1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 | /* * This program is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program. If not, see <http://www.gnu.org/licenses/>. */ /******************************************************************** * Description: * Author: jason,,, <> * Created at: Fri Jan 13 08:30:53 AEDT 2023 * Computer: jason-Lenovo-H50-55 * System: Linux 5.15.0-57-generic on x86_64 * * Copyright (c) 2023 jason,,, All rights reserved. * ********************************************************************/ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> void error(const char *msg) { perror(msg); exit(1); } int main(int argc, char *argv[]) { int sockfd, newsockfd, portno; socklen_t clilen; char buffer[256]; struct sockaddr_in serv_addr, cli_addr; int n; // create a socket sockfd = socket(AF_INET, SOCK_STREAM, 0); if (sockfd < 0) error("ERROR opening socket"); // clear address structure bzero((char *) &serv_addr, sizeof(serv_addr)); // set port number portno = 5080; // setup the host_addr structure for use in bind call serv_addr.sin_family = AF_INET; serv_addr.sin_addr.s_addr = INADDR_ANY; serv_addr.sin_port = htons(portno); // bind the socket to the specified port if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) error("ERROR on binding"); // listen for clients listen(sockfd, 5); clilen = sizeof(cli_addr); // accept a client newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen); if (newsockfd < 0) error("ERROR on accept"); // read data from client bzero(buffer, 256); n = read(newsockfd, buffer, 255); if (n < 0) error("ERROR reading from socket"); printf("Here is the message: %s\n", buffer); // write data to client n = write(newsockfd, "I got your message", 18); if (n < 0) error("ERROR writing to socket"); // close sockets close(newsockfd); close(sockfd); return 0; } |
This is a very useful little program. This opens port 5080 and listens for a netcat connection.
Then it will print a text message when a text string is sent to it. As shown below, I connected to the remote netcat server and then sent “Hello.” as a message and it was acknowledged automatically.
┌──(john㉿DESKTOP-PF01IEE)-[~] └─$ netcat 192.168.1.2 5080 Hello. I got your message |
This works perfectly.
Scanning the system with Nmap, I can see the open port on 5080. Once the connection is established and the message is sent, the program closes. But this could be used for a variety of things.
┌──(john㉿DESKTOP-PF01IEE)-[~] └─$ nmap -p 3000-6000 192.168.1.2 Starting Nmap 7.93 ( https://nmap.org ) at 2023-01-13 08:51 AEDT Nmap scan report for 192.168.1.2 Host is up (0.85s latency). Not shown: 2999 closed tcp ports (conn-refused) PORT STATE SERVICE 3389/tcp open ms-wbt-server 5080/tcp open onscreen Nmap done: 1 IP address (1 host up) scanned in 2.14 seconds |