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.


#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:

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

  an atom which is named from the Posix error codes used in
  Unix, and in the runtime libraries of most C compilers

  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 can be passed to gethostbyaddr/1 either as the string "" or as the tuple {150, 236, 20, 73}.

IPv4 address examples:

Address          ip_address()
-------          ------------        {127,0,0,1}     {192,168,42,2}

IPv6 address examples:

Address          ip_address()
-------          ------------
::1             {0,0,0,0,0,0,0,1}
::  {0,0,0,0,0,0,(192 bsl 8) bor 168,(42 bsl 8) bor 2}
                {16#FFFF,0,0,0,0,0,(192 bsl 8) bor 168,(42 bsl 8) bor 2}

A function that may be useful is inet_parse:address/1:

1> inet_parse:address("").
2> inet_parse:address("FFFF::").


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-adress, 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:

\011get_tcpi_sacked(Sock) -> 
\011    {ok,[{raw,_,_,Info}]} = inet:getopts(Sock,[{raw,6,11,92}]),
\011    <<_:28/binary,TcpiSacked:32/native,_/binary>> = Info,

Preferably, you would check the machine type, the OS and the kernel version prior to executing anything similar to the code above.

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 is false (passive mode), the process must explicitly receive incoming data by calling gen_tcp:recv/2,3 or gen_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 the exit_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 is false.

{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 the header option is specified, the first Size 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 example Size == 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, the TCP_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; the order of the bytes is big-endian. Each send operation will generate the header, and the header will be stripped off on each receive operation.

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.


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} from gen_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 occured. Default is infinity.

{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.\011

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:


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:


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 long eacces - permission denied eaddrinuse - address already in use eaddrnotavail - cannot assign requested address eadv - advertise error eafnosupport - address family not supported by protocol family eagain - resource temporarily unavailable ealign - EALIGN ealready - operation already in progress ebade - bad exchange descriptor ebadf - bad file number ebadfd - file descriptor in bad state ebadmsg - not a data message ebadr - bad request descriptor ebadrpc - RPC structure is bad ebadrqc - bad request code ebadslt - invalid slot ebfont - bad font file format ebusy - file busy echild - no children echrng - channel number out of range ecomm - communication error on send econnaborted - software caused connection abort econnrefused - connection refused econnreset - connection reset by peer edeadlk - resource deadlock avoided edeadlock - resource deadlock avoided edestaddrreq - destination address required edirty - mounting a dirty fs w/o force edom - math argument out of range edotdot - cross mount point edquot - disk quota exceeded eduppkg - duplicate package name eexist - file already exists efault - bad address in system call argument efbig - file too large ehostdown - host is down ehostunreach - host is unreachable eidrm - identifier removed einit - initialization error einprogress - operation now in progress eintr - interrupted system call einval - invalid argument eio - I/O error eisconn - socket is already connected eisdir - illegal operation on a directory eisnam - is a named file el2hlt - level 2 halted el2nsync - level 2 not synchronized el3hlt - level 3 halted el3rst - level 3 reset elbin - ELBIN elibacc - cannot access a needed shared library elibbad - accessing a corrupted shared library elibexec - cannot exec a shared library directly elibmax - attempting to link in more shared libraries than system limit elibscn - .lib section in a.out corrupted elnrng - link number out of range eloop - too many levels of symbolic links emfile - too many open files emlink - too many links emsgsize - message too long emultihop - multihop attempted enametoolong - file name too long enavail - not available enet - ENET enetdown - network is down enetreset - network dropped connection on reset enetunreach - network is unreachable enfile - file table overflow enoano - anode table overflow enobufs - no buffer space available enocsi - no CSI structure available enodata - no data available enodev - no such device enoent - no such file or directory enoexec - exec format error enolck - no locks available enolink - link has be severed enomem - not enough memory enomsg - no message of desired type enonet - machine is not on the network enopkg - package not installed enoprotoopt - bad proocol option enospc - no space left on device enosr - out of stream resources or not a stream device enosym - unresolved symbol name enosys - function not implemented enotblk - block device required enotconn - socket is not connected enotdir - not a directory enotempty - directory not empty enotnam - not a named file enotsock - socket operation on non-socket enotsup - operation not supported enotty - inappropriate device for ioctl enotuniq - name not unique on network enxio - no such device or address eopnotsupp - operation not supported on socket eperm - not owner epfnosupport - protocol family not supported epipe - broken pipe eproclim - too many processes eprocunavail - bad procedure for program eprogmismatch - program version wrong eprogunavail - RPC program not available eproto - protocol error eprotonosupport - protocol not supported eprototype - protocol wrong type for socket erange - math result unrepresentable erefused - EREFUSED eremchg - remote address changed eremdev - remote device eremote - pathname hit remote file system eremoteio - remote i/o error eremoterelease - EREMOTERELEASE erofs - read-only file system erpcmismatch - RPC version is wrong erremote - object is remote eshutdown - cannot send after socket shutdown esocktnosupport - socket type not supported espipe - invalid seek esrch - no such process esrmnt - srmount error estale - stale remote file handle esuccess - Error 0 etime - timer expired etimedout - connection timed out etoomanyrefs - too many references etxtbsy - text file or pseudo-device busy euclean - structure needs cleaning eunatch - protocol driver not attached eusers - too many users eversion - version mismatch ewouldblock - operation would block exdev - cross-domain link exfull - message tables full nxdomain - the hostname or domain name could not be found
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