+ * Generic/fallback MTU values
+ *
+ * These are taken from RFC 1122, RFC 2460, and RFC 5405.
+ * - RFC 1122, 3.3.3 defines EMTU_S ("Effective MTU for sending") and recommends
+ * to use an EMTU_S size of of 576 bytes if the IPv4 path MTU is unknown;
+ * - RFC 2460, 5. requires a minimum IPv6 MTU of 1280 bytes;
+ * - RFC 5405, 3.2 recommends that if path MTU discovery is not done,
+ * UDP senders should use the respective minimum values of EMTU_S.
+ */
+static inline int generic_mtu(const int af_type)
+{
+ return af_type == AF_INET6 ? 1280 : 576;
+}
+
+/** Crude approximation of IP header overhead - neglecting options. */
+static inline int estimated_header_overhead(const int af_type)
+{
+ return af_type == AF_INET6 ? 40 : 20;
+}
+
+/**
+ * Get the maximum transport-layer message size (MMS_S).
+ *
+ * \param sockfd The socket file descriptor.
+ *
+ * The socket must be connected. See RFC 1122, 3.3.3. If the protocol familiy
+ * could not be determined, \p AF_INET is assumed.
+ *
+ * \return The maximum message size of the address family type.
+ */
+int generic_max_transport_msg_size(int sockfd)
+{
+ struct sockaddr_storage ss;
+ socklen_t sslen = sizeof(ss);
+ int af_type = AF_INET;
+
+ if (getpeername(sockfd, (struct sockaddr *)&ss, &sslen) < 0) {
+ PARA_ERROR_LOG("can not determine remote address type: %s\n",
+ strerror(errno));
+ } else if (!SS_IS_ADDR_V4MAPPED(&ss)) {
+ af_type = ss.ss_family;
+ }
+ return generic_mtu(af_type) - estimated_header_overhead(af_type);
+}
+
+/**
+ * Look up the local or remote side of a connected socket structure.
+ *
+ * \param fd The socket descriptor of the connected socket.
+ * \param getname Either \p getsockname() for local, or \p getpeername() for
+ * remote side.
+ *
+ * \return A static character string identifying hostname and port of the
+ * chosen side in numeric host:port format.
+ *
+ * \sa getsockname(2), getpeername(2), parse_url(), getnameinfo(3),
+ * services(5), nsswitch.conf(5).
+ */
+static char *__get_sock_name(int fd, typeof(getsockname) getname)
+{
+ struct sockaddr_storage ss;
+ const struct sockaddr *sa;
+ socklen_t sslen = sizeof(ss);
+ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
+ static char output[sizeof(hbuf) + sizeof(sbuf) + 4];
+ int ret;
+
+ if (getname(fd, (struct sockaddr *)&ss, &sslen) < 0) {
+ PARA_ERROR_LOG("can not determine address from fd %d: %s\n",
+ fd, strerror(errno));
+ snprintf(output, sizeof(output), "(unknown)");
+ return output;
+ }
+ sa = normalize_ip_address(&ss);
+ ret = getnameinfo(sa, salen(sa), hbuf, sizeof(hbuf), sbuf,
+ sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV);
+ if (ret) {
+ PARA_WARNING_LOG("hostname lookup error (%s).\n",
+ gai_strerror(ret));
+ snprintf(output, sizeof(output), "(lookup error)");
+ } else if (sa->sa_family == AF_INET6)
+ snprintf(output, sizeof(output), "[%s]:%s", hbuf, sbuf);
+ else
+ snprintf(output, sizeof(output), "%s:%s", hbuf, sbuf);
+ return output;
+}
+
+/**
+ * Look up the local side of a connected socket structure.
+ *
+ * \param sockfd The file descriptor of the socket.
+ *
+ * \return A pointer to a static buffer containing hostname an port. This
+ * buffer must not be freed by the caller.
+ *
+ * \sa remote_name().
+ */
+char *local_name(int sockfd)
+{
+ return __get_sock_name(sockfd, getsockname);
+}
+
+/**
+ * Look up the remote side of a connected socket structure.
+ *
+ * \param sockfd The file descriptor of the socket.
+ *
+ * \return Analogous to the return value of \ref local_name() but for the
+ * remote side.
+ *
+ * \sa local_name().
+ */
+char *remote_name(int sockfd)
+{
+ return __get_sock_name(sockfd, getpeername);
+}
+
+/**
+ * Extract IPv4 or IPv6-mapped-IPv4 address from sockaddr_storage.
+ * \param ss Container of IPv4/6 address
+ * \return Extracted IPv4 address (different from 0) or 0 if unsuccessful.
+ *
+ * \sa RFC 3493
+ */
+struct in_addr extract_v4_addr(const struct sockaddr_storage *ss)
+{
+ struct in_addr ia = {.s_addr = 0};
+ const struct sockaddr *sa = normalize_ip_address(ss);
+
+ if (sa->sa_family == AF_INET)
+ ia = ((struct sockaddr_in *)sa)->sin_addr;
+ return ia;
+}
+
+/**
+ * Send a binary buffer.