Proxy for one website and it injects javascript (and probably doesn't work)

proxy.c

#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <netdb.h>
#include <poll.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

#define BACKLOG         64
#define BUF_LEN         (1024*1024)
#define CLIENTS_MAX     256
#define NFDS            (2 * CLIENTS_MAX + 1)

#define LOCAL_PORT      8081
#define REMOTE          "bash.im"
#define REMOTE_LEN      (sizeof(REMOTE) - 1)
#define REMOTE_PORT     80

#define PAYLOAD         "<script>alert(\"hOI!\")</script>"
#define PAYLOAD_LEN     (sizeof(PAYLOAD) - 1)

#define HEADER_HOST     "Host: localhost"
#define HEADER_HOST_LEN (sizeof(HEADER_HOST) - 1)
#define HEADER_ENC      "Accept-Encoding: "
#define HEADER_ENC_LEN  (sizeof(HEADER_ENC) - 1)
#define END_BODY        "</head>"
#define END_BODY_LEN    (sizeof(END_BODY) - 1)

void shit_happened(const char *s) {
    if (s) {
        fprintf(stderr, "%s\n", s);
    } else {
        fprintf(stderr, "%s (%d)\n", strerror(errno), errno);
    }
    exit(1);
}

int make_tcp_socket(const char *host, uint16_t port) {
    int ret = socket(AF_INET, SOCK_STREAM, 0);
    if (ret == -1) shit_happened(NULL);

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(struct sockaddr_in));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    in_addr_t host_ip;
    if (host) {
        struct hostent *hostent = gethostbyname(host);
        if (!hostent) shit_happened(NULL);
        host_ip = *(in_addr_t *) hostent->h_addr_list[0];
    } else {
        host_ip = htonl(INADDR_ANY);
    }
    addr.sin_addr.s_addr = host_ip;

    if (host) {
        if (connect(ret, (struct sockaddr *) &addr,
            sizeof(struct sockaddr_in)) == -1) shit_happened(NULL);
    } else {
        if (bind(ret, (struct sockaddr *) &addr,
            sizeof(struct sockaddr_in)) == -1) shit_happened(NULL);
        if (listen(ret, BACKLOG) != 0) shit_happened(NULL);
    }

    return ret;
}

void nonblock(int fd) {
    int on = 1;
    if (ioctl(fd, FIONBIO, (char *) &on) == -1) shit_happened(NULL);
}

struct pollfd fds[NFDS];

void close_all(void) {
    for (int i = 0; i < NFDS; ++i) {
        if (fds[i].fd != 0) {
            close(fds[i].fd);
        }
    }
}

int main() {
    atexit(close_all);
    int sock_listen = make_tcp_socket(NULL, LOCAL_PORT);
    nonblock(sock_listen);

    memset(&fds, 0, sizeof(fds));
    fds[0].fd = sock_listen;
    fds[0].events = POLLIN;
    char buf[BUF_LEN];
    for (;;) {
        int evs = poll(fds, NFDS, 3 * 60 * 1000);
        if (evs == -1) shit_happened(NULL);
        if (evs == 0) continue;
        if (fds[0].revents == POLLIN) {
            int new_client = accept(fds[0].fd, NULL, NULL);
            if (new_client == -1) shit_happened(NULL);
            int new_client_pos = -1;
            for (int i = 1; i < NFDS; i += 2) {
                if (fds[i].fd == 0) {
                    new_client_pos = i;
                    break;
                }
            }
            if (new_client_pos == -1) {
                shit_happened("Too many clients");
            }
            printf("client %d new\n", new_client_pos / 2 + 1);
            fds[new_client_pos].fd = new_client;
            fds[new_client_pos].events = POLLIN;
            fds[new_client_pos + 1].fd = make_tcp_socket(REMOTE, REMOTE_PORT);
            fds[new_client_pos + 1].events = POLLIN;
            nonblock(fds[new_client_pos + 1].fd);
        }
        for (int i = 1; i < NFDS; i += 2) {
            char closed = 0;
            if (fds[i].revents & POLLIN) {
                printf("client %d send\n", i / 2 + 1);
                int len;
                if ((len = read(fds[i].fd, buf, BUF_LEN)) == -1)
                    shit_happened(NULL);
                if (len == 0)
                    goto close_conn;
                buf[len] = 0;
                char modbuf[BUF_LEN * 2];
                int modbuf_ptr = 0;
                for (int p = 0; p < len;) {
                    if (strncmp(buf + p, HEADER_HOST, HEADER_HOST_LEN) == 0) {
                        printf("client %d fixing Host\n", i / 2 + 1);
                        strcpy(modbuf + modbuf_ptr, "Host: " REMOTE);
                        modbuf_ptr += strlen("Host: " REMOTE);
                        p += HEADER_HOST_LEN;
                    } else if (strncmp(buf + p, HEADER_ENC,
                            HEADER_ENC_LEN) == 0) {
                        printf("client %d fixing Accept-Encoding\n", i / 2 + 1);
                        strcpy(modbuf + modbuf_ptr, HEADER_ENC "identity");
                        modbuf_ptr += strlen(HEADER_ENC "identity");
                        modbuf[modbuf_ptr++] = '\r';
                        for (; buf[p] != '\n'; ++p);
                    } else {
                        modbuf[modbuf_ptr++] = buf[p++];
                    }
                }
                if (write(fds[i + 1].fd, modbuf, modbuf_ptr) == -1)
                    shit_happened(NULL);
            }
            if (fds[i + 1].revents & POLLIN) {
                int len;
                printf("client %d recv\n", i / 2 + 1);
                if ((len = read(fds[i + 1].fd, buf, BUF_LEN)) == -1)
                    shit_happened(NULL);
                buf[len] = 0;
                if (len == 0)
                    goto close_conn;
                char modbuf[BUF_LEN + PAYLOAD_LEN + 1];
                int modbuf_ptr = 0;
                for (int p = 0; p < len;) {
                    if (strncmp(buf + p, END_BODY, END_BODY_LEN) == 0) {
                        printf("client %d injecting payload\n", i / 2 + 1);
                        strcpy(modbuf + modbuf_ptr, PAYLOAD);
                        modbuf_ptr += PAYLOAD_LEN;
                    }
                    modbuf[modbuf_ptr++] = buf[p++];
                }
                if (write(fds[i].fd, modbuf, modbuf_ptr) == -1)
                    shit_happened(NULL);
            }
            if (closed) {
                close_conn:
                close(fds[i].fd);
                close(fds[i + 1].fd);
                memset(fds + i, 0, 2 * sizeof(struct pollfd));
                printf("client %d term\n", i / 2 + 1);
            }
        }
    }
}