PING(4)

NAME
ping - ping protocol

SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ping.h>
int
socket(AF_INET, SOCK_DGRAM, IPPROTO_PING);

DESCRIPTION
The Ping Protocol uses the Echo Request and Echo Reply messages of the Internet Control Message Protocol (ICMP) to provide a connectionless best-effort echo of datagrams. A cooperating host will send back a Echo Reply message containing the same data as any Echo Request messages it receives. It is designed for packet-switched networks and provides multiplexing with a 16-bit port number (using the identifier field of the Echo Request and Echo Reply messages), and basic data integrity checks (16-bit ones' complement sum), and broadcasting. It does not provide a guarantee of delivery, avoidance of delivering multiple times, ordering, out of band data, nor flow control.

Ping sockets allow only sending Echo Request messages and receiving Echo Reply messages. The kernel will automatically send Echo Reply messages in response to any received Echo Request Messages.

The structure of ping datagrams is a 4 bytes sequence number (in big endian) followed by 0 or more bytes of an optional payload. Ping datagrams are sent inside a Echo Request message (with that sequence number) and received inside a Echo Reply message (also containing a sequence number).

Ping sockets are made with socket(2) by passing an appropriate domain (AF_INET), SOCK_DGRAM as the type, and IPPROTO_PING as the protocol. Initially a socket is not bound, it won't receive datagrams, and it does not have a remote address and port set.

A Ping socket has the following state:

•   The address family it belongs to.
•   The network interface it is bound to (if any) (SO_BINDTODEVICE and SO_BINDTOINDEX) (initially none).
•   The local address and port (when bound) (initially none).
•   The remote address and port (when connected) (initially none).
•   A receive queue (initially empty).
•   Whether the socket has been shutdown(2) for read and/or write (initially neither).
•   A single pending asynchronous error (if any) (SO_ERROR) (initially none).
•   Whether broadcast datagrams can be sent (SO_BROADCAST) (initially no).
•   Whether binding to the any address and a port doesn't conflict with binding to another address on the same port (SO_REUSEADDR) (initially no).
•   Limits on the size of the receive and send queues (SO_RCVBUF and SO_SNDBUF).

Datagrams are sent as a packet with a header and the datagram itself. The header contains the port and the checksum. The header is 8 bytes.

Port numbers are 16-bit and range from 1 to 65535. Port 0 is not valid. Binding to port 0 will assign an available port on the requested address. Sending or connecting to port 0 will fail with EADDRNOTAVAIL. Received Echo Reply packets whose port number is port 0 will be silently dropped. Ping ports are distinct from ports in other transport layer protocols.

Packets contain a 16-bit ones' complement checksum. A received packet will be silently discarded if its checksum does not match its contents.

Sockets can be bound to a local address and port with bind(2) (if not already bound), or an local address and port will be automatically assigned on the first send or connect operation. The local address and port can be read with getsockname(2). If the socket hasn't been bound, the local address and port is reported as the any address on port 0. There are no ports that require superuser privileges.

Sockets can be bound to the any address, the broadcast address, the address of a network interface, or the broadcast address of a network interface. Binding to port 0 will automatically assign an available port on the requested local address or fail with EAGAIN if no port is available. No two sockets can bind to the same local address and port. No two sockets can be bound such that one is bound to the any address and a port, and the other socket is bound to another address and the same port; unless both sockets had the SO_REUSEADDR socket option set when the second socket was bound, and the current user is the same that bound the first socket or the current user has superuser privileges.

A socket bound to a local address and port will receive an incoming datagram of the following conditions hold:

•   The datagram belongs to the socket's address family and the protocol is Ping.
•   The datagram's checksum matches the datagram.
•   The datagram is an Echo Reply message.
•   The datagram's port number is not port 0.
•   The datagram is sent to the address or broadcast address of the network interface it is received on, or the datagram was sent to the broadcast address;
•   The socket is either bound to the receiving network interface, or the socket is not bound to a network interface;
•   The datagram is sent to the socket's local port;
•   The datagram is sent to the socket's local address, or the socket's local address is the any address (and no other socket is bound to the datagram's address and that port);
•   The socket is connected and the datagram was sent from the remote address and the remote port, or the socket is not connected; and
•   The socket is not shut down for reading.

If so, the datagram is added to the socket's receive queue, otherwise it is discarded. The receive queue contains incoming packets waiting to be received. Incoming packets are dropped if the receive queue is full. Shrinking the receive queue limit drops packets as needed to stay below the limit.

The remote address and port can be set multiple times with connect(2), after which the socket is said to be connected, but Ping is connectionless and no handshake is sent. The remote port must not be port 0 or the connection will fail with EADDRNOTAVAIL. If the socket is not bound, connect(2) will determine which network interface will be used to send to the remote address, and then bind to the address of that network interface together with an available port. connect(2) will fail if there is no route from the local address to the requested remote address. A connected socket only receive datagrams from the remote address and port. connect(2) will drop datagrams in the receive queue that don't originate from the requested remote address. The send(2), write(2), and writev(2) functions can be used on a connected socket and they send to the remote address and port by default. If the socket is connected, the destination given to sendto(2) and sendmsg(2) must be NULL. The remote address and port can be read with getpeername(2).

The socket can be disconnected by connecting to a socket address with the family value set to AF_UNSPEC, which resets the remote address and port (if set), and otherwise has no effect.

Datagrams can be sent with sendmsg(2) and sendto(2). Sending on a socket not bound to a local address and port will bind to the any address and an available port, or fail with EAGAIN if no port is available. Datagrams can be received with recvmsg(2), recvfrom(2), recv(2), read(2), and readv(2). If an asynchronous error is pending, the next send and receive operation will fail with that error and clear the asynchronous eror, so the next operation can succeed. Asynchronous errors can arise from network problems. There is no send queue at the Ping level and datagrams are directly forwarded to the network layer. It is an error to use any of the flags MSG_CMSG_CLOEXEC, MSG_CMSG_CLOFORK, MSG_EOR, MSG_OOB, and MSG_WAITALL.

The condition of the socket can be tested with poll(2) where POLLIN signifies a packet has been received (or the socket is shut down for reading), POLLOUT signifies a packet can be sent now (and the socket is not shut down for writing), POLLHUP signifies the socket is shut down for writing, and POLLERR signifies an asynchronous error is pending.

The socket can be shut down for receiving and/or sending with shutdown(2). The receive queue is emptied when shut down for receive (asynchronous errors are preserved) and receive operations will succeed with an end of file condition, but any pending asynchronous errors will take precedence and be delivered instead. Sending when shut down for writing will raise SIGPIPE and fail with EPIPE (regardless of a pending asynchronous error).

Socket options can be set with setsockopt(2) and read with getsockopt(2) and exist on the IPPROTO_PING level as well as applicable underlying protocol levels.

Broadcast Echo Requests can be sent by setting the SO_BROADCAST socket option with setsockopt(2) and sending to a broadcast address of the network layer. RFC 1122 3.2.2.6 allows hosts to ignore broadcast Echo Requests.

SOCKET OPTIONS
Ping sockets support these setsockopt(2) / getsockopt(2) options at level SOL_SOCKET:

SO_BINDTODEVICE char[]
    Bind to a network interface by its name. (Described in if(4))

SO_BINDTOINDEX unsigned int
    Bind to a network interface by its index number. (Described in if(4))

SO_BROADCAST int
    Whether sending to a broadcast address is allowed. (Described in if(4))

SO_DEBUG int
    Whether the socket is in debug mode. This option is not implemented and is initially 0. Attempting to set it to non-zero will fail with EPERM. (Described in if(4))

SO_DOMAIN sa_family_t
    The socket domain (the address family). This option can only be read. (Described in if(4))

SO_DONTROUTE int
    Whether to bypass the routing table and only send on the local network. This option is not implemented and is initially 0. Attempting to set it to non-zero will fail with EPERM. (Described in if(4))

SO_ERROR int
    The asynchronous pending error (an errno(3) value). Cleared to 0 when read. This option can only be read. (Described in if(4))

SO_PROTOCOL int
    The socket protocol (IPPROTO_PING). This option can only be read. (Described in if(4))

SO_RCVBUF int
    How many bytes the receive queue can use (default is 64 pages, max 4096 pages). (Described in if(4))

SO_REUSEADDR int
    Whether binding to the any address on a port doesn't conflict with binding to another address and the same port, if both sockets have this option set and the user binding the second socket is the same that bound the first socket or the user binding the second socket has superuser privileges. (Described in if(4))

SO_SNDBUF int
    How many bytes the send queue can use (default is 64 pages, max 4096 pages). (Described in if(4))

SO_TYPE int
    The socket type (SOCK_DGRAM). This option can only be read. (Described in if(4))

IMPLEMENTATION NOTES
Received broadcast echo requests are ignored as permitted by RFC 1122 3.2.2.6.

Each packet currently use a page of memory, which counts towards the receive queue limit.

If no specific port is requested, one is randomly selected in the dynamic port range 32768 (inclusive) through 61000 (exclusive).

EXAMPLES
This example sends a Echo Request and blocks indefinitely until it receives a Echo Reply. remote is the remote socket address and remote_len is the size of remote. The remote and remote_len values should all be chosen according to the address family and network layer.

    sa_family_t af = /* ... */;
    const struct sockaddr *remote = /* ... */;
    socklen_t remote_len = /* ... */;
    int fd = socket(af, SOCK_DGRAM, IPPROTO_PING);
    if (fd < 0)
            err(1, "socket");
    if (connect(fd, remote, remote_len) < 0)
            err(1, "connect");
    unsigned char request[56];
    arc4random_buf(request, sizeof(request));
    if (send(fd, request, sizeof(request), 0) < 0)
            err(1, "send");
    unsigned char reply[56 + 1 /* detect too large reply */];
    ssize_t amount = recv(fd, reply, sizeof(reply), 0);
    if (amount < 0 )
            err(1, "recv");
    if (amount == sizeof(request) && !memcmp(request, reply, sizeof(request)))
            printf("correct echo reply\n");
    else
            printf("incorrect echo reply\n");

ERRORS
Socket operations can fail due to these error conditions, in addition to the error conditions of the network and link layer, and the error conditions of the invoked function.

[EACCES]
    A datagram was sent to a broadcast address, but SO_BROADCAST is turned off.

[EADDRINUSE]
    The socket cannot be bound to the requested address and port because another socket was already bound to 1) the same address and port 2) the any address and the same port (and SO_REUSEADDR was not set on both sockets), or 3) some address and the same port but the requested address was the any address (and SO_REUSEADDR was not set on both sockets).

[EADDRNOTAVAIL]
    The socket cannot be bound to the requested address because no network interface had that address or broadcast address.

[EAGAIN]
    A port could not be assigned because each port in the dynamic port range had already been bound to a socket in a conflicting manner.

[ECONNREFUSED]
    The destination host of a datagram was not listening on the port. This error can happen asynchronously.

[EHOSTDOWN]
    The destination host of a datagram is not up. This error can happen asynchronously.

[EHOSTUNREACH]
    The destination host of a datagram was unreachable. This error can happen asynchronously.

[EISCONN]
    A destination address and port was specified when sending a datagram, but the socket has already been connected to a remote address and port.

[EMSGSIZE]
    The datagram was too large to be sent because it exceeded the maximum transmission unit (MTU) on the path between the local and remote address. This error can happen asynchronously.

[ENETDOWN]
    The network interface used to deliver a datagram isn't up. This error can happen asynchronously.

[ENETUNREACH]
    The destination network of a datagram was unreachable. This error can happen asynchronously.

[ENETUNREACH]
    The remote address could not be connected because there was no route from the local address to the remote address.

[ENOBUFS]
    There was not enough memory available for network packets.

[EPERM]
    One of the unimplemented SO_DEBUG and SO_DONTROUTE socket options was attempted to be set to a non-zero value.

SEE ALSO
bind(2), connect(2), getpeername(2), getsockname(2), getsockopt(2), poll(2), recvfrom(2), recvmsg(2), sendmsg(2), sendto(2), setsockopt(2), shutdown(2), socket(2), icmp(4), if(4), inet(4), ip(4), kernel(7), ping(8)

STANDARDS
J. Postel (ed.), September 1981, STD 5, RFC 792, Internet Control Message Protocol - DARPA Internet Program Protocol Specification, USC/Information Sciences Institute.

Internet Engineering Task Force, R. Braden (ed.), October 1989, STD 3, RFC 1122, Requirements for Internet Hosts -- Communication Layers, USC/Information Sciences Institute.

HISTORY
Ping sockets originally appeared in Sortix 1.1.

BUGS
The handling of SO_REUSEADDR in bind(2) does not yet enforce the two sockets to be bound by the same user or the second socket to be bound by a user with superuser privileges. The requirement that both sockets have SO_REUSEADDR set might be relaxed to only the second socket having it set when this permission check is implemented.

The integration with the network layer is inadequate and the asynchronous errors ECONNREFUSED, EHOSTDOWN, EHOSTUNREACH, and ENETUNREACH are never delivered asynchronously from the network.

Ping sockets does not yet provide access to IP header values such as the Time To Live and does not yet report ICMP error messages.

The send(2) flag MSG_DONTROUTE and the SO_DONTROUTE socket option are not implemented yet.

The SO_SNDBUF socket option is currently not used and the send queue is not limited at the socket level.

The automatic assignment of ports is random, but is statistically biased. A random port is picked, and if it is taken, the search sequentially iterates ports in ascending order until an available port is found or the search terminates.