lewdfingerd/lewdfingerd.c

#define _GNU_SOURCE
#include <netdb.h>
#include <poll.h>
#include <stdarg.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

struct pollfd *listens = NULL;
size_t num_listens = 0;

void log_error(const char * restrict format, ...) {
	va_list args;

	va_start(args, format);

	vfprintf(stderr, format, args);

	va_end(args);
}

void setup_listen(void) {
	struct addrinfo hints;
	struct addrinfo *getaddrinfo_result;

	// AF_UNSPEC: either IPv4 or IPv6
	// SOCK_STREAM: TCP
	// AI_PASSIVE: fill out my IP for me
	// AI_ADDRCONFIG: only return addresses I have a configured interface for
	memset(&hints, 0, sizeof(hints));
	hints.ai_family = AF_UNSPEC;
	hints.ai_socktype = SOCK_STREAM;
	hints.ai_flags = AI_PASSIVE | AI_ADDRCONFIG;

	int status = getaddrinfo(NULL, "79", &hints, &getaddrinfo_result);

	if(status != 0) {
		log_error("getaddrinfo failed: %s\n", gai_strerror(status));
		exit(1);
	}

	for(struct addrinfo *res = getaddrinfo_result; res != NULL; res = res->ai_next) {
		int yes = 1;

		// Add corresponding interface to table of sockets
		// Create socket
		int sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
		if(sock == -1) {
			perror("socket");
			exit(1);
		}

		// Disable the IPv4 over IPv6, as that results in IPv4 and IPv6 sockets conflicting and one of them not being able to be set up
		if(res->ai_family == AF_INET6) {
			if(setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, &yes, sizeof(yes)) == -1) {
				perror("setsockopt");
				exit(1);
			}
		}

		// Set reuseaddr
		if(setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) == -1) {
			perror("setsockopt");
			exit(1);
		}

		// Bind onto given address
		if(bind(sock, res->ai_addr, res->ai_addrlen) == -1) {
			perror("bind");
			exit(1);
		}

		// Listen for incoming connections
		if(listen(sock, 1) == -1) {
			perror("listen");
			exit(1);
		}

		// Grow the table of sockets
		size_t index = num_listens++;
		if(SIZE_MAX / sizeof(struct pollfd) < num_listens){
			log_error("Too many interfaces to listen on\n");
			exit(1);
		}

		listens = realloc(listens, num_listens * sizeof(struct pollfd));

		if(listens == NULL) {
			perror("realloc");
			exit(1);
		}

		// Add socket to the table
		struct pollfd new_pollobj = {.fd = sock, .events = POLLIN};
		listens[index] = new_pollobj;
	}

	freeaddrinfo(getaddrinfo_result);
}

void drop_privileges(void) {
	uid_t uid = getuid();
	gid_t gid = getgid();

	if(setresgid(gid, gid, gid) != 0) {
		perror("setresgid");
		exit(1);
	}
	if(setresuid(uid, uid, uid) != 0) {
		perror("setresuid");
		exit(1);
	}
}

ssize_t writeall(int fd, const char *buf, size_t amount) {
	size_t written = 0;
	while(written < amount) {
		ssize_t r = write(fd, buf + written, amount - written);
		if(r < 0) {
			return r;
		}

		written += r;
	}

	return written;
}

void handle_connection(int sock) {
	size_t request_size = 0;
	char *request = NULL;

	for(;;) {
		ssize_t amount_read;
		char iobuf[1024];

		amount_read = read(sock, &iobuf, sizeof(iobuf));

		if(amount_read == 0) {
			log_error("Client hung up\n");
			shutdown(sock, SHUT_RDWR);
			close(sock);
			return;
		} else if(amount_read < 0) {
			perror("read");
			shutdown(sock, SHUT_RDWR);
			close(sock);
			return;
		}

		size_t index = request_size;
		request_size += amount_read;
		if((request = realloc(request, request_size)) == NULL) {
			perror("realloc");
			exit(1);
		}
		memmove(request + index, iobuf, amount_read);

		if(request_size >= 2 && request[request_size-2] == '\r' && request[request_size-1] == '\n') {
			// Request read
			break;
		}
	}

	request[request_size-2] = '\0';

	if(!strcmp(request, "")) {
		const char *response = "Who do you want to finger?\r\n";
		if(writeall(sock, response, strlen(response)) < 0) {
			perror("write");
		}
	} else {
		const char *response = "Lewd.\r\n";
		if(writeall(sock, response, strlen(response)) < 0) {
			perror("write");
		}
	}

	shutdown(sock, SHUT_RDWR);
	close(sock);
}

int main(void) {
	drop_privileges();
	setup_listen();

	for(;;) {
		int amount_ready = poll(listens, num_listens, -1);
		if(amount_ready < 0) {
			perror("poll");
			exit(1);
		}

		for(size_t i = 0; i < num_listens && amount_ready > 0; i++) {
			if(listens[i].revents & POLLIN) {
				struct sockaddr_storage client_addr;
				socklen_t addr_size = sizeof(client_addr);

				int sock = accept(listens[i].fd, (struct sockaddr *)&client_addr, &addr_size);

				handle_connection(sock);

				amount_ready--;
			}
		}
	}
}