The Linux Programming Interface

Sockets: Internet Domains 1213

The protocol is typically either tcp or udp. The optional (space-delimited) aliases specify

alternative names for the service. In addition to the above, lines may include com-

ments starting with the # character.

As noted previously, a given port number refers to distinct entities for UDP

and TCP, but IANA policy assigns both port numbers to a service, even if that service

uses only one protocol. For example, telnet, ssh, HTTP, and SMTP all use TCP, but

the corresponding UDP port is also assigned to these services. Conversely, NTP

uses only UDP, but the TCP port 123 is also assigned to this service. In some cases,

a service uses both UDP and TCP; DNS and echo are examples of such services.

Finally, there are a very few cases where the UDP and TCP ports with the same

number are assigned to different services; for example, rsh uses TCP port 514,

while the syslog daemon (Section 37.5) uses UDP port 514. This is because these

port numbers were assigned before the adoption of the present IANA policy.

The /etc/services file is merely a record of name-to-number mappings. It is not

a reservation mechanism: the appearance of a port number in /etc/services

doesn’t guarantee that it will actually be available for binding by a particular

service.

59.10 Protocol-Independent Host and Service Conversion

The getaddrinfo() function converts host and service names to IP addresses and port

numbers. It was defined in POSIX.1g as the (reentrant) successor to the obsolete

gethostbyname() and getservbyname() functions. (Replacing the use of gethostbyname()

with getaddrinfo() allows us to eliminate IPv4-versus-IPv6 dependencies from our

programs.)

The getnameinfo() function is the converse of getaddrinfo(). It translates a socket

address structure (either IPv4 or IPv6) to strings containing the corresponding host

and service name. This function is the (reentrant) equivalent of the obsolete

gethostbyaddr() and getservbyport() functions.

Chapter 11 of [Stevens et al., 2004] describes getaddrinfo() and getnameinfo() in

detail, and provides implementations of these functions. These functions are

also described in RFC 3493.

59.10.1 The getaddrinfo() Function

Given a host name and a service name, getaddrinfo() returns a list of socket address

structures, each of which contains an IP address and port number.

#include <sys/socket.h>

#include <netdb.h>

int getaddrinfo(const char *host, const char *service,

const struct addrinfo *hints, struct addrinfo **result);

Returns 0 on success, or nonzero on error