Initial import

.gitlab-ci.yml

+image: docker:latest
+
+stages:
+  - build
+  - release
+
+services:
+  - docker:dind
+
+variables:
+  IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME
+  RELEASE_TAG: $CI_REGISTRY_IMAGE:latest
+
+before_script:
+  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN registry.git.autistici.org
+
+build:
+  stage: build
+  script:
+    - docker build --pull -t $IMAGE_TAG .
+    - docker push $IMAGE_TAG
+
+release:
+  stage: release
+  script:
+    - docker pull $IMAGE_TAG
+    - docker tag $IMAGE_TAG $RELEASE_TAG
+    - docker push $RELEASE_TAG
+  only:
+    - master

Dockerfile

+FROM golang:latest AS build
+COPY sendmail.go /sendmail.go
+RUN go build -tags netgo -o /sendmail /sendmail.go && strip /sendmail
+
+FROM scratch
+COPY --from=build /sendmail /

README.md

+sendmail-go
+===
+
+A minimal */usr/sbin/sendmail* drop-in replacement that will accept
+emails on standard input and will forward them via a relay host using
+(authenticated) SMTP.
+
+# Usage
+
+The tool supports the most common sendmail command-line options. It
+will ignore most of them except for *-f* (to set the envelope sender),
+and *-t* (to extract recipients from the message itself). If *-t* is
+not specified, the tool will expect recipients to be passed as
+command-line arguments.
+
+The tool will read a configuration file (default */etc/sendmail.json*,
+it can be overridden with the *SENDMAIL_CONFIG* environment variable
+or by using the *-C* option) containing relay and authentication
+parameters. The configuration file should be encoded in JSON format,
+as a dictionary containing the following attributes:
+
+* `smtp_server` server (host:port) to connect to (default
+  "localhost:25"). The tool will automatically switch to TLS if
+  supported.
+* `smtp_user`, `smtp_pass`: credentials for authentication
+
+# Building
+
+You can build a static *sendmail* executable by running:
+
+```shell
+$ go build -tags netgo -o sendmail sendmail.go
+```
+
+Or reuse our Docker image within a multi-stage Dockerfile:
+
+```
+FROM registry.git.autistici.org/ai3/tools/sendmail-go:master AS sendmail
+
+[...]
+
+COPY --from=sendmail /sendmail /usr/sbin/sendmail
+```
+

sendmail.go

+// A minimal 'sendmail' clone that forwards all email remotely over
+// (authenticated) SMTP.
+//
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"flag"
+	"io/ioutil"
+	"log"
+	"net/mail"
+	"net/smtp"
+	"os"
+)
+
+var (
+	configPath        = flag.String("C", getenvDefault("SENDMAIL_CONFIG", "/etc/sendmail.json"), "location of the configuration file")
+	envelopeSender    = flag.String("f", "", "envelope sender")
+	recipientsFromMsg = flag.Bool("t", false, "extract recipients from message")
+
+	// Some common sendmail options that we wish to ignore,
+	// luckily the Go flag parser will accept the "short flag"
+	// format for them. Ignoring these options makes it less
+	// likely that we'll have to modify the existing sendmail
+	// commands used inside applications.
+	ignoredFlagI   = flag.Bool("i", false, "ignored")
+	ignoredFlagM   = flag.Bool("m", false, "ignored")
+	ignoredFlagN   = flag.Bool("n", false, "ignored")
+	ignoredFlagO   = flag.String("O", "", "ignored")
+	ignoredFlagOEE = flag.Bool("oee", false, "ignored")
+	ignoredFlagOEM = flag.Bool("oem", false, "ignored")
+	ignoredFlagOEP = flag.Bool("oep", false, "ignored")
+	ignoredFlagOEQ = flag.Bool("oeq", false, "ignored")
+	ignoredFlagOEW = flag.Bool("oew", false, "ignored")
+	ignoredFlagOI  = flag.Bool("oi", false, "ignored")
+	ignoredFlagOM  = flag.Bool("om", false, "ignored")
+	ignoredFlagOO  = flag.Bool("oo", false, "ignored")
+	ignoredFlagV   = flag.Bool("v", false, "ignored")
+
+	defaultSMTPServer = "localhost:25"
+)
+
+// The configuration is stored in a JSON-encoded file.
+type config struct {
+	Server string `json:"smtp_server"`
+	User   string `json:"smtp_user"`
+	Pass   string `json:"smtp_pass"`
+}
+
+func readConfig(path string) (server string, auth smtp.Auth, err error) {
+	var data []byte
+	data, err = ioutil.ReadFile(path)
+	if err != nil {
+		return
+	}
+
+	var config config
+	if err = json.Unmarshal(data, &config); err != nil {
+		return
+	}
+
+	server = config.Server
+	if config.User != "" {
+		auth = smtp.PlainAuth("", config.User, config.Pass, "")
+	}
+	return
+}
+
+func uniqueRecipients(rcpts []string) []string {
+	tmp := make(map[string]struct{})
+	for _, r := range rcpts {
+		tmp[r] = struct{}{}
+	}
+	out := make([]string, 0, len(tmp))
+	for r := range tmp {
+		out = append(out, r)
+	}
+	return out
+}
+
+func getenvDefault(key, dflt string) string {
+	if s := os.Getenv(key); s != "" {
+		return s
+	}
+	return dflt
+}
+
+func main() {
+	log.SetFlags(0)
+	flag.Parse()
+
+	smtpServer, smtpAuth, err := readConfig(*configPath)
+	if err != nil {
+		log.Fatalf("error reading configuration: %v", err)
+	}
+	if smtpServer == "" {
+		smtpServer = defaultSMTPServer
+	}
+
+	// Read the whole message from stdin, and parse it.
+	msgData, err := ioutil.ReadAll(os.Stdin)
+	if err != nil {
+		log.Fatal(err)
+	}
+	msg, err := mail.ReadMessage(bytes.NewReader(msgData))
+	if err != nil {
+		log.Fatal(err)
+	}
+	header := msg.Header
+
+	// Find envelope sender, from args or from message.
+	sender := *envelopeSender
+	if sender == "" {
+		senderAddr, err := mail.ParseAddress(header.Get("From"))
+		if err != nil {
+			log.Fatal("could not parse envelope sender")
+		}
+		sender = senderAddr.Address
+	}
+
+	// Find recipients, from args or from message.
+	recipients := flag.Args()
+	if *recipientsFromMsg {
+		for _, rcptHdr := range []string{"To", "Cc", "Bcc"} {
+			rcptAddrs, err := header.AddressList(rcptHdr)
+			if err != nil && err != mail.ErrHeaderNotPresent {
+				log.Fatalf("could not parse recipients: %v", err)
+			}
+			for _, addr := range rcptAddrs {
+				recipients = append(recipients, addr.Address)
+			}
+		}
+	}
+	recipients = uniqueRecipients(recipients)
+	if len(recipients) == 0 {
+		log.Fatal("message has no recipients")
+	}
+
+	// Send the message using the net/smtp package.
+	err = smtp.SendMail(smtpServer, smtpAuth, sender, recipients, msgData)
+	if err != nil {
+		log.Fatalf("error sending message: %v", err)
+	}
+}