1. networks and the internet 2. network programming

1. I/O

2. Networks and the Internet

I/O Unix I/O RIO (robust I/O) package Metadata, sharing, and redirection Standard I/O Conclusions and examples

Unix Files A Unix file is a sequence of m bytes:

B0 , B1 , .... , Bk , .... , Bm-1

All I/O devices are represented as files: /dev/sda2 (/usr disk partition) /dev/tty2 (terminal)

Even the kernel is represented as a file: /dev/kmem (kernel memory image) /proc (kernel data structures)

Unix File Types Regular file

File containing user/app data (binary, text, whatever) OS does not know anything about the format

other than “sequence of bytes”, akin to main memory Directory file

A file that contains the names and locations of other files Character special and block special files

Terminals (character special) and disks (block special) FIFO (named pipe)

A file type used for inter-process communication Socket

A file type used for network communication between processes

Unix I/O Key Features

Elegant mapping of files to devices allows kernel to export simple interface called Unix I/O

Important idea: All input and output is handled in a consistent and uniform way

Basic Unix I/O operations (system calls): Opening and closing files

open()and close() Reading and writing a file

read() and write() Changing the current file position (seek)

indicates next offset into file to read or write lseek()

B0 B1 • • • Bk-1 Bk Bk+1 • • •

Current file position = k

Opening Files Opening a file informs the kernel that you are getting ready to

access that file

Returns a small identifying integer file descriptor fd == -1 indicates that an error occurred

Each process created by a Unix shell begins life with three open files associated with a terminal: 0: standard input 1: standard output 2: standard error

int fd; /* file descriptor */

if ((fd = open("/etc/hosts", O_RDONLY)) < 0) { perror("open"); exit(1);}

Closing Files Closing a file informs the kernel that you are finished

accessing that file

Closing an already closed file is a recipe for disaster in threaded programs (more on this later)

Moral: Always check return codes, even for seemingly benign functions such as close()

int fd; /* file descriptor */int retval; /* return value */

if ((retval = close(fd)) < 0) { perror("close"); exit(1);}

Reading Files Reading a file copies bytes from the current file position to

memory, and then updates file position

Returns number of bytes read from file fd into buf Return type ssize_t is signed integer nbytes < 0 indicates that an error occurred Short counts (nbytes < sizeof(buf) ) are possible and are not

errors!

char buf[512];int fd; /* file descriptor */int nbytes; /* number of bytes read */

/* Open file fd ... *//* Then read up to 512 bytes from file fd */if ((nbytes = read(fd, buf, sizeof(buf))) < 0) { perror("read"); exit(1);}

Writing Files Writing a file copies bytes from memory to the current file

position, and then updates current file position

Returns number of bytes written from buf to file fd nbytes < 0 indicates that an error occurred As with reads, short counts are possible and are not errors!

char buf[512];int fd; /* file descriptor */int nbytes; /* number of bytes read */

/* Open the file fd ... *//* Then write up to 512 bytes from buf to file fd */if ((nbytes = write(fd, buf, sizeof(buf)) < 0) { perror("write"); exit(1);}

Simple Unix I/O example Copying standard in to standard out, one byte at a time

#include "csapp.h"

int main(void){ char c;

while(Read(STDIN_FILENO, &c, 1) != 0)Write(STDOUT_FILENO, &c, 1);

exit(0);}

Note the use of error handling wrappers for read and write (Appendix A).

cpstdin.c

Dealing with Short Counts Short counts can occur in these situations:

Encountering (end-of-file) EOF on reads Reading text lines from a terminal Reading and writing network sockets or Unix pipes

Short counts never occur in these situations: Reading from disk files (except for EOF) Writing to disk files

One way to deal with short counts in your code: Use the RIO (Robust I/O) package from your textbook’s csapp.c

file (Appendix B)

The RIO Package RIO is a set of wrappers that provide efficient and robust I/O

in apps, such as network programs that are subject to short counts

RIO provides two different kinds of functions Unbuffered input and output of binary data

rio_readn and rio_writen Buffered input of binary data and text lines

rio_readlineb and rio_readnb Buffered RIO routines are thread-safe and can be interleaved

arbitrarily on the same descriptor

Unbuffered RIO Input and Output Same interface as Unix read and write Especially useful for transferring data on network sockets

rio_readn returns short count only if it encounters EOF Only use it when you know how many bytes to read

rio_writen never returns a short count Calls to rio_readn and rio_writen can be interleaved arbitrarily on

the same descriptor

#include "csapp.h"

ssize_t rio_readn(int fd, void *usrbuf, size_t n);ssize_t rio_writen(int fd, void *usrbuf, size_t n);

Return: num. bytes transferred if OK, 0 on EOF (rio_readn only), -1 on error

Implementation of rio_readn/* * rio_readn - robustly read n bytes (unbuffered) */ssize_t rio_readn(int fd, void *usrbuf, size_t n) { size_t nleft = n; ssize_t nread; char *bufp = usrbuf;

while (nleft > 0) {if ((nread = read(fd, bufp, nleft)) < 0) { if (errno == EINTR) /* interrupted by sig handler return */

nread = 0; /* and call read() again */ else

return -1; /* errno set by read() */ } else if (nread == 0) break; /* EOF */nleft -= nread;bufp += nread;

} return (n - nleft); /* return >= 0 */} csapp.c

Buffered I/O: Motivation Applications often read/write one character at a time

getc, putc, ungetc gets, fgets

Read line of text on character at a time, stopping at newline Implementing as Unix I/O calls expensive

read and write require Unix kernel calls > 10,000 clock cycles

Solution: Buffered read Use Unix read to grab block of bytes User input functions take one byte at a time from buffer

Refill buffer when empty

unreadalready readBuffer

unread

Buffered I/O: Implementation For reading from file File has associated buffer to hold bytes that have been read

from file but not yet read by user code

Layered on Unix file:

already readBuffer

rio_bufrio_bufptr

rio_cnt

unreadalready readnot in buffer unseen

Current File Position

Buffered Portion

Buffered I/O: Declaration All information contained in struct

typedef struct { int rio_fd; /* descriptor for this internal buf */ int rio_cnt; /* unread bytes in internal buf */ char *rio_bufptr; /* next unread byte in internal buf */ char rio_buf[RIO_BUFSIZE]; /* internal buffer */} rio_t;

unreadalready readBuffer

rio_bufrio_bufptr

rio_cnt

Buffered RIO Input Functions Efficiently read text lines and binary data from a file partially

cached in an internal memory buffer

rio_readlineb reads a text line of up to maxlen bytes from file fd and stores the line in usrbuf

Especially useful for reading text lines from network sockets Stopping conditions

maxlen bytes read EOF encountered Newline (‘\n’) encountered

#include "csapp.h"

void rio_readinitb(rio_t *rp, int fd);

ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);

Return: num. bytes read if OK, 0 on EOF, -1 on error

Buffered RIO Input Functions (cont)

rio_readnb reads up to n bytes from file fd Stopping conditions

maxlen bytes read EOF encountered

Calls to rio_readlineb and rio_readnb can be interleaved arbitrarily on the same descriptor

Warning: Don’t interleave with calls to rio_readn

#include "csapp.h"

void rio_readinitb(rio_t *rp, int fd);

ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n);

Return: num. bytes read if OK, 0 on EOF, -1 on error

RIO Example Copying the lines of a text file from standard input to

standard output

#include "csapp.h"

int main(int argc, char **argv) { int n; rio_t rio; char buf[MAXLINE];

Rio_readinitb(&rio, STDIN_FILENO); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0)

Rio_writen(STDOUT_FILENO, buf, n); exit(0);} cpfile.c

Today Unix I/O RIO (robust I/O) package Metadata, sharing, and redirection Standard I/O Conclusions and examples

File Metadata Metadata is data about data, in this case file data Per-file metadata maintained by kernel

accessed by users with the stat and fstat functions

/* Metadata returned by the stat and fstat functions */struct stat { dev_t st_dev; /* device */ ino_t st_ino; /* inode */ mode_t st_mode; /* protection and file type */ nlink_t st_nlink; /* number of hard links */ uid_t st_uid; /* user ID of owner */ gid_t st_gid; /* group ID of owner */ dev_t st_rdev; /* device type (if inode device) */ off_t st_size; /* total size, in bytes */ unsigned long st_blksize; /* blocksize for filesystem I/O */ unsigned long st_blocks; /* number of blocks allocated */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last modification */ time_t st_ctime; /* time of last change */};

Example of Accessing File Metadata/* statcheck.c - Querying and manipulating a file’s meta data */#include "csapp.h"

int main (int argc, char **argv) { struct stat stat; char *type, *readok; Stat(argv[1], &stat); if (S_ISREG(stat.st_mode))

type = "regular"; else if (S_ISDIR(stat.st_mode))

type = "directory"; else

type = "other"; if ((stat.st_mode & S_IRUSR)) /* OK to read?*/

readok = "yes"; else

readok = "no";

printf("type: %s, read: %s\n", type, readok); exit(0);}

unix> ./statcheck statcheck.ctype: regular, read: yesunix> chmod 000 statcheck.cunix> ./statcheck statcheck.ctype: regular, read: nounix> ./statcheck ..type: directory, read: yesunix> ./statcheck /dev/kmemtype: other, read: yes

statcheck.c

How the Unix Kernel Represents Open Files Two descriptors referencing two distinct open disk files.

Descriptor 1 (stdout) points to terminal, and descriptor 4 points to open disk file

fd 0fd 1fd 2fd 3fd 4

Descriptor table[one table per process]

Open file table [shared by all processes]

v-node table[shared by all processes]

File posrefcnt=1

...

File posrefcnt=1

...

stderrstdoutstdin File access

...

File sizeFile type

File access

...

File sizeFile type

File A (terminal)

File B (disk)

Info in stat struct

File Sharing Two distinct descriptors sharing the same disk file through

two distinct open file table entries E.g., Calling open twice with the same filename argument





File posrefcnt=1

...

File posrefcnt=1

...


...

File sizeFile type

File A (disk)

File B (disk)

How Processes Share Files: Fork() A child process inherits its parent’s open files

Note: situation unchanged by exec functions (use fcntl to change) Before fork() call:





File posrefcnt=1

...

File posrefcnt=1

...


...

File sizeFile type

File access

...

File sizeFile type

File A (terminal)

File B (disk)

How Processes Share Files: Fork() A child process inherits its parent’s open files After fork():

Child’s table same as parent’s, and +1 to each refcnt





File posrefcnt=2

...

File posrefcnt=2

...

File access

...

File sizeFile type

File access

...

File sizeFile type

File A (terminal)

File B (disk)fd 0fd 1fd 2fd 3fd 4

Parent

Child

I/O Redirection Question: How does a shell implement I/O redirection?

unix> ls > foo.txt

Answer: By calling the dup2(oldfd, newfd) function Copies (per-process) descriptor table entry oldfd to entry newfd

a

b


Descriptor tablebefore dup2(4,1)

b

b


Descriptor tableafter dup2(4,1)

I/O Redirection Example Step #1: open file to which stdout should be redirected

Happens in child executing shell code, before exec





File posrefcnt=1

...

File posrefcnt=1

...


...

File sizeFile type

File access

...

File sizeFile type

File A

File B

I/O Redirection Example (cont.) Step #2: call dup2(4,1)

cause fd=1 (stdout) to refer to disk file pointed at by fd=4





File posrefcnt=0

...

File posrefcnt=2

...


...

File sizeFile type

File access

...

File sizeFile type

File A

File B

Fun with File Descriptors (1)

What would this program print for file containing “abcde”?

#include "csapp.h"int main(int argc, char *argv[]){ int fd1, fd2, fd3; char c1, c2, c3; char *fname = argv[1]; fd1 = Open(fname, O_RDONLY, 0); fd2 = Open(fname, O_RDONLY, 0); fd3 = Open(fname, O_RDONLY, 0); Dup2(fd2, fd3); Read(fd1, &c1, 1); Read(fd2, &c2, 1); Read(fd3, &c3, 1); printf("c1 = %c, c2 = %c, c3 = %c\n", c1, c2, c3); return 0;} ffiles1.c


What would this program print for file containing “abcde”?

#include "csapp.h"int main(int argc, char *argv[]){ int fd1; int s = getpid() & 0x1; char c1, c2; char *fname = argv[1]; fd1 = Open(fname, O_RDONLY, 0); Read(fd1, &c1, 1); if (fork()) { /* Parent */ sleep(s); Read(fd1, &c2, 1); printf("Parent: c1 = %c, c2 = %c\n", c1, c2); } else { /* Child */ sleep(1-s); Read(fd1, &c2, 1); printf("Child: c1 = %c, c2 = %c\n", c1, c2); } return 0;} ffiles2.c


What would be the contents of the resulting file?

#include "csapp.h"int main(int argc, char *argv[]){ int fd1, fd2, fd3; char *fname = argv[1]; fd1 = Open(fname, O_CREAT|O_TRUNC|O_RDWR, S_IRUSR|S_IWUSR); Write(fd1, "pqrs", 4); fd3 = Open(fname, O_APPEND|O_WRONLY, 0); Write(fd3, "jklmn", 5); fd2 = dup(fd1); /* Allocates descriptor */ Write(fd2, "wxyz", 4); Write(fd3, "ef", 2); return 0;} ffiles3.c

Standard I/O Functions The C standard library (libc.so) contains a collection of

higher-level standard I/O functions Documented in Appendix B of K&R.

Examples of standard I/O functions: Opening and closing files (fopen and fclose) Reading and writing bytes (fread and fwrite) Reading and writing text lines (fgets and fputs) Formatted reading and writing (fscanf and fprintf)

Standard I/O Streams Standard I/O models open files as streams

Abstraction for a file descriptor and a buffer in memory. Similar to buffered RIO

C programs begin life with three open streams (defined in stdio.h) stdin (standard input) stdout (standard output) stderr (standard error)

#include <stdio.h>extern FILE *stdin; /* standard input (descriptor 0) */extern FILE *stdout; /* standard output (descriptor 1) */extern FILE *stderr; /* standard error (descriptor 2) */

int main() { fprintf(stdout, "Hello, world\n");}

Buffering in Standard I/O Standard I/O functions use buffered I/O

Buffer flushed to output fd on “\n” or fflush() call

printf("h");

h e l l o \n . .

printf("e");printf("l");

printf("l");printf("o");

printf("\n");

fflush(stdout);

buf

write(1, buf, 6);

Standard I/O Buffering in Action You can see this buffering in action for yourself, using the

always fascinating Unix strace program:

linux> strace ./helloexecve("./hello", ["hello"], [/* ... */])....write(1, "hello\n", 6) = 6...exit_group(0) = ?

#include <stdio.h>

int main(){ printf("h"); printf("e"); printf("l"); printf("l"); printf("o"); printf("\n"); fflush(stdout); exit(0);}

I/O Unix I/O RIO (robust I/O) package Metadata, sharing, and redirection Standard I/O Conclusions

Unix I/O vs. Standard I/O vs. RIO Standard I/O and RIO are implemented using low-level

Unix I/O

Which ones should you use in your programs?

Unix I/O functions (accessed via system calls)

Standard I/O functions

C application program

fopen fdopenfread fwrite fscanf fprintf sscanf sprintf fgets fputs fflush fseekfclose

open readwrite lseekstat close

rio_readnrio_writenrio_readinitbrio_readlinebrio_readnb

RIOfunctions

Pros and Cons of Unix I/O Pros

Unix I/O is the most general and lowest overhead form of I/O. All other I/O packages are implemented using Unix I/O

functions. Unix I/O provides functions for accessing file metadata. Unix I/O functions are async-signal-safe and can be used safely in

signal handlers.

Cons Dealing with short counts is tricky and error prone. Efficient reading of text lines requires some form of buffering, also

tricky and error prone. Both of these issues are addressed by the standard I/O and RIO

packages.

Pros and Cons of Standard I/O Pros:

Buffering increases efficiency by decreasing the number of read and write system calls

Short counts are handled automatically Cons:

Provides no function for accessing file metadata Standard I/O functions are not async-signal-safe, and not

appropriate for signal handlers. Standard I/O is not appropriate for input and output on network

sockets There are poorly documented restrictions on streams that

interact badly with restrictions on sockets (CS:APP2e, Sec 10.9)

Choosing I/O Functions General rule: use the highest-level I/O functions you can

Many C programmers are able to do all of their work using the standard I/O functions

When to use standard I/O When working with disk or terminal files

When to use raw Unix I/O Inside signal handlers, because Unix I/O is async-signal-safe. In rare cases when you need absolute highest performance.

When to use RIO When you are reading and writing network sockets. Avoid using standard I/O on sockets.

For Further Information The Unix bible:

W. Richard Stevens & Stephen A. Rago, Advanced Programming in the Unix Environment, 2nd Edition, Addison Wesley, 2005

Updated from Stevens’s 1993 classic text.

Networks and the Internet

A Client-Server Transaction

Clientprocess

Serverprocess

1. Client sends request

2. Server handlesrequest

3. Server sends response4. Client handles

response

Resource

Most network applications are based on the client-server model: A server process and one or more client processes Server manages some resource Server provides service by manipulating resource for clients Server activated by request from client (vending machine analogy)

Note: clients and servers are processes running on hosts (can be the same or different hosts)

Hardware Organization of a Network Host

mainmemory

I/O bridgeMI

ALU

register fileCPU chip

system bus memory bus

disk controller

graphicsadapter

USBcontroller

mouse keyboard monitordisk

I/O bus

Expansion slots

networkadapter

network

Computer Networks A network is a hierarchical system of boxes and wires

organized by geographical proximity SAN (System Area Network) spans cluster or machine room

Switched Ethernet, Quadrics QSW, … LAN (Local Area Network) spans a building or campus

Ethernet is most prominent example WAN (Wide Area Network) spans country or world

Typically high-speed point-to-point phone lines

An internetwork (internet) is an interconnected set of networks The Global IP Internet (uppercase “I”) is the most famous example

of an internet (lowercase “i”)

Let’s see how an internet is built from the ground up

Lowest Level: Ethernet Segment

Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub

Spans room or floor in a building Operation

Each Ethernet adapter has a unique 48-bit address (MAC address) E.g., 00:16:ea:e3:54:e6

Hosts send bits to any other host in chunks called frames Hub slavishly copies each bit from each port to every other port

Every host sees every bit Note: Hubs are on their way out. Bridges (switches, routers) became cheap enough

to replace them (means no more broadcasting)

host host host

hub100 Mb/s100 Mb/s

port

Next Level: Bridged Ethernet Segment

Spans building or campus Bridges cleverly learn which hosts are reachable from which

ports and then selectively copy frames from port to port

host host host host host

hub

hub

bridge

100 Mb/s 100 Mb/s

host host

hub

100 Mb/s 100 Mb/s

1 Gb/s

host host host

bridge

hosthost

hub

A B

C

X

Y

Conceptual View of LANs For simplicity, hubs, bridges, and wires are often shown as a

collection of hosts attached to a single wire:

host host host...

Next Level: internets Multiple incompatible LANs can be physically connected by

specialized computers called routers The connected networks are called an internet

host host host... host host host...

WAN WAN

LAN 1 and LAN 2 might be completely different, totally incompatible (e.g., Ethernet and Wifi, 802.11*, T1-links, DSL, …)

router router routerLAN LAN

Logical Structure of an internet

Ad hoc interconnection of networks No particular topology Vastly different router & link capacities

Send packets from source to destination by hopping through networks Router forms bridge from one network to another Different packets may take different routes

router

router

routerrouter

router

router

hosthost

The Notion of an internet Protocol How is it possible to send bits across incompatible LANs

and WANs?

Solution: protocol software running on each host and router smooths out the differences between the different networks

Implements an internet protocol (i.e., set of rules) governs how hosts and routers should cooperate when they

transfer data from network to network TCP/IP is the protocol for the global IP Internet

What Does an internet Protocol Do? Provides a naming scheme

An internet protocol defines a uniform format for host addresses Each host (and router) is assigned at least one of these internet

addresses that uniquely identifies it

Provides a delivery mechanism An internet protocol defines a standard transfer unit (packet) Packet consists of header and payload

Header: contains info such as packet size, source and destination addresses

Payload: contains data bits sent from source host

LAN2

Transferring Data Over an internet

protocolsoftware

client

LAN1adapter

Host ALAN1

data(1)

data PH FH1(4)

data PH FH2(6)

data(8)

data PH FH2 (5)

LAN2 frame

protocolsoftware

LAN1adapter

LAN2adapter

Routerdata PH(3) FH1

data PH FH1(2)

internet packet

LAN1 frame

(7) data PH FH2

protocolsoftware

server

LAN2adapter

Host B

PH: Internet packet headerFH: LAN frame header

Other Issues We are glossing over a number of important questions:

What if different networks have different maximum frame sizes? (segmentation)

How do routers know where to forward frames? How are routers informed when the network topology changes? What if packets get lost?

These (and other) questions are addressed by the area of systems known as computer networking

Global IP Internet Most famous example of an internet

Based on the TCP/IP protocol family IP (Internet protocol) :

Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host

UDP (Unreliable Datagram Protocol) Uses IP to provide unreliable datagram delivery from

process-to-process TCP (Transmission Control Protocol)

Uses IP to provide reliable byte streams from process-to-process over connections

Accessed via a mix of Unix file I/O and functions from the sockets interface

Hardware and Software Organization of an Internet Application

TCP/IP

Client

Networkadapter

Global IP Internet

TCP/IP

Server

Networkadapter

Internet client host Internet server host

Sockets interface(system calls)

Hardware interface(interrupts)

User code

Kernel code

Hardwareand firmware

A Programmer’s View of the Internet Hosts are mapped to a set of 32-bit IP addresses

140.192.36.43

The set of IP addresses is mapped to a set of identifiers called Internet domain names 140.192.36.43 is mapped to cdmlinux.cdm.depaul.edu

IP Addresses 32-bit IP addresses are stored in an IP address struct

IP addresses are always stored in memory in network byte order (big-endian byte order)

True in general for any integer transferred in a packet header from one machine to another.

E.g., the port number used to identify an Internet connection.

/* Internet address structure */struct in_addr { unsigned int s_addr; /* network byte order (big-endian) */};

Useful network byte-order conversion functions (“l” = 32 bits, “s” = 16 bits)

htonl: convert uint32_t from host to network byte orderhtons: convert uint16_t from host to network byte orderntohl: convert uint32_t from network to host byte orderntohs: convert uint16_t from network to host byte order

Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented

by its decimal value and separated by a period IP address: 0x8002C2F2 = 128.2.194.242

Functions for converting between binary IP addresses and dotted decimal strings: inet_aton: dotted decimal string → IP address in network byte order inet_ntoa: IP address in network byte order → dotted decimal string

“n” denotes network representation “a” denotes application representation

Internet Domain Names

.net .edu .gov .com

depaul berkeleysmith

cti ece

ctilinux3140.192.36.43

cstsis

unnamed root

reed140.192.32.110

amazon

www207.171.166.252

First-level domain names

Second-level domain names

Third-level domain names

Domain Naming System (DNS) The Internet maintains a mapping between IP addresses and

domain names in a huge worldwide distributed database called DNS Conceptually, programmers can view the DNS database as a collection of

millions of host entry structures:

Functions for retrieving host entries from DNS: gethostbyname: query key is a DNS domain name. gethostbyaddr: query key is an IP address.

/* DNS host entry structure */ struct hostent { char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* null-terminated array of in_addr structs */ };

Properties of DNS Host Entries Each host entry is an equivalence class of domain names and

IP addresses Each host has a locally defined domain name localhost

which always maps to the loopback address 127.0.0.1 Different kinds of mappings are possible:

Simple case: one-to-one mapping between domain name and IP address: reed.cs.depaul.edu maps to 140.192.32.110

Multiple domain names mapped to the same IP address: eecs.mit.edu and cs.mit.edu both map to 18.62.1.6

Multiple domain names mapped to multiple IP addresses: google.com maps to multiple IP addresses

A Program That Queries DNSint main(int argc, char **argv) { /* argv[1] is a domain name */ char **pp; /* or dotted decimal IP addr */ struct in_addr addr; struct hostent *hostp;

if (inet_aton(argv[1], &addr) != 0) hostp = Gethostbyaddr((const char *)&addr, sizeof(addr), AF_INET); else hostp = Gethostbyname(argv[1]); printf("official hostname: %s\n", hostp->h_name); for (pp = hostp->h_aliases; *pp != NULL; pp++) printf("alias: %s\n", *pp);

for (pp = hostp->h_addr_list; *pp != NULL; pp++) { addr.s_addr = ((struct in_addr *)*pp)->s_addr; printf("address: %s\n", inet_ntoa(addr)); }}

Using DNS Program

$ ./hostinfo reed.cs.depaul.eduofficial hostname: reed.cti.depaul.edualias: reed.cs.depaul.eduaddress: 140.192.39.42$ ./hostinfo 140.192.39.42official hostname: reed.cti.depaul.eduaddress: 140.192.39.42$ ./hostinfo www.google.comofficial hostname: www.google.comaddress: 173.194.73.104address: 173.194.73.105address: 173.194.73.99address: 173.194.73.147address: 173.194.73.103address: 173.194.73.106

Querying DIG Domain Information Groper (dig) provides a scriptable

command line interface to DNS

$ dig +short reed.cs.depaul.edureed.cti.depaul.edu.140.192.39.42$ dig +short -x 140.192.39.42reed.cti.depaul.edu.baf346.cstcis.cti.depaul.edu.$ dig +short www.google.com173.194.73.99173.194.73.147173.194.73.103173.194.73.106173.194.73.104173.194.73.105

Internet Connections Clients and servers communicate by sending streams of bytes

over connections: Point-to-point, full-duplex (2-way communication), and reliable.

A socket is an endpoint of a connection Socket address is an IPaddress:port pair

A port is a 16-bit integer that identifies a process: Ephemeral port: Assigned automatically on client when client makes a

connection request Well-known port: Associated with some service provided by a server

(e.g., port 80 is associated with Web servers)

A connection is uniquely identified by the socket addresses of its endpoints (socket pair) (cliaddr:cliport, servaddr:servport)

Putting it all Together: Anatomy of an Internet Connection

Connection socket pair(128.2.194.242:51213, 208.216.181.15:80)

Server(port 80)Client

Client socket address128.2.194.242:51213

Server socket address208.216.181.15:80

Client host address128.2.194.242

Server host address208.216.181.15

Clients Examples of client programs

Web browsers, ftp, telnet, ssh

How does a client find the server? The IP address in the server socket address identifies the host

(more precisely, an adapter on the host) The (well-known) port in the server socket address identifies the

service, and thus implicitly identifies the server process that performs that service.

Examples of well know ports Port 7: Echo server Port 23: Telnet server Port 25: Mail server Port 80: Web server

Using Ports to Identify Services

Web server(port 80)

Client host

Server host 128.2.194.242

Echo server(port 7)

Service request for128.2.194.242:80

(i.e., the Web server)

Web server(port 80)

Echo server(port 7)

Service request for128.2.194.242:7

(i.e., the echo server)

Kernel

Kernel

Client

Client

Servers Servers are long-running processes (daemons)

Created at boot-time (typically) by the init process (process 1) Run continuously until the machine is turned off

Each server waits for requests to arrive on a well-known port associated with a particular service Port 7: echo server Port 23: telnet server Port 25: mail server Port 80: HTTP server

A machine that runs a server process is also often referred to as a “server”

Server Examples Web server (port 80)

Resource: files/compute cycles (CGI programs) Service: retrieves files and runs CGI programs on behalf of the client

FTP server (20, 21) Resource: files Service: stores and retrieve files

Telnet server (23) Resource: terminal Service: proxies a terminal on the server machine

Mail server (25) Resource: email “spool” file Service: stores mail messages in spool file

See /etc/services for a comprehensive list of the port mappings on a Linux machine

1. networks and the internet 2. network programming

Documents

file fdnbytes

open file fd

file copies bytes

pipea file type

unix filesa unix file

sequence of bytes

closeint fd

standard errorint fd