BIO_s_datagram.3ossl - Man Page

Network BIO with datagram semantics


 #include <openssl/bio.h>

 BIO_METHOD *BIO_s_datagram(void);
 BIO *BIO_new_dgram(int fd, int close_flag);

 int BIO_ctrl_dgram_connect(BIO *bio, const BIO_ADDR *peer);
 int BIO_ctrl_set_connected(BIO *bio, const BIO_ADDR *peer);
 int BIO_dgram_recv_timedout(BIO *bio);
 int BIO_dgram_send_timedout(BIO *bio);
 int BIO_dgram_get_peer(BIO *bio, BIO_ADDR *peer);
 int BIO_dgram_set_peer(BIO *bio, const BIO_ADDR *peer);
 int BIO_dgram_get_mtu_overhead(BIO *bio);
 int BIO_dgram_detect_peer_addr(BIO *bio, BIO_ADDR *peer);


BIO_s_datagram() is a BIO implementation designed for use with network sockets which provide datagram semantics, such as UDP sockets. It is suitable for use with DTLSv1 or QUIC.

Because BIO_s_datagram() has datagram semantics, a single BIO_write() call sends a single datagram and a single BIO_read() call receives a single datagram. If the size of the buffer passed to BIO_read() is inadequate, the datagram is silently truncated.

For a memory-based BIO which provides datagram semantics identical to those of BIO_s_datagram(), see BIO_s_dgram_pair(3).

This BIO supports the BIO_sendmmsg(3) and BIO_recvmmsg(3) functions.

When using BIO_s_datagram(), it is important to note that:

Various controls are available for configuring the BIO_s_datagram() using BIO_ctrl(3):

BIO_ctrl_dgram_connect (BIO_CTRL_DGRAM_CONNECT)

This is equivalent to calling BIO_dgram_set_peer(3).

Despite its name, this function does not cause the underlying socket to become connected.

BIO_ctrl_set_connected (BIO_CTRL_SET_CONNECTED)

This informs the BIO_s_datagram() whether the underlying socket has been connected, and therefore how the BIO_s_datagram() should attempt to use the socket.

If the peer argument is non-NULL, BIO_s_datagram() assumes that the underlying socket has been connected and will attempt to use the socket using OS APIs which do not specify peer addresses (for example, send(3) and recv(3) or similar). The peer argument should specify the peer address to which the socket is connected.

If the peer argument is NULL, BIO_s_datagram() assumes that the underlying socket is not connected and will attempt to use the socket using an OS APIs which specify peer addresses (for example, sendto(3) and recvfrom(3)).

This control does not affect the operation of BIO_sendmmsg(3) or BIO_recvmmsg(3).

BIO_dgram_get_peer (BIO_CTRL_DGRAM_GET_PEER)

This outputs a BIO_ADDR which specifies one of the following values, whichever happened most recently:

  • The peer address last passed to BIO_dgram_set_peer(), BIO_ctrl_dgram_connect() or BIO_ctrl_set_connected().
  • The peer address of the datagram last received by a call to BIO_read().
BIO_dgram_set_peer (BIO_CTRL_DGRAM_SET_PEER)

Sets the peer address to be used for subsequent writes to this BIO.

Warning: When used with an unconnected network socket, the value set may be modified by future calls to BIO_read(3), making use of BIO_s_datagram() hazardous when used with unconnected network sockets; see above.

This does not affect the operation of BIO_sendmmsg(3). BIO_recvmmsg(3) does not affect the value set by BIO_dgram_set_peer().

BIO_dgram_detect_peer_addr (BIO_CTRL_DGRAM_DETECT_PEER_ADDR)

This is similar to BIO_dgram_get_peer() except that if the peer address has not been set on the BIO object, an OS call such as getpeername(2) will be attempted to try and autodetect the peer address to which the underlying socket is connected. Other BIOs may also implement this control if they are capable of sensing a peer address, without necessarily also implementing BIO_dgram_set_peer() and BIO_dgram_get_peer().


Returns 1 if the last I/O operation performed on the BIO (for example, via a call to BIO_read(3)) may have been caused by a receive timeout.


Returns 1 if the last I/O operation performed on the BIO (for example, via a call to BIO_write(3)) may have been caused by a send timeout.

BIO_dgram_get_mtu_overhead (BIO_CTRL_DGRAM_GET_MTU_OVERHEAD)

Returns a quantity in bytes which is a rough estimate of the number of bytes of overhead which should typically be added to a datagram payload size in order to estimate the final size of the Layer 3 (e.g. IP) packet which will contain the datagram. In most cases, the maximum datagram payload size which can be transmitted can be determined by determining the link MTU in bytes and subtracting the value returned by this call.

The value returned by this call depends on the network layer protocol being used.

The value returned is not fully reliable because datagram overheads can be higher in atypical network configurations, for example where IPv6 extension headers or IPv4 options are used.


If num is nonzero, configures the underlying network socket to enable Don't Fragment mode, in which datagrams will be set with the IP Don't Fragment (DF) bit set. If num is zero, Don't Fragment mode is disabled.


Queries the OS for its assessment of the Path MTU for the destination to which the underlying network socket, and returns that Path MTU in bytes. This control can only be used with a connected socket.

This is not supported on all platforms and depends on OS support being available. Returns 0 on failure.


This control requests that Path MTU discovery be enabled on the underlying network socket.


Returns the estimated minimum size of datagram payload which should always be supported on the BIO. This size is determined by the minimum MTU required to be supported by the applicable underlying network layer. Use of datagrams of this size may lead to suboptimal performance, but should be routable in all circumstances. The value returned is the datagram payload size in bytes and does not include the size of layer 3 or layer 4 protocol headers.


Returns 1 if the last attempted write to the BIO failed due to the size of the attempted write exceeding the applicable MTU.


Accepts a pointer to a struct timeval. If the time specified is zero, disables receive timeouts. Otherwise, configures the specified time interval as the receive timeout for the socket for the purposes of future BIO_read(3) calls.


If num is nonzero, enables peek mode; otherwise, disables peek mode. Where peek mode is enabled, calls to BIO_read(3) read datagrams from the underlying network socket in peek mode, meaning that a future call to BIO_read(3) will yield the same datagram until peek mode is disabled.

BIO_recvmmsg(3) is not affected by this control.

BIO_new_dgram() is a helper function which instantiates a BIO_s_datagram() and sets the BIO to use the socket given in fd by calling BIO_set_fd().

Return Values

BIO_s_datagram() returns a BIO method.

BIO_new_dgram() returns a BIO on success and NULL on failure.

BIO_ctrl_dgram_connect(), BIO_ctrl_set_connected() and BIO_dgram_set_peer() return 1 on success and 0 on failure.

BIO_dgram_get_peer() and BIO_dgram_detect_peer_addr() return 0 on failure and the number of bytes for the outputted address representation (a positive value) on success.

BIO_dgram_recv_timedout() and BIO_dgram_send_timedout() return 0 or 1 depending on the circumstance; see discussion above.

BIO_dgram_get_mtu_overhead() returns a value in bytes.

See Also

BIO_sendmmsg(3), BIO_s_dgram_pair(3), DTLSv1_listen(3), bio(7)

Referenced By

BIO_s_connect.3ossl(3), DTLSv1_listen.3ossl(3), openssl-quic.7ossl(7), ossl-guide-quic-client-block.7ossl(7), ossl-guide-quic-introduction.7ossl(7).

The man pages BIO_ctrl_dgram_connect.3ossl(3), BIO_ctrl_set_connected.3ossl(3), BIO_dgram_detect_peer_addr.3ossl(3), BIO_dgram_get_mtu_overhead.3ossl(3), BIO_dgram_get_peer.3ossl(3), BIO_dgram_recv_timedout.3ossl(3), BIO_dgram_send_timedout.3ossl(3), BIO_dgram_set_peer.3ossl(3) and BIO_new_dgram.3ossl(3) are aliases of BIO_s_datagram.3ossl(3).

2024-02-09 3.2.1 OpenSSL