From 8daa2dfbfdc6713b81fa4310fcc9d22e6ad4b284 Mon Sep 17 00:00:00 2001
From: Mario Fetka <mario.fetka@gmail.com>
Date: Tue, 28 Jul 2026 13:30:11 +0200
Subject: [PATCH] smtp: deliver IPv4 address literals directly

---
 src/agents/smtp/smtpc.c | 51 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/src/agents/smtp/smtpc.c b/src/agents/smtp/smtpc.c
index f5016c5..37185a6 100755
--- a/src/agents/smtp/smtpc.c
+++ b/src/agents/smtp/smtpc.c
@@ -499,6 +499,8 @@ Connection *LookupRemoteMX(RecipStruct *Recipient, char *hostname,
     XplDns_MxLookup *mx = NULL;
     XplDns_IpList *list = NULL;
     Connection *Result=NULL;
+    struct in_addr literalAddress;
+    BOOL addressLiteral = FALSE;
 
     if (hostname == NULL || hostnameSize == 0 || canonicalHostname == NULL ||
         canonicalHostnameSize == 0 || nextHopDomain == NULL ||
@@ -533,7 +535,13 @@ Connection *LookupRemoteMX(RecipStruct *Recipient, char *hostname,
             *(Recipient->localPart-1) = '@';
             return Result;
         }
-        // FIXME: assumes our DNS routines will accept IP address.
+        if (inet_pton(AF_INET, Host, &literalAddress) != 1) {
+            Recipient->Result = DELIVER_BOGUS_NAME;
+            *end = ']';
+            *(Recipient->localPart-1) = '@';
+            return Result;
+        }
+        addressLiteral = TRUE;
         *end = ']';
     } else {
         size_t hostLength = strlen(Recipient->SortField);
@@ -549,6 +557,47 @@ Connection *LookupRemoteMX(RecipStruct *Recipient, char *hostname,
     /* make sure to reset the SortField as it was previously */
     *(Recipient->localPart-1) = '@';
 
+    /*
+     * RFC 5321 address literals bypass MX and address lookups.  Besides
+     * avoiding a meaningless DNS query, this preserves the literal as the
+     * next-hop identity and prevents DNSSEC state from being inferred for it.
+     */
+    if (addressLiteral) {
+        int status;
+
+        snprintf(hostname, hostnameSize, "%s", Host);
+        snprintf(canonicalHostname, canonicalHostnameSize, "%s", Host);
+        snprintf(nextHopDomain, nextHopDomainSize, "%s", Host);
+        Result = ConnAlloc(TRUE);
+        if (Result == NULL) {
+            Recipient->Result = DELIVER_INTERNAL_ERROR;
+            goto finish;
+        }
+        Result->socketAddress.sin_addr = literalAddress;
+        Result->socketAddress.sin_family = AF_INET;
+        Result->socketAddress.sin_port = htons(25);
+        status = ConnConnect(Result, NULL, 0, NULL);
+        if (status <= 0) {
+            switch (errno) {
+                case ETIMEDOUT:
+                    Recipient->Result = DELIVER_TIMEOUT;
+                    break;
+                case ECONNREFUSED:
+                    Recipient->Result = DELIVER_REFUSED;
+                    break;
+                case ENETUNREACH:
+                    Recipient->Result = DELIVER_UNREACHABLE;
+                    break;
+                default:
+                    Recipient->Result = DELIVER_TRY_LATER;
+                    break;
+            }
+            ConnFree(Result);
+            Result = NULL;
+        }
+        goto finish;
+    }
+
     /* DNS and the normalized SMTP envelope both use the same IDNA2008
      * A-label.  This also validates ASCII domains before lookup. */
     if (!BongoDomainToASCII(Host, asciiHost, sizeof(asciiHost))) {