The ucspi-tcp distribution contains tcpserver and tcpclient, which are easy-to-use command-line tools for building TCP client-server applications. tcpserver waits for incoming connections and, for each connection, runs a program of your choice. tcpclient makes a TCP connection and runs a program of your choice. This package includes several sample clients built on top of tcpclient: who@, date@, finger@, http@, tcpcat, and mconnect. tcpserver and tcpclient conform to UCSPI, the UNIX Client-Server Program Interface, using the TCP protocol. UCSPI tools are available for several different networks.
udhcp Server/Client is an embedded DHCP client and server that can be compiled seperately (19k and 17k respectively), or as one combined binary (29k). The server stores its lease file periodically, or by a signal (for systems with flash memory) in terms of absolute time of lease expiration, or time remaining in a lease (for systems with no clocks). The client calls helper scripts to do the work of configuring the interfaces for more flexibility. udhcp has been included in BusyBox and is no longer maintained as a separate project.
UDPCast allows to send data simultaneously to many destinations on a local net. This can for instance be used to install entire classrooms of PCs at once. The advantage over using other methods (nfs, ftp, whatever) is that udpcast uses Ethernet's multicast abilities: it won't take longer to install 15 machines than it would to install 2. The tool comes with a busybox bootdisk for easy loading of the tool. However, udpcast can also be started from the command line of an already installed system, and can be used for other purposes than just system installation.
udpeq balances UDP traffic over parallel routes, useful if you want to connect two endpoints through several slow or unreliable channels. For example, if you have three modem lines and a one-way satellite link, you might want to bond all of these together to form a faster, more reliable "virtual" connection.
udpfwd is a stateful userspace UDP port forwarder that can accomplish SNAT/DNAT. It listens for packets received on a given port, changes their source IP addresses to the address of the local host ("SNAT"), and forwards them to the destination (dest_ip:dest_port) from a randomly chosen local sending port. Once chosen, the source_ip:source_port:send_port triplet is unique, allowing for connection tracking. Conversely, when a packet is received on send_port, its destination is changed to source_ip:source_port ("DNAT"), and forwarded onwards.
uidentd (mu-identd) is a fast and secure identd/auth server. It runs on Linux and any other OS that provides the virtual files, /proc/net/tcp and /proc/net/tcp6. It supports IPv6 natively. It is launched from a superserver such as xinetd or tcpserver and is supposedly resistant to buffer overflows since it is written in Perl. It supports configurable connection timeouts and multi-query as defined by RFC 1413.