PATCH: mis-configured luser_relay becomes black hole

[Wietse Venema]

Wietse Venema:
> However, when luser_relay handes mail for a non-existent recipient,
> and it is configured with a non-existent local user, then the local
> delivery agent will go through the same code path twice with the
> same address, in which case the duplicate suppressor will silently
> terminate the recursion like it does for all other deliveries.
> 
> This may or may not be easy to fix. For now, don't configure
> luser_relay with a non-existent address, just like postmaster, if
> you don't want errors to go unreported.

Easy enough: I was able to fix this on the train. If luser_relay
specifies a non-existent local address, then the luser_relay feature
becomes a black hole. This was introduced three months after the
first public release, on 19990302. This patch should apply to every
supported Postfix release.

        Wietse


diff '--exclude=man' '--exclude=html' '--exclude=README_FILES' 
'--exclude=INSTALL' '--exclude=.indent.pro' '--exclude=Makefile.in' -r -ur 
/var/tmp/postfix-3.4-20180222/src/local/unknown.c src/local/unknown.c
--- /var/tmp/postfix-3.4-20180222/src/local/unknown.c   2015-01-11 
15:30:20.000000000 -0500
+++ src/local/unknown.c 2018-03-06 17:06:17.477496641 -0500
@@ -78,6 +78,8 @@
 
 #include "local.h"
 
+#define STREQ(x,y) (strcasecmp((x),(y)) == 0)
+
 /* deliver_unknown - delivery for unknown recipients */
 
 int     deliver_unknown(LOCAL_STATE state, USER_ATTR usr_attr)
@@ -140,6 +142,13 @@
        state.msg_attr.unmatched = 0;
        expand_luser = vstring_alloc(100);
        local_expand(expand_luser, var_luser_relay, &state, &usr_attr, (void *) 
0);
+       /* Assumes that the address resolver will not change the address. */
+       if (STREQ(vstring_str(expand_luser), state.msg_attr.rcpt.address)) {
+           dsb_simple(state.msg_attr.why, "5.1.1",
+                      "unknown user: \"%s\"", state.msg_attr.user);
+           return (bounce_append(BOUNCE_FLAGS(state.request),
+                                 BOUNCE_ATTR(state.msg_attr)));
+       }
        status = deliver_resolve_addr(state, usr_attr, STR(expand_luser));
        vstring_free(expand_luser);
        return (status);
@@ -149,8 +158,6 @@
      * If no alias was found for a required reserved name, toss the message
      * into the bit bucket, and issue a warning instead.
      */
-#define STREQ(x,y) (strcasecmp(x,y) == 0)
-
     if (STREQ(state.msg_attr.user, MAIL_ADDR_MAIL_DAEMON)
        || STREQ(state.msg_attr.user, MAIL_ADDR_POSTMASTER)) {
        msg_warn("required alias not found: %s", state.msg_attr.user);