sudo wireshark
curl http://neverssl.com

tcp
udp
dns
http
icmp
arp
ip.addr == 8.8.8.8
tcp.port == 80
tcp.flags.syn==1
tls.handshake.extensions_server_name

sudo apt install -y wireshark

no
enable
configure terminal
interface fastEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 no shutdown
 exit
interface serial2/0
 ip address 10.0.0.1 255.0.0.0
 clock rate 64000
 no shutdown
 exit

no
enable
configure terminal
interface fastEthernet0/0
 ip address 192.168.2.1 255.255.255.0
 no shutdown
 exit
interface serial2/0
 ip address 10.0.0.2 255.0.0.0
 no shutdown
 exit

ip route 192.168.2.0 255.255.255.0 10.0.0.2
end

ip route 192.168.1.0 255.255.255.0 10.0.0.1
end

router rip
 version 2
 network 192.168.1.0
 network 10.0.0.0
 end

router rip
 version 2
 network 192.168.2.0
 network 10.0.0.0
 end

show ip route

/* EXP 7: TCP Server - receives a sentence, sends back its reverse (port 5777) */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 5777
#define MAX 1024

int main(void) {
    int listenfd, connfd, n, i;
    char line[MAX], rev[MAX];
    struct sockaddr_in servaddr, cliaddr;
    socklen_t clilen = sizeof(cliaddr);

    listenfd = socket(AF_INET, SOCK_STREAM, 0);            /* 1. socket  */
    int opt = 1;
    setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(PORT);
    if (bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {  /* 2. bind */
        perror("bind"); exit(1);
    }
    listen(listenfd, 5);                                      /* 3. listen  */
    printf("Server waiting on port %d...\n", PORT);

    connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &clilen);  /* 4. accept */
    printf("Connected to client %s\n", inet_ntoa(cliaddr.sin_addr));

    while ((n = read(connfd, line, MAX - 1)) > 0) {          /* 5. read */
        line[n] = '\0';
        if (line[n - 1] == '\n') line[--n] = '\0';
        printf("Received : %s\n", line);
        for (i = 0; i < n; i++) rev[i] = line[n - 1 - i];    /* reverse */
        rev[n] = '\0';
        write(connfd, rev, n + 1);                           /* 6. write */
        printf("Sent back: %s\n", rev);
    }
    printf("Client disconnected\n");
    close(connfd);
    close(listenfd);                                         /* 7. close */
    return 0;
}

/* EXP 7: TCP Client - sends a sentence, prints the reversed reply (port 5777) */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 5777
#define MAX 1024

int main(void) {
    int sockfd, n;
    char sendline[MAX], revline[MAX];
    struct sockaddr_in servaddr;

    sockfd = socket(AF_INET, SOCK_STREAM, 0);               /* 1. socket  */
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(PORT);
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");       /* server IP */
    /* 2. connect */
    if (connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {
        perror("connect"); exit(1);
    }
    printf("Connected. Enter sentences (Ctrl+D to quit):\n");
    while (fgets(sendline, MAX, stdin) != NULL) {
        write(sockfd, sendline, strlen(sendline));          /* 3. write */
        n = read(sockfd, revline, MAX - 1);                 /* 4. read  */
        if (n <= 0) break;
        revline[n] = '\0';
        printf("Reverse of the given sentence is: %s\n", revline);
    }
    close(sockfd);                                          /* 5. close */
    return 0;
}

/* EXP 8: UDP Server - two-way chat with client until "exit" (port 5777) */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 5777
#define MAX 1024

int main(void) {
    int sockfd, n;
    char buf[MAX];
    struct sockaddr_in servaddr, cliaddr;
    socklen_t len = sizeof(cliaddr);

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);                /* 1. socket (UDP) */
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(PORT);
    if (bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {  /* 2. bind */
        perror("bind"); exit(1);
    }
    printf("UDP server waiting on port %d...\n", PORT);

    while (1) {
        /* 3. recvfrom */
        n = recvfrom(sockfd, buf, MAX - 1, 0, (struct sockaddr *)&cliaddr, &len);
        buf[n] = '\0';
        printf("Client: %s\n", buf);
        if (strncmp(buf, "exit", 4) == 0) break;

        printf("Server: ");
        if (!fgets(buf, MAX, stdin)) break;
        buf[strcspn(buf, "\n")] = '\0';
        /* 4. sendto */
        sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&cliaddr, len);
        if (strncmp(buf, "exit", 4) == 0) break;
    }
    printf("Chat ended\n");
    close(sockfd);
    return 0;
}

/* EXP 8: UDP Client - two-way chat with server until "exit" (127.0.0.1:5777) */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 5777
#define MAX 1024

int main(void) {
    int sockfd, n;
    char buf[MAX];
    struct sockaddr_in servaddr;
    socklen_t len = sizeof(servaddr);

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);                /* 1. socket (UDP) */
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(PORT);
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    while (1) {
        printf("Client: ");
        if (!fgets(buf, MAX, stdin)) break;
        buf[strcspn(buf, "\n")] = '\0';
        /* 2. sendto */
        sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&servaddr, len);
        if (strncmp(buf, "exit", 4) == 0) break;

        /* 3. recvfrom */
        n = recvfrom(sockfd, buf, MAX - 1, 0, NULL, NULL);
        buf[n] = '\0';
        printf("Server: %s\n", buf);
        if (strncmp(buf, "exit", 4) == 0) break;
    }
    printf("Chat ended\n");
    close(sockfd);
    return 0;
}

sudo apt install -y net-tools traceroute dnsutils

ifconfig
ip a
ping -c 4 google.com
traceroute google.com
netstat -r
netstat -tuln
arp -a
nslookup google.com
dig google.com

ping -c 5 google.com
ping -i 2 -c 4 google.com
ping -s 1000 -c 4 google.com
ping -t 3 -c 3 google.com

netstat -rn
route -n
ip route show

sudo route add -net 10.10.10.0 netmask 255.255.255.0 gw 192.168.0.1 dev enp3s0
route -n
sudo route del -net 10.10.10.0 netmask 255.255.255.0 gw 192.168.0.1 dev enp3s0
route -n

sudo route del default gw 192.168.0.1
sudo route add default gw 192.168.0.254 dev enp3s0
route -n

sudo route del default gw 192.168.0.254
sudo route add default gw 192.168.0.1 dev enp3s0

nslookup google.com
ping -c 10 8.8.8.8
ip route | grep default
ping -c 2 192.168.0.1
curl http://neverssl.com
sudo apt install -y nmap
sudo nmap -sS 127.0.0.1

dns
icmp
arp
http
tcp.flags.syn==1
arp.duplicate-address-detected

sudo sysctl -w net.ipv4.ip_forward=1
sudo iptables -L -n -v
sudo iptables -A INPUT -p icmp -j DROP      # block ping
ping -c 2 127.0.0.1                         # now fails
sudo iptables -L -n --line-numbers
sudo iptables -D INPUT 1                    # remove the rule
ping -c 2 127.0.0.1