+/** \cond Userland defines for Linux DCCP support. */
+#ifndef IPPROTO_DCCP
+#define IPPROTO_DCCP 33 /**< IANA assigned value */
+#define SOCK_DCCP 6 /**< Linux socket type */
+#define SOL_DCCP 269 /**< Linux socket level */
+#endif
+/** \endcond */
+
+
+/**
+ * Functions to parse and validate (parts of) URLs.
+ */
+extern char *parse_url(const char *url,
+ char *host, ssize_t hostlen, int32_t *port);
+/**
+ * Ensure that string conforms to the IPv4 address format.
+ *
+ * \param address The address string to check.
+ *
+ * \return 1 if \a address conforms to the IPv4 address format, else 0.
+ */
+_static_inline_ bool is_valid_ipv4_address(const char *address)
+{
+ struct in_addr test_it;
+
+ return inet_pton(AF_INET, address, &test_it) != 0;
+}
+
+/**
+ * Ensure that string conforms to IPv6 address format.
+ *
+ * \param address The address string to check.
+ *
+ * \return 1 if string has a valid IPv6 address syntax, 0 if not.
+ * \sa RFC 4291
+ */
+_static_inline_ bool is_valid_ipv6_address(const char *address)
+{
+ struct in6_addr test_it;
+
+ return inet_pton(AF_INET6, address, &test_it) != 0;
+}
+
+/**
+ * Generic socket creation (passive and active sockets).
+ */
+extern int makesock(unsigned l3type, unsigned l4type, int passive,
+ const char *host, unsigned short port_number);
+extern struct in_addr extract_v4_addr(const struct sockaddr_storage *ss);
+
+/**
+ * Functions to support listening sockets.
+ */
+/** How many pending connections queue of a listening server will hold. */
+#define BACKLOG 10
+extern int para_listen(unsigned l3type, unsigned l4type, unsigned short port);
+
+/** Pretty-printing of IPv4/6 socket addresses */
+extern char *local_name(int sockfd);
+extern char *remote_name(int sockfd);
+
+/** used to crypt the communication between para_server and para_client */