Proof-of-concept for dlundquist#371

Usage:

listen 0.0.0.0 443 {
    proto tls
    embed fd46:1::
}

If the inbound connection is IPv4, and the outbound connection is IPv6,
then the IPv4 address is embedded in the low 32 bits of the source.

src/config.c

@@ -125,6 +125,10 @@ static const struct Keyword listener_stanza_grammar[] = {
         .keyword="source",
         .parse_arg=(int(*)(void *, const char *))accept_listener_source_address,
     },
+    {
+        .keyword="embed",
+        .parse_arg=(int(*)(void *, const char *))accept_listener_embed_address,
+    },
     {
         .keyword="access_log",
         .create=(void *(*)())new_logger_builder,

src/connection.c

@@ -653,6 +653,35 @@ initiate_server_connect(struct Connection *con, struct ev_loop *loop) {
             abort_connection(con);
             return;
         }
+    } else if (con->listener->embed_address
+                && con->client.addr.ss_family == AF_INET
+                && con->server.addr.ss_family == AF_INET6) {
+        struct sockaddr_in6 embedded_source;
+#ifdef IP_FREEBIND
+        int on = 1;
+        int result = setsockopt(sockfd, SOL_IP, IP_FREEBIND, &on, sizeof(on));
+        if (result < 0) {
+            warn("setsockopt IP_FREEBIND failed: %s", strerror(errno));
+            // May not be necessary if user has set sysctl net.ipv6.ip_nonlocal_bind
+        }
+#else
+        int result = -EPERM;
+#endif
+
+	memcpy(&embedded_source,
+                address_sa(con->listener->embed_address),
+                sizeof(embedded_source));
+        memcpy(&(embedded_source.sin6_addr.s6_addr[12]),
+                &((struct sockaddr_in*)(&con->client.addr))->sin_addr, 4);
+        // XXX: maybe optionally the source port in bits 80-95
+
+        result = bind(sockfd, &embedded_source, sizeof(embedded_source));
+        if (result < 0) {
+            err("bind failed: %s", strerror(errno));
+            close(sockfd);
+            abort_connection(con);
+            return;
+        }
     } else if (con->listener->source_address) {
         int on = 1;
         int result = setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));

src/listener.c

@@ -176,6 +176,10 @@ listener_update(struct Listener *existing_listener, struct Listener *new_listene
     existing_listener->source_address = new_listener->source_address;
     new_listener->source_address = NULL;
 
+    free(existing_listener->embed_address);
+    existing_listener->embed_address = new_listener->embed_address;
+    new_listener->embed_address = NULL;
+
     existing_listener->protocol = new_listener->protocol;
 
     free(existing_listener->table_name);
@@ -212,6 +216,7 @@ new_listener() {
     listener->address = NULL;
     listener->fallback_address = NULL;
     listener->source_address = NULL;
+    listener->embed_address = NULL;
     listener->protocol = tls_protocol;
     listener->table_name = NULL;
     listener->access_log = NULL;
@@ -406,6 +411,40 @@ accept_listener_source_address(struct Listener *listener, const char *source) {
     return 1;
 }
 
+int
+accept_listener_embed_address(struct Listener *listener, const char *embed) {
+    if (listener->embed_address != NULL) {
+        err("Duplicate embed address: %s", embed);
+        return 0;
+    }
+
+    listener->embed_address = new_address(embed);
+    if (listener->embed_address == NULL) {
+        err("Unable to parse embed address: %s", embed);
+        return 0;
+    }
+    if (!address_is_sockaddr(listener->embed_address)) {
+        err("Only embed socket addresses permitted");
+        free(listener->embed_address);
+        listener->embed_address = NULL;
+        return 0;
+    }
+    if (address_sa(listener->embed_address)->sa_family != AF_INET6) {
+        err("Embed address must be IPv6 prefix");
+        free(listener->embed_address);
+        listener->embed_address = NULL;
+        return 0;
+    }
+    if (address_port(listener->embed_address) != 0) {
+        err("Port on embed address not permitted");
+        free(listener->embed_address);
+        listener->embed_address = NULL;
+        return 0;
+    }
+
+    return 1;
+}
+
 int
 accept_listener_bad_request_action(struct Listener *listener, const char *action) {
     if (strncmp("log", action, strlen(action)) == 0) {
@@ -707,6 +746,11 @@ print_listener_config(FILE *file, const struct Listener *listener) {
                 display_address(listener->source_address,
                     address, sizeof(address)));
 
+    if (listener->embed_address)
+        fprintf(file, "\tembed %s\n",
+                display_address(listener->embed_address,
+                    address, sizeof(address)));
+
     if (listener->reuseport)
         fprintf(file, "\treuseport on\n");
 
@@ -732,6 +776,7 @@ free_listener(struct Listener *listener) {
     free(listener->address);
     free(listener->fallback_address);
     free(listener->source_address);
+    free(listener->embed_address);
     free(listener->table_name);
 
     table_ref_put(listener->table);

src/listener.h

@@ -35,7 +35,7 @@ SLIST_HEAD(Listener_head, Listener);
 
 struct Listener {
     /* Configuration fields */
-    struct Address *address, *fallback_address, *source_address;
+    struct Address *address, *fallback_address, *source_address, *embed_address;
     const struct Protocol *protocol;
     char *table_name;
     struct Logger *access_log;
@@ -57,6 +57,7 @@ int accept_listener_arg(struct Listener *, const char *);
 int accept_listener_table_name(struct Listener *, const char *);
 int accept_listener_fallback_address(struct Listener *, const char *);
 int accept_listener_source_address(struct Listener *, const char *);
+int accept_listener_embed_address(struct Listener *, const char *);
 int accept_listener_protocol(struct Listener *, const char *);
 int accept_listener_reuseport(struct Listener *, const char *);
 int accept_listener_ipv6_v6only(struct Listener *, const char *);