111 lines
2.4 KiB
Go
111 lines
2.4 KiB
Go
package trysmtp
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import (
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"crypto/tls"
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"fmt"
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"net"
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"net/smtp"
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"strings"
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)
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// SMTPAddrs priority list
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var SMTPAddrs = []string{":25", ":587", ":465"}
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// Connect to SMTP server and call MAIL and RCPT commands
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// NOTE: check if client is not nil, because it can return non-fatal errors with initialized client
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func Connect(from, to string) (*smtp.Client, error) {
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localname := strings.SplitN(from, "@", 2)[1]
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hostname := strings.SplitN(to, "@", 2)[1]
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client, cerr := initClient(localname, hostname)
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if client == nil {
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return nil, cerr
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}
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err := client.Mail(from)
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if err != nil {
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client.Close()
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return nil, err
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}
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err = client.Rcpt(to)
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if err != nil {
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client.Close()
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return nil, err
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}
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return client, cerr
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}
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func unwrapErrors(errs []error) error {
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if len(errs) == 0 {
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return nil
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}
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var text strings.Builder
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last := len(errs) - 1
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for i := 0; i < len(errs); i++ {
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text.WriteString(errs[i].Error())
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if i < last {
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text.WriteString("; ")
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}
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}
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return fmt.Errorf(text.String())
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}
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func initClient(localname, hostname string) (*smtp.Client, error) {
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mxs, err := net.LookupMX(hostname)
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if err != nil {
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return nil, err
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}
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cerrs := []error{}
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var client *smtp.Client
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for _, mx := range mxs {
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if mx.Host == "." {
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continue // no records case
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}
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for _, addr := range SMTPAddrs {
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client, err = trySMTP(localname, strings.TrimSuffix(mx.Host, "."), addr)
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if err != nil {
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cerrs = append(cerrs, err)
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}
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if client != nil {
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return client, unwrapErrors(cerrs)
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}
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}
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}
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// If there are no MX records, according to https://datatracker.ietf.org/doc/html/rfc5321#section-5.1,
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// we're supposed to try talking directly to the host.
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for _, addr := range SMTPAddrs {
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client, err = trySMTP(localname, hostname, addr)
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if err != nil {
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cerrs = append(cerrs, err)
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}
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if client != nil {
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return client, unwrapErrors(cerrs)
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}
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}
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return nil, unwrapErrors(cerrs)
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}
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func trySMTP(localname, mxhost, addr string) (*smtp.Client, error) {
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target := mxhost + addr
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conn, err := smtp.Dial(target)
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if err != nil {
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return nil, fmt.Errorf("%s: %w", target, err)
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}
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err = conn.Hello(localname)
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if err != nil {
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return nil, fmt.Errorf("%s: %w", target, err)
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}
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if ok, _ := conn.Extension("STARTTLS"); ok {
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config := &tls.Config{ServerName: mxhost}
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conn.StartTLS(config) //nolint:errcheck // if it doesn't work - we can't do anything anyway
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}
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return conn, nil
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}
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