequireTLS == RequireTLSNo {

218 sb.WriteString("TLS-Required: No\r\n")

219 }

220 msg := sb.String()

221

222 if recipient == "" {

223 log.Fatalf("no recipient")

224 }

225

226 // Message seems acceptable. We'll try to deliver it from here. If that fails, we

227 // store the message in the users home directory.

228 // Must only use xsavecheckf for error checking in the code below.

229

230 xsavecheckf := func(err error, format string, args ...any) {

231 if err == nil {

232 return

233 }

234 log.Printf("submit failed: %s: %s", fmt.Sprintf(format, args...), err)

235 homedir, err := os.UserHomeDir()

236 xcheckf(err, "finding homedir for storing message after failed delivery")

237 maildir := filepath.Join(homedir, "moxsubmit.failures")

238 os.Mkdir(maildir, 0700)

239 f, err := os.CreateTemp(maildir, "newmsg.")

240 xcheckf(err, "creating temp file for storing message after failed delivery")

241 // note: not removing the partial file if writing/closing below fails.

242 _, err = f.Write([]byte(msg))

243 xcheckf(err, "writing message to temp file after failed delivery")

244 name := f.Name()

245 err = f.Close()

246 xcheckf(err, "closing message in temp file after failed delivery")

247 f = nil

248 log.Printf("saved message in %s", name)

249 os.Exit(1)

250 }

251

252 addr := net.JoinHostPort(submitconf.Host, fmt.Sprintf("%d", submitconf.Port))

253 d := net.Dialer{Timeout: 30 * time.Second}

254 conn, err := d.Dial("tcp", addr)

255 xsavecheckf(err, "dial submit server")

256

257 auth := func(mechanisms []string, cs *tls.ConnectionState) (sasl.Client, error) {

258 // Check explicitly configured mechanisms.

259 switch submitconf.AuthMethod {

260 case "SCRAM-SHA-256-PLUS":

261 if cs == nil {

262 return nil, fmt.Errorf("scram plus authentication mechanism requires tls")

263 }

264 return sasl.NewClientSCRAMSHA256PLUS(submitconf.Username, submitconf.Password, *cs), nil

265 case "SCRAM-SHA-256":

266 return sasl.NewClientSCRAMSHA256(submitconf.Username, submitconf.Password, false), nil

267 case "SCRAM-SHA-1-PLUS":

268 if cs == nil {

269 return nil, fmt.Errorf("scram plus authentication mechanism requires tls")

270 }

271 return sasl.NewClientSCRAMSHA1PLUS(submitconf.Username, submitconf.Password, *cs), nil

272 case "SCRAM-SHA-1":

273 return sasl.NewClientSCRAMSHA1(submitconf.Username, submitconf.Password, false), nil

274 case "CRAM-MD5":

275 return sasl.NewClientCRAMMD5(submitconf.Username, submitconf.Password), nil

276 case "PLAIN":

277 return sasl.NewClientPlain(submitconf.Username, submitconf.Password), nil

278 }

279

280 // Try the defaults, from more to less secure.

281 if cs != nil && slices.Contains(mechanisms, "SCRAM-SHA-256-PLUS") {

282 return sasl.NewClientSCRAMSHA256PLUS(submitconf.Username, submitconf.Password, *cs), nil

283 } else if slices.Contains(mechanisms, "SCRAM-SHA-256") {

284 return sasl.NewClientSCRAMSHA256(submitconf.Username, submitconf.Password, true), nil

285 } else if cs != nil && slices.Contains(mechanisms, "SCRAM-SHA-1-PLUS") {

286 return sasl.NewClientSCRAMSHA1PLUS(submitconf.Username, submitconf.Password, *cs), nil

287 } else if slices.Contains(mechanisms, "SCRAM-SHA-1") {

288 return sasl.NewClientSCRAMSHA1(submitconf.Username, submitconf.Password, true), nil

289 } else if slices.Contains(mechanisms, "CRAM-MD5") {

290 return sasl.NewClientCRAMMD5(submitconf.Username, submitconf.Password), nil

291 } else if slices.Contains(mechanisms, "PLAIN") {

292 return sasl.NewClientPlain(submitconf.Username, submitconf.Password), nil

293 }

294 // No mutually supported mechanism.

295 return nil, nil

296 }

297

298 ctx, cancel := context.WithTimeout(context.Background(), 3*time.Minute)

299 defer cancel()

300 tlsMode := smtpclient.TLSSkip

301 tlsPKIX := false

302 if submitconf.TLS {

303 tlsMode = smtpclient.TLSImmediate

304 tlsPKIX = true

305 } else if submitconf.STARTTLS {

306 tlsMode = smtpclient.TLSRequiredStartTLS

307 tlsPKIX = true

308 } else if submitconf.RequireTLS == RequireTLSYes {

309 xsavecheckf(errors.New("cannot submit with requiretls enabled without tls to submission server"), "checking tls configuration")

310 }

311

312 ourHostname, err := dns.ParseDomain(submitconf.LocalHostname)

313 xsavecheckf(err, "parsing our local hostname")

314

315 var remoteHostname dns.Domain

316 if net.ParseIP(submitconf.Host) == nil {

317 remoteHostname, err = dns.ParseDomain(submitconf.Host)

318 xsavecheckf(err, "parsing remote hostname")

319 }

320

321 // todo: implement SRV and DANE, allowing for a simpler config file (just the email address & password)

322 opts := smtpclient.Opts{

323 Auth: auth,

324 RootCAs: mox.Conf.Static.TLS.CertPool,

325 }

326 client, err := smtpclient.New(ctx, c.log.Logger, conn, tlsMode, tlsPKIX, ourHostname, remoteHostname, opts)

327 xsavecheckf(err, "open smtp session")

328

329 err = client.Deliver(ctx, submitconf.From, recipient, int64(len(msg)), strings.NewReader(msg), true, false, submitconf.RequireTLS == RequireTLSYes)

330 xsavecheckf(err, "submit message")

331

332 if err := client.Close(); err != nil {

333 log.Printf("closing smtp session after message was sent: %v", err)

334 }

335}

336