Socket options
A way for network applications to
‘tweak’ the processing done at
lower-levels of the TCP/IP stack
The TCP/IP stack
Application layer
FTP, HTTP, SSH, DHCP, etc
The sockets API
Transport layer
TCP, UDP, etc
Network layer
IP, ICMP, etc
Link layer
ARP, RARP, etc
Physical layer
DSL, FDDI, etc
Encapsulation
message
data
Application
segment
UDP
Header
data
Transport
data
Network
datagram
IP
Header
UDP
Header
frame
Frame
Header
IP
Header
UDP
Header
data
Frame
CRC
data
Frame
CRC
Link
stream-of-bits
preamble
Frame
Headerd
IP
Header
UDP
Header
Inter-Frame
Gap
Key library functions
•
•
•
•
•
•
•
int
int
int
int
int
int
int
socket()
bind(), int connect()
listen(), int accept()
write(), int read()
send(), int recv()
sendto(), int recvfrom()
shutdown(), int close()
Simple program-flow example
int sock = socket( AF_INET, SOCK_DGRAM, IPPROTO_UDP );
connect( sock, (sockaddr*)&saddr, sizeof( saddr ) );
write( sock, message, sizeof( message ) );
close( sock );
socket-address object
struct sockaddr_in
socklen_t
bzero( &saddr, salen );
saddr;
salen = sizeof( saddr );
saddr.sin_family = AF_INET;
saddr.sin_port = htons( port_number );
saddr.sin_addr.s_addr = htonl( peer_ip_address );
Using the sockets API requires allocating and initializing data-objects known
as ‘socket-addresses’ (aka ‘socket-names’), with some numeric fields which
require using the Internet’s standard ‘big-endian’ byte-order, rather than the
‘little-endian’ byte-order employed within Intel’s x86 processors. But helperfunctions, like htons() and htonl(), will convert host-order into network-order.
Using socket ‘options’
• The default behavior of the kernel routines
for the lower-layers of the TCP/IP protocol
stack may not be fully suitable for the aims
of particular network application programs
• But applications usually can’t alter code in
an operating system’s protected kernel
• The sockets API offers a ‘workaround’ for
such situations, i.e.: int setsockopt()
Function prototypes
int getsockopt( int sd, int level, int optname, void *optval, socklen_t *optlen );
int setsockopt( int sd, int level, int optname, void *optval, socklen_t optlen );
There are various socket options, which apply to various levels in the
networking system’s software hierarchy, and which selectively apply
to various types of sockets -- and which are implemented to varying
degrees within different versions of popular operating systems.
We will demonstrate use of two socket options available in Linux for
datagram sockets: SO_BROADCAST and SO_BINDTODEVICE.
Our demo also illustrate the use of ‘write()’ for a connected socket.
Demo: ‘bindtoif.cpp’
• This program lets a privileged user write a
‘broadcast’ message to all of the hosts on
our classroom’s Local Area Network – as
you can confirm using our ‘nicwatch’ tool
• And -- the user can choose the interface!
• Normally an application wouldn’t be able
to send a ‘broadcast’ message, nor be
able to select which interface gets used
Overview
Get the name of the desired network interface from the command-line
Open an internet datagram socket
Turn on the ‘SO_BROADCAST’ socket-option
Bind the chosen network interface to that socket
Connect the socket to the network‘s broadcast address
Write a message to the connected socket
Show the user a confirmation message
Programming details
• Now we take a ‘timeout’ from these slides
to look carefully line-by-line at the sourcecode which will implement those steps
• Then we will be ready to compile and run
our ‘bindtoif’ demo-program (using ‘sudo’)
• You should all be able to watch the arrival
of the broadcast message at your desktop!
‘ifconfig’
• Remember that your classroom computer
leaves the ‘eth1’ interface disabled after a
reboot – so you will need to use ‘ifconfig’
to enable that interface and also assign it
an appropriate Internet Protocol address:
$ sudo /sbin/ifconfig eth1 192.168.1.xxx up