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Author image Paul Evans


IO::Socket::IP - Family-neutral IP socket supporting both IPv4 and IPv6


 use IO::Socket::IP;

 my $sock = IO::Socket::IP->new(
    PeerHost => "www.google.com",
    PeerPort => "http",
    Type     => SOCK_STREAM,
 ) or die "Cannot construct socket - $@";

 my $familyname = ( $sock->sockdomain == PF_INET6 ) ? "IPv6" :
                  ( $sock->sockdomain == PF_INET  ) ? "IPv4" :

 printf "Connected to google via %s\n", $familyname;


This module provides a protocol-independent way to use IPv4 and IPv6 sockets, intended as a replacement for IO::Socket::INET. Most constructor arguments and methods are provided in a backward-compatible way. For a list of known differences, see the IO::Socket::INET INCOMPATIBILITES section below.

It uses the getaddrinfo(3) function to convert hostnames and service names or port numbers into sets of possible addresses to connect to or listen on. This allows it to work for IPv6 where the system supports it, while still falling back to IPv4-only on systems which don't.


By placing -register in the import list to IO::Socket::IP, it will register itself with IO::Socket as the class that handles PF_INET. It will also ask to handle PF_INET6 as well, provided that constant is available.

Changing IO::Socket's default behaviour means that calling the IO::Socket constructor with either PF_INET or PF_INET6 as the Domain parameter will yield an IO::Socket::IP object.

 use IO::Socket::IP -register;

 my $sock = IO::Socket->new(
    Domain    => PF_INET6,
    LocalHost => "::1",
    Listen    => 1,
 ) or die "Cannot create socket - $@\n";

 print "Created a socket of type " . ref($sock) . "\n";

Note that -register is a global setting that applies to the entire program; it cannot be applied only for certain callers, removed, or limited by lexical scope.



   $sock = IO::Socket::IP->new( %args )

Creates a new IO::Socket::IP object, containing a newly created socket handle according to the named arguments passed. The recognised arguments are:

PeerHost => STRING
PeerService => STRING

Hostname and service name for the peer to connect() to. The service name may be given as a port number, as a decimal string.

PeerAddr => STRING
PeerPort => STRING

For symmetry with the accessor methods and compatibility with IO::Socket::INET, these are accepted as synonyms for PeerHost and PeerService respectively.

PeerAddrInfo => ARRAY

Alternate form of specifying the peer to connect() to. This should be an array of the form returned by Socket::getaddrinfo.

This parameter takes precedence over the Peer*, Family, Type and Proto arguments.

LocalHost => STRING
LocalService => STRING

Hostname and service name for the local address to bind() to.

LocalAddr => STRING
LocalPort => STRING

For symmetry with the accessor methods and compatibility with IO::Socket::INET, these are accepted as synonyms for LocalHost and LocalService respectively.

LocalAddrInfo => ARRAY

Alternate form of specifying the local address to bind() to. This should be an array of the form returned by Socket::getaddrinfo.

This parameter takes precedence over the Local*, Family, Type and Proto arguments.

Family => INT

The address family to pass to getaddrinfo (e.g. AF_INET, AF_INET6). Normally this will be left undefined, and getaddrinfo will search using any address family supported by the system.

Type => INT

The socket type to pass to getaddrinfo (e.g. SOCK_STREAM, SOCK_DGRAM). Normally defined by the caller; if left undefined getaddrinfo may attempt to infer the type from the service name.

Proto => STRING or INT

The IP protocol to use for the socket (e.g. 'tcp', IPPROTO_TCP, 'udp',IPPROTO_UDP). Normally this will be left undefined, and either getaddrinfo or the kernel will choose an appropriate value. May be given either in string name or numeric form.

GetAddrInfoFlags => INT

More flags to pass to the getaddrinfo() function. If not supplied, a default of AI_ADDRCONFIG will be used.

These flags will be combined with AI_PASSIVE if the Listen argument is given. For more information see the documentation about getaddrinfo() in the Socket module.

Listen => INT

If defined, puts the socket into listening mode where new connections can be accepted using the accept method. The value given is used as the listen(2) queue size.

ReuseAddr => BOOL

If true, set the SO_REUSEADDR sockopt

ReusePort => BOOL

If true, set the SO_REUSEPORT sockopt (not all OSes implement this sockopt)

Broadcast => BOOL

If true, set the SO_BROADCAST sockopt

Sockopts => ARRAY

An optional array of other socket options to apply after the three listed above. The value is an ARRAY containing 2- or 3-element ARRAYrefs. Each inner array relates to a single option, giving the level and option name, and an optional value. If the value element is missing, it will be given the value of a platform-sized integer 1 constant (i.e. suitable to enable most of the common boolean options).

For example, both options given below are equivalent to setting ReuseAddr.

 Sockopts => [
    [ SOL_SOCKET, SO_REUSEADDR, pack( "i", 1 ) ],
V6Only => BOOL

If defined, set the IPV6_V6ONLY sockopt when creating PF_INET6 sockets to the given value. If true, a listening-mode socket will only listen on the AF_INET6 addresses; if false it will also accept connections from AF_INET addresses.

If not defined, the socket option will not be changed, and default value set by the operating system will apply. For repeatable behaviour across platforms it is recommended this value always be defined for listening-mode sockets.

Note that not all platforms support disabling this option. Some, at least OpenBSD and MirBSD, will fail with EINVAL if you attempt to disable it. To determine whether it is possible to disable, you may use the class method

 if( IO::Socket::IP->CAN_DISABLE_V6ONLY ) {
 else {

If your platform does not support disabling this option but you still want to listen for both AF_INET and AF_INET6 connections you will have to create two listening sockets, one bound to each protocol.


This IO::Socket::INET-style argument is ignored, except if it is defined but false. See the IO::Socket::INET INCOMPATIBILITES section below.

However, the behaviour it enables is always performed by IO::Socket::IP.

Blocking => BOOL

If defined but false, the socket will be set to non-blocking mode. Otherwise it will default to blocking mode. See the NON-BLOCKING section below for more detail.

Timeout => NUM

If defined, gives a maximum time in seconds to block per connect() call when in blocking mode. If missing, no timeout is applied other than that provided by the underlying operating system. When in non-blocking mode this parameter is ignored.

Note that if the hostname resolves to multiple address candidates, the same timeout will apply to each connection attempt individually, rather than to the operation as a whole. Further note that the timeout does not apply to the initial hostname resolve operation, if connecting by hostname.

This behviour is copied inspired by IO::Socket::INET; for more fine grained control over connection timeouts, consider performing a nonblocking connect directly.

If neither Type nor Proto hints are provided, a default of SOCK_STREAM and IPPROTO_TCP respectively will be set, to maintain compatibility with IO::Socket::INET. Other named arguments that are not recognised are ignored.

If neither Family nor any hosts or addresses are passed, nor any *AddrInfo, then the constructor has no information on which to decide a socket family to create. In this case, it performs a getaddinfo call with the AI_ADDRCONFIG flag, no host name, and a service name of "0", and uses the family of the first returned result.

If the constructor fails, it will set $@ to an appropriate error message; this may be from $! or it may be some other string; not every failure necessarily has an associated errno value.

new (one arg)

   $sock = IO::Socket::IP->new( $peeraddr )

As a special case, if the constructor is passed a single argument (as opposed to an even-sized list of key/value pairs), it is taken to be the value of the PeerAddr parameter. This is parsed in the same way, according to the behaviour given in the PeerHost AND LocalHost PARSING section below.


As well as the following methods, this class inherits all the methods in IO::Socket and IO::Handle.


   ( $host, $service ) = $sock->sockhost_service( $numeric )

Returns the hostname and service name of the local address (that is, the socket address given by the sockname method).

If $numeric is true, these will be given in numeric form rather than being resolved into names.

The following four convenience wrappers may be used to obtain one of the two values returned here. If both host and service names are required, this method is preferable to the following wrappers, because it will call getnameinfo(3) only once.


   $addr = $sock->sockhost

Return the numeric form of the local address as a textual representation


   $port = $sock->sockport

Return the numeric form of the local port number


   $host = $sock->sockhostname

Return the resolved name of the local address


   $service = $sock->sockservice

Return the resolved name of the local port number


   $addr = $sock->sockaddr

Return the local address as a binary octet string


   ( $host, $service ) = $sock->peerhost_service( $numeric )

Returns the hostname and service name of the peer address (that is, the socket address given by the peername method), similar to the sockhost_service method.

The following four convenience wrappers may be used to obtain one of the two values returned here. If both host and service names are required, this method is preferable to the following wrappers, because it will call getnameinfo(3) only once.


   $addr = $sock->peerhost

Return the numeric form of the peer address as a textual representation


   $port = $sock->peerport

Return the numeric form of the peer port number


   $host = $sock->peerhostname

Return the resolved name of the peer address


   $service = $sock->peerservice

Return the resolved name of the peer port number


   $addr = $peer->peeraddr

Return the peer address as a binary octet string


   $inet = $sock->as_inet

Returns a new IO::Socket::INET instance wrapping the same filehandle. This may be useful in cases where it is required, for backward-compatibility, to have a real object of IO::Socket::INET type instead of IO::Socket::IP. The new object will wrap the same underlying socket filehandle as the original, so care should be taken not to continue to use both objects concurrently. Ideally the original $sock should be discarded after this method is called.

This method checks that the socket domain is PF_INET and will throw an exception if it isn't.


If the constructor is passed a defined but false value for the Blocking argument then the socket is put into non-blocking mode. When in non-blocking mode, the socket will not be set up by the time the constructor returns, because the underlying connect(2) syscall would otherwise have to block.

The non-blocking behaviour is an extension of the IO::Socket::INET API, unique to IO::Socket::IP, because the former does not support multi-homed non-blocking connect.

When using non-blocking mode, the caller must repeatedly check for writeability on the filehandle (for instance using select or IO::Poll). Each time the filehandle is ready to write, the connect method must be called, with no arguments. Note that some operating systems, most notably MSWin32 do not report a connect() failure using write-ready; so you must also select() for exceptional status.

While connect returns false, the value of $! indicates whether it should be tried again (by being set to the value EINPROGRESS, or EWOULDBLOCK on MSWin32), or whether a permanent error has occurred (e.g. ECONNREFUSED).

Once the socket has been connected to the peer, connect will return true and the socket will now be ready to use.

Note that calls to the platform's underlying getaddrinfo(3) function may block. If IO::Socket::IP has to perform this lookup, the constructor will block even when in non-blocking mode.

To avoid this blocking behaviour, the caller should pass in the result of such a lookup using the PeerAddrInfo or LocalAddrInfo arguments. This can be achieved by using Net::LibAsyncNS, or the getaddrinfo(3) function can be called in a child process.

 use IO::Socket::IP;

 my @peeraddrinfo = ... # Caller must obtain the getaddinfo result here

 my $socket = IO::Socket::IP->new(
    PeerAddrInfo => \@peeraddrinfo,
    Blocking     => 0,
 ) or die "Cannot construct socket - $@";

 while( !$socket->connect and ( $! == EINPROGRESS || $! == EWOULDBLOCK ) ) {
    my $wvec = '';
    vec( $wvec, fileno $socket, 1 ) = 1;
    my $evec = '';
    vec( $evec, fileno $socket, 1 ) = 1;

    select( undef, $wvec, $evec, undef ) or die "Cannot select - $!";

 die "Cannot connect - $!" if $!;


The example above uses select(), but any similar mechanism should work analogously. IO::Socket::IP takes care when creating new socket filehandles to preserve the actual file descriptor number, so such techniques as poll or epoll should be transparent to its reallocation of a different socket underneath, perhaps in order to switch protocol family between PF_INET and PF_INET6.

For another example using IO::Poll and Net::LibAsyncNS, see the examples/nonblocking_libasyncns.pl file in the module distribution.

PeerHost AND LocalHost PARSING

To support the IO::Socket::INET API, the host and port information may be passed in a single string rather than as two separate arguments.

If either LocalHost or PeerHost (or their ...Addr synonyms) have any of the following special forms then special parsing is applied.

The value of the ...Host argument will be split to give both the hostname and port (or service name):

 hostname.example.org:http    # Host name                 # IPv4 address
 [2001:db8::1]:80             # IPv6 address

In each case, the port or service name (e.g. 80) is passed as the LocalService or PeerService argument.

Either of LocalService or PeerService (or their ...Port synonyms) can be either a service name, a decimal number, or a string containing both a service name and number, in a form such as


In this case, the name (http) will be tried first, but if the resolver does not understand it then the port number (80) will be used instead.

If the ...Host argument is in this special form and the corresponding ...Service or ...Port argument is also defined, the one parsed from the ...Host argument will take precedence and the other will be ignored.


   ( $host, $port ) = IO::Socket::IP->split_addr( $addr )

Utility method that provides the parsing functionality described above. Returns a 2-element list, containing either the split hostname and port description if it could be parsed, or the given address and undef if it was not recognised.

 IO::Socket::IP->split_addr( "hostname:http" )
                              # ( "hostname",  "http" )

 IO::Socket::IP->split_addr( "" )
                              # ( "", "80"   )

 IO::Socket::IP->split_addr( "[2001:db8::1]:80" )
                              # ( "2001:db8::1", "80" )

 IO::Socket::IP->split_addr( "something.else" )
                              # ( "something.else", undef )


   $addr = IO::Socket::IP->join_addr( $host, $port )

Utility method that performs the reverse of split_addr, returning a string formed by joining the specified host address and port number. The host address will be wrapped in [] brackets if required (because it is a raw IPv6 numeric address).

This can be especially useful when combined with the sockhost_service or peerhost_service methods.

 say "Connected to ", IO::Socket::IP->join_addr( $sock->peerhost_service );


  • The behaviour enabled by MultiHomed is in fact implemented by IO::Socket::IP as it is required to correctly support searching for a useable address from the results of the getaddrinfo(3) call. The constructor will ignore the value of this argument, except if it is defined but false. An exception is thrown in this case, because that would request it disable the getaddrinfo(3) search behaviour in the first place.

  • IO::Socket::IP implements both the Blocking and Timeout parameters, but it implements the interaction of both in a different way.

    In ::INET, supplying a timeout overrides the non-blocking behaviour, meaning that the connect() operation will still block despite that the caller asked for a non-blocking socket. This is not explicitly specified in its documentation, nor does this author believe that is a useful behaviour - it appears to come from a quirk of implementation.

    In ::IP therefore, the Blocking parameter takes precedence - if a non-blocking socket is requested, no operation will block. The Timeout parameter here simply defines the maximum time that a blocking connect() call will wait, if it blocks at all.

    In order to specifically obtain the "blocking connect then non-blocking send and receive" behaviour of specifying this combination of options to ::INET when using ::IP, perform first a blocking connect, then afterwards turn the socket into nonblocking mode.

     my $sock = IO::Socket::IP->new(
        PeerHost => $peer,
        Timeout => 20,
     ) or die "Cannot connect - $@";
     $sock->blocking( 0 );

    This code will behave identically under both IO::Socket::INET and IO::Socket::IP.


  • Investigate whether POSIX::dup2 upsets BSD's kqueue watchers, and if so, consider what possible workarounds might be applied.


Paul Evans <leonerd@leonerd.org.uk>