Socket(3pm)

💖 NAME 💖

Socket(3pm) from the Perl Programmers Reference Guide — the module itself is "Socket" — and it is literally your low-level hookup for networking constants and support functions. That is the whole vibe, bestie: socket goodies, constants, conversions, resolver stuff. ✨

🦋 SYNOPSIS 🦋

Okay so "Socket" is a low-level module used by, among other things, the IO::Socket family of modules. The examples below show some low-level uses, but a practical program would likely use the higher-level API provided by IO::Socket or similar instead. No cap.

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>;

Also peep the EXAMPLES section. 👀

🌸 DESCRIPTION 🌸

This module serves up a whole 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. Very demure, very mindful, very socket-coded. 🫶

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 explicitly list all the symbols required. Like, do not make your future self guess, bestie. 💅

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. Iconic little export tag, honestly. 🎀

👑 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. So yeah — vibe check your platform, iykyk. 🌈

🧚 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 constants for the IP_MTU_DISCOVER socket option.

🌸 IPTOS_LOWDELAY, IPTOS_THROUGHPUT, IPTOS_RELIABILITY, ... 🌸

Socket option value constants for the 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 ✨

Okay bestie, this next batch is literally the translator squad 💅 — these functions flip between lists of Perl values and packed binary strings that represent structures. Like, one direction you've got your cute readable list, the other direction you've got the raw bytes that the socket syscalls actually want. Both iconic, both necessary.

---

🎀 sockaddr_family

$family = sockaddr_family $sockaddr;

So you've got a packed socket address in your hands — maybe it came from pack_sockaddr_in(), maybe from pack_sockaddr_un(), or maybe from the perl builtin functions getsockname() and getpeername(). Either way, this one hands you back the address family tag. 🏷️

That tag is gonna be one of the "AF_*" constants — like "AF_INET" if it's a sockaddr_in address, or "AF_UNIX" if it's a sockaddr_un. And the whole point? You use it to figure out which unpack to reach for when you've got a sockaddr of unknown type. Detective work, but make it slay 🔍✨

---

🎀 pack_sockaddr_in

$sockaddr = pack_sockaddr_in $port, $ip_address;

Two arguments, babe: a port number and an opaque string (the kind inet_aton() gives you, or a v-string works too). You get back the sockaddr_in structure with both of those packed in and "AF_INET" filled in for free. 🎁

For Internet domain sockets, this structure is normally exactly what you need for the arguments in bind(), connect(), and send(). Like, this is the one. 💖

Heads up: an undefined $port argument is taken as zero, but an undefined $ip_address is considered a fatal error 😭 no skipping that one.

---

🎀 unpack_sockaddr_in

($port, $ip_address) = unpack_sockaddr_in $sockaddr;

Feed it a sockaddr_in structure — the kind from pack_sockaddr_in(), getpeername() or recv() — and you get back a list of two elements: the port, and an opaque string representing the IP address. (Pro tip: use inet_ntoa() to turn that address into the four-dotted numeric format, iykyk 🌈)

It will croak if the structure doesn't represent an "AF_INET" address. Rude of it, but fair 💀

Oh and — in scalar context it returns just the IP address. Cute, right? 🫶

---

🎀 sockaddr_in

$sockaddr = sockaddr_in $port, $ip_address;

($port, $ip_address) = sockaddr_in $sockaddr;

A wrapper of pack_sockaddr_in() or unpack_sockaddr_in() — it decides based on context, main character energy honestly 👑

In list context, it unpacks its argument and returns a list of the port and IP address. In scalar context, it packs its port and IP address arguments as a sockaddr_in and returns that.

Ngl, this one's provided largely for legacy compatibility. It's better to use pack_sockaddr_in() or unpack_sockaddr_in() explicitly. Be specific, bestie 💅

---

🎀 pack_sockaddr_in6

$sockaddr = pack_sockaddr_in6 $port, $ip6_address,
    $scope_id = 0, $flowinfo = 0;

The IPv6 glow-up of pack_sockaddr_in() ✨ 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. You get back the sockaddr_in6 structure with all of that packed in and "AF_INET6" filled in. 🦋

Same rules as its IPv4 sibling: an undefined $port argument is taken as zero, and an undefined $ip6_address is considered a fatal error 😭

---

🎀 unpack_sockaddr_in6

($port, $ip6_address, $scope_id, $flowinfo) = unpack_sockaddr_in6 $sockaddr;

Takes a sockaddr_in6 structure and gives you back a list of four elements: the port number, an opaque string representing the IPv6 address, the scope ID, and the flow label. Four whole things, we love a generous return value 🎀

(You can use inet_ntop() to convert the address to the usual string format, in case you were wondering 🌸)

It will croak if the structure doesn't represent an "AF_INET6" address. No cap, it checks 💀

And in scalar context? Just the IP address. Keeping it simple 🧚

---

🎀 sockaddr_in6

$sockaddr = sockaddr_in6 $port, $ip6_address,
    $scope_id = 0, $flowinfo = 0;

($port, $ip6_address, $scope_id, $flowinfo) = sockaddr_in6 $sockaddr;

A wrapper of pack_sockaddr_in6() or unpack_sockaddr_in6(). In list context, it unpacks its argument according to unpack_sockaddr_in6(). In scalar context, it packs its arguments according to pack_sockaddr_in6(). Same energy, different direction 💫

Again: provided largely for legacy compatibility, and it's better to use pack_sockaddr_in6() or unpack_sockaddr_in6() explicitly. We stan being explicit 🫶

---

🎀 pack_sockaddr_un

$sockaddr = pack_sockaddr_un $path;

One argument, a pathname. You get back the sockaddr_un structure with that path packed in and "AF_UNIX" filled in. 🐚

For "PF_UNIX" sockets, this structure is normally what you need for the arguments in bind(), connect(), and send(). Local sockets, local vibes ✨

---

🎀 unpack_sockaddr_un

($path) = unpack_sockaddr_un $sockaddr;

Takes a sockaddr_un structure (as returned by pack_sockaddr_un(), getpeername() or recv()) and returns a list of one element: the pathname. Just the one, babe 🌸

It will croak if the structure doesn't represent an "AF_UNIX" address. It's picky like that 💅

---

🎀 sockaddr_un

$sockaddr = sockaddr_un $path;

($path) = sockaddr_un $sockaddr;

A wrapper of pack_sockaddr_un() or unpack_sockaddr_un(). In a list context, it unpacks its argument and returns a list consisting of the pathname. In a scalar context, it packs its pathname as a sockaddr_un and returns it. 🎀

Provided largely for legacy compatibility; it's better to use pack_sockaddr_un() or unpack_sockaddr_un() explicitly. (Seeing a pattern here? 😅)

Lowkey important: these are only supported if your system has <sys/un.h>. If not, well, no UNIX sockets for you 😭

---

🎀 pack_ip_mreq

$ip_mreq = pack_ip_mreq $multiaddr, $interface;

Takes an IPv4 multicast address and optionally an interface address (or "INADDR_ANY"). You get back the ip_mreq structure with those arguments packed in — perfect for using with the "IP_ADD_MEMBERSHIP" and "IP_DROP_MEMBERSHIP" sockopts. 📡

---

🎀 unpack_ip_mreq

($multiaddr, $interface) = unpack_ip_mreq $ip_mreq;

Takes an ip_mreq structure and returns a list of two elements: the IPv4 multicast address and the interface address. Two for two, slay ✌️

---

🎀 pack_ip_mreq_source

$ip_mreq_source = pack_ip_mreq_source $multiaddr, $source, $interface;

Takes an IPv4 multicast address, a source address, and optionally an interface address (or "INADDR_ANY"). You get back the ip_mreq_source structure with everything packed in — suitable for the "IP_ADD_SOURCE_MEMBERSHIP" and "IP_DROP_SOURCE_MEMBERSHIP" sockopts. Source-specific multicast, we see you 👀🔥

---

🎀 unpack_ip_mreq_source

($multiaddr, $source, $interface) = unpack_ip_mreq_source $ip_mreq;

Takes an ip_mreq_source structure and returns a list of three elements: the IPv4 multicast address, the source address, and the interface address. Three whole elements, we're being spoiled 💖

---

🎀 pack_ipv6_mreq

$ipv6_mreq = pack_ipv6_mreq $multiaddr6, $ifindex;

Takes an IPv6 multicast address and an interface number. You get back the ipv6_mreq structure with those arguments packed in — suitable for use with the "IPV6_ADD_MEMBERSHIP" and "IPV6_DROP_MEMBERSHIP" sockopts. IPv6 multicast, six-ified and gorgeous 🦋📡

---

🎀 unpack_ipv6_mreq

($multiaddr6, $ifindex) = unpack_ipv6_mreq $ipv6_mreq;

Takes an ipv6_mreq structure and returns a list of two elements: the IPv6 address and an interface number. And that's the whole structure-manipulator lineup, bestie — every single one accounted for ✨👑

✨ FUNCTIONS ✨

💖 inet_aton

$ip_address = inet_aton $string;

Okay so inet_aton takes a string that's either a host name or a textual representation of an IP address, and translates it into a packed binary address structure suitable to pass to pack_sockaddr_in(). If you pass a hostname that cannot be resolved, it returns undef. For multi-homed hosts (hosts with more than one address), the first address found is returned.

Portability check, bestie: do not assume that the result of inet_aton() is 32 bits wide — in other words, don't assume 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.

🦋 inet_ntoa

$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. That's the normal human-readable four dotted number notation for Internet addresses, iykyk.

This IPv4-only function is provided largely for legacy reasons. Newly-written code should use getnameinfo() or inet_ntop() instead for IPv6 support.

🌸 inet_pton

$address = inet_pton $family, $string;

Takes an address family (such as AF_INET or AF_INET6) plus a string containing a textual representation of an address in that family, and translates that into a packed binary address structure.

See also getaddrinfo() for a more powerful and flexible function to look up socket addresses given hostnames or textual addresses.

🎀 inet_ntop

$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.

🔥 getaddrinfo

($err, @result) = getaddrinfo $host, $service;

($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.

🌈 getnameinfo

($err, $hostname, $servicename) = getnameinfo $sockaddr,
    $flags = 0, $xflags = 0;

Given a packed socket address (such as from getsockname(), getpeername(), or returned by getaddrinfo() in an 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()

Okay so getaddrinfo() converts a hostname and a service name into a list of structures, and every single structure contains a potential way to connect() to the named service on the named host. It's giving "here are all your candidates, bestie" energy.

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. Like speed-dating but for sockets, and way less awkward.

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. Human-readable? Obsessed, actually.

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. Skip the extra work, queen.

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. Round trip, but make it cute.

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. Not the triplicate era, bestie.

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(). Options on options on options.

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. Convenient and iconic, as usual.

---

👑 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@leonerd.org.uk>.

---

This iconic page is Socket(3pm) — source/version perl v5.42.3, dated 2026-11-03. Mwah 💋