Socket(3) User Contributed Perl Documentation Socket(3)
NAME
"Socket" - networking constants and support functions
SYNOPSIS
"Socket" a low-level module used by, among other things, the IO::Socket
family of modules. The following examples demonstrate some low-level
uses but a practical program would likely use the higher-level API
provided by "IO::Socket" or similar instead.
use Socket qw(PF_INET SOCK_STREAM pack_sockaddr_in inet_aton);
socket(my $socket, PF_INET, SOCK_STREAM, 0)
or die "socket: $!";
my $port = getservbyname "echo", "tcp";
connect($socket, pack_sockaddr_in($port, inet_aton("localhost")))
or die "connect: $!";
print $socket "Hello, world!\n";
print <$socket>;
See also the "EXAMPLES" section.
DESCRIPTION
This module provides a variety of constants, structure manipulators and
other functions related to socket-based networking. The values and
functions provided are useful when used in conjunction with Perl core
functions such as socket(), setsockopt() and bind(). It also provides
several other support functions, mostly for dealing with conversions of
network addresses between human-readable and native binary forms, and
for hostname resolver operations.
Some constants and functions are exported by default by this module;
but for backward-compatibility any recently-added symbols are not
exported by default and must be requested explicitly. When an import
list is provided to the "use Socket" line, the default exports are not
automatically imported. It is therefore best practice to always to
explicitly list all the symbols required.
Also, some common socket "newline" constants are provided: the
constants "CR", "LF", and "CRLF", as well as $CR, $LF, and $CRLF, which
map to "\015", "\012", and "\015\012". If you do not want to use the
literal characters in your programs, then use the constants provided
here. They are not exported by default, but can be imported
individually, and with the ":crlf" export tag:
use Socket qw(:DEFAULT :crlf);
$sock->print("GET / HTTP/1.0$CRLF");
The entire getaddrinfo() subsystem can be exported using the tag
":addrinfo"; this exports the getaddrinfo() and getnameinfo()
functions, and all the "AI_*", "NI_*", "NIx_*" and "EAI_*" constants.
CONSTANTS
In each of the following groups, there may be many more constants
provided than just the ones given as examples in the section heading.
If the heading ends "..." then this means there are likely more; the
exact constants provided will depend on the OS and headers found at
compile-time.
PF_INET, PF_INET6, PF_UNIX, ...
Protocol family constants to use as the first argument to socket() or
the value of the "SO_DOMAIN" or "SO_FAMILY" socket option.
AF_INET, AF_INET6, AF_UNIX, ...
Address family constants used by the socket address structures, to pass
to such functions as inet_pton() or getaddrinfo(), or are returned by
such functions as sockaddr_family().
SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, ...
Socket type constants to use as the second argument to socket(), or the
value of the "SO_TYPE" socket option.
SOCK_NONBLOCK. SOCK_CLOEXEC
Linux-specific shortcuts to specify the "O_NONBLOCK" and "FD_CLOEXEC"
flags during a socket(2) call.
socket( my $sockh, PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, 0 )
SOL_SOCKET
Socket option level constant for setsockopt() and getsockopt().
SO_ACCEPTCONN, SO_BROADCAST, SO_ERROR, ...
Socket option name constants for setsockopt() and getsockopt() at the
"SOL_SOCKET" level.
IP_OPTIONS, IP_TOS, IP_TTL, ...
Socket option name constants for IPv4 socket options at the
"IPPROTO_IP" level.
IP_PMTUDISC_WANT, IP_PMTUDISC_DONT, ...
Socket option value contants for "IP_MTU_DISCOVER" socket option.
IPTOS_LOWDELAY, IPTOS_THROUGHPUT, IPTOS_RELIABILITY, ...
Socket option value constants for "IP_TOS" socket option.
MSG_BCAST, MSG_OOB, MSG_TRUNC, ...
Message flag constants for send() and recv().
SHUT_RD, SHUT_RDWR, SHUT_WR
Direction constants for shutdown().
INADDR_ANY, INADDR_BROADCAST, INADDR_LOOPBACK, INADDR_NONE
Constants giving the special "AF_INET" addresses for wildcard,
broadcast, local loopback, and invalid addresses.
Normally equivalent to inet_aton('0.0.0.0'),
inet_aton('255.255.255.255'), inet_aton('localhost') and
inet_aton('255.255.255.255') respectively.
IPPROTO_IP, IPPROTO_IPV6, IPPROTO_TCP, ...
IP protocol constants to use as the third argument to socket(), the
level argument to getsockopt() or setsockopt(), or the value of the
"SO_PROTOCOL" socket option.
TCP_CORK, TCP_KEEPALIVE, TCP_NODELAY, ...
Socket option name constants for TCP socket options at the
"IPPROTO_TCP" level.
IN6ADDR_ANY, IN6ADDR_LOOPBACK
Constants giving the special "AF_INET6" addresses for wildcard and
local loopback.
Normally equivalent to inet_pton(AF_INET6, "::") and
inet_pton(AF_INET6, "::1") respectively.
IPV6_ADD_MEMBERSHIP, IPV6_MTU, IPV6_V6ONLY, ...
Socket option name constants for IPv6 socket options at the
"IPPROTO_IPV6" level.
STRUCTURE MANIPULATORS
The following functions convert between lists of Perl values and packed
binary strings representing structures.
$family = sockaddr_family $sockaddr
Takes a packed socket address (as returned by pack_sockaddr_in(),
pack_sockaddr_un() or the perl builtin functions getsockname() and
getpeername()). Returns the address family tag. This will be one of the
"AF_*" constants, such as "AF_INET" for a "sockaddr_in" addresses or
"AF_UNIX" for a "sockaddr_un". It can be used to figure out what unpack
to use for a sockaddr of unknown type.
$sockaddr = pack_sockaddr_in $port, $ip_address
Takes two arguments, a port number and an opaque string (as returned by
inet_aton(), or a v-string). Returns the "sockaddr_in" structure with
those arguments packed in and "AF_INET" filled in. For Internet domain
sockets, this structure is normally what you need for the arguments in
bind(), connect(), and send().
An undefined $port argument is taken as zero; an undefined $ip_address
is considered a fatal error.
($port, $ip_address) = unpack_sockaddr_in $sockaddr
Takes a "sockaddr_in" structure (as returned by pack_sockaddr_in(),
getpeername() or recv()). Returns a list of two elements: the port and
an opaque string representing the IP address (you can use inet_ntoa()
to convert the address to the four-dotted numeric format). Will croak
if the structure does not represent an "AF_INET" address.
In scalar context will return just the IP address.
$sockaddr = sockaddr_in $port, $ip_address
($port, $ip_address) = sockaddr_in $sockaddr
A wrapper of pack_sockaddr_in() or unpack_sockaddr_in(). In list
context, unpacks its argument and returns a list consisting of the port
and IP address. In scalar context, packs its port and IP address
arguments as a "sockaddr_in" and returns it.
Provided largely for legacy compatibility; it is better to use
pack_sockaddr_in() or unpack_sockaddr_in() explicitly.
$sockaddr = pack_sockaddr_in6 $port, $ip6_address, [$scope_id, [$flowinfo]]
Takes two to four arguments, a port number, an opaque string (as
returned by inet_pton()), optionally a scope ID number, and optionally
a flow label number. Returns the "sockaddr_in6" structure with those
arguments packed in and "AF_INET6" filled in. IPv6 equivalent of
pack_sockaddr_in().
An undefined $port argument is taken as zero; an undefined $ip6_address
is considered a fatal error.
($port, $ip6_address, $scope_id, $flowinfo) = unpack_sockaddr_in6 $sockaddr
Takes a "sockaddr_in6" structure. Returns a list of four elements: the
port number, an opaque string representing the IPv6 address, the scope
ID, and the flow label. (You can use inet_ntop() to convert the address
to the usual string format). Will croak if the structure does not
represent an "AF_INET6" address.
In scalar context will return just the IP address.
$sockaddr = sockaddr_in6 $port, $ip6_address, [$scope_id, [$flowinfo]]
($port, $ip6_address, $scope_id, $flowinfo) = sockaddr_in6 $sockaddr
A wrapper of pack_sockaddr_in6() or unpack_sockaddr_in6(). In list
context, unpacks its argument according to unpack_sockaddr_in6(). In
scalar context, packs its arguments according to pack_sockaddr_in6().
Provided largely for legacy compatibility; it is better to use
pack_sockaddr_in6() or unpack_sockaddr_in6() explicitly.
$sockaddr = pack_sockaddr_un $path
Takes one argument, a pathname. Returns the "sockaddr_un" structure
with that path packed in with "AF_UNIX" filled in. For "PF_UNIX"
sockets, this structure is normally what you need for the arguments in
bind(), connect(), and send().
($path) = unpack_sockaddr_un $sockaddr
Takes a "sockaddr_un" structure (as returned by pack_sockaddr_un(),
getpeername() or recv()). Returns a list of one element: the pathname.
Will croak if the structure does not represent an "AF_UNIX" address.
$sockaddr = sockaddr_un $path
($path) = sockaddr_un $sockaddr
A wrapper of pack_sockaddr_un() or unpack_sockaddr_un(). In a list
context, unpacks its argument and returns a list consisting of the
pathname. In a scalar context, packs its pathname as a "sockaddr_un"
and returns it.
Provided largely for legacy compatibility; it is better to use
pack_sockaddr_un() or unpack_sockaddr_un() explicitly.
These are only supported if your system has <sys/un.h>.
$ip_mreq = pack_ip_mreq $multiaddr, $interface
Takes an IPv4 multicast address and optionally an interface address (or
"INADDR_ANY"). Returns the "ip_mreq" structure with those arguments
packed in. Suitable for use with the "IP_ADD_MEMBERSHIP" and
"IP_DROP_MEMBERSHIP" sockopts.
($multiaddr, $interface) = unpack_ip_mreq $ip_mreq
Takes an "ip_mreq" structure. Returns a list of two elements; the IPv4
multicast address and interface address.
$ip_mreq_source = pack_ip_mreq_source $multiaddr, $source, $interface
Takes an IPv4 multicast address, source address, and optionally an
interface address (or "INADDR_ANY"). Returns the "ip_mreq_source"
structure with those arguments packed in. Suitable for use with the
"IP_ADD_SOURCE_MEMBERSHIP" and "IP_DROP_SOURCE_MEMBERSHIP" sockopts.
($multiaddr, $source, $interface) = unpack_ip_mreq_source $ip_mreq
Takes an "ip_mreq_source" structure. Returns a list of three elements;
the IPv4 multicast address, source address and interface address.
$ipv6_mreq = pack_ipv6_mreq $multiaddr6, $ifindex
Takes an IPv6 multicast address and an interface number. Returns the
"ipv6_mreq" structure with those arguments packed in. Suitable for use
with the "IPV6_ADD_MEMBERSHIP" and "IPV6_DROP_MEMBERSHIP" sockopts.
($multiaddr6, $ifindex) = unpack_ipv6_mreq $ipv6_mreq
Takes an "ipv6_mreq" structure. Returns a list of two elements; the
IPv6 address and an interface number.
FUNCTIONS
$ip_address = inet_aton $string
Takes a string giving the name of a host, or a textual representation
of an IP address and translates that to an packed binary address
structure suitable to pass to pack_sockaddr_in(). If passed a hostname
that cannot be resolved, returns "undef". For multi-homed hosts (hosts
with more than one address), the first address found is returned.
For portability do not assume that the result of inet_aton() is 32 bits
wide, in other words, that it would contain only the IPv4 address in
network order.
This IPv4-only function is provided largely for legacy reasons. Newly-
written code should use getaddrinfo() or inet_pton() instead for IPv6
support.
$string = inet_ntoa $ip_address
Takes a packed binary address structure such as returned by
unpack_sockaddr_in() (or a v-string representing the four octets of the
IPv4 address in network order) and translates it into a string of the
form "d.d.d.d" where the "d"s are numbers less than 256 (the normal
human-readable four dotted number notation for Internet addresses).
This IPv4-only function is provided largely for legacy reasons. Newly-
written code should use getnameinfo() or inet_ntop() instead for IPv6
support.
$address = inet_pton $family, $string
Takes an address family (such as "AF_INET" or "AF_INET6") and a string
containing a textual representation of an address in that family and
translates that to an packed binary address structure.
See also getaddrinfo() for a more powerful and flexible function to
look up socket addresses given hostnames or textual addresses.
$string = inet_ntop $family, $address
Takes an address family and a packed binary address structure and
translates it into a human-readable textual representation of the
address; typically in "d.d.d.d" form for "AF_INET" or "hhhh:hhhh::hhhh"
form for "AF_INET6".
See also getnameinfo() for a more powerful and flexible function to
turn socket addresses into human-readable textual representations.
($err, @result) = getaddrinfo $host, $service, [$hints]
Given both a hostname and service name, this function attempts to
resolve the host name into a list of network addresses, and the service
name into a protocol and port number, and then returns a list of
address structures suitable to connect() to it.
Given just a host name, this function attempts to resolve it to a list
of network addresses, and then returns a list of address structures
giving these addresses.
Given just a service name, this function attempts to resolve it to a
protocol and port number, and then returns a list of address structures
that represent it suitable to bind() to. This use should be combined
with the "AI_PASSIVE" flag; see below.
Given neither name, it generates an error.
If present, $hints should be a reference to a hash, where the following
keys are recognised:
flags => INT
A bitfield containing "AI_*" constants; see below.
family => INT
Restrict to only generating addresses in this address family
socktype => INT
Restrict to only generating addresses of this socket type
protocol => INT
Restrict to only generating addresses for this protocol
The return value will be a list; the first value being an error
indication, followed by a list of address structures (if no error
occurred).
The error value will be a dualvar; comparable to the "EAI_*" error
constants, or printable as a human-readable error message string. If no
error occurred it will be zero numerically and an empty string.
Each value in the results list will be a hash reference containing the
following fields:
family => INT
The address family (e.g. "AF_INET")
socktype => INT
The socket type (e.g. "SOCK_STREAM")
protocol => INT
The protocol (e.g. "IPPROTO_TCP")
addr => STRING
The address in a packed string (such as would be returned by
pack_sockaddr_in())
canonname => STRING
The canonical name for the host if the "AI_CANONNAME" flag was
provided, or "undef" otherwise. This field will only be present on
the first returned address.
The following flag constants are recognised in the $hints hash. Other
flag constants may exist as provided by the OS.
AI_PASSIVE
Indicates that this resolution is for a local bind() for a passive
(i.e. listening) socket, rather than an active (i.e. connecting)
socket.
AI_CANONNAME
Indicates that the caller wishes the canonical hostname
("canonname") field of the result to be filled in.
AI_NUMERICHOST
Indicates that the caller will pass a numeric address, rather than
a hostname, and that getaddrinfo() must not perform a resolve
operation on this name. This flag will prevent a possibly-slow
network lookup operation, and instead return an error if a hostname
is passed.
($err, $hostname, $servicename) = getnameinfo $sockaddr, [$flags,
[$xflags]]
Given a packed socket address (such as from getsockname(),
getpeername(), or returned by getaddrinfo() in a "addr" field), returns
the hostname and symbolic service name it represents. $flags may be a
bitmask of "NI_*" constants, or defaults to 0 if unspecified.
The return value will be a list; the first value being an error
condition, followed by the hostname and service name.
The error value will be a dualvar; comparable to the "EAI_*" error
constants, or printable as a human-readable error message string. The
host and service names will be plain strings.
The following flag constants are recognised as $flags. Other flag
constants may exist as provided by the OS.
NI_NUMERICHOST
Requests that a human-readable string representation of the numeric
address be returned directly, rather than performing a name resolve
operation that may convert it into a hostname. This will also avoid
potentially-blocking network IO.
NI_NUMERICSERV
Requests that the port number be returned directly as a number
representation rather than performing a name resolve operation that
may convert it into a service name.
NI_NAMEREQD
If a name resolve operation fails to provide a name, then this flag
will cause getnameinfo() to indicate an error, rather than
returning the numeric representation as a human-readable string.
NI_DGRAM
Indicates that the socket address relates to a "SOCK_DGRAM" socket,
for the services whose name differs between TCP and UDP protocols.
The following constants may be supplied as $xflags.
NIx_NOHOST
Indicates that the caller is not interested in the hostname of the
result, so it does not have to be converted. "undef" will be
returned as the hostname.
NIx_NOSERV
Indicates that the caller is not interested in the service name of
the result, so it does not have to be converted. "undef" will be
returned as the service name.
getaddrinfo() / getnameinfo() ERROR CONSTANTS
The following constants may be returned by getaddrinfo() or
getnameinfo(). Others may be provided by the OS.
EAI_AGAIN
A temporary failure occurred during name resolution. The operation
may be successful if it is retried later.
EAI_BADFLAGS
The value of the "flags" hint to getaddrinfo(), or the $flags
parameter to getnameinfo() contains unrecognised flags.
EAI_FAMILY
The "family" hint to getaddrinfo(), or the family of the socket
address passed to getnameinfo() is not supported.
EAI_NODATA
The host name supplied to getaddrinfo() did not provide any usable
address data.
EAI_NONAME
The host name supplied to getaddrinfo() does not exist, or the
address supplied to getnameinfo() is not associated with a host
name and the "NI_NAMEREQD" flag was supplied.
EAI_SERVICE
The service name supplied to getaddrinfo() is not available for the
socket type given in the $hints.
EXAMPLES
Lookup for connect()
The getaddrinfo() function converts a hostname and a service name into
a list of structures, each containing a potential way to connect() to
the named service on the named host.
use IO::Socket;
use Socket qw(SOCK_STREAM getaddrinfo);
my %hints = (socktype => SOCK_STREAM);
my ($err, @res) = getaddrinfo("localhost", "echo", \%hints);
die "Cannot getaddrinfo - $err" if $err;
my $sock;
foreach my $ai (@res) {
my $candidate = IO::Socket->new();
$candidate->socket($ai->{family}, $ai->{socktype}, $ai->{protocol})
or next;
$candidate->connect($ai->{addr})
or next;
$sock = $candidate;
last;
}
die "Cannot connect to localhost:echo" unless $sock;
$sock->print("Hello, world!\n");
print <$sock>;
Because a list of potential candidates is returned, the "while" loop
tries each in turn until it finds one that succeeds both the socket()
and connect() calls.
This function performs the work of the legacy functions
gethostbyname(), getservbyname(), inet_aton() and pack_sockaddr_in().
In practice this logic is better performed by IO::Socket::IP.
Making a human-readable string out of an address
The getnameinfo() function converts a socket address, such as returned
by getsockname() or getpeername(), into a pair of human-readable
strings representing the address and service name.
use IO::Socket::IP;
use Socket qw(getnameinfo);
my $server = IO::Socket::IP->new(LocalPort => 12345, Listen => 1) or
die "Cannot listen - $@";
my $socket = $server->accept or die "accept: $!";
my ($err, $hostname, $servicename) = getnameinfo($socket->peername);
die "Cannot getnameinfo - $err" if $err;
print "The peer is connected from $hostname\n";
Since in this example only the hostname was used, the redundant
conversion of the port number into a service name may be omitted by
passing the "NIx_NOSERV" flag.
use Socket qw(getnameinfo NIx_NOSERV);
my ($err, $hostname) = getnameinfo($socket->peername, 0, NIx_NOSERV);
This function performs the work of the legacy functions
unpack_sockaddr_in(), inet_ntoa(), gethostbyaddr() and getservbyport().
In practice this logic is better performed by IO::Socket::IP.
Resolving hostnames into IP addresses
To turn a hostname into a human-readable plain IP address use
getaddrinfo() to turn the hostname into a list of socket structures,
then getnameinfo() on each one to make it a readable IP address again.
use Socket qw(:addrinfo SOCK_RAW);
my ($err, @res) = getaddrinfo($hostname, "", {socktype => SOCK_RAW});
die "Cannot getaddrinfo - $err" if $err;
while( my $ai = shift @res ) {
my ($err, $ipaddr) = getnameinfo($ai->{addr}, NI_NUMERICHOST, NIx_NOSERV);
die "Cannot getnameinfo - $err" if $err;
print "$ipaddr\n";
}
The "socktype" hint to getaddrinfo() filters the results to only
include one socket type and protocol. Without this most OSes return
three combinations, for "SOCK_STREAM", "SOCK_DGRAM" and "SOCK_RAW",
resulting in triplicate output of addresses. The "NI_NUMERICHOST" flag
to getnameinfo() causes it to return a string-formatted plain IP
address, rather than reverse resolving it back into a hostname.
This combination performs the work of the legacy functions
gethostbyname() and inet_ntoa().
Accessing socket options
The many "SO_*" and other constants provide the socket option names for
getsockopt() and setsockopt().
use IO::Socket::INET;
use Socket qw(SOL_SOCKET SO_RCVBUF IPPROTO_IP IP_TTL);
my $socket = IO::Socket::INET->new(LocalPort => 0, Proto => 'udp')
or die "Cannot create socket: $@";
$socket->setsockopt(SOL_SOCKET, SO_RCVBUF, 64*1024) or
die "setsockopt: $!";
print "Receive buffer is ", $socket->getsockopt(SOL_SOCKET, SO_RCVBUF),
" bytes\n";
print "IP TTL is ", $socket->getsockopt(IPPROTO_IP, IP_TTL), "\n";
As a convenience, IO::Socket's setsockopt() method will convert a
number into a packed byte buffer, and getsockopt() will unpack a byte
buffer of the correct size back into a number.
AUTHOR
This module was originally maintained in Perl core by the Perl 5
Porters.
It was extracted to dual-life on CPAN at version 1.95 by Paul Evans
<leonerd AT leonerd.uk>
perl v5.26.3 2018-01-12 Socket(3)
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