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package alps
import (
"fmt"
"net"
"net/url"
"strings"
)
func discoverTCP(service, name string) (string, error) {
_, addrs, err := net.LookupSRV(service, "tcp", name)
if dnsErr, ok := err.(*net.DNSError); ok {
if dnsErr.IsTemporary {
return "", err
}
} else if err != nil {
return "", err
}
if len(addrs) == 0 {
return "", nil
}
addr := addrs[0]
target := strings.TrimSuffix(addr.Target, ".")
if target == "" {
return "", nil
}
return fmt.Sprintf("%v:%v", target, addr.Port), nil
}
// discoverIMAP performs a DNS-based IMAP service discovery, as defined in
// RFC 6186 section 3.2.
func discoverIMAP(domain string) (*url.URL, error) {
imapsHost, err := discoverTCP("imaps", domain)
if err != nil {
return nil, err
}
if imapsHost != "" {
return &url.URL{Scheme: "imaps", Host: imapsHost}, nil
}
imapHost, err := discoverTCP("imap", domain)
if err != nil {
return nil, err
}
if imapHost != "" {
return &url.URL{Scheme: "imap", Host: imapHost}, nil
}
return nil, fmt.Errorf("IMAP service discovery not configured for domain %q", domain)
}
// discoverSMTP performs a DNS-based SMTP submission service discovery, as
// defined in RFC 6186 section 3.1. RFC 8314 section 5.1 adds a new service for
// SMTP submission with implicit TLS.
func discoverSMTP(domain string) (*url.URL, error) {
smtpsHost, err := discoverTCP("submissions", domain)
if err != nil {
return nil, err
}
if smtpsHost != "" {
return &url.URL{Scheme: "smtps", Host: smtpsHost}, nil
}
smtpHost, err := discoverTCP("submission", domain)
if err != nil {
return nil, err
}
if smtpHost != "" {
return &url.URL{Scheme: "smtp", Host: smtpHost}, nil
}
return nil, fmt.Errorf("SMTP service discovery not configured for domain %q", domain)
}
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