usr.bin/nc/netcat.c

	netcat.c revision 63c99f9316851ebfab1e6d91ff1807739e55bd5b
/* $OpenBSD: netcat.c,v 1.89 2007/02/20 14:11:17 jmc Exp $ */
/*
 * Copyright (c) 2001 Eric Jackson <ericj@monkey.org>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Re-written nc(1) for OpenBSD. Original implementation by
 * *Hobbit* <hobbit@avian.org>.
 */

/*
 * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma ident   "%Z%%M% %I% %E% SMI"

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/un.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/tcp.h>
#include <netinet/ip.h>
#include <arpa/telnet.h>

#include <err.h>
#include <errno.h>
#include <netdb.h>
#include <poll.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <limits.h>
#include <signal.h>

#include "atomicio.h"
#include "strtonum.h"

#ifndef SUN_LEN
#define SUN_LEN(su) \
    (sizeof (*(su)) - sizeof ((su)->sun_path) + strlen((su)->sun_path))
#endif

#define PORT_MIN    1
#define PORT_MAX    65535
#define PORT_MAX_LEN    6

/* Command Line Options */
int dflag;      /* detached, no stdin */
unsigned int iflag; /* Interval Flag */
int kflag;      /* More than one connect */
int lflag;      /* Bind to local port */
int nflag;      /* Don't do name lookup */
char    *Pflag;     /* Proxy username */
char    *pflag;     /* Localport flag */
int rflag;      /* Random ports flag */
char    *sflag;     /* Source Address */
int tflag;      /* Telnet Emulation */
int uflag;      /* UDP - Default to TCP */
int vflag;      /* Verbosity */
int xflag;      /* Socks proxy */
int Xflag;      /* indicator of Socks version set */
int zflag;      /* Port Scan Flag */
int Dflag;      /* sodebug */
int Tflag = -1; /* IP Type of Service */

int timeout = -1;
int family = AF_UNSPEC;
char    *portlist[PORT_MAX+1];

void    atelnet(int, unsigned char *, unsigned int);
void    build_ports(char *);
void    help(void);
int local_listen(char *, char *, struct addrinfo);
void    readwrite(int);
int remote_connect(const char *, const char *, struct addrinfo);
int socks_connect(const char *, const char *,
        const char *, const char *, struct addrinfo, int, const char *);
int udptest(int);
int unix_connect(char *);
int unix_listen(char *);
void    set_common_sockopts(int);
int parse_iptos(char *);
void    usage(int);
char    *print_addr(char *, size_t, struct sockaddr *, int, int);

int
main(int argc, char *argv[])
{
    int ch, s, ret, socksv;
    char *host, *uport, *proxy;
    struct addrinfo hints;
    struct servent *sv;
    socklen_t len;
    struct sockaddr_storage cliaddr;
    const char *errstr, *proxyhost = "", *proxyport = NULL;
    struct addrinfo proxyhints;

    ret = 1;
    s = 0;
    socksv = 5;
    host = NULL;
    uport = NULL;
    sv = NULL;

    while ((ch = getopt(argc, argv,
        "46Ddhi:klnP:p:rs:T:tUuvw:X:x:z")) != -1) {
        switch (ch) {
        case '4':
            family = AF_INET;
            break;
        case '6':
            family = AF_INET6;
            break;
        case 'U':
            family = AF_UNIX;
            break;
        case 'X':
            Xflag = 1;
            if (strcasecmp(optarg, "connect") == 0)
                socksv = -1; /* HTTP proxy CONNECT */
            else if (strcmp(optarg, "4") == 0)
                socksv = 4; /* SOCKS v.4 */
            else if (strcmp(optarg, "5") == 0)
                socksv = 5; /* SOCKS v.5 */
            else
                errx(1, "unsupported proxy protocol");
            break;
        case 'd':
            dflag = 1;
            break;
        case 'h':
            help();
            break;
        case 'i':
            iflag = strtonum(optarg, 0, UINT_MAX, &errstr);
            if (errstr)
                errx(1, "interval %s: %s", errstr, optarg);
            break;
        case 'k':
            kflag = 1;
            break;
        case 'l':
            lflag = 1;
            break;
        case 'n':
            nflag = 1;
            break;
        case 'P':
            Pflag = optarg;
            break;
        case 'p':
            pflag = optarg;
            break;
        case 'r':
            rflag = 1;
            break;
        case 's':
            sflag = optarg;
            break;
        case 't':
            tflag = 1;
            break;
        case 'u':
            uflag = 1;
            break;
        case 'v':
            vflag = 1;
            break;
        case 'w':
            timeout = strtonum(optarg, 0, INT_MAX / 1000, &errstr);
            if (errstr)
                errx(1, "timeout %s: %s", errstr, optarg);
            timeout *= 1000;
            break;
        case 'x':
            xflag = 1;
            if ((proxy = strdup(optarg)) == NULL)
                err(1, NULL);
            break;
        case 'z':
            zflag = 1;
            break;
        case 'D':
            Dflag = 1;
            break;
        case 'T':
            Tflag = parse_iptos(optarg);
            break;
        default:
            usage(1);
        }
    }
    argc -= optind;
    argv += optind;

    /* Cruft to make sure options are clean, and used properly. */
    if (argv[0] && !argv[1] && family == AF_UNIX) {
        if (uflag)
            errx(1, "cannot use -u and -U");
        host = argv[0];
        uport = NULL;
    } else if (argv[0] && !argv[1]) {
        if (!lflag)
            usage(1);
        uport = argv[0];
        host = NULL;
    } else if (argv[0] && argv[1]) {
        if (family == AF_UNIX)
            usage(1);
        host = argv[0];
        uport = argv[1];
    } else {
        if (!(lflag && pflag))
            usage(1);
    }

    if (argc > 2)
        usage(1);

    if (lflag && sflag)
        errx(1, "cannot use -s and -l");
    if (lflag && rflag)
        errx(1, "cannot use -r and -l");
    if (lflag && (timeout >= 0))
        warnx("-w has no effect with -l");
    if (lflag && pflag) {
        if (uport)
            usage(1);
        uport = pflag;
    }
    if (lflag && zflag)
        errx(1, "cannot use -z and -l");
    if (!lflag && kflag)
        errx(1, "must use -l with -k");
    if (lflag && (Pflag || xflag || Xflag))
        errx(1, "cannot use -l with -P, -X or -x");

    /* Initialize addrinfo structure. */
    if (family != AF_UNIX) {
        (void) memset(&hints, 0, sizeof (struct addrinfo));
        hints.ai_family = family;
        hints.ai_socktype = uflag ? SOCK_DGRAM : SOCK_STREAM;
        hints.ai_protocol = uflag ? IPPROTO_UDP : IPPROTO_TCP;
        if (nflag)
            hints.ai_flags |= AI_NUMERICHOST;
    }

    if (xflag) {
        if (uflag)
            errx(1, "no proxy support for UDP mode");

        if (lflag)
            errx(1, "no proxy support for listen");

        if (family == AF_UNIX)
            errx(1, "no proxy support for unix sockets");

        if (family == AF_INET6)
            errx(1, "no proxy support for IPv6");

        if (sflag)
            errx(1, "no proxy support for local source address");

        if ((proxyhost = strtok(proxy, ":")) == NULL)
            errx(1, "missing port specification");
        proxyport = strtok(NULL, ":");

        (void) memset(&proxyhints, 0, sizeof (struct addrinfo));
        proxyhints.ai_family = family;
        proxyhints.ai_socktype = SOCK_STREAM;
        proxyhints.ai_protocol = IPPROTO_TCP;
        if (nflag)
            proxyhints.ai_flags |= AI_NUMERICHOST;
    }

    if (lflag) {
        int connfd;
        ret = 0;

        if (family == AF_UNIX)
            s = unix_listen(host);

        /* Allow only one connection at a time, but stay alive. */
        for (;;) {
            if (family != AF_UNIX)
                s = local_listen(host, uport, hints);
            if (s < 0)
                err(1, NULL);
            /*
             * For UDP, we will use recvfrom() initially
             * to wait for a caller, then use the regular
             * functions to talk to the caller.
             */
            if (uflag) {
                int rv, plen;
                char buf[8192];
                struct sockaddr_storage z;

                len = sizeof (z);
                plen = 1024;
                rv = recvfrom(s, buf, plen, MSG_PEEK,
                    (struct sockaddr *)&z, &len);
                if (rv < 0)
                    err(1, "recvfrom");

                rv = connect(s, (struct sockaddr *)&z, len);
                if (rv < 0)
                    err(1, "connect");

                connfd = s;
            } else {
                len = sizeof (cliaddr);
                connfd = accept(s, (struct sockaddr *)&cliaddr,
                    &len);
                if ((connfd != -1) && vflag) {
                    char ntop[NI_MAXHOST + NI_MAXSERV];
                    (void) fprintf(stderr,
                        "Received connection from %s\n",
                        print_addr(ntop, sizeof (ntop),
                        (struct sockaddr *)&cliaddr, len,
                        nflag ? NI_NUMERICHOST : 0));
                }
            }

            readwrite(connfd);
            (void) close(connfd);
            if (family != AF_UNIX)
                (void) close(s);

            if (!kflag)
                break;
        }
    } else if (family == AF_UNIX) {
        ret = 0;

        if ((s = unix_connect(host)) > 0 && !zflag) {
            readwrite(s);
            (void) close(s);
        } else
            ret = 1;

        exit(ret);

    } else {    /* AF_INET or AF_INET6 */
        int i = 0;

        /* Construct the portlist[] array. */
        build_ports(uport);

        /* Cycle through portlist, connecting to each port. */
        for (i = 0; portlist[i] != NULL; i++) {
            if (s)
                (void) close(s);

            if (xflag)
                s = socks_connect(host, portlist[i],
                    proxyhost, proxyport, proxyhints, socksv,
                    Pflag);
            else
                s = remote_connect(host, portlist[i], hints);

            if (s < 0)
                continue;

            ret = 0;
            if (vflag || zflag) {
                /* For UDP, make sure we are connected. */
                if (uflag) {
                    if (udptest(s) == -1) {
                        ret = 1;
                        continue;
                    }
                }

                /* Don't look up port if -n. */
                if (nflag)
                    sv = NULL;
                else {
                    sv = getservbyport(
                        ntohs(atoi(portlist[i])),
                        uflag ? "udp" : "tcp");
                }

                (void) fprintf(stderr, "Connection to %s %s "
                    "port [%s/%s] succeeded!\n",
                    host, portlist[i], uflag ? "udp" : "tcp",
                    sv ? sv->s_name : "*");
            }
            if (!zflag)
                readwrite(s);
        }
    }

    if (s)
        (void) close(s);

    return (ret);
}

/*
 * print IP address and (optionally) a port
 */
char *
print_addr(char *ntop, size_t ntlen, struct sockaddr *addr, int len, int flags)
{
    char port[NI_MAXSERV];
    int e;

    /* print port always as number */
    if ((e = getnameinfo(addr, len, ntop, ntlen,
        port, sizeof (port), flags|NI_NUMERICSERV)) != 0) {
        return ((char *)gai_strerror(e));
    }

    (void) snprintf(ntop, ntlen, "%s port %s", ntop, port);

    return (ntop);
}

/*
 * unix_connect()
 * Returns a socket connected to a local unix socket. Returns -1 on failure.
 */
int
unix_connect(char *path)
{
    struct sockaddr_un sunaddr;
    int s;

    if ((s = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)
        return (-1);

    (void) memset(&sunaddr, 0, sizeof (struct sockaddr_un));
    sunaddr.sun_family = AF_UNIX;

    if (strlcpy(sunaddr.sun_path, path, sizeof (sunaddr.sun_path)) >=
        sizeof (sunaddr.sun_path)) {
        (void) close(s);
        errno = ENAMETOOLONG;
        return (-1);
    }
    if (connect(s, (struct sockaddr *)&sunaddr, SUN_LEN(&sunaddr)) < 0) {
        (void) close(s);
        return (-1);
    }
    return (s);
}

/*
 * unix_listen()
 * Create a unix domain socket, and listen on it.
 */
int
unix_listen(char *path)
{
    struct sockaddr_un sunaddr;
    int s;

    /* Create unix domain socket. */
    if ((s = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)
        return (-1);

    (void) memset(&sunaddr, 0, sizeof (struct sockaddr_un));
    sunaddr.sun_family = AF_UNIX;

    if (strlcpy(sunaddr.sun_path, path, sizeof (sunaddr.sun_path)) >=
        sizeof (sunaddr.sun_path)) {
        (void) close(s);
        errno = ENAMETOOLONG;
        return (-1);
    }

    if (bind(s, (struct sockaddr *)&sunaddr, SUN_LEN(&sunaddr)) < 0) {
        (void) close(s);
        return (-1);
    }

    if (listen(s, 5) < 0) {
        (void) close(s);
        return (-1);
    }
    return (s);
}

/*
 * remote_connect()
 * Returns a socket connected to a remote host. Properly binds to a local
 * port or source address if needed. Returns -1 on failure.
 */
int
remote_connect(const char *host, const char *port, struct addrinfo hints)
{
    struct addrinfo *res, *res0;
    int s, error;

    if ((error = getaddrinfo(host, port, &hints, &res)))
        errx(1, "getaddrinfo: %s", gai_strerror(error));

    res0 = res;
    do {
        if ((s = socket(res0->ai_family, res0->ai_socktype,
            res0->ai_protocol)) < 0) {
            warn("failed to create socket");
            continue;
        }

        /* Bind to a local port or source address if specified. */
        if (sflag || pflag) {
            struct addrinfo ahints, *ares;

            (void) memset(&ahints, 0, sizeof (struct addrinfo));
            ahints.ai_family = res0->ai_family;
            ahints.ai_socktype = uflag ? SOCK_DGRAM : SOCK_STREAM;
            ahints.ai_protocol = uflag ? IPPROTO_UDP : IPPROTO_TCP;
            ahints.ai_flags = AI_PASSIVE;
            if ((error = getaddrinfo(sflag, pflag, &ahints, &ares)))
                errx(1, "getaddrinfo: %s", gai_strerror(error));

            if (bind(s, (struct sockaddr *)ares->ai_addr,
                ares->ai_addrlen) < 0)
                errx(1, "bind failed: %s", strerror(errno));
            freeaddrinfo(ares);

            if (vflag && !lflag) {
                if (sflag != NULL)
                    (void) fprintf(stderr,
                        "Using source address: %s\n",
                        sflag);
                if (pflag != NULL)
                    (void) fprintf(stderr,
                        "Using source port: %s\n", pflag);
            }
        }

        set_common_sockopts(s);

        if (connect(s, res0->ai_addr, res0->ai_addrlen) == 0)
            break;
        else if (vflag) {
            char ntop[NI_MAXHOST + NI_MAXSERV];
            warn("connect to %s [host %s] (%s) failed",
                print_addr(ntop, sizeof (ntop),
                res0->ai_addr, res0->ai_addrlen, NI_NUMERICHOST),
                host, uflag ? "udp" : "tcp");
        }

        (void) close(s);
        s = -1;
    } while ((res0 = res0->ai_next) != NULL);

    freeaddrinfo(res);

    return (s);
}

/*
 * local_listen()
 * Returns a socket listening on a local port, binds to specified source
 * address. Returns -1 on failure.
 */
int
local_listen(char *host, char *port, struct addrinfo hints)
{
    struct addrinfo *res, *res0;
    int s, ret, x = 1;
    int error;

    /* Allow nodename to be null. */
    hints.ai_flags |= AI_PASSIVE;

    if ((error = getaddrinfo(host, port, &hints, &res)))
        errx(1, "getaddrinfo: %s", gai_strerror(error));

    res0 = res;
    do {
        if ((s = socket(res0->ai_family, res0->ai_socktype,
            res0->ai_protocol)) < 0) {
            warn("failed to create socket");
            continue;
        }

        ret = setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &x, sizeof (x));
        if (ret == -1)
            err(1, NULL);

        set_common_sockopts(s);

        if (bind(s, (struct sockaddr *)res0->ai_addr,
            res0->ai_addrlen) == 0)
            break;

        (void) close(s);
        s = -1;
    } while ((res0 = res0->ai_next) != NULL);

    if (!uflag && s != -1) {
        if (listen(s, 1) < 0)
            err(1, "listen");
    }

    freeaddrinfo(res);

    return (s);
}

/*
 * readwrite()
 * Loop that polls on the network file descriptor and stdin.
 */
void
readwrite(int nfd)
{
    struct pollfd pfd[2];
    unsigned char buf[8192];
    int n, wfd = fileno(stdin);
    int lfd = fileno(stdout);
    int plen;

    plen = 1024;

    /* Setup Network FD */
    pfd[0].fd = nfd;
    pfd[0].events = POLLIN;

    /* Set up STDIN FD. */
    pfd[1].fd = wfd;
    pfd[1].events = POLLIN;

    while (pfd[0].fd != -1) {
        if (iflag)
            (void) sleep(iflag);

        if ((n = poll(pfd, 2 - dflag, timeout)) < 0) {
            (void) close(nfd);
            err(1, "Polling Error");
        }

        if (n == 0)
            return;

        if (pfd[0].revents & (POLLIN|POLLHUP)) {
            if ((n = read(nfd, buf, plen)) < 0)
                return;
            else if (n == 0) {
                (void) shutdown(nfd, SHUT_RD);
                pfd[0].fd = -1;
                pfd[0].events = 0;
            } else {
                if (tflag)
                    atelnet(nfd, buf, n);
                if (atomicio(vwrite, lfd, buf, n) != n)
                    return;
            }
        }

        /*
         * handle the case of disconnected pipe: after pipe
         * is closed (indicated by POLLHUP) there may still
         * be some data lingering (POLLIN). After we read
         * the data, only POLLHUP remains, read() returns 0
         * and we are finished.
         */
        if (!dflag && (pfd[1].revents & (POLLIN|POLLHUP))) {
            if ((n = read(wfd, buf, plen)) < 0)
                return;
            else if (n == 0) {
                (void) shutdown(nfd, SHUT_WR);
                pfd[1].fd = -1;
                pfd[1].events = 0;
            } else {
                if (atomicio(vwrite, nfd, buf, n) != n)
                    return;
            }
        }
    }
}

/* Deal with RFC 854 WILL/WONT DO/DONT negotiation. */
void
atelnet(int nfd, unsigned char *buf, unsigned int size)
{
    unsigned char *p, *end;
    unsigned char obuf[4];

    end = buf + size;
    obuf[0] = '\0';

    for (p = buf; p < end; p++) {
        if (*p != IAC)
            break;

        obuf[0] = IAC;
        obuf[1] = 0;
        p++;
        /* refuse all options */
        if ((*p == WILL) || (*p == WONT))
            obuf[1] = DONT;
        if ((*p == DO) || (*p == DONT))
            obuf[1] = WONT;
        if (obuf[1]) {
            p++;
            obuf[2] = *p;
            obuf[3] = '\0';
            if (atomicio(vwrite, nfd, obuf, 3) != 3)
                warn("Write Error!");
            obuf[0] = '\0';
        }
    }
}

/*
 * build_ports()
 * Build an array of ports in portlist[], listing each port
 * that we should try to connect to.
 */
void
build_ports(char *p)
{
    const char *errstr;
    char *n;
    int hi, lo, cp;
    int x = 0;

    if ((n = strchr(p, '-')) != NULL) {
        if (lflag)
            errx(1, "Cannot use -l with multiple ports!");

        *n = '\0';
        n++;

        /* Make sure the ports are in order: lowest->highest. */
        hi = strtonum(n, PORT_MIN, PORT_MAX, &errstr);
        if (errstr)
            errx(1, "port number %s: %s", errstr, n);
        lo = strtonum(p, PORT_MIN, PORT_MAX, &errstr);
        if (errstr)
            errx(1, "port number %s: %s", errstr, p);

        if (lo > hi) {
            cp = hi;
            hi = lo;
            lo = cp;
        }

        /* Load ports sequentially. */
        for (cp = lo; cp <= hi; cp++) {
            portlist[x] = calloc(1, PORT_MAX_LEN);
            if (portlist[x] == NULL)
                err(1, NULL);
            (void) snprintf(portlist[x], PORT_MAX_LEN, "%d", cp);
            x++;
        }

        /* Randomly swap ports. */
        if (rflag) {
            int y;
            char *c;

            srandom(time((time_t *)0));

            for (x = 0; x <= (hi - lo); x++) {
                y = (random() & 0xFFFF) % (hi - lo);
                c = portlist[x];
                portlist[x] = portlist[y];
                portlist[y] = c;
            }
        }
    } else {
        hi = strtonum(p, PORT_MIN, PORT_MAX, &errstr);
        if (errstr)
            errx(1, "port number %s: %s", errstr, p);
        portlist[0] = calloc(1, PORT_MAX_LEN);
        if (portlist[0] == NULL)
            err(1, NULL);
        portlist[0] = p;
    }
}

/*
 * udptest()
 * Do a few writes to see if the UDP port is there.
 * XXX - Better way of doing this? Doesn't work for IPv6.
 * Also fails after around 100 ports checked.
 */
int
udptest(int s)
{
    int i, ret;

    for (i = 0; i <= 3; i++) {
        if (write(s, "X", 1) == 1)
            ret = 1;
        else
            ret = -1;
    }
    return (ret);
}

void
set_common_sockopts(int s)
{
    int x = 1;

    if (Dflag) {
        if (setsockopt(s, SOL_SOCKET, SO_DEBUG, &x, sizeof (x)) == -1)
            err(1, NULL);
    }
    if (Tflag != -1) {
        if (setsockopt(s, IPPROTO_IP, IP_TOS, &Tflag,
            sizeof (Tflag)) == -1)
            err(1, "set IP ToS");
    }
}

int
parse_iptos(char *s)
{
    int tos = -1;

    if (strcmp(s, "lowdelay") == 0)
        return (IPTOS_LOWDELAY);
    if (strcmp(s, "throughput") == 0)
        return (IPTOS_THROUGHPUT);
    if (strcmp(s, "reliability") == 0)
        return (IPTOS_RELIABILITY);

    if (sscanf(s, "0x%x", (unsigned int *) &tos) != 1 ||
        tos < 0 || tos > 0xff)
        errx(1, "invalid IP Type of Service");
    return (tos);
}

void
help(void)
{
    usage(0);
    (void) fprintf(stderr, "\tCommand Summary:\n\
    \t-4        Use IPv4\n\
    \t-6        Use IPv6\n\
    \t-D        Enable the debug socket option\n\
    \t-d        Detach from stdin\n\
    \t-h        This help text\n\
    \t-i secs\t Delay interval for lines sent, ports scanned\n\
    \t-k        Keep inbound sockets open for multiple connects\n\
    \t-l        Listen mode, for inbound connects\n\
    \t-n        Suppress name/port resolutions\n\
    \t-P proxyuser\tUsername for proxy authentication\n\
    \t-p port\t Specify local port or listen port\n\
    \t-r        Randomize remote ports\n\
    \t-s addr\t Local source address\n\
    \t-T ToS\t  Set IP Type of Service\n\
    \t-t        Answer TELNET negotiation\n\
    \t-U        Use UNIX domain socket\n\
    \t-u        UDP mode\n\
    \t-v        Verbose\n\
    \t-w secs\t Timeout for connects and final net reads\n\
    \t-X proto  Proxy protocol: \"4\", \"5\" (SOCKS) or \"connect\"\n\
    \t-x addr[:port]\tSpecify proxy address and port\n\
    \t-z        Zero-I/O mode [used for scanning]\n\
    Port numbers can be individual or ranges: lo-hi [inclusive]\n");
    exit(1);
}

void
usage(int ret)
{
    (void) fprintf(stderr,
        "usage: nc [-46DdhklnrtUuvz] [-i interval] [-P proxy_username]"
        " [-p port]\n");
    (void) fprintf(stderr,
        "\t  [-s source_ip_address] [-T ToS] [-w timeout]"
        " [-X proxy_protocol]\n");
    (void) fprintf(stderr,
        "\t  [-x proxy_address[:port]] [hostname]"
        " [port[s]]\n");
    if (ret)
        exit(1);
}