getaddrinfo(...) public

Return address information for host and port. The remaining arguments are hints that limit the address information returned.

This method corresponds closely to the POSIX.1g getaddrinfo() definition.


  • host is a host name or an address string (dotted decimal for IPv4, or a hex string for IPv6) for which to return information. A nil is also allowed, its meaning depends on flags, see below.

  • service is a service name (“http”, “ssh”, …), or a port number (80, 22, …), see Socket.getservbyname for more information. A nil is also allowed, meaning zero.

  • family limits the output to a specific address family, one of the Socket::AF_* constants. Socket::AF_INET (IPv4) and Socket::AF_INET6 (IPv6) are the most commonly used families. You will usually pass either nil or Socket::AF_UNSPEC, allowing the IPv6 information to be returned first if host is reachable via IPv6, and IPv4 information otherwise. The two strings “AF_INET” or “AF_INET6” are also allowed, they are converted to their respective Socket::AF_* constants.

  • socktype limits the output to a specific type of socket, one of the Socket::SOCK_* constants. Socket::SOCK_STREAM (for TCP) and Socket::SOCK_DGRAM (for UDP) are the most commonly used socket types. If nil, then information for all types of sockets supported by service will be returned. You will usually know what type of socket you intend to create, and should pass that socket type in.

  • protocol limits the output to a specific protocol numpber, one of the Socket::IPPROTO_* constants. It is usually implied by the socket type (Socket::SOCK_STREAM => Socket::IPPROTO_TCP, …), if you pass other than nil you already know what this is for.

  • flags is one of the Socket::AI_* constants. They mean:

    • Socket::AI_PASSIVE: when set, if host is nil the ‘any’ address will be returned, Socket::INADDR_ANY or 0 for IPv4, “0::0” or “::” for IPv6. This address is suitable for use by servers that will bind their socket and do a passive listen, thus the name of the flag. Otherwise the local or loopback address will be returned, this is “” for IPv4 and “::1’ for IPv6.


Returns an array of arrays, where each subarray contains:

  • address family, a string like “AF_INET” or “AF_INET6”

  • port number, the port number for service

  • host name, either a canonical name for host, or it’s address in presentation format if the address could not be looked up.

  • host IP, the address of host in presentation format

  • address family, as a numeric value (one of the Socket::AF_* constants).

  • socket type, as a numeric value (one of the Socket::SOCK_* constants).

  • protocol number, as a numeric value (one of the Socket::IPPROTO_* constants).

The first four values are identical to what is commonly returned as an address array, see IPSocket for more information.


Not all input combinations are valid, and while there are many combinations, only a few cases are common.

A typical client will call getaddrinfo with the host and service it wants to connect to. It knows that it will attempt to connect with either TCP or UDP, and specifies socktype accordingly. It loops through all returned addresses, and try to connect to them in turn:

addrinfo = Socket::getaddrinfo('www.example.com', 'www', nil, Socket::SOCK_STREAM)
addrinfo.each do |af, port, name, addr|
    sock = TCPSocket.new(addr, port)
    # ...
    exit 1

With UDP you don’t know if connect suceeded, but if communication fails, the next address can be tried.

A typical server will call getaddrinfo with a host of nil, the service it listens to, and a flags of Socket::AI_PASSIVE. It will listen for connections on the first returned address:

addrinfo = Socket::getaddrinfo(nil, 'www', nil, Socket::SOCK_STREAM, nil, Socket::AI_PASSIVE)
af, port, name, addr = addrinfo.first
sock = TCPServer(addr, port)
while( client = s.accept )
  # ...
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