pptpd/pptpgre.c

/*
 * pptpgre.c
 *
 * originally by C. S. Ananian
 * Modified for PoPToP
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#ifdef __linux__
#define _GNU_SOURCE 1           /* broken arpa/inet.h */
#endif

#include "our_syslog.h"

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/stat.h>
#include <time.h>
#include <sys/time.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#ifdef HAVE_SYS_UIO_H
#include <sys/uio.h>
#endif
#ifdef VRF
#include <vrf.h>
#endif

#include "ppphdlc.h"
#include "pptpgre.h"
#include "pptpdefs.h"
#include "pptpctrl.h"
#include "defaults.h"
#include "pqueue.h"

#ifndef HAVE_STRERROR
#include "compat.h"
#endif

#define PACKET_MAX 8196

typedef int (*callback_t)(int cl, void *pack, unsigned int len);

/* test for a 32 bit counter overflow */
#define WRAPPED( curseq, lastseq) \
    ((((curseq) & 0xffffff00) == 0) && \
     (((lastseq) & 0xffffff00 ) == 0xffffff00))

static struct gre_state gre; 
gre_stats_t stats; 

static uint64_t time_now_usecs()
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    return (tv.tv_sec * 1000000) + tv.tv_usec;
}

int pptp_gre_init(u_int32_t call_id_pair, int pty_fd, struct in_addr *inetaddrs)
{
        struct sockaddr_in addr;
        int gre_fd;

        /* Open IP protocol socket */
        gre_fd = vrf_socket(vrf, AF_INET, SOCK_RAW, PPTP_PROTO);
        if (gre_fd < 0) {
                syslog(LOG_ERR, "GRE: socket() failed");
                return -1;
        }

        memset(&addr, 0, sizeof(addr));

        addr.sin_family = AF_INET;
        addr.sin_addr = inetaddrs[0];
        addr.sin_port = 0;
        if (bind(gre_fd, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
                syslog(LOG_ERR, "GRE: bind() failed: %s", strerror(errno));
                syslog(LOG_ERR, "GRE: continuing, but may not work if multi-homed");
        }

        addr.sin_family = AF_INET;
        addr.sin_addr = inetaddrs[1];
        addr.sin_port = 0;
        if (connect(gre_fd, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
                syslog(LOG_ERR, "GRE: connect() failed: %s", strerror(errno));
                close(gre_fd);
                return -1;
        }

        gre.seq_sent = 0;
        gre.ack_sent = gre.ack_recv = gre.seq_recv = 0xFFFFFFFF;
        /* seq_recv is -1, therefore next packet expected is seq 0,
           to comply with RFC 2637: 'The sequence number for each
           user session is set to zero at session startup.' */
                                      
        gre.call_id_pair = call_id_pair;               /* network byte order */
        return gre_fd;
}

/* ONE blocking read per call; dispatches all packets possible */
/* returns 0 on success, or <0 on read failure                 */
int decaps_hdlc(int fd, int (*cb) (int cl, void *pack, unsigned len), int cl)
{
        static unsigned char buffer[PACKET_MAX], copy[PACKET_MAX];
        static unsigned start = 0, end = 0;
        static unsigned len = 0, escape = 0;
        static u_int16_t fcs = PPPINITFCS16;
        static unsigned char err = 0;
        unsigned char c;
        int status;

        /* we do one read only, since it may block.  and only if the
         * buffer is empty (start == end)
         */

        if (fd == -1) {
                if(cb == NULL) {
                        /* peek mode */
                        return err ? -1 : 0;
                } else if (!err) {
                        /* re-xmit and nothing queued */
                        syslog(LOG_ERR, "GRE: Re-xmit called with nothing queued");
                        return -1;
                }
        }

        if (!err) {
                /* All known data is processed.  This true unless the last
                 * network write failed.
                 */
                if ((status = read(fd, buffer, sizeof(buffer))) <= 0) {
                        syslog(LOG_ERR, "GRE: read(fd=%d,buffer=%lx,len=%d) from PTY failed: status = %d error = %s%s",
                               fd, (unsigned long) buffer,
                               (int) sizeof(buffer),
                               status, status ? strerror(errno) : "No error",
                               errno != EIO ? "" : ", usually caused by unexpected termination of pppd, check option syntax and pppd logs");
                        /* FAQ: mistakes in pppd option spelling in
                         * /etc/ppp/options.pptpd often cause EIO,
                         * with pppd not reporting the problem to any
                         * logs.  Termination of pppd by signal can
                         * *also* cause this situation. -- James Cameron
                         */
                        return -1;
                }
                end = status;
                start = 0;
        } else {
                /* We're here because of a network write failure.  Try again.
                 * Then do what we would do normally and enter the loop as if
                 * just continuing the while(1).  Not sure that this ever
                 * really happens, but since we error-check status then we
                 * should have the code to handle an error :-)
                 */
                err = 0;
                if ((status = cb(cl, copy, len)) < 0) {
                        syslog(LOG_ERR, "GRE: re-xmit failed from decaps_hdlc: %s", strerror(errno));
                        err = 1;
                        return status;  /* return error */
                }
                /* Great!  Let's do more! */
                fcs = PPPINITFCS16;
                len = 0;
                escape = 0;
        }

        while (1) {
                /* Infinite loop, we return when we're out of data */

                /* Check if out of data */
                if (start == end)
                        return 0;

                /* Add to the packet up till the next HDLC_FLAG (start/end of
                 * packet marker).  Copy to 'copy', un-escape and checksum as we go.
                 */
                while (buffer[start] != HDLC_FLAG) {

                        /* Dispose of 'too long' packets */
                        if (len >= PACKET_MAX) {
                                syslog(LOG_ERR, "GRE: Received too long packet from pppd.");
                                while (buffer[start] != HDLC_FLAG && start < end)
                                        start++;
                                if (start < end) {
                                        goto newpacket;
                                } else
                                        return 0;
                        }
                        /* Read a character, un-escaping if needed */
                        if (buffer[start] == HDLC_ESCAPE && !escape)
                                escape = 1;
                        else {
                                if (escape) {
                                        copy[len] = c = buffer[start] ^ 0x20;
                                        escape = 0;
                                } else
                                        copy[len] = c = buffer[start];
                                fcs = (fcs >> 8) ^ fcstab[(fcs ^ c) & 0xff];
                                len++;
                        }
                        start++;

                        /* Check if out of data */
                        if (start == end)
                                return 0;
                }

                /* Found flag.  Skip past it */
                start++;

                /* Check for over-short packets and silently discard, as per RFC1662 */
                if ((len < 4) || (escape == 1)) {
                        /* len == 0 is possible, we generate it :-) [using HDLC_ESCAPE at
                         * start and end of packet].  Others are worth recording.
                         */
                        if (len && len < 4)
                                syslog(LOG_ERR, "GRE: Received too short packet from pppd.");
                        if (escape)
                                syslog(LOG_ERR, "GRE: Received bad packet from pppd.");
                        goto newpacket;
                }
                /* Check, then remove the 16-bit FCS checksum field */
                if (fcs != PPPGOODFCS16) {
                        syslog(LOG_ERR, "GRE: Bad checksum from pppd.");
                        goto newpacket;
                }
                len -= sizeof(u_int16_t);

                /* So now we have a packet of length 'len' in 'copy' */
                if ((status = cb(cl, copy, len)) < 0) {
                        syslog(LOG_ERR, "GRE: xmit failed from decaps_hdlc: %s", strerror(errno));
                        err = 1;
                        return status;  /* return error */
                }
              newpacket:
                /* Great!  Let's do more! */
                fcs = PPPINITFCS16;
                len = 0;
                escape = 0;
        }
}

#define seq_greater(A,B) ((A)>(B) || \
                         (((u_int32_t)(A)<0xff) && ((~((u_int32_t)(B)))<0xff)))

/* Macro used in encaps_hdlc().  add "val" to "dest" at position "pos",
 * incrementing "pos" to point after the added value.  set "tmp" to "val"
 * as a side-effect.
 */
#define ADD_CHAR(dest, pos, val, tmp) \
        tmp = (val); \
        if ((tmp<0x20) || (tmp==HDLC_FLAG) || (tmp==HDLC_ESCAPE)) { \
                dest[pos++]=HDLC_ESCAPE; \
                dest[pos++]=tmp^0x20; \
        } else \
                dest[pos++]=tmp

/* Make stripped packet into HDLC packet */
int encaps_hdlc(int fd, void *pack, unsigned len)
{
        unsigned char *source = (unsigned char *) pack;
        /* largest expansion possible - double all + double fcs + 2 flags */
        static unsigned char dest[2 * PACKET_MAX + 6];
        unsigned pos = 1, i;
        u_int16_t fcs;
        unsigned char c;

        fcs = PPPINITFCS16;

        /* make sure overflow is impossible so we don't have to bounds check
         * in loop.  drop large packets.
         */
        if (len > PACKET_MAX) {
                syslog(LOG_ERR, "GRE: Asked to encapsulate too large packet (len = %d)", len);
                return -1;
        }
        /* start character */
        dest[0] = HDLC_FLAG;

        /* escape the payload */
        for (i = 0; i < len; i++) {
                ADD_CHAR(dest, pos, source[i], c);
                fcs = (fcs >> 8) ^ fcstab[(fcs ^ c) & 0xff];
        }

        fcs ^= 0xFFFF;

        ADD_CHAR(dest, pos, fcs & 0xFF, c);
        ADD_CHAR(dest, pos, fcs >> 8, c);

        /* tack on the end-flag */
        dest[pos++] = HDLC_FLAG;

        /* now write this packet */
        return write(fd, dest, pos);
}

#undef ADD_CHAR


static int dequeue_gre (callback_t callback, int cl)
{
        pqueue_t *head;
        int status;
        /* process packets in the queue that either are expected or
           have timed out. */
        head = pqueue_head();
        while ( head != NULL &&
                ( (head->seq == gre.seq_recv + 1) || /* wrap-around safe */
                  (pqueue_expiry_time(head) <= 0)
                  )
                ) {
                /* if it is timed out... */
                if (head->seq != gre.seq_recv + 1 ) {  /* wrap-around safe */
                        stats.rx_lost += head->seq - gre.seq_recv - 1;
                        if (pptpctrl_debug)
                                syslog(LOG_DEBUG, 
                                       "GRE: timeout waiting for %d packets", 
                                       head->seq - gre.seq_recv - 1);        
                }
                if (pptpctrl_debug)
                        syslog(LOG_DEBUG, "GRE: accepting #%d from queue", 
                               head->seq);
                gre.seq_recv = head->seq;
                status = callback(cl, head->packet, head->packlen);
                pqueue_del(head);
                if (status < 0) return status;
                head = pqueue_head();
        }
        return 0;
}


int decaps_gre(int fd, int (*cb) (int cl, void *pack, unsigned len), int cl)
{
        static unsigned char buffer[PACKET_MAX + 64 /*ip header */ ];
        struct pptp_gre_header *header;
        int status, ip_len = 0;

        dequeue_gre(cb, cl);
        if ((status = read(fd, buffer, sizeof(buffer))) <= 0) {
                syslog(LOG_ERR, "GRE: read(fd=%d,buffer=%lx,len=%d) from network failed: status = %d error = %s",
                       fd, (unsigned long) buffer, (int) sizeof(buffer),
                       status, status ? strerror(errno) : "No error");
                stats.rx_errors++;
                return -1;
        }
        /* strip off IP header, if present */
        if ((buffer[0] & 0xF0) == 0x40)
                ip_len = (buffer[0] & 0xF) * 4;
        header = (struct pptp_gre_header *) (buffer + ip_len);

        /* verify packet (else discard) */
        if (((ntoh8(header->ver) & 0x7F) != PPTP_GRE_VER) ||    /* version should be 1   */
            (ntoh16(header->protocol) != PPTP_GRE_PROTO) ||     /* GRE protocol for PPTP */
            PPTP_GRE_IS_C(ntoh8(header->flags)) ||      /* flag C should be clear   */
            PPTP_GRE_IS_R(ntoh8(header->flags)) ||      /* flag R should be clear   */
            (!PPTP_GRE_IS_K(ntoh8(header->flags))) ||   /* flag K should be set     */
            ((ntoh8(header->flags) & 0xF) != 0)) {      /* routing and recursion ctrl = 0  */
                /* if invalid, discard this packet */
                syslog(LOG_ERR, "GRE: Discarding packet by header check");
                stats.rx_invalid++;
                return 0;
        }
        if (header->call_id != GET_VALUE(PAC, gre.call_id_pair)) {
                /*
                 * Discard silently to allow more than one GRE tunnel from
                 * the same IP address in case clients are behind the
                 * firewall.
                 *
                 * syslog(LOG_ERR, "GRE: Discarding for incorrect call");
                 */
                return 0;
        }
        if (PPTP_GRE_IS_A(ntoh8(header->ver))) {        /* acknowledgement present */
                u_int32_t ack = (PPTP_GRE_IS_S(ntoh8(header->flags))) ?
                        ntoh32(header->ack) : ntoh32(header->seq);
                        /* ack in different place if S=0 */

                if (seq_greater(ack, gre.ack_recv))
                        gre.ack_recv = ack;

                /* also handle sequence number wrap-around  */
                if (WRAPPED(ack,gre.ack_recv)) gre.ack_recv = ack;
                if (gre.ack_recv == stats.pt.seq) {
                        int rtt = time_now_usecs() - stats.pt.time;
                        stats.rtt = (stats.rtt + rtt) / 2;
                }
        }
        if (PPTP_GRE_IS_S(ntoh8(header->flags))) {      /* payload present */
                unsigned headersize = sizeof(*header);
                unsigned payload_len = ntoh16(header->payload_len);
                u_int32_t seq = ntoh32(header->seq);

                if (!PPTP_GRE_IS_A(ntoh8(header->ver)))
                        headersize -= sizeof(header->ack);
                /* check for incomplete packet (length smaller than expected) */
                if (status - headersize < payload_len) {
                        stats.rx_truncated++;
                        return 0;
                }
                /* check for out-of-order sequence number
                 * N.B.: some client implementations violate RFC 2637
                 * and start their sequence numbers at 1 instead of 0,
                 * so we have to introduce a kludge to deal with it. 
                 * on wrap we may allow an out of order packet to pass
                 */
                if (seq == gre.seq_recv + 1 || seq == 1) {
                        if (pptpctrl_debug)
                                syslog(LOG_DEBUG, "GRE: accepting packet #%d", 
                                       seq);
                        stats.rx_accepted++;
                        gre.seq_recv = seq;
                        return cb(cl, buffer + ip_len + headersize, payload_len);
                } else if (!seq_greater(seq, gre.seq_recv)) {
                        if (pptpctrl_debug)
                                syslog(LOG_DEBUG,
                                       "GRE: discarding duplicate or old packet #%d (expecting #%d)", 
                                       seq, gre.seq_recv + 1);
                        return 0;       /* discard duplicate packets */
                } else {
                        stats.rx_buffered++;
                        if (pptpctrl_debug)
                                syslog(LOG_DEBUG,
                                       "GRE: buffering packet #%d (expecting #%d, lost or reordered)",
                                       seq, gre.seq_recv + 1);
                        pqueue_add(seq, buffer + ip_len + headersize, payload_len);
                        return 0;       /* discard out-of-order packets */
                }
        }
        return 0;               /* ack, but no payload */
}


int encaps_gre(int fd, void *pack, unsigned len)
{
        static union {
                struct pptp_gre_header header;
                unsigned char buffer[PACKET_MAX + sizeof(struct pptp_gre_header)];
        } u;
        unsigned header_len;
        ssize_t status;
#ifdef HAVE_WRITEV
        struct iovec iovec[2];
#endif

        if(fd == -1)
                /* peek mode */
                return (gre.ack_sent == gre.seq_recv) ? 0 : -1;

        /* package this up in a GRE shell. */
        u.header.flags = hton8(PPTP_GRE_FLAG_K);
        u.header.ver = hton8(PPTP_GRE_VER);
        u.header.protocol = hton16(PPTP_GRE_PROTO);
        u.header.payload_len = hton16(len);
        u.header.call_id = GET_VALUE(PNS, gre.call_id_pair);

        /* special case ACK with no payload */
        if (pack == NULL) {
                if (gre.ack_sent != gre.seq_recv) {
                        u.header.ver |= hton8(PPTP_GRE_FLAG_A);
                        u.header.payload_len = hton16(0);
                        u.header.seq = hton32(gre.seq_recv);    /* ack is in odd place because S=0 */
                        gre.ack_sent = gre.seq_recv;
                        /* don't sent ACK field, ACK is in SYN field */
                        return write(fd, u.buffer, sizeof(u.header) - sizeof(u.header.ack));
                } else
                        return 0;       /* we don't need to send ACK */
        }
        /* send packet with payload */
        u.header.flags |= hton8(PPTP_GRE_FLAG_S);
        u.header.seq = hton32(gre.seq_sent);
        gre.seq_sent++;
        if (gre.ack_sent != gre.seq_recv) {     /* send ack with this message */
                u.header.ver |= hton8(PPTP_GRE_FLAG_A);
                u.header.ack = hton32(gre.seq_recv);
                gre.ack_sent = gre.seq_recv;
                header_len = sizeof(u.header);
        } else {        /* don't send ack */
                header_len = sizeof(u.header) - sizeof(u.header.ack);
        }
        if (len > PACKET_MAX) {
                syslog(LOG_ERR, "GRE: packet is too large %d", len);
                stats.tx_oversize++;
                return 0;       /* drop this, it's too big */
        }
#ifdef HAVE_WRITEV
        /* write header and buffer without copying. */
        iovec[0].iov_base = u.buffer;
        iovec[0].iov_len = header_len;
        iovec[1].iov_base = pack;
        iovec[1].iov_len = len;
        status = writev(fd, iovec, 2);
#else
        /* copy payload into buffer */
        memcpy(u.buffer + header_len, pack, len);
        /* record and increment sequence numbers */
        /* write this baby out to the net */
        status = write(fd, u.buffer, header_len + len);
#endif
        /* if ENOBUFS, do not close the connection */
        if ((status < 0) && (errno == ENOBUFS)) {
                gre.seq_sent--;
                status = 0;
        }
        return status;
}