inet
Access to TCP/IP Protocols
Provides access to TCP/IP protocols.
See also ERTS User's Guide, Inet configuration for more information on how to configure an Erlang runtime system for IP communication.
Two Kernel configuration parameters affect the behaviour of all
sockets opened on an Erlang node:
inet_default_connect_options
can contain a list of default
options used for all sockets returned when doing connect
,
and inet_default_listen_options
can contain a list of
default options used when issuing a listen
call. When
accept
is issued, the values of the listensocket options
are inherited, why no such application variable is needed for
accept
.
Using the Kernel configuration parameters mentioned above, one
can set default options for all TCP sockets on a node. This should
be used with care, but options like {delay_send,true}
might be specified in this way. An example of starting an Erlang
node with all sockets using delayed send could look like this:
$erl -sname test -kernel \\
inet_default_connect_options '[{delay_send,true}]' \\
inet_default_listen_options '[{delay_send,true}]'
Note that the default option {active, true}
currently
cannot be changed, for internal reasons.
DATA TYPES
#hostent{h_addr_list = [ip_address()] % list of addresses for this host h_addrtype = inet | inet6 h_aliases = [hostname()] % list of aliases h_length = int() % length of address in bytes h_name = hostname() % official name for host The record is defined in the Kernel include file "inet.hrl" Add the following directive to the module: -include_lib("kernel/include/inet.hrl"). hostname() = atom() | string() ip_address() = {N1,N2,N3,N4} % IPv4 | {K1,K2,K3,K4,K5,K6,K7,K8} % IPv6 Ni = 0..255 Ki = 0..65535 posix() an atom which is named from the Posix error codes used in Unix, and in the runtime libraries of most C compilers socket() see gen_tcp(3), gen_udp(3)
Addresses as inputs to functions can be either a string or a
tuple. For instance, the IP address 150.236.20.73 can be passed to
gethostbyaddr/1
either as the string "150.236.20.73"
or as the tuple {150, 236, 20, 73}
.
IPv4 address examples:
Address ip_address() ------- ------------ 127.0.0.1 {127,0,0,1} 192.168.42.2 {192,168,42,2}
IPv6 address examples:
Address ip_address() ------- ------------ ::1 {0,0,0,0,0,0,0,1} ::192.168.42.2 {0,0,0,0,0,0,(192 bsl 8) bor 168,(42 bsl 8) bor 2} FFFF::192.168.42.2 {16#FFFF,0,0,0,0,0,(192 bsl 8) bor 168,(42 bsl 8) bor 2} 3ffe:b80:1f8d:2:204:acff:fe17:bf38 {16#3ffe,16#b80,16#1f8d,16#2,16#204,16#acff,16#fe17,16#bf38} fe80::204:acff:fe17:bf38 {16#fe80,0,0,0,0,16#204,16#acff,16#fe17,16#bf38}
A function that may be useful is inet_parse:address/1
:
1>inet_parse:address("192.168.42.2").
{ok,{192,168,42,2}} 2>inet_parse:address("FFFF::192.168.42.2").
{ok,{65535,0,0,0,0,0,49320,10754}}
Functions
close(Socket) -> ok
Socket = socket()
Closes a socket of any type.
get_rc() -> [{Par, Val}]
Par, Val -- see below
Returns the state of the Inet configuration database in form of a list of recorded configuration parameters. (See the ERTS User's Guide, Inet configuration, for more information). Only parameters with other than default values are returned.
format_error(Posix) -> string()
Posix = posix()
Returns a diagnostic error string. See the section below
for possible Posix
values and the corresponding
strings.
getaddr(Host, Family) -> {ok, Address} | {error, posix()}
Host = ip_address() | string() | atom()
Family = inet | inet6
Address = ip_address()
posix() = term()
Returns the IP-address for Host
as a tuple of
integers. Host
can be an IP-address, a single hostname
or a fully qualified hostname.
getaddrs(Host, Family) -> {ok, Addresses} | {error, posix()}
Host = ip_address() | string() | atom()
Addresses = [ip_address()]
Family = inet | inet6
Returns a list of all IP-addresses for Host
.
Host
can be an IP-address, a single hostname or a fully
qualified hostname.
gethostbyaddr(Address) -> {ok, Hostent} | {error, posix()}
Address = string() | ip_address()
Hostent = #hostent{}
Returns a hostent
record given an address.
gethostbyname(Name) -> {ok, Hostent} | {error, posix()}
Hostname = hostname()
Hostent = #hostent{}
Returns a hostent
record given a hostname.
gethostbyname(Name, Family) -> {ok, Hostent} | {error, posix()}
Hostname = hostname()
Family = inet | inet6
Hostent = #hostent{}
Returns a hostent
record given a hostname, restricted
to the given address family.
gethostname() -> {ok, Hostname}
Hostname = string()
Returns the local hostname. Will never fail.
getopts(Socket, Options) -> OptionValues | {error, posix()}
Socket = term()
Options = [Opt | RawOptReq]
Opt = atom()
RawOptReq = {raw, Protocol, OptionNum, ValueSpec}
Protocol = int()
OptionNum = int()
ValueSpec = ValueSize | ValueBin
ValueSize = int()
ValueBin = binary()
OptionValues = [{Opt, Val} | {raw, Protocol, OptionNum, ValueBin}]
Gets one or more options for a socket. See setopts/2 for a list of available options.
The number of elements in the returned OptionValues
list does not necessarily correspond to the number of options
asked for. If the operating system fails to support an option,
it is simply left out in the returned list. An error tuple is only
returned when getting options for the socket is impossible
(i.e. the socket is closed or the buffer size in a raw request
is too large). This behavior is kept for backward
compatibility reasons.
A RawOptReq
can be used to get information about
socket options not (explicitly) supported by the emulator. The
use of raw socket options makes the code non portable, but
allows the Erlang programmer to take advantage of unusual features
present on the current platform.
The RawOptReq
consists of the tag raw
followed
by the protocol level, the option number and either a binary
or the size, in bytes, of the
buffer in which the option value is to be stored. A binary
should be used when the underlying getsockopt
requires
input
in the argument field, in which case the size of the binary
should correspond to the required buffer
size of the return value. The supplied values in a RawOptReq
correspond to the second, third and fourth/fifth parameters to the
getsockopt
call in the C socket API. The value stored
in the buffer is returned as a binary ValueBin
where all values are coded in the native endianess.
Asking for and inspecting raw socket options require low level information about the current operating system and TCP stack.
As an example, consider a Linux machine where the
TCP_INFO
option could be used to collect TCP statistics
for a socket. Lets say we're interested in the
tcpi_sacked
field of the struct tcp_info
filled in when asking for TCP_INFO
. To
be able to access this information, we need to know both the
numeric value of the protocol level IPPROTO_TCP
, the
numeric value of the option TCP_INFO
, the size of the
struct tcp_info
and the size and offset of
the specific field. By inspecting the headers or writing a small C
program, we found IPPROTO_TCP
to be 6,
TCP_INFO
to be 11, the structure size to be 92 (bytes),
the offset of tcpi_sacked
to be 28 bytes and the actual
value to be a 32 bit integer. We could use the following
code to retrieve the value:
get_tcpi_sacked(Sock) -> {ok,[{raw,_,_,Info}]} = inet:getopts(Sock,[{raw,6,11,92}]), <<_:28/binary,TcpiSacked:32/native,_/binary>> = Info, TcpiSacked.
Preferably, you would check the machine type, the OS and the kernel version prior to executing anything similar to the code above.
getstat(Socket)
getstat(Socket, Options) -> {ok, OptionValues} | {error, posix()}
Socket = term()
Options = [Opt]
OptionValues = [{Opt, Val}]
Opt, Val -- see below
Gets one or more statistic options for a socket.
getstat(Socket)
is equivalent to
getstat(Socket, [recv_avg, recv_cnt, recv_dvi, recv_max, recv_oct, send_avg, send_cnt, send_dvi, send_max, send_oct])
The following options are available:
recv_avg
-
Average size of packets in bytes received to the socket.
recv_cnt
-
Number of packets received to the socket.
recv_dvi
-
Average packet size deviation in bytes received to the socket.
recv_max
-
The size of the largest packet in bytes received to the socket.
recv_oct
-
Number of bytes received to the socket.
send_avg
-
Average size of packets in bytes sent from the socket.
send_cnt
-
Number of packets sent from the socket.
send_dvi
-
Average packet size deviation in bytes received sent from the socket.
send_max
-
The size of the largest packet in bytes sent from the socket.
send_oct
-
Number of bytes sent from the socket.
peername(Socket) -> {ok, {Address, Port}} | {error, posix()}
Socket = socket()
Address = ip_address()
Port = int()
Returns the address and port for the other end of a connection.
port(Socket) -> {ok, Port}
Socket = socket()
Port = int()
Returns the local port number for a socket.
sockname(Socket) -> {ok, {Address, Port}} | {error, posix()}
Socket = socket()
Address = ip_address()
Port = int()
Returns the local address and port number for a socket.
setopts(Socket, Options) -> ok | {error, posix()}
Socket = term()
Options = [{Opt, Val} | {raw, Protocol, Option, ValueBin}]
Protocol = int()
OptionNum = int()
ValueBin = binary()
Opt, Val -- see below
Sets one or more options for a socket. The following options are available:
{active, true | false | once}
-
If the value is
true
, which is the default, everything received from the socket will be sent as messages to the receiving process. If the value isfalse
(passive mode), the process must explicitly receive incoming data by callinggen_tcp:recv/2,3
orgen_udp:recv/2,3
(depending on the type of socket).If the value is
once
({active, once}
), one data message from the socket will be sent to the process. To receive one more message,setopts/2
must be called again with the{active, once}
option.When using
{active, once}
, the socket changes behaviour automatically when data is received. This can sometimes be confusing in combination with connection oriented sockets (i.e.gen_tcp
) as a socket with{active, false}
behaviour reports closing differently than a socket with{active, true}
behaviour. To make programming easier, a socket where the peer closed and this was detected while in{active, false}
mode, will still generate the message{tcp_closed,Socket}
when set to{active, once}
or{active, true}
mode. It is therefore safe to assume that the message{tcp_closed,Socket}
, possibly followed by socket port termination (depending on theexit_on_close
option) will eventually appear when a socket changes back and forth between{active, true}
and{active, false}
mode. However, when peer closing is detected is all up to the underlying TCP/IP stack and protocol.Note that
{active,true}
mode provides no flow control; a fast sender could easily overflow the receiver with incoming messages. Use active mode only if your high-level protocol provides its own flow control (for instance, acknowledging received messages) or the amount of data exchanged is small.{active,false}
mode or use of the{active, once}
mode provides flow control; the other side will not be able send faster than the receiver can read. {broadcast, Boolean}
(UDP sockets)-
Enable/disable permission to send broadcasts.
{delay_send, Boolean}
-
Normally, when an Erlang process sends to a socket, the driver will try to immediately send the data. If that fails, the driver will use any means available to queue up the message to be sent whenever the operating system says it can handle it. Setting
{delay_send, true}
will make all messages queue up. This makes the messages actually sent onto the network be larger but fewer. The option actually affects the scheduling of send requests versus Erlang processes instead of changing any real property of the socket. Needless to say it is an implementation specific option. Default isfalse
. {dontroute, Boolean}
-
Enable/disable routing bypass for outgoing messages.
{exit_on_close, Boolean}
-
By default this option is set to
true
.The only reason to set it to
false
is if you want to continue sending data to the socket after a close has been detected, for instance if the peer has used gen_tcp:shutdown/2 to shutdown the write side. {header, Size}
-
This option is only meaningful if the
binary
option was specified when the socket was created. If theheader
option is specified, the firstSize
number bytes of data received from the socket will be elements of a list, and the rest of the data will be a binary given as the tail of the same list. If for exampleSize == 2
, the data received will match[Byte1,Byte2|Binary]
. {keepalive, Boolean}
(TCP/IP sockets)-
Enables/disables periodic transmission on a connected socket, when no other data is being exchanged. If the other end does not respond, the connection is considered broken and an error message will be sent to the controlling process. Default disabled.
{nodelay, Boolean}
(TCP/IP sockets)-
If
Boolean == true
, theTCP_NODELAY
option is turned on for the socket, which means that even small amounts of data will be sent immediately. {packet, PacketType}
(TCP/IP sockets)-
Defines the type of packets to use for a socket. The following values are valid:
raw | 0
-
No packaging is done.
1 | 2 | 4
-
Packets consist of a header specifying the number of bytes in the packet, followed by that number of bytes. The length of header can be one, two, or four bytes; containing an unsigned integer in big-endian byte order. Each send operation will generate the header, and the header will be stripped off on each receive operation.
In current implementation the 4-byte header is limited to 2Gb.
asn1 | cdr | sunrm | fcgi | tpkt | line
-
These packet types only have effect on receiving. When sending a packet, it is the responsibility of the application to supply a correct header. On receiving, however, there will be one message sent to the controlling process for each complete packet received, and, similarly, each call to
gen_tcp:recv/2,3
returns one complete packet. The header is not stripped off.The meanings of the packet types are as follows:
asn1
- ASN.1 BER,
sunrm
- Sun's RPC encoding,
cdr
- CORBA (GIOP 1.1),
fcgi
- Fast CGI,
tpkt
- TPKT format [RFC1006],
line
- Line mode, a packet is a line terminated with newline, lines longer than the receive buffer are truncated. http | http_bin
-
The Hypertext Transfer Protocol. The packets are returned with the format according to
HttpPacket
described in erlang:decode_packet/3. A socket in passive mode will return{ok, HttpPacket}
fromgen_tcp:recv
while an active socket will send messages like{http, Socket, HttpPacket}
.Note that the packet type
httph
is not needed when reading from a socket.
{packet_size, Integer}
(TCP/IP sockets)-
Sets the max allowed length of the packet body. If the packet header indicates that the length of the packet is longer than the max allowed length, the packet is considered invalid. The same happens if the packet header is too big for the socket receive buffer.
{read_packets, Integer}
(UDP sockets)-
Sets the max number of UDP packets to read without intervention from the socket when data is available. When this many packets have been read and delivered to the destination process, new packets are not read until a new notification of available data has arrived. The default is 5, and if this parameter is set too high the system can become unresponsive due to UDP packet flooding.
{recbuf, Integer}
-
Gives the size of the receive buffer to use for the socket.
{reuseaddr, Boolean}
-
Allows or disallows local reuse of port numbers. By default, reuse is disallowed.
{send_timeout, Integer}
-
Only allowed for connection oriented sockets.
Specifies a longest time to wait for a send operation to be accepted by the underlying TCP stack. When the limit is exceeded, the send operation will return
{error,timeout}
. How much of a packet that actually got sent is unknown, why the socket should be closed whenever a timeout has occurred (seesend_timeout_close
). Default isinfinity
. {send_timeout_close, Boolean}
-
Only allowed for connection oriented sockets.
Used together with
send_timeout
to specify whether the socket will be automatically closed when the send operation returns{error,timeout}
. The recommended setting istrue
which will automatically close the socket. Default isfalse
due to backward compatibility. {sndbuf, Integer}
-
Gives the size of the send buffer to use for the socket.
{priority, Integer}
-
Sets the SO_PRIORITY socket level option on platforms where this is implemented. The behaviour and allowed range varies on different systems. The option is ignored on platforms where the option is not implemented. Use with caution.
{tos, Integer}
-
Sets IP_TOS IP level options on platforms where this is implemented. The behaviour and allowed range varies on different systems. The option is ignored on platforms where the option is not implemented. Use with caution.
In addition to the options mentioned above, raw
option specifications can be used. The raw options are
specified as a tuple of arity four, beginning with the tag
raw
, followed by the protocol level, the option number
and the actual option value specified as a binary. This
corresponds to the second, third and fourth argument to the
setsockopt
call in the C socket API. The option value
needs to be coded in the native endianess of the platform and,
if a structure is required, needs to follow the struct
alignment conventions on the specific platform.
Using raw socket options require detailed knowledge about the current operating system and TCP stack.
As an example of the usage of raw options, consider a Linux
system where you want to set the TCP_LINGER2
option on
the IPPROTO_TCP
protocol level in the stack. You know
that on this particular system it defaults to 60 (seconds),
but you would like to lower it to 30 for a particular
socket. The TCP_LINGER2
option is not explicitly
supported by inet, but you know that the protocol level
translates to the number 6, the option number to the number 8
and the value is to be given as a 32 bit integer. You can use
this line of code to set the option for the socket named
Sock
:
inet:setopts(Sock,[{raw,6,8,<<30:32/native>>}]),
As many options are silently discarded by the stack if they are given out of range, it could be a good idea to check that a raw option really got accepted. This code places the value in the variable TcpLinger2:
{ok,[{raw,6,8,<<TcpLinger2:32/native>>}]}=inet:getopts(Sock,[{raw,6,8,4}]),
Code such as the examples above is inherently non portable, even different versions of the same OS on the same platform may respond differently to this kind of option manipulation. Use with care.
Note that the default options for TCP/IP sockets can be changed with the Kernel configuration parameters mentioned in the beginning of this document.
POSIX Error Codes
e2big
- argument list too longeacces
- permission deniedeaddrinuse
- address already in useeaddrnotavail
- cannot assign requested addresseadv
- advertise erroreafnosupport
- address family not supported by protocol familyeagain
- resource temporarily unavailableealign
- EALIGNealready
- operation already in progressebade
- bad exchange descriptorebadf
- bad file numberebadfd
- file descriptor in bad stateebadmsg
- not a data messageebadr
- bad request descriptorebadrpc
- RPC structure is badebadrqc
- bad request codeebadslt
- invalid slotebfont
- bad font file formatebusy
- file busyechild
- no childrenechrng
- channel number out of rangeecomm
- communication error on sendeconnaborted
- software caused connection aborteconnrefused
- connection refusedeconnreset
- connection reset by peeredeadlk
- resource deadlock avoidededeadlock
- resource deadlock avoidededestaddrreq
- destination address requirededirty
- mounting a dirty fs w/o forceedom
- math argument out of rangeedotdot
- cross mount pointedquot
- disk quota exceedededuppkg
- duplicate package nameeexist
- file already existsefault
- bad address in system call argumentefbig
- file too largeehostdown
- host is downehostunreach
- host is unreachableeidrm
- identifier removedeinit
- initialization erroreinprogress
- operation now in progresseintr
- interrupted system calleinval
- invalid argumenteio
- I/O erroreisconn
- socket is already connectedeisdir
- illegal operation on a directoryeisnam
- is a named fileel2hlt
- level 2 haltedel2nsync
- level 2 not synchronizedel3hlt
- level 3 haltedel3rst
- level 3 resetelbin
- ELBINelibacc
- cannot access a needed shared libraryelibbad
- accessing a corrupted shared libraryelibexec
- cannot exec a shared library directlyelibmax
- attempting to link in more shared libraries than system limitelibscn
- .lib section in a.out corruptedelnrng
- link number out of rangeeloop
- too many levels of symbolic linksemfile
- too many open filesemlink
- too many linksemsgsize
- message too longemultihop
- multihop attemptedenametoolong
- file name too longenavail
- not availableenet
- ENETenetdown
- network is downenetreset
- network dropped connection on resetenetunreach
- network is unreachableenfile
- file table overflowenoano
- anode table overflowenobufs
- no buffer space availableenocsi
- no CSI structure availableenodata
- no data availableenodev
- no such deviceenoent
- no such file or directoryenoexec
- exec format errorenolck
- no locks availableenolink
- link has be severedenomem
- not enough memoryenomsg
- no message of desired typeenonet
- machine is not on the networkenopkg
- package not installedenoprotoopt
- bad protocol optionenospc
- no space left on deviceenosr
- out of stream resources or not a stream deviceenosym
- unresolved symbol nameenosys
- function not implementedenotblk
- block device requiredenotconn
- socket is not connectedenotdir
- not a directoryenotempty
- directory not emptyenotnam
- not a named fileenotsock
- socket operation on non-socketenotsup
- operation not supportedenotty
- inappropriate device for ioctlenotuniq
- name not unique on networkenxio
- no such device or addresseopnotsupp
- operation not supported on socketeperm
- not ownerepfnosupport
- protocol family not supportedepipe
- broken pipeeproclim
- too many processeseprocunavail
- bad procedure for programeprogmismatch
- program version wrongeprogunavail
- RPC program not availableeproto
- protocol erroreprotonosupport
- protocol not supportedeprototype
- protocol wrong type for socketerange
- math result unrepresentableerefused
- EREFUSEDeremchg
- remote address changederemdev
- remote deviceeremote
- pathname hit remote file systemeremoteio
- remote i/o erroreremoterelease
- EREMOTERELEASEerofs
- read-only file systemerpcmismatch
- RPC version is wrongerremote
- object is remoteeshutdown
- cannot send after socket shutdownesocktnosupport
- socket type not supportedespipe
- invalid seekesrch
- no such processesrmnt
- srmount errorestale
- stale remote file handleesuccess
- Error 0etime
- timer expiredetimedout
- connection timed outetoomanyrefs
- too many referencesetxtbsy
- text file or pseudo-device busyeuclean
- structure needs cleaningeunatch
- protocol driver not attachedeusers
- too many userseversion
- version mismatchewouldblock
- operation would blockexdev
- cross-domain linkexfull
- message tables fullnxdomain
- the hostname or domain name could not be found