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netintro(7) Miscellaneous Information Manual netintro(7) NAME netintro, networking - Introduction to socket networking facilities SYNOPSIS #include <sys/socket.h> #include <net/route.h> #include <net/if.h> DESCRIPTION This section is a general introduction to the networking facilities available in the system. Documentation in this part of Section 7 is broken up into three areas: protocol families (domains), protocols, and network interfaces. All network protocols are associated with a specific protocol family. A protocol family provides basic services to the protocol implementa- tion to allow it to function within a specific network environment. These services may include packet fragmentation and reassembly, rout- ing, addressing, and basic transport. A protocol family may support multiple methods of addressing, though the current protocol implementa- tions do not. A protocol family is normally comprised of a number of protocols, one per socket type. It is not required that a protocol family support all socket types. A protocol family may contain multi- ple protocols supporting the same socket abstraction. A protocol supports one of the socket abstractions detailed in the ref- erence page for the socket() function. A specific protocol may be ac- cessed either by creating a socket of the appropriate type and protocol family, or by requesting the protocol explicitly when creating a socket. Protocols normally accept only one type of address format, usually determined by the addressing structure inherent in the design of the protocol family and network architecture. Certain semantics of the basic socket abstractions are protocol specific. All protocols are expected to support the basic model for their particular socket type, but may, in addition, provide nonstandard facilities or extensions to a mechanism. For example, a protocol supporting the SOCK_STREAM abstrac- tion may allow more than one byte of out-of-band data to be transmitted per out-of-band message. A network interface is similar to a device interface. Network inter- faces comprise the lowest layer of the networking subsystem, interact- ing with the actual transport hardware. An interface may support one or more protocol families, address formats, or both. The SYNOPSIS sec- tion of each network interface entry gives a sample specification of the related drivers for use in providing a system description to the config program. The ERRORS section lists messages which may appear on the console and/or in the system error log, /var/log/messages (see the syslogd function), due to errors in device operation. The system currently supports the DARPA Internet protocols. Raw socket interfaces are provided to the IP layer of the DARPA Internet. Consult the appropriate manual pages in this section for more information re- garding this support. Addressing Associated with each protocol family is an address format. All network address adhere to a general structure, called a sockaddr. However, each protocol imposes finer and more specific structure, generally re- naming the variant. Both the 4.3BSD and 4.4BSD sockaddr structures are supported by DIGITAL UNIX. The default sockaddr structure is the 4.3BSD structure, which is as follows: struct sockaddr { u_short sa_family; char sa_data[14]; }; If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h header file is included, however, the 4.4BSD sockaddr structure is defined, which is as follows: struct sockaddr { u_char sa_len; u_char sa_family; char sa_data[14]; }; The 4.4BSD sockaddr structure provides for a sa_len field, which con- tains the total length of the structure. Unlike the 4.3BSD sockaddr structure, this length may exceed 16 bytes. The following address values for sa_family are known to the system (and additional formats are defined for possible future implementation): #define AF_UNIX 1 /* local to host (pipes, portals) */ #de- fine AF_INET 2 /* internetwork: UDP, TCP, etc. */ Routing The UNIX operating system provides packet routing facilities. The ker- nel maintains a routing information database, which is used in select- ing the appropriate network interface when transmitting packets. A user process (or possibly multiple cooperating processes) maintains this database by sending messages over a special kind of socket. This supplants fixed size ioctl's used in earlier releases. This facility is described in the files reference page for the route function. Interfaces Each network interface in a system corresponds to a path through which messages may be sent and received. A network interface usually has a hardware device associated with it, though certain interfaces such as the loopback interface, lo, do not. The following ioctl calls may be used to manipulate network interfaces. The ioctl is made on a socket (typically of type SOCK_DGRAM) in the de- sired domain. Most of the requests supported in earlier releases take an ifreq structure as its parameter. This structure has the following form: struct ifreq { #define IFNAMSIZ 16 char ifr_name[IFNAMSIZE]; /*if name, e.g. "en0"*/ union { struct sockaddr ifru_addr; struct sockaddr ifru_dstaddr; struct sockaddr ifru_broadaddr; short ifru_flags; int ifru_metric; caddr_t ifru_data; } ifr_ifru; #define ifr_addr ifr_ifru.ifru_addr /* address */ #define ifr_dstaddr ifr_ifru.ifru_dstaddr /* other end of p- to-p link */ #define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */ #define ifr_flags ifr_ifru.ifru_flags /* flags */ #define ifr_metric ifr_ifru.ifru_metric /* metric */ #de- fine ifr_data ifr_ifru.ifru_data /* for use by interface */ }; Calls which are now deprecated are: Set interface address for protocol family. Following the address assignment, the ``initialization'' rou- tine for the interface is called. Set point to point address for pro- tocol family and interface. Set broadcast address for protocol family and interface. All ioctl requests to obtain addresses and requests both to set and re- trieve other data are still fully supported and use the ifreq struc- ture: Get interface address for protocol family. Get point to point address for protocol family and interface. Get broadcast address for protocol family and interface. Set interface flags field. If the in- terface is marked down, any processes currently routing packets through the interface are notified; some interfaces may be reset so that incom- ing packets are no longer received. When marked up again, the inter- face is reinitialized. Get interface flags. Set interface routing metric. The metric is used only by user-level routers. Get interface metric. There are three requests that make use of a new structure: An interface may have more than one address associated with it in some protocols. This request provides a means to add additional addresses (or modify characteristics of the primary address if the default address for the address family is specified). Rather than making separate calls to set destination addresses, broadcast addresses, or network masks (now an integral feature of multiple protocols) a separate structure is used to specify all three facets simultaneously: struct ifaliasreq { char ifra_name[IFNAMSIZ]; /* if name, e.g. "en0" */ struct sockaddr ifra_addr; struct sock- addr ifra_broadaddr; struct sockaddr ifra_mask; }; One would use a slightly tailored version of this struct are specific to each family (replacing each sockaddr by one of the family-specific type). Where the sockaddr itself is larger than the default size, one needs to modify the ioctl identifier itself to include the total size. This request deletes the specified address from the list associated with an interface. It uses the if_aliasreq structure to permit proto- cols to allow multiple masks or destination addresses, and it adopts the convention that specification of the default address means to delete the first address for the interface belonging to the address family in which the original socket was opened. Get interface configu- ration list. This request takes an ifconf structure (see below) as a value-result parameter. The ifc_len field should be initially set to the size of the buffer pointed to by ifc_buf. On return it contains the length, in bytes, of the configuration list. /* * Structure used in SIOCGIFCONF request. * Used to retrieve interface configuration * for machine (useful for programs which * must know all networks accessible). */ struct ifconf { int ifc_len; /* size of associated buffer */ union { caddr_t ifcu_buf; struct ifreq *ifcu_req; } ifc_ifcu; #de- fine ifc_buf ifc_ifcu.ifcu_buf /* buffer address */ #de- fine ifc_req ifc_ifcu.ifcu_req /* array of structures returned */ }; RELATED INFORMATION Functions: socket(2), ioctl(2) Files: config(8), routed(8) delim off netintro(7)