SMTPUTF8: convert UTF-8 domains to IDNA

This isn't entirely correct according to RFC 6531 as it's better to not
convert at all. However this would require we additionally add the UTF-8
form to qmails local recipient users/domains.

Plus our SMTPUTF8 implementation doesn't convert outgoing UTF-8 mails if
receiving MTA doesn't support SMTPUTF8. So mail forwarding might generate
a bounce the user very likely doesn't understand.

As MTAs who support SMTPUTF8 most likely also support the IDNA form,
always converting is probably a good trade-off.

2 files changed, 31 insertions, 2 deletions

diff --git a/Makefile b/Makefile
index a13882b..9ffde73 100644
--- a/Makefile
+++ b/Makefile
@@ -1626,7 +1626,7 @@ socket.lib dns.lib
         tls.o ssl_timeoutio.o ndelay.a -L/usr/local/ssl/lib -lssl -lcrypto \
         cdb.a fd.a wait.a datetime.a getln.a open.a sig.a case.a env.a \
         stralloc.a alloc.a strerr.a substdio.a error.a str.a fs.a \
-        `cat socket.lib` `cat dns.lib`
+        `cat socket.lib` `cat dns.lib` -lidn2
 
 qmail-smtpd.0: \
 qmail-smtpd.8
diff --git a/qmail-smtpd.c b/qmail-smtpd.c
index db0e16b..95993eb 100644
--- a/qmail-smtpd.c
+++ b/qmail-smtpd.c
@@ -77,6 +77,11 @@ stralloc proto = {0};
 int tls_before_auth = 0;
 #endif
 
+#ifdef SMTPUTF8
+# include <idn2.h>
+int smtputf8 = 0;
+#endif
+
 int safewrite(fd,buf,len) int fd; char *buf; int len;
 {
   int r;
@@ -385,6 +390,25 @@ char *arg;
   return 1;
 }
 
+#ifdef SMTPUTF8
+static int idn_addr_convert(stralloc *addr)
+{
+  int i = byte_rchr(addr->s, addr->len, '@') + 1;
+  if (i < addr->len) {
+    char *asciihost = NULL;
+    switch (idn2_lookup_u8(addr->s + i, (uint8_t **)&asciihost, IDN2_NFC_INPUT)) {
+      case IDN2_OK: break;
+      case IDN2_MALLOC: die_nomem();
+      default: err_smf(); return 0;
+    }
+    addr->len = i;
+    if (!stralloc_cats(addr, asciihost)) die_nomem();
+    if (!stralloc_0(addr)) die_nomem();
+  }
+  return 1;
+}
+#endif
+
 static void log_deny(m,f,t) char *m,*f,*t;
 {
   enew(); eout(m); eout(" check failed ("); eout(f); eout(") -> (");
@@ -515,7 +539,6 @@ stralloc mailfrom = {0};
 stralloc rcptto = {0};
 stralloc fuser = {0};
 stralloc mfparms = {0};
-int smtputf8 = 0;
 int recipcount;
 
 int mailfrom_size(arg) char *arg;
@@ -630,6 +653,9 @@ void smtp_mail(arg) char *arg;
   flagsize = 0;
   mailfrom_parms(arg);
   if (flagsize) { err_size(); return; }
+#ifdef SMTPUTF8
+  if (smtputf8) { if (!idn_addr_convert(&addr)) return; }
+#endif
   flagbarfbmf = 0; /* bmcheck is skipped for empty envelope senders */
   if ((bmfok) && (addr.len != 1)) flagbarfbmf = bmcheck(BMCHECK_BMF);
   if ((!flagbarfbmf) && (bmfnrok) && (addr.len != 1) && (!relayclient)) {
@@ -673,6 +699,9 @@ void smtp_rcpt(arg) char *arg; {
     err_bmf();
     return;
   }
+#ifdef SMTPUTF8
+  if (smtputf8) { if (!idn_addr_convert(&addr)) return; }
+#endif
   if (bmtok) flagbarfbmt = bmcheck(BMCHECK_BMT);
   if ((!flagbarfbmt) && (bmtnrok) && (!relayclient)) {
     flagbarfbmt = bmcheck(BMCHECK_BMTNR);