ComputerNetworks unipd2018
1.0.0
輕鬆通過帕多瓦電腦網路實作考試（“Reti di calcolatori”）的提示和程式碼。您可以透過man以及各種 RFC 找到本自述文件中的所有內容。我這樣做只是為了快速參考。
乙太網路訊框（資料鏈結層）包含 IP 資料封包（網路層），該資料封包可以包含以下內容之一：{ tcp_segment（傳輸層）、icmp_packet }（出於本考試的目的）。實現這一點的一個簡單方法是：
eth = ( struct eth_frame * ) buffer ;
ip = ( struct ip_datagram * ) eth -> payload ;
tcp = ( struct tcp_segment * ) ip -> payload ;
// or
icmp = ( struct icmp_packet * ) ip -> payload ;（取決於架構，但您可以假設以下內容對於本次考試來說是正確的）
unsigned char ：1 個位元組unsigned short ：2 個位元組unsiged int ：4 位元組在網路上傳輸使用的是Big endian。大多數 intel 的 cpu 都是小端位元組序。要進行轉換，請使用這兩個函數來自動了解是否需要轉換：
htonl(x)或htons(x)將 x 從主機轉換為網路字節序， l如果必須轉換 4 位元組變量，則 s轉換為 2 位元組變數。ntohl(x)或ntohs(x)則相反。 （你可能會注意到htonx和ntohx的實作是相同的） // Frame Ethernet
struct eth_frame {
unsigned char dst [ 6 ]; // mac address
unsigned char src [ 6 ]; // mac address
unsigned short type ; // 0x0800 = ip, 0x0806 = arp
char payload [ 1500 ]; //ARP or IP
};借助該type我們可以了解在下一個層級將其轉發到何處（2 個範例是 ip 或 arp）
標頭長度：檢查ver_ihl屬性的後半部。範例：如果是“5”，則標頭長度為4 * 5 = 20 位元組。
//待辦事項新增圖像
// Datagramma IP
struct ip_datagram {
unsigned char ver_ihl ; // first 4 bits: version, second 4 bits: (lenght header)/8
unsigned char tos ; //type of service
unsigned short totlen ; // len header + payload
unsigned short id ; // useful in case of fragmentation
unsigned short flags_offs ; //offset/8 related to the original ip package
unsigned char ttl ;
unsigned char protocol ; // TCP = 6, ICMP = 1
unsigned short checksum ; // only header checksum (not of payload). Must be at 0 before the calculation.
unsigned int src ; // ip address
unsigned int dst ; // ip address
unsigned char payload [ 1500 ];
};標頭（如此處定義）長度： 20
struct tcp_segment {
unsigned short s_port ;
unsigned short d_port ;
unsigned int seq ; // offset in bytes from the start of the tcp segment in the stream (from initial sequance n)
unsigned int ack ; // useful only if ACK flag is 1. Next seq that sender expect
unsigned char d_offs_res ; // first 4 bits: (header len/8)
unsigned char flags ; // check rfc
unsigned short win ; // usually initially a 0 (?)
unsigned short checksum ; // use tcp_pseudo to calculate it. Must be at 0 before the calculation.
unsigned short urgp ;
unsigned char payload [ 1000 ];
};計算 TCP 段的校驗和對於定義附加結構很有用（檢查相關 RFC）。它的大小，不包括 tcp_segment 部分
struct tcp_pseudo {
unsigned int ip_src , ip_dst ;
unsigned char zeroes ;
unsigned char proto ; // ip datagram protocol field (tcp = 6, ip = 1)
unsigned short entire_len ; // tcp length (header + data)
unsigned char tcp_segment [ 20 /*to set appropriatly */ ]; // entire tcp packet pointer
};要計算整個 tcp 段（或 icmp）的大小，或更一般的 ip 負載的大小：
unsigned short ip_total_len = ntohs ( ip -> totlen );
unsigned short ip_header_dim = ( ip -> ver_ihl & 0x0F ) * 4 ;
int ip_payload_len = ip_total_len - ip_header_dim ;我們可以將此函數用於 IP 資料封包和 TCP 段，但我們必須注意len參數。
unsigned short checksum ( unsigned char * buffer , int len ){
int i ;
unsigned short * p ;
unsigned int tot = 0 ;
p = ( unsigned short * ) buffer ;
for ( i = 0 ; i < len / 2 ; i ++ ){
tot = tot + htons ( p [ i ]);
if ( tot & 0x10000 ) tot = ( tot & 0xFFFF ) + 1 ;
}
return ( unsigned short ) 0xFFFF - tot ;
}這2個案例是：
ip->checksum=htons(checksum((unsigned char*) ip, 20)); ` int TCP_TOTAL_LEN = 20 ;
struct tcp_pseudo pseudo ; // size of this: 12
memcpy ( pseudo . tcp_segment , tcp , TCP_TOTAL_LEN );
pseudo . zeroes = 0 ;
pseudo . ip_src = ip -> src ;
pseudo . ip_dst = ip -> dst ;
pseudo . proto = 6 ;
pseudo . entire_len = htons ( TCP_TOTAL_LEN ); // may vary
tcp -> checksum = htons ( checksum (( unsigned char * ) & pseudo , TCP_TOTAL_LEN + 12 )); #include <arpa/inet.h>
void print_ip ( unsigned int ip ){
struct in_addr ip_addr ;
ip_addr . s_addr = ip ;
printf ( "%sn" , inet_ntoa ( ip_addr ));
}我建議VIM。請縮排您的程式碼。
:wq儲存並退出。esc兩次/query搜尋“query”， n和N搜尋上一個/下一個結果 " auto reformat when you pres F7
map <F7> mzgg=G`z
" F8 to save and compile creating np executable
map <F8> :w <CR> :!gcc % -o np -g <CR>
" F9 to execute
map <F9> :!./np <CR>
" make your code look nicer
set tabstop=3
set shiftwidth=3
set softtabstop=0 expandtab
set incsearch
set cindent
" Ctrl+shift+up/down to swap the line up or doen
nnoremap <C-S-Up> <Up>"add"ap<Up>
nnoremap <C-S-Down> "add"ap
" ctrl+h to hilight the last search
nnoremap <C-h> :set hlsearch!<CR>
set number
set cursorline
set mouse=a
set foldmethod=indent
set foldlevelstart=99
let mapleader="<space>"
nnoremap <leader>b :make <CR> :cw <CR>
首先，在包含要編譯的檔案的目錄中建立一個makefile ，如下所示：
np : ws18.c
gcc -o np ws18.c注意在「gcc」之前放置一個製表符，而不是空格（如果您在 vim 中啟用了 Expandtab，請使用ctrl=v tab ）。這裡np是您想要產生的檔案（執行檔）， ws18.c是要編譯的檔案。下面一行是每次在 vim 中編寫:make時呼叫的命令。然後，使用上面提供的.vimrc ，按space （釋放它）和b ( b uild)。該命令將被執行，您將在程式碼底部看到錯誤清單。您可以透過在每個項目中按 Enter 快速跳到正確的行。若要在頂部和底部分割之間移動，請按CTRL+W W 。若要關閉底部視圖（快速修復） :q或:cw 。
您可以在本自述文件開頭的網站中找到完整的考試說明。完整的程式碼在資料夾中。
實作 TCP 三向握手（ACK+SYN）。
提示：您可以使用wireshark檢查您的TCP校驗和是否正確。
僅對特定大小的 icmp 請求實現回顯回复
提示：您可以透過以下方式計算 icmp 訊息的大小：
unsigned short dimension = ntohs ( ip -> totlen );
unsigned short header_dim = ( ip -> ver_ihl & 0x0F ) * 4 ;
int icmp_dimension = dimension - header_dim ;實作一個 HTTP 伺服器：
提示：大多數網頁瀏覽器都會忽略Retry-After標頭，因此重定向不會在 10 秒後發生，而是立即發生。在解決方案中，有一個陣列保存每個 ip 的連線狀態。
實施 ICMP“目標不可達”，表示連接埠不可用
提示：您必須傳送資料包以回應 tcp 連線。 icmp->type = 3 ， icmp->code=3 。並記住將 icmp 原始有效負載的內容複製到有效負載中。
攔截第一個收到的連接，並列印它們的序列和確認號。然後在 2 個不同的緩衝區中重建 2 個流，並列印它們的內容。
提示：要攔截連線結束，只需檢查某個包是否包含 FIN 位元為 1 的包（過濾完所有包後，僅保留屬於第一個連接的包）。使用 tcp 序列欄位將 contnet 複製到 2 個緩衝區中的正確偏移處。不要重複代碼。
修改代理程式以僅允許來自 IP 位址池的請求，並且僅允許傳輸文字或 html 檔案。
提示：最好先在緩衝區中接收來自伺服器的回應，然後將此內容複製到另一個緩衝區以像往常一樣提取標頭。這是因為標頭提取過程修改了緩衝區。如果滿足 Content-type 的條件，則僅轉送初始緩衝區的 contnet。
發送帶有分塊正文的 HTTP 回應。
提示：將Content-Type: text/plainrnTransfer-Encoding: chunkedrn新增至 HTTP 標頭。然後，要建立要傳送的每個區塊，您可以使用類似以下內容的內容：
int build_chunk ( char * s , int len ){
sprintf ( chunk_buffer , "%xrn" , len ); // size in hex
// debug printf("%d in hex: %s",len,chunk_buffer);
int from = strlen ( chunk_buffer );
int i = 0 ;
for (; i < len ; i ++ )
chunk_buffer [ from + i ] = s [ i ];
chunk_buffer [ from + ( i ++ )] = 'r' ;
chunk_buffer [ from + ( i ++ )] = 'n' ;
chunk_buffer [ i + from ] = 0 ;
return i + from ;
}實作 HTTP/1.0 的Last-Modified標頭
提示：雜項部分中有一些有用的時間轉換函數。它也可以在不需要這些轉換的情況下完成。 HTTP 日期格式為%a, %d %b %Y %H:%M:%S %Z
1：內容長度（已實現）2：追蹤（??）
修改 icmp echo 將請求拆分為兩個 IP 資料報，一個資料報的負載大小為 16 字節，另一個資料報的負載大小為請求的負載大小。
課程期間會佈置一些作業（非強制性）。即使它們不是考試，它們的難度也或多或少相同。
實施追蹤路由程序，在到達目的地之前追蹤封包訪問的每個節點。提示：當生存時間變為 0 時，節點將丟棄資料包並向資料包的來源 IP 位址發送「逾時訊息」（請參閱 RFC793）。
使用 HTTP 標頭WWW-Authenticate來要求客戶端提供存取憑證。
實現一個接受分塊內容的客戶端。
HTTP 日期格式為%a, %d %b %Y %H:%M:%S %Z
char date_buf [ 1000 ];
char * getNowHttpDate (){
time_t now = time ( 0 );
struct tm tm = * gmtime ( & now );
strftime ( date_buf , sizeof date_buf , "%a, %d %b %Y %H:%M:%S %Z" , & tm );
printf ( "Time is: [%s]n" , date_buf );
return date_buf ;
}
// parse time and convert it to millisecond from epoch
time_t httpTimeToEpoch ( char * time ){
struct tm tm ;
char buf [ 255 ];
memset ( & tm , 0 , sizeof ( struct tm ));
strptime ( time , "%a, %d %b %Y %H:%M:%S %Z" , & tm );
return mktime ( & tm );
}
// returns 1 if d1 < d2
unsigned char compareHttpDates ( char * d1 , char * d2 ){
return httpTimeToEpoch ( d1 ) < httpTimeToEpoch ( d2 );
}
unsigned char expired ( char * uri , char * last_modified ){
char * complete_name = uriToCachedFile ( uri );
FILE * fp = fopen ( complete_name , "r" );
if ( fp == NULL ) return 1 ;
//read the first line
char * line = 0 ; size_t len = 0 ;
getline ( & line , & len , fp );
if ( compareHttpDates ( last_modified , line )) return 0 ;
return 1 ;
//todo read First line and compare
}rewind(FILE*)將遊標設定在開頭
FILE * fin ;
if (( fin = fopen ( uri + 1 , "rt" )) == NULL ) { // the t is useless
printf ( "File %s non aperton" , uri + 1 );
sprintf ( response , "HTTP/1.1 404 File not foundrnrn<html>File non trovato</html>" );
t = write ( s2 , response , strlen ( response ));
if ( t == -1 ) {
perror ( "write fallita" );
return -1 ;
}
} else {
content_length = 0 ;
while (( c = fgetc ( fin )) != EOF ) content_length ++ ; // get file lenght
sprintf ( response , "HTTP/1.1 200 OKrnConnection: keep-alivernContent-Length: %drnrn" , content_length );
printf ( "Response: %sn" , response );
//send header
t = write ( s2 , response , strlen ( response ));
//rewind the file
rewind ( fin );
//re-read the file, char per char
while (( c = fgetc ( fin )) != EOF ) {
//printf("%c", c);
//sending the file, char per char
if ( write ( s2 , ( unsigned char * ) & c , 1 ) != 1 ) {
perror ( "Write fallita" );
}
}
fclose ( fin );
} char car ;
while ( read ( s3 , & car , 1 )) {
write ( s2 , & car , 1 );
// printf("%c",car);
} unsigned char targetip [ 4 ] = { 147 , 162 , 2 , 100 };
unsigned int netmask = 0x00FFFFFF ;
if (( * (( unsigned int * ) targetip ) & netmask ) == ( * (( unsigned int * ) myip ) & netmask ))
nexthop = targetip ;
else
nexthop = gateway ;從主機名稱（如 www.google.it）到 IP 位址
/**
struct hostent {
char *h_name; // official name of host
char **h_aliases; // alias list
int h_addrtype; // host address type
int h_length; // length of address
char **h_addr_list; // list of addresses
}
#define h_addr h_addr_list[0] // for backward compatibility
*/
struct hostent * he ;
he = gethostbyname ( hostname );
printf ( "Indirizzo di %s : %d.%d.%d.%dn" , hostname ,
( unsigned char )( he -> h_addr [ 0 ]), ( unsigned char )( he -> h_addr [ 1 ]),
( unsigned char )( he -> h_addr [ 2 ]), ( unsigned char )( he -> h_addr [ 3 ]));供聆聽：
int s = socket ( AF_INET , // domain: ipv4
/*
SOCK_STREAM Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data
transmission mechanism may be supported.
SOCK_DGRAM Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
SOCK_RAW Provides raw network protocol access.
*/
SOCK_STREAM , // type: stream
0 ); // protocol (0=ip), check /etc/protocols
if ( s == -1 ) {
perror ( "Socket Fallita" );
return 1 ;
}
// https://stackoverflow.com/questions/3229860/what-is-the-meaning-of-so-reuseaddr-setsockopt-option-linux
// SO_REUSEADDR allows your server to bind to an address which is in a TIME_WAIT state.
int yes = 1 ;
if ( setsockopt ( s , SOL_SOCKET , SO_REUSEADDR , & yes , sizeof ( int )) == -1 ) {
perror ( "setsockopt" );
return 1 ;
} struct sockaddr_in indirizzo ;
indirizzo . sin_family = AF_INET ;
indirizzo . sin_port = htons ( 8987 );
indirizzo . sin_addr . s_addr = 0 ;
t = bind ( s , ( struct sockaddr * ) & indirizzo , sizeof ( struct sockaddr_in ));
if ( t == -1 ) {
perror ( "Bind fallita" );
return 1 ;
}
t = listen ( s ,
// backlog defines the maximum length for the queue of pending connections.
10 );
if ( t == -1 ) {
perror ( "Listen Fallita" );
return 1 ;
} int lunghezza = sizeof ( struct sockaddr_in );
// the remote address will be placed in indirizzo_remoto
s2 = accept ( s , ( struct sockaddr * ) & indirizzo_remoto , & lunghezza );
if ( s2 == -1 ) {
perror ( "Accept Fallita" );
return 1 ;
}
// now we can read in this way:
char buffer [ 10000 ];
int i ;
for ( i = 0 ; ( t = read ( s2 , buffer + i , 1 )) > 0 ; i ++ ); // ps. it's not a good way
// if the previous read returned -1
if ( t == -1 ) {
perror ( "Read Fallita" );
return 1 ;
}最後，記得用close(s)關閉所有套接字（其中 s 位於要關閉的套接字中）
int s = socket (
//AF_PACKET Low level packet interface packet(7)
AF_PACKET ,
//SOCK_RAW Provides raw network protocol access.
SOCK_RAW ,
// When protocol is set to htons(ETH_P_ALL), then all protocols are received.
htons ( ETH_P_ALL ));
unsigned char buffer [ 1500 ];
bzero ( & sll , sizeof ( struct sockaddr_ll ));
struct sockaddr_ll sll ;
sll . sll_ifindex = if_nametoindex ( "eth0" );
len = sizeof ( sll );
int t = sendto ( s , //socket
buffer , //things to send
14 + 20 + 28 , // len datagram
0 , //flags
( struct sockaddr * ) & sll , // destination addr
len // dest addr len
);
// to receive
t = recvfrom ( s , buffer , 1500 , 0 , ( struct sockaddr * ) & sll , & len );
if ( t == -1 ) {
perror ( "recvfrom fallita" );
return 1 ;
} void stampa_eth ( struct eth_frame * e ){
printf ( "nn ***** PACCHETTO Ethernet *****n" );
printf ( "Mac destinazione: %x:%x:%x:%x:%x:%xn" , e -> dst [ 0 ], e -> dst [ 1 ], e -> dst [ 2 ], e -> dst [ 3 ], e -> dst [ 4 ], e -> dst [ 5 ] );
printf ( "Mac sorgente: %x:%x:%x:%x:%x:%xn" , e -> src [ 0 ], e -> src [ 1 ], e -> src [ 2 ], e -> src [ 3 ], e -> src [ 4 ], e -> src [ 5 ] );
printf ( "EtherType: 0x%xn" , htons ( e -> type ) );
} void stampa_ip ( struct ip_datagram * i ){
unsigned int ihl = ( i -> ver_ihl & 0x0F ) * 4 ; // Lunghezza header IP
unsigned int totlen = htons ( i -> totlen ); // Lunghezza totale pacchetto
unsigned int opt_len = ihl - 20 ; // Lunghezza campo opzioni
printf ( "nn ***** PACCHETTO IP *****n" );
printf ( "Version: %dn" , i -> ver_ihl & 0xF0 );
printf ( "IHL (bytes 60max): %dn" , ihl );
printf ( "TOS: %dn" , i -> tos );
printf ( "Lunghezza totale: %dn" , totlen );
printf ( "ID: %xn" , htons ( i -> id ) );
unsigned char flags = ( unsigned char )( htons ( i -> flag_offs ) >> 13 );
printf ( "Flags: %d | %d | %d n" , flags & 4 , flags & 2 , flags & 1 );
printf ( "Fragment Offset: %dn" , htons ( i -> flag_offs ) & 0x1FFF );
printf ( "TTL: %dn" , i -> ttl );
printf ( "Protocol: %dn" , i -> proto );
printf ( "Checksum: %xn" , htons ( i -> checksum ) );
unsigned char * saddr = ( unsigned char * ) & i -> saddr ;
unsigned char * daddr = ( unsigned char * ) & i -> daddr ;
printf ( "IP Source: %d.%d.%d.%dn" , saddr [ 0 ], saddr [ 1 ], saddr [ 2 ], saddr [ 3 ] );
printf ( "IP Destination: %d.%d.%d.%dn" , daddr [ 0 ], daddr [ 1 ], daddr [ 2 ], daddr [ 3 ] );
if ( ihl > 20 ){
// Stampa opzioni
printf ( "Options: " );
for ( int j = 0 ; j < opt_len ; j ++ ){
printf ( "%.3d(%.2x) " , i -> payload [ j ], i -> payload [ j ]);
}
printf ( "n" );
}
} void stampa_arp ( struct arp_packet * a ){
printf ( "nn ***** PACCHETTO ARP *****n" );
printf ( "Hardware type: %dn" , htons ( a -> htype ) );
printf ( "Protocol type: %xn" , htons ( a -> ptype ) );
printf ( "Hardware Addr len: %dn" , a -> hlen );
printf ( "Protocol Addr len: %dn" , a -> plen );
printf ( "Operation: %dn" , htons ( a -> op ) );
printf ( "HW Addr sorgente: %x:%x:%x:%x:%x:%xn" , a -> hsrc [ 0 ], a -> hsrc [ 1 ], a -> hsrc [ 2 ], a -> hsrc [ 3 ], a -> hsrc [ 4 ], a -> hsrc [ 5 ] );
printf ( "IP Source: %d.%d.%d.%dn" , a -> psrc [ 0 ], a -> psrc [ 1 ], a -> psrc [ 2 ], a -> psrc [ 3 ] );
printf ( "HW Addr Destinazione: %x:%x:%x:%x:%x:%xn" , a -> hdst [ 0 ], a -> hdst [ 1 ], a -> hdst [ 2 ], a -> hdst [ 3 ], a -> hdst [ 4 ], a -> hdst [ 5 ] );
printf ( "IP Dest: %d.%d.%d.%dn" , a -> pdst [ 0 ], a -> pdst [ 1 ], a -> pdst [ 2 ], a -> pdst [ 3 ] );
} void stampa_icmp ( struct icmp_packet * i ){
printf ( "nn ***** PACCHETTO ICMP *****n" );
printf ( "Type: %dn" , i -> type );
printf ( "Code: %dn" , i -> code );
printf ( "Code: 0x%xn" , htons ( i -> checksum ) );
printf ( "ID: %dn" , htons ( i -> id ) );
printf ( "Sequence: %dn" , htons ( i -> seq ) );
} void stampa_tcp ( struct tcp_segment * t ){
printf ( "nn ***** PACCHETTO TCP *****n" );
printf ( "Source Port: %dn" , htons ( t -> s_port ) );
printf ( "Source Port: %dn" , htons ( t -> d_port ) );
printf ( "Sequence N: %dn" , ntohl ( t -> seq ) );
printf ( "ACK: %dn" , ntohl ( t -> ack ) );
printf ( "Data offset (bytes): %dn" , ( t -> d_offs_res >> 4 ) * 4 );
printf ( "Flags: " );
printf ( "CWR=%d | " , ( t -> flags & 0x80 ) >> 7 );
printf ( "ECE=%d | " , ( t -> flags & 0x40 ) >> 6 );
printf ( "URG=%d | " , ( t -> flags & 0x20 ) >> 5 );
printf ( "ACK=%d | " , ( t -> flags & 0x10 ) >> 4 );
printf ( "PSH=%d | " , ( t -> flags & 0x08 ) >> 3 );
printf ( "RST=%d | " , ( t -> flags & 0x04 ) >> 2 );
printf ( "SYN=%d | " , ( t -> flags & 0x02 ) >> 1 );
printf ( "FIN=%dn" , ( t -> flags & 0x01 ) );
printf ( "Windows size: %dn" , htons ( t -> win ) );
printf ( "Checksum: 0x%xn" , htons ( t -> checksum ) );
printf ( "Urgent pointer: %dn" , htons ( t -> urgp ) );
}不太有用，但是..
// es. tcp.c
printf ( "%.4d. // delta_sec (unsigned int)
% .6d // delta_usec
% .5d -> % .5d // ports (unsigned short)
% .2 x // tcp flags (unsigned char) in hex: es: "12"
% .10u // seq (unsigned int)
% .10u // ack
% .5u //tcp win
% 4.2f n ", delta_sec , delta_usec , htons ( tcp -> s_port ), htons ( tcp -> d_port ), tcp -> flags , htonl ( tcp -> seq ) - seqzero , htonl ( tcp -> ack ) - ackzero , htons ( tcp -> win ), ( htonl ( tcp -> ack ) - ackzero ) / ( double )( delta_sec * 1000000 + delta_usec ));/etc/services ：了解應用程式層級可用的所有 TCP 連接埠。/etc/protocolsnslookup <URL> ：尋找指定 URL 的 IP 位址（例如：www.google.com）netstat -rn顯示路由表traceroute透過列印決定丟棄低 TTL（生存時間，每跳遞減）計數偽造的資料包的每個網關的 IP，來路由 ip 資料包所經過的路徑。
