src/tftp.c

/* dnsmasq is Copyright (c) 2000-2020 Simon Kelley

   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; version 2 dated June, 1991, or
   (at your option) version 3 dated 29 June, 2007.
 
   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/>.
*/

#include "dnsmasq.h"

#ifdef HAVE_TFTP

static void handle_tftp(time_t now, struct tftp_transfer *transfer, ssize_t len);
static struct tftp_file *check_tftp_fileperm(ssize_t *len, char *prefix);
static void free_transfer(struct tftp_transfer *transfer);
static ssize_t tftp_err(int err, char *packet, char *message, char *file);
static ssize_t tftp_err_oops(char *packet, char *file);
static ssize_t get_block(char *packet, struct tftp_transfer *transfer);
static char *next(char **p, char *end);
static void sanitise(char *buf);

#define OP_RRQ  1
#define OP_WRQ  2
#define OP_DATA 3
#define OP_ACK  4
#define OP_ERR  5
#define OP_OACK 6

#define ERR_NOTDEF 0
#define ERR_FNF    1
#define ERR_PERM   2
#define ERR_FULL   3
#define ERR_ILL    4

void tftp_request(struct listener *listen, time_t now)
{
  ssize_t len;
  char *packet = daemon->packet;
  char *filename, *mode, *p, *end, *opt;
  union mysockaddr addr, peer;
  struct msghdr msg;
  struct iovec iov;
  struct ifreq ifr;
  int is_err = 1, if_index = 0, mtu = 0;
  struct iname *tmp;
  struct tftp_transfer *transfer = NULL, **up;
  int port = daemon->start_tftp_port; /* may be zero to use ephemeral port */
#if defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_DONT)
  int mtuflag = IP_PMTUDISC_DONT;
#endif
  char namebuff[IF_NAMESIZE];
  char *name = NULL;
  char *prefix = daemon->tftp_prefix;
  struct tftp_prefix *pref;
  union all_addr addra;
  /* Can always get recvd interface for IPv6 */
  int check_dest = !option_bool(OPT_NOWILD) || listen->family == AF_INET6;
  union {
    struct cmsghdr align; /* this ensures alignment */
    char control6[CMSG_SPACE(sizeof(struct in6_pktinfo))];
#if defined(HAVE_LINUX_NETWORK)
    char control[CMSG_SPACE(sizeof(struct in_pktinfo))];
#elif defined(HAVE_SOLARIS_NETWORK)
    char control[CMSG_SPACE(sizeof(struct in_addr)) +
		 CMSG_SPACE(sizeof(unsigned int))];
#elif defined(IP_RECVDSTADDR) && defined(IP_RECVIF)
    char control[CMSG_SPACE(sizeof(struct in_addr)) +
		 CMSG_SPACE(sizeof(struct sockaddr_dl))];
#endif
  } control_u; 

  msg.msg_controllen = sizeof(control_u);
  msg.msg_control = control_u.control;
  msg.msg_flags = 0;
  msg.msg_name = &peer;
  msg.msg_namelen = sizeof(peer);
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;

  iov.iov_base = packet;
  iov.iov_len = daemon->packet_buff_sz;

  /* we overwrote the buffer... */
  daemon->srv_save = NULL;

  if ((len = recvmsg(listen->tftpfd, &msg, 0)) < 2)
    return;

  /* Can always get recvd interface for IPv6 */
  if (!check_dest)
    {
      if (listen->iface)
	{
	  addr = listen->iface->addr;
	  name = listen->iface->name;
	  mtu = listen->iface->mtu;
	  if (daemon->tftp_mtu != 0 && daemon->tftp_mtu < mtu)
	    mtu = daemon->tftp_mtu;
	}
      else
	{
	  /* we're listening on an address that doesn't appear on an interface,
	     ask the kernel what the socket is bound to */
	  socklen_t tcp_len = sizeof(union mysockaddr);
	  if (getsockname(listen->tftpfd, (struct sockaddr *)&addr, &tcp_len) == -1)
	    return;
	}
    }
  else
    {
      struct cmsghdr *cmptr;

      if (msg.msg_controllen < sizeof(struct cmsghdr))
        return;
      
      addr.sa.sa_family = listen->family;
      
#if defined(HAVE_LINUX_NETWORK)
      if (listen->family == AF_INET)
	for (cmptr = CMSG_FIRSTHDR(&msg); cmptr; cmptr = CMSG_NXTHDR(&msg, cmptr))
	  if (cmptr->cmsg_level == IPPROTO_IP && cmptr->cmsg_type == IP_PKTINFO)
	    {
	      union {
		unsigned char *c;
		struct in_pktinfo *p;
	      } p;
	      p.c = CMSG_DATA(cmptr);
	      addr.in.sin_addr = p.p->ipi_spec_dst;
	      if_index = p.p->ipi_ifindex;
	    }
      
#elif defined(HAVE_SOLARIS_NETWORK)
      if (listen->family == AF_INET)
	for (cmptr = CMSG_FIRSTHDR(&msg); cmptr; cmptr = CMSG_NXTHDR(&msg, cmptr))
	  {
	    union {
	      unsigned char *c;
	      struct in_addr *a;
	      unsigned int *i;
	    } p;
	    p.c = CMSG_DATA(cmptr);
	    if (cmptr->cmsg_level == IPPROTO_IP && cmptr->cmsg_type == IP_RECVDSTADDR)
	    addr.in.sin_addr = *(p.a);
	    else if (cmptr->cmsg_level == IPPROTO_IP && cmptr->cmsg_type == IP_RECVIF)
	    if_index = *(p.i);
	  }
      
#elif defined(IP_RECVDSTADDR) && defined(IP_RECVIF)
      if (listen->family == AF_INET)
	for (cmptr = CMSG_FIRSTHDR(&msg); cmptr; cmptr = CMSG_NXTHDR(&msg, cmptr))
	  {
	    union {
	      unsigned char *c;
	      struct in_addr *a;
	      struct sockaddr_dl *s;
	    } p;
	    p.c = CMSG_DATA(cmptr);
	    if (cmptr->cmsg_level == IPPROTO_IP && cmptr->cmsg_type == IP_RECVDSTADDR)
	      addr.in.sin_addr = *(p.a);
	    else if (cmptr->cmsg_level == IPPROTO_IP && cmptr->cmsg_type == IP_RECVIF)
	      if_index = p.s->sdl_index;
	  }
	  
#endif

      if (listen->family == AF_INET6)
        {
          for (cmptr = CMSG_FIRSTHDR(&msg); cmptr; cmptr = CMSG_NXTHDR(&msg, cmptr))
            if (cmptr->cmsg_level == IPPROTO_IPV6 && cmptr->cmsg_type == daemon->v6pktinfo)
              {
                union {
                  unsigned char *c;
                  struct in6_pktinfo *p;
                } p;
                p.c = CMSG_DATA(cmptr);
                  
                addr.in6.sin6_addr = p.p->ipi6_addr;
                if_index = p.p->ipi6_ifindex;
              }
        }
      
      if (!indextoname(listen->tftpfd, if_index, namebuff))
	return;

      name = namebuff;
      
      addra.addr4 = addr.in.sin_addr;

      if (listen->family == AF_INET6)
	addra.addr6 = addr.in6.sin6_addr;

      if (daemon->tftp_interfaces)
	{
	  /* dedicated tftp interface list */
	  for (tmp = daemon->tftp_interfaces; tmp; tmp = tmp->next)
	    if (tmp->name && wildcard_match(tmp->name, name))
	      break;

	  if (!tmp)
	    return;
	}
      else
	{
	  /* Do the same as DHCP */
	  if (!iface_check(listen->family, &addra, name, NULL))
	    {
	      if (!option_bool(OPT_CLEVERBIND))
		enumerate_interfaces(0); 
	      if (!loopback_exception(listen->tftpfd, listen->family, &addra, name) &&
		  !label_exception(if_index, listen->family, &addra))
		return;
	    }
	  
#ifdef HAVE_DHCP      
	  /* allowed interfaces are the same as for DHCP */
	  for (tmp = daemon->dhcp_except; tmp; tmp = tmp->next)
	    if (tmp->name && wildcard_match(tmp->name, name))
	      return;
#endif
	}

      safe_strncpy(ifr.ifr_name, name, IF_NAMESIZE);
      if (ioctl(listen->tftpfd, SIOCGIFMTU, &ifr) != -1)
	{
	  mtu = ifr.ifr_mtu;  
	  if (daemon->tftp_mtu != 0 && daemon->tftp_mtu < mtu)
	    mtu = daemon->tftp_mtu;    
	}
    }

  /* Failed to get interface mtu - can use configured value. */
  if (mtu == 0)
    mtu = daemon->tftp_mtu;

  /* data transfer via server listening socket */
  if (option_bool(OPT_SINGLE_PORT))
    {
      int tftp_cnt;

      for (tftp_cnt = 0, transfer = daemon->tftp_trans, up = &daemon->tftp_trans; transfer; up = &transfer->next, transfer = transfer->next)
	{
	  tftp_cnt++;

	  if (sockaddr_isequal(&peer, &transfer->peer))
	    {
	      if (ntohs(*((unsigned short *)packet)) == OP_RRQ)
		{
		  /* Handle repeated RRQ or abandoned transfer from same host and port 
		     by unlinking and reusing the struct transfer. */
		  *up = transfer->next;
		  break;
		}
	      else
		{
		  handle_tftp(now, transfer, len);
		  return;
		}
	    }
	}
      
      /* Enforce simultaneous transfer limit. In non-single-port mode
	 this is doene by not listening on the server socket when
	 too many transfers are in progress. */
      if (!transfer && tftp_cnt >= daemon->tftp_max)
	return;
    }
  
  if (name)
    {
      /* check for per-interface prefix */ 
      for (pref = daemon->if_prefix; pref; pref = pref->next)
	if (strcmp(pref->interface, name) == 0)
	  prefix = pref->prefix;  
    }

  if (listen->family == AF_INET)
    {
      addr.in.sin_port = htons(port);
#ifdef HAVE_SOCKADDR_SA_LEN
      addr.in.sin_len = sizeof(addr.in);
#endif
    }
  else
    {
      addr.in6.sin6_port = htons(port);
      addr.in6.sin6_flowinfo = 0;
      addr.in6.sin6_scope_id = 0;
#ifdef HAVE_SOCKADDR_SA_LEN
      addr.in6.sin6_len = sizeof(addr.in6);
#endif
    }

  /* May reuse struct transfer from abandoned transfer in single port mode. */
  if (!transfer && !(transfer = whine_malloc(sizeof(struct tftp_transfer))))
    return;
  
  if (option_bool(OPT_SINGLE_PORT))
    transfer->sockfd = listen->tftpfd;
  else if ((transfer->sockfd = socket(listen->family, SOCK_DGRAM, 0)) == -1)
    {
      free(transfer);
      return;
    }
  
  transfer->peer = peer;
  transfer->source = addra;
  transfer->if_index = if_index;
  transfer->timeout = now + 2;
  transfer->backoff = 1;
  transfer->block = 1;
  transfer->blocksize = 512;
  transfer->offset = 0;
  transfer->file = NULL;
  transfer->opt_blocksize = transfer->opt_transize = 0;
  transfer->netascii = transfer->carrylf = 0;
 
  prettyprint_addr(&peer, daemon->addrbuff);
  
  /* if we have a nailed-down range, iterate until we find a free one. */
  while (!option_bool(OPT_SINGLE_PORT))
    {
      if (bind(transfer->sockfd, &addr.sa, sa_len(&addr)) == -1 ||
#if defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_DONT)
	  setsockopt(transfer->sockfd, IPPROTO_IP, IP_MTU_DISCOVER, &mtuflag, sizeof(mtuflag)) == -1 ||
#endif
	  !fix_fd(transfer->sockfd))
	{
	  if (errno == EADDRINUSE && daemon->start_tftp_port != 0)
	    {
	      if (++port <= daemon->end_tftp_port)
		{ 
		  if (listen->family == AF_INET)
		    addr.in.sin_port = htons(port);
		  else
		    addr.in6.sin6_port = htons(port);
		  
		  continue;
		}
	      my_syslog(MS_TFTP | LOG_ERR, _("unable to get free port for TFTP"));
	    }
	  free_transfer(transfer);
	  return;
	}
      break;
    }
  
  p = packet + 2;
  end = packet + len;
  
  if (ntohs(*((unsigned short *)packet)) != OP_RRQ ||
      !(filename = next(&p, end)) ||
      !(mode = next(&p, end)) ||
      (strcasecmp(mode, "octet") != 0 && strcasecmp(mode, "netascii") != 0))
    {
      len = tftp_err(ERR_ILL, packet, _("unsupported request from %s"), daemon->addrbuff);
      is_err = 1;
    }
  else
    {
      if (strcasecmp(mode, "netascii") == 0)
	transfer->netascii = 1;
      
      while ((opt = next(&p, end)))
	{
	  if (strcasecmp(opt, "blksize") == 0)
	    {
	      if ((opt = next(&p, end)) && !option_bool(OPT_TFTP_NOBLOCK))
		{
		  /* 32 bytes for IP, UDP and TFTP headers, 52 bytes for IPv6 */
		  int overhead = (listen->family == AF_INET) ? 32 : 52;
		  transfer->blocksize = atoi(opt);
		  if (transfer->blocksize < 1)
		    transfer->blocksize = 1;
		  if (transfer->blocksize > (unsigned)daemon->packet_buff_sz - 4)
		    transfer->blocksize = (unsigned)daemon->packet_buff_sz - 4;
		  if (mtu != 0 && transfer->blocksize > (unsigned)mtu - overhead)
		    transfer->blocksize = (unsigned)mtu - overhead;
		  transfer->opt_blocksize = 1;
		  transfer->block = 0;
		}
	    }
	  else if (strcasecmp(opt, "tsize") == 0 && next(&p, end) && !transfer->netascii)
	    {
	      transfer->opt_transize = 1;
	      transfer->block = 0;
	    }
	}

      /* cope with backslashes from windows boxen. */
      for (p = filename; *p; p++)
	if (*p == '\\')
	  *p = '/';
	else if (option_bool(OPT_TFTP_LC))
	  *p = tolower(*p);
		
      strcpy(daemon->namebuff, "/");
      if (prefix)
	{
	  if (prefix[0] == '/')
	    daemon->namebuff[0] = 0;
	  strncat(daemon->namebuff, prefix, (MAXDNAME-1) - strlen(daemon->namebuff));
	  if (prefix[strlen(prefix)-1] != '/')
	    strncat(daemon->namebuff, "/", (MAXDNAME-1) - strlen(daemon->namebuff));

	  if (option_bool(OPT_TFTP_APREF_IP))
	    {
	      size_t oldlen = strlen(daemon->namebuff);
	      struct stat statbuf;
	      
	      strncat(daemon->namebuff, daemon->addrbuff, (MAXDNAME-1) - strlen(daemon->namebuff));
	      strncat(daemon->namebuff, "/", (MAXDNAME-1) - strlen(daemon->namebuff));
	      
	      /* remove unique-directory if it doesn't exist */
	      if (stat(daemon->namebuff, &statbuf) == -1 || !S_ISDIR(statbuf.st_mode))
		daemon->namebuff[oldlen] = 0;
	    }
	  
	  if (option_bool(OPT_TFTP_APREF_MAC))
	    {
	      unsigned char *macaddr = NULL;
	      unsigned char macbuf[DHCP_CHADDR_MAX];
	      
#ifdef HAVE_DHCP
	      if (daemon->dhcp && peer.sa.sa_family == AF_INET)
	        {
		  /* Check if the client IP is in our lease database */
		  struct dhcp_lease *lease = lease_find_by_addr(peer.in.sin_addr);
		  if (lease && lease->hwaddr_type == ARPHRD_ETHER && lease->hwaddr_len == ETHER_ADDR_LEN)
		    macaddr = lease->hwaddr;
		}
#endif
	      
	      /* If no luck, try to find in ARP table. This only works if client is in same (V)LAN */
	      if (!macaddr && find_mac(&peer, macbuf, 1, now) > 0)
		macaddr = macbuf;
	      
	      if (macaddr)
	        {
		  size_t oldlen = strlen(daemon->namebuff);
		  struct stat statbuf;

		  snprintf(daemon->namebuff + oldlen, (MAXDNAME-1) - oldlen, "%.2x-%.2x-%.2x-%.2x-%.2x-%.2x/",
			   macaddr[0], macaddr[1], macaddr[2], macaddr[3], macaddr[4], macaddr[5]);
		  
		  /* remove unique-directory if it doesn't exist */
		  if (stat(daemon->namebuff, &statbuf) == -1 || !S_ISDIR(statbuf.st_mode))
		    daemon->namebuff[oldlen] = 0;
		}
	    }
	  
	  /* Absolute pathnames OK if they match prefix */
	  if (filename[0] == '/')
	    {
	      if (strstr(filename, daemon->namebuff) == filename)
		daemon->namebuff[0] = 0;
	      else
		filename++;
	    }
	}
      else if (filename[0] == '/')
	daemon->namebuff[0] = 0;
      strncat(daemon->namebuff, filename, (MAXDNAME-1) - strlen(daemon->namebuff));
      
      /* check permissions and open file */
      if ((transfer->file = check_tftp_fileperm(&len, prefix)))
	{
	  if ((len = get_block(packet, transfer)) == -1)
	    len = tftp_err_oops(packet, daemon->namebuff);
	  else
	    is_err = 0;
	}
    }

  send_from(transfer->sockfd, !option_bool(OPT_SINGLE_PORT), packet, len, &peer, &addra, if_index);
  
  if (is_err)
    free_transfer(transfer);
  else
    {
      transfer->next = daemon->tftp_trans;
      daemon->tftp_trans = transfer;
    }
}
 
static struct tftp_file *check_tftp_fileperm(ssize_t *len, char *prefix)
{
  char *packet = daemon->packet, *namebuff = daemon->namebuff;
  struct tftp_file *file;
  struct tftp_transfer *t;
  uid_t uid = geteuid();
  struct stat statbuf;
  int fd = -1;

  /* trick to ban moving out of the subtree */
  if (prefix && strstr(namebuff, "/../"))
    goto perm;
  
  if ((fd = open(namebuff, O_RDONLY)) == -1)
    {
      if (errno == ENOENT)
	{
	  *len = tftp_err(ERR_FNF, packet, _("file %s not found"), namebuff);
	  return NULL;
	}
      else if (errno == EACCES)
	goto perm;
      else
	goto oops;
    }
  
  /* stat the file descriptor to avoid stat->open races */
  if (fstat(fd, &statbuf) == -1)
    goto oops;
  
  /* running as root, must be world-readable */
  if (uid == 0)
    {
      if (!(statbuf.st_mode & S_IROTH))
	goto perm;
    }
  /* in secure mode, must be owned by user running dnsmasq */
  else if (option_bool(OPT_TFTP_SECURE) && uid != statbuf.st_uid)
    goto perm;
      
  /* If we're doing many transfers from the same file, only 
     open it once this saves lots of file descriptors 
     when mass-booting a big cluster, for instance. 
     Be conservative and only share when inode and name match
     this keeps error messages sane. */
  for (t = daemon->tftp_trans; t; t = t->next)
    if (t->file->dev == statbuf.st_dev && 
	t->file->inode == statbuf.st_ino &&
	strcmp(t->file->filename, namebuff) == 0)
      {
	close(fd);
	t->file->refcount++;
	return t->file;
      }
  
  if (!(file = whine_malloc(sizeof(struct tftp_file) + strlen(namebuff) + 1)))
    {
      errno = ENOMEM;
      goto oops;
    }

  file->fd = fd;
  file->size = statbuf.st_size;
  file->dev = statbuf.st_dev;
  file->inode = statbuf.st_ino;
  file->refcount = 1;
  strcpy(file->filename, namebuff);
  return file;
  
 perm:
  errno = EACCES;
  *len =  tftp_err(ERR_PERM, packet, _("cannot access %s: %s"), namebuff);
  if (fd != -1)
    close(fd);
  return NULL;

 oops:
  *len =  tftp_err_oops(packet, namebuff);
  if (fd != -1)
    close(fd);
  return NULL;
}

void check_tftp_listeners(time_t now)
{
  struct tftp_transfer *transfer, *tmp, **up;
  
  /* In single port mode, all packets come via port 69 and tftp_request() */
  if (!option_bool(OPT_SINGLE_PORT))
    for (transfer = daemon->tftp_trans; transfer; transfer = transfer->next)
      if (poll_check(transfer->sockfd, POLLIN))
	{
	  /* we overwrote the buffer... */
	  daemon->srv_save = NULL;
	  handle_tftp(now, transfer, recv(transfer->sockfd, daemon->packet, daemon->packet_buff_sz, 0));
	}

  for (transfer = daemon->tftp_trans, up = &daemon->tftp_trans; transfer; transfer = tmp)
    {
      tmp = transfer->next;
      
      if (difftime(now, transfer->timeout) >= 0.0)
	{
	  int endcon = 0;
	  ssize_t len;

	  /* timeout, retransmit */
	  transfer->timeout += 1 + (1<<(transfer->backoff/2));
	  	  
	  /* we overwrote the buffer... */
	  daemon->srv_save = NULL;
	 
	  if ((len = get_block(daemon->packet, transfer)) == -1)
	    {
	      len = tftp_err_oops(daemon->packet, transfer->file->filename);
	      endcon = 1;
	    }
	  else if (++transfer->backoff > 7)
	    {
	      /* don't complain about timeout when we're awaiting the last
		 ACK, some clients never send it */
	      if ((unsigned)len == transfer->blocksize + 4)
		endcon = 1;
	      len = 0;
	    }

	  if (len != 0)
	    send_from(transfer->sockfd, !option_bool(OPT_SINGLE_PORT), daemon->packet, len,
		      &transfer->peer, &transfer->source, transfer->if_index);
	  	  
	  if (endcon || len == 0)
	    {
	      strcpy(daemon->namebuff, transfer->file->filename);
	      sanitise(daemon->namebuff);
	      prettyprint_addr(&transfer->peer, daemon->addrbuff);
	      my_syslog(MS_TFTP | LOG_INFO, endcon ? _("failed sending %s to %s") : _("sent %s to %s"), daemon->namebuff, daemon->addrbuff);
	      /* unlink */
	      *up = tmp;
	      if (endcon)
		free_transfer(transfer);
	      else
		{
		  /* put on queue to be sent to script and deleted */
		  transfer->next = daemon->tftp_done_trans;
		  daemon->tftp_done_trans = transfer;
		}
	      continue;
	    }
	}

      up = &transfer->next;
    }    
}
	  
/* packet in daemon->packet as this is called. */
static void handle_tftp(time_t now, struct tftp_transfer *transfer, ssize_t len)
{
  struct ack {
    unsigned short op, block;
  } *mess = (struct ack *)daemon->packet;
  
  if (len >= (ssize_t)sizeof(struct ack))
    {
      if (ntohs(mess->op) == OP_ACK && ntohs(mess->block) == (unsigned short)transfer->block) 
	{
	  /* Got ack, ensure we take the (re)transmit path */
	  transfer->timeout = now;
	  transfer->backoff = 0;
	  if (transfer->block++ != 0)
	    transfer->offset += transfer->blocksize - transfer->expansion;
	}
      else if (ntohs(mess->op) == OP_ERR)
	{
	  char *p = daemon->packet + sizeof(struct ack);
	  char *end = daemon->packet + len;
	  char *err = next(&p, end);
	  
	  prettyprint_addr(&transfer->peer, daemon->addrbuff);
	  
	  /* Sanitise error message */
	  if (!err)
	    err = "";
	  else
	    sanitise(err);
	  
	  my_syslog(MS_TFTP | LOG_ERR, _("error %d %s received from %s"),
		    (int)ntohs(mess->block), err, 
		    daemon->addrbuff);	
	  
	  /* Got err, ensure we take abort */
	  transfer->timeout = now;
	  transfer->backoff = 100;
	}
    }
}

static void free_transfer(struct tftp_transfer *transfer)
{
  if (!option_bool(OPT_SINGLE_PORT))
    close(transfer->sockfd);

  if (transfer->file && (--transfer->file->refcount) == 0)
    {
      close(transfer->file->fd);
      free(transfer->file);
    }
  
  free(transfer);
}

static char *next(char **p, char *end)
{
  char *ret = *p;
  size_t len;

  if (*(end-1) != 0 || 
      *p == end ||
      (len = strlen(ret)) == 0)
    return NULL;

  *p += len + 1;
  return ret;
}

static void sanitise(char *buf)
{
  unsigned char *q, *r;
  for (q = r = (unsigned char *)buf; *r; r++)
    if (isprint((int)*r))
      *(q++) = *r;
  *q = 0;

}

#define MAXMESSAGE 500 /* limit to make packet < 512 bytes and definitely smaller than buffer */ 
static ssize_t tftp_err(int err, char *packet, char *message, char *file)
{
  struct errmess {
    unsigned short op, err;
    char message[];
  } *mess = (struct errmess *)packet;
  ssize_t len, ret = 4;
  char *errstr = strerror(errno);
  
  memset(packet, 0, daemon->packet_buff_sz);
  sanitise(file);
  
  mess->op = htons(OP_ERR);
  mess->err = htons(err);
  len = snprintf(mess->message, MAXMESSAGE,  message, file, errstr);
  ret += (len < MAXMESSAGE) ? len + 1 : MAXMESSAGE; /* include terminating zero */
  
  my_syslog(MS_TFTP | LOG_ERR, "%s", mess->message);
  
  return  ret;
}

static ssize_t tftp_err_oops(char *packet, char *file)
{
  /* May have >1 refs to file, so potentially mangle a copy of the name */
  strcpy(daemon->namebuff, file);
  return tftp_err(ERR_NOTDEF, packet, _("cannot read %s: %s"), daemon->namebuff);
}

/* return -1 for error, zero for done. */
static ssize_t get_block(char *packet, struct tftp_transfer *transfer)
{
  memset(packet, 0, daemon->packet_buff_sz);
  
  if (transfer->block == 0)
    {
      /* send OACK */
      char *p;
      struct oackmess {
	unsigned short op;
	char data[];
      } *mess = (struct oackmess *)packet;
      
      p = mess->data;
      mess->op = htons(OP_OACK);
      if (transfer->opt_blocksize)
	{
	  p += (sprintf(p, "blksize") + 1);
	  p += (sprintf(p, "%u", transfer->blocksize) + 1);
	}
      if (transfer->opt_transize)
	{
	  p += (sprintf(p,"tsize") + 1);
	  p += (sprintf(p, "%u", (unsigned int)transfer->file->size) + 1);
	}

      return p - packet;
    }
  else
    {
      /* send data packet */
      struct datamess {
	unsigned short op, block;
	unsigned char data[];
      } *mess = (struct datamess *)packet;
      
      size_t size = transfer->file->size - transfer->offset; 
      
      if (transfer->offset > transfer->file->size)
	return 0; /* finished */
      
      if (size > transfer->blocksize)
	size = transfer->blocksize;
      
      mess->op = htons(OP_DATA);
      mess->block = htons((unsigned short)(transfer->block));
      
      if (lseek(transfer->file->fd, transfer->offset, SEEK_SET) == (off_t)-1 ||
	  !read_write(transfer->file->fd, mess->data, size, 1))
	return -1;
      
      transfer->expansion = 0;
      
      /* Map '\n' to CR-LF in netascii mode */
      if (transfer->netascii)
	{
	  size_t i;
	  int newcarrylf;

	  for (i = 0, newcarrylf = 0; i < size; i++)
	    if (mess->data[i] == '\n' && ( i != 0 || !transfer->carrylf))
	      {
		transfer->expansion++;

		if (size != transfer->blocksize)
		  size++; /* room in this block */
		else  if (i == size - 1)
		  newcarrylf = 1; /* don't expand LF again if it moves to the next block */
		  
		/* make space and insert CR */
		memmove(&mess->data[i+1], &mess->data[i], size - (i + 1));
		mess->data[i] = '\r';
		
		i++;
	      }
	  transfer->carrylf = newcarrylf;
	  
	}

      return size + 4;
    }
}


int do_tftp_script_run(void)
{
  struct tftp_transfer *transfer;

  if ((transfer = daemon->tftp_done_trans))
    {
      daemon->tftp_done_trans = transfer->next;
#ifdef HAVE_SCRIPT
      queue_tftp(transfer->file->size, transfer->file->filename, &transfer->peer);
#endif
      free_transfer(transfer);
      return 1;
    }

  return 0;
}
#endif