First version of the TFTP server. Basic functionality is there.

1 files changed, 865 insertions, 0 deletions

diff --git a/tests/server/tftpd.c b/tests/server/tftpd.c
new file mode 100644
index 000000000..f122c9b1e
--- /dev/null
+++ b/tests/server/tftpd.c
@@ -0,0 +1,865 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * $Id$
+ *
+ * Trivial file transfer protocol server.
+ *
+ * This code includes many modifications by Jim Guyton <guyton@rand-unix>
+ *
+ * This source file was started based on netkit-tftpd 0.17
+ * Heavily modified for curl's test suite
+ */
+
+/*
+ * Copyright (c) 1983 Regents of the University of California.
+ * All rights reserved.
+ *
+ * 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. All advertising materials mentioning features or use of this software
+ *    must display the following acknowledgement:
+ *      This product includes software developed by the University of
+ *      California, Berkeley and its contributors.
+ * 4. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``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 REGENTS OR CONTRIBUTORS 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.
+ */
+
+#include "setup.h" /* portability help from the lib directory */
+#include <sys/types.h>
+#include <sys/ioctl.h>
+#include <sys/stat.h>
+#include <signal.h>
+#include <fcntl.h>
+
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/tftp.h>
+#include <netdb.h>
+
+#include <setjmp.h>
+#include <stdio.h>
+#include <errno.h>
+#include <ctype.h>
+#include <string.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <pwd.h>
+#include <grp.h>
+
+#include "getpart.h"
+#include "util.h"
+
+struct testcase {
+  char *buffer;   /* holds the file data to send to the client */
+  size_t bufsize; /* size of the data in buffer */
+  char *rptr;     /* read pointer into the buffer */
+  size_t rcount;  /* amount of data left to read of the file */
+  long num;       /* test case number */
+  int ofile;      /* file descriptor for output file when uploading to us */
+  FILE *server;   /* write input "protocol" there for client verification */
+};
+
+static int synchnet(int);
+static struct tftphdr *r_init(void);
+static struct tftphdr *w_init(void);
+static int readit(struct testcase *test, struct tftphdr **dpp, int convert);
+static int writeit(struct testcase *test, struct tftphdr **dpp, int ct,
+                   int convert);
+static void mysignal(int, void (*func)(int));
+
+
+#define TIMEOUT         5
+
+#define PKTSIZE SEGSIZE+4
+
+struct formats;
+static int tftp(struct testcase *test, struct tftphdr *tp, int size);
+static void nak(int error);
+static void sendfile(struct testcase *test, struct formats *pf);
+static void recvfile(struct testcase *test, struct formats *pf);
+static int validate_access(struct testcase *test, const char *, int);
+
+static curl_socket_t peer;
+static int rexmtval = TIMEOUT;
+static int maxtimeout = 5*TIMEOUT;
+
+static char buf[PKTSIZE];
+static char ackbuf[PKTSIZE];
+static struct sockaddr_in from;
+static socklen_t fromlen;
+
+struct bf {
+  int counter;            /* size of data in buffer, or flag */
+  char buf[PKTSIZE];      /* room for data packet */
+} bfs[2];
+
+                                /* Values for bf.counter  */
+#define BF_ALLOC -3             /* alloc'd but not yet filled */
+#define BF_FREE  -2             /* free */
+/* [-1 .. SEGSIZE] = size of data in the data buffer */
+
+static int nextone;     /* index of next buffer to use */
+static int current;     /* index of buffer in use */
+
+                        /* control flags for crlf conversions */
+int newline = 0;        /* fillbuf: in middle of newline expansion */
+int prevchar = -1;      /* putbuf: previous char (cr check) */
+
+static void read_ahead(struct testcase *test,
+                       int convert /* if true, convert to ascii */);
+static int write_behind(struct testcase *test, int convert);
+static struct tftphdr *rw_init(int);
+static struct tftphdr *w_init(void) { return rw_init(0); } /* write-behind */
+static struct tftphdr *r_init(void) { return rw_init(1); } /* read-ahead */
+
+static struct tftphdr *
+rw_init(int x)              /* init for either read-ahead or write-behind */
+{                           /* zero for write-behind, one for read-head */
+  newline = 0;            /* init crlf flag */
+  prevchar = -1;
+  bfs[0].counter =  BF_ALLOC;     /* pass out the first buffer */
+  current = 0;
+  bfs[1].counter = BF_FREE;
+  nextone = x;                    /* ahead or behind? */
+  return (struct tftphdr *)bfs[0].buf;
+}
+
+
+/* Have emptied current buffer by sending to net and getting ack.
+   Free it and return next buffer filled with data.
+ */
+static int readit(struct testcase *test, struct tftphdr **dpp,
+           int convert /* if true, convert to ascii */)
+{
+  struct bf *b;
+
+  bfs[current].counter = BF_FREE; /* free old one */
+  current = !current;             /* "incr" current */
+
+  b = &bfs[current];              /* look at new buffer */
+  if (b->counter == BF_FREE)      /* if it's empty */
+    read_ahead(test, convert);      /* fill it */
+
+  *dpp = (struct tftphdr *)b->buf;        /* set caller's ptr */
+  return b->counter;
+}
+
+#define MIN(x,y) ((x)<(y)?(x):(y));
+
+/*
+ * fill the input buffer, doing ascii conversions if requested
+ * conversions are  lf -> cr,lf  and cr -> cr, nul
+ */
+static void read_ahead(struct testcase *test,
+                       int convert /* if true, convert to ascii */)
+{
+  int i;
+  char *p;
+  int c;
+  struct bf *b;
+  struct tftphdr *dp;
+
+  b = &bfs[nextone];              /* look at "next" buffer */
+  if (b->counter != BF_FREE)      /* nop if not free */
+    return;
+  nextone = !nextone;             /* "incr" next buffer ptr */
+
+  dp = (struct tftphdr *)b->buf;
+
+  if (convert == 0) {
+    /* The former file reading code did this:
+       b->counter = read(fileno(file), dp->th_data, SEGSIZE); */
+    int copy_n = MIN(SEGSIZE, test->rcount);
+    memcpy(dp->th_data, test->rptr, copy_n);
+
+    /* decrease amount, advance pointer */
+    test->rcount -= copy_n;
+    test->rptr += copy_n;
+    b->counter = copy_n;
+    return;
+  }
+
+  p = dp->th_data;
+  for (i = 0 ; i < SEGSIZE; i++) {
+    if (newline) {
+      if (prevchar == '\n')
+        c = '\n';       /* lf to cr,lf */
+      else
+        c = '\0';       /* cr to cr,nul */
+      newline = 0;
+    }
+    else {
+      if(test->rcount) {
+        c=test->rptr[0];
+        test->rptr++;
+        test->rcount--;
+      }
+      else
+        break;
+      if (c == '\n' || c == '\r') {
+        prevchar = c;
+        c = '\r';
+        newline = 1;
+      }
+    }
+    *p++ = c;
+  }
+  b->counter = (int)(p - dp->th_data);
+}
+
+/* Update count associated with the buffer, get new buffer from the queue.
+   Calls write_behind only if next buffer not available.
+ */
+static int writeit(struct testcase *test, struct tftphdr **dpp,
+                   int ct, int convert)
+{
+  bfs[current].counter = ct;      /* set size of data to write */
+  current = !current;             /* switch to other buffer */
+  if (bfs[current].counter != BF_FREE)     /* if not free */
+    write_behind(test, convert);     /* flush it */
+  bfs[current].counter = BF_ALLOC;        /* mark as alloc'd */
+  *dpp =  (struct tftphdr *)bfs[current].buf;
+  return ct;                      /* this is a lie of course */
+}
+
+/*
+ * Output a buffer to a file, converting from netascii if requested.
+ * CR,NUL -> CR  and CR,LF => LF.
+ * Note spec is undefined if we get CR as last byte of file or a
+ * CR followed by anything else.  In this case we leave it alone.
+ */
+static int write_behind(struct testcase *test, int convert)
+{
+  char *buf;
+  int count;
+  int ct;
+  char *p;
+  int c;                          /* current character */
+  struct bf *b;
+  struct tftphdr *dp;
+
+  b = &bfs[nextone];
+  if (b->counter < -1)            /* anything to flush? */
+    return 0;                     /* just nop if nothing to do */
+
+  if(!test->ofile) {
+    char outfile[256];
+    snprintf(outfile, sizeof(outfile), "log/upload.%ld", test->num);
+    test->ofile=open(outfile, O_CREAT|O_RDWR, 0777);
+    if(test->ofile == -1) {
+      logmsg("Couldn't create and/or open file %s for upload!", outfile);
+      return -1; /* failure! */
+    }
+  }
+
+  count = b->counter;             /* remember byte count */
+  b->counter = BF_FREE;           /* reset flag */
+  dp = (struct tftphdr *)b->buf;
+  nextone = !nextone;             /* incr for next time */
+  buf = dp->th_data;
+
+  if (count <= 0)
+    return -1;                    /* nak logic? */
+
+  if (convert == 0)
+    return write(test->ofile, buf, count);
+
+  p = buf;
+  ct = count;
+  while (ct--) {                  /* loop over the buffer */
+    c = *p++;                     /* pick up a character */
+    if (prevchar == '\r') {       /* if prev char was cr */
+      if (c == '\n')              /* if have cr,lf then just */
+        lseek(test->ofile, -1, SEEK_CUR); /* smash lf on top of the cr */
+      else
+        if (c == '\0')            /* if have cr,nul then */
+          goto skipit;            /* just skip over the putc */
+      /* else just fall through and allow it */
+    }
+    /* formerly
+       putc(c, file); */
+    write(test->ofile, &c, 1);
+    skipit:
+    prevchar = c;
+  }
+  return count;
+}
+
+
+/* When an error has occurred, it is possible that the two sides are out of
+ * synch.  Ie: that what I think is the other side's response to packet N is
+ * really their response to packet N-1.
+ *
+ * So, to try to prevent that, we flush all the input queued up for us on the
+ * network connection on our host.
+ *
+ * We return the number of packets we flushed (mostly for reporting when trace
+ * is active).
+ */
+
+static int synchnet(curl_socket_t f /* socket to flush */)
+{
+  int i, j = 0;
+  char rbuf[PKTSIZE];
+  struct sockaddr_in from;
+  socklen_t fromlen;
+
+  while (1) {
+    (void) ioctl(f, FIONREAD, &i);
+    if (i) {
+      j++;
+      fromlen = sizeof from;
+      (void) recvfrom(f, rbuf, sizeof (rbuf), 0,
+                      (struct sockaddr *)&from, &fromlen);
+    }
+    else
+      return j;
+  }
+}
+
+/*
+ * Like signal(), but with well-defined semantics.
+ */
+static void mysignal(int sig, void (*handler)(int))
+{
+  struct sigaction sa;
+  memset(&sa, 0, sizeof(sa));
+  sa.sa_handler = handler;
+  sigaction(sig, &sa, NULL);
+}
+
+
+#ifndef DEFAULT_LOGFILE
+#define DEFAULT_LOGFILE "log/tftpd.log"
+#endif
+
+#define DEFAULT_PORT 8999 /* UDP */
+const char *serverlogfile = DEFAULT_LOGFILE;
+
+#define REQUEST_DUMP  "log/server.input"
+
+char use_ipv6=FALSE;
+
+int main(int argc, char **argv)
+{
+  struct sockaddr_in me;
+#ifdef ENABLE_IPV6
+  struct sockaddr_in6 me6;
+#endif /* ENABLE_IPV6 */
+
+  struct tftphdr *tp;
+  int n = 0;
+  int arg = 1;
+  FILE *pidfile;
+  char *pidname= (char *)".tftpd.pid";
+  unsigned short port = DEFAULT_PORT;
+  curl_socket_t sock;
+  int flag;
+  int rc;
+  struct testcase test;
+
+  while(argc>arg) {
+    if(!strcmp("--version", argv[arg])) {
+      printf("tftpd IPv4%s\n",
+#ifdef ENABLE_IPV6
+             "/IPv6"
+#else
+             ""
+#endif
+             );
+      return 0;
+    }
+    else if(!strcmp("--pidfile", argv[arg])) {
+      arg++;
+      if(argc>arg)
+        pidname = argv[arg++];
+    }
+    else if(!strcmp("--ipv6", argv[arg])) {
+#ifdef ENABLE_IPV6
+      use_ipv6=TRUE;
+#endif
+      arg++;
+    }
+    else if(argc>arg) {
+
+      if(atoi(argv[arg]))
+        port = (unsigned short)atoi(argv[arg++]);
+
+      if(argc>arg)
+        path = argv[arg++];
+    }
+  }
+
+#if defined(WIN32) && !defined(__GNUC__) || defined(__MINGW32__)
+  win32_init();
+  atexit(win32_cleanup);
+#endif
+
+#ifdef ENABLE_IPV6
+  if(!use_ipv6)
+#endif
+    sock = socket(AF_INET, SOCK_DGRAM, 0);
+#ifdef ENABLE_IPV6
+  else
+    sock = socket(AF_INET6, SOCK_DGRAM, 0);
+#endif
+
+  if (sock < 0) {
+    perror("opening stream socket");
+    logmsg("Error opening socket");
+    exit(1);
+  }
+
+  flag = 1;
+  if (setsockopt
+      (sock, SOL_SOCKET, SO_REUSEADDR, (const void *) &flag,
+       sizeof(int)) < 0) {
+    perror("setsockopt(SO_REUSEADDR)");
+  }
+
+#ifdef ENABLE_IPV6
+  if(!use_ipv6) {
+#endif
+    me.sin_family = AF_INET;
+    me.sin_addr.s_addr = INADDR_ANY;
+    me.sin_port = htons(port);
+    rc = bind(sock, (struct sockaddr *) &me, sizeof(me));
+#ifdef ENABLE_IPV6
+  }
+  else {
+    memset(&me6, 0, sizeof(struct sockaddr_in6));
+    me6.sin6_family = AF_INET6;
+    me6.sin6_addr = in6addr_any;
+    me6.sin6_port = htons(port);
+    rc = bind(sock, (struct sockaddr *) &me6, sizeof(me6));
+  }
+#endif /* ENABLE_IPV6 */
+  if(rc < 0) {
+    perror("binding stream socket");
+    logmsg("Error binding socket");
+    exit(1);
+  }
+
+  pidfile = fopen(pidname, "w");
+  if(pidfile) {
+    fprintf(pidfile, "%d\n", (int)getpid());
+    fclose(pidfile);
+  }
+  else
+    fprintf(stderr, "Couldn't write pid file\n");
+
+  logmsg("Running IPv%d version on port UDP/%d",
+#ifdef ENABLE_IPV6
+         (use_ipv6?6:4)
+#else
+         4
+#endif
+         , port );
+
+  do {
+    FILE *server;
+
+    server = fopen(REQUEST_DUMP, "ab");
+    if(!server)
+      break;
+
+    fromlen = sizeof(from);
+    n = recvfrom(sock, buf, sizeof (buf), 0,
+                 (struct sockaddr *)&from, &fromlen);
+    if (n < 0) {
+      logmsg("recvfrom:\n");
+      return 3;
+    }
+
+    from.sin_family = AF_INET;
+
+    peer = socket(AF_INET, SOCK_DGRAM, 0);
+    if (peer < 0) {
+      logmsg("socket:\n");
+      return 2;
+    }
+
+    if (connect(peer, (struct sockaddr *)&from, sizeof(from)) < 0) {
+      logmsg("connect: fail\n");
+      return 1;
+    }
+
+    tp = (struct tftphdr *)buf;
+    tp->th_opcode = ntohs(tp->th_opcode);
+    if (tp->th_opcode == RRQ || tp->th_opcode == WRQ) {
+      memset(&test, 0, sizeof(test));
+      test.server = server;
+      tftp(&test, tp, n);
+    }
+    fclose(server);
+    sclose(peer);
+  } while(1);
+  return 0;
+}
+
+struct formats {
+  const char *f_mode;
+  int f_convert;
+} formats[] = {
+  { "netascii",   1 },
+  { "octet",      0 },
+  { NULL,         0 }
+};
+
+/*
+ * Handle initial connection protocol.
+ */
+static int tftp(struct testcase *test, struct tftphdr *tp, int size)
+{
+  char *cp;
+  int first = 1, ecode;
+  struct formats *pf;
+  char *filename, *mode = NULL;
+
+  /* store input protocol */
+  fprintf(test->server, "opcode: %x\n", tp->th_opcode);
+
+  filename = cp = tp->th_stuff;
+again:
+  while (cp < buf + size) {
+    if (*cp == '\0')
+      break;
+    cp++;
+  }
+  if (*cp) {
+    nak(EBADOP);
+    return 3;
+  }
+  if (first) {
+    mode = ++cp;
+    first = 0;
+    goto again;
+  }
+  /* store input protocol */
+  fprintf(test->server, "filename: %s\n", filename);
+
+  for (cp = mode; *cp; cp++)
+    if (isupper(*cp))
+      *cp = tolower(*cp);
+
+  /* store input protocol */
+  fprintf(test->server, "mode: %s\n", mode);
+
+  for (pf = formats; pf->f_mode; pf++)
+    if (strcmp(pf->f_mode, mode) == 0)
+      break;
+  if (!pf->f_mode) {
+    nak(EBADOP);
+    return 2;
+  }
+  ecode = validate_access(test, filename, tp->th_opcode);
+  if (ecode) {
+    nak(ecode);
+    return 1;
+  }
+  if (tp->th_opcode == WRQ)
+    recvfile(test, pf);
+  else
+    sendfile(test, pf);
+  return 0;
+}
+
+/*
+ * Validate file access.
+ */
+static int validate_access(struct testcase *test,
+                           const char *filename, int mode)
+{
+  char *ptr;
+  long testno;
+
+  logmsg("trying to get file: %s mode %x", filename, mode);
+
+  if(!strncmp("/verifiedserver", filename, 15)) {
+    char weare[128];
+    size_t count = sprintf(weare, "WE ROOLZ: %d\r\n", (int)getpid());
+
+    logmsg("Are-we-friendly question received");
+    test->buffer = strdup(weare);
+    test->rptr = test->buffer; /* set read pointer */
+    test->bufsize = count;    /* set total count */
+    test->rcount = count;     /* set data left to read */
+    return 0; /* fine */
+  }
+
+  /* find the last slash */
+  ptr = strrchr(filename, '/');
+
+  if(ptr) {
+    char *file;
+
+    ptr++; /* skip the slash */
+
+    /* skip all non-numericals following the slash */
+    while(*ptr && !isdigit((int)*ptr))
+      ptr++;
+
+    /* get the number */
+    testno = strtol(ptr, &ptr, 10);
+
+    logmsg("requested test number %d", testno);
+
+    test->num = testno;
+
+    file = test2file(testno);
+
+    if(file) {
+      FILE *stream=fopen(file, "rb");
+      if(!stream) {
+        logmsg("Couldn't open test file: %s", file);
+        return EACCESS;
+      }
+      else {
+        size_t count;
+        test->buffer = (char *)spitout(stream, "reply", "data", &count);
+        fclose(stream);
+        if(test->buffer) {
+          test->rptr = test->buffer; /* set read pointer */
+          test->bufsize = count;    /* set total count */
+          test->rcount = count;     /* set data left to read */
+        }
+        else
+          return EACCESS;
+      }
+
+    }
+    else
+      return EACCESS;
+  }
+  else {
+    logmsg("no slash found in path");
+    return EACCESS; /* failure */
+  }
+
+  return 0;
+}
+
+int timeout;
+sigjmp_buf timeoutbuf;
+
+static void timer(int signum)
+{
+  (void)signum;
+
+  timeout += rexmtval;
+  if (timeout >= maxtimeout)
+    exit(1);
+  siglongjmp(timeoutbuf, 1);
+}
+
+/*
+ * Send the requested file.
+ */
+static void sendfile(struct testcase *test, struct formats *pf)
+{
+  struct tftphdr *dp;
+  struct tftphdr *ap;    /* ack packet */
+  u_int16_t block = 1;
+  int size, n;
+
+  mysignal(SIGALRM, timer);
+  dp = r_init();
+  ap = (struct tftphdr *)ackbuf;
+  do {
+    size = readit(test, &dp, pf->f_convert);
+    if (size < 0) {
+      nak(errno + 100);
+      goto abort;
+    }
+    dp->th_opcode = htons((u_short)DATA);
+    dp->th_block = htons((u_short)block);
+    timeout = 0;
+    (void) sigsetjmp(timeoutbuf, 1);
+
+    send_data:
+    if (send(peer, dp, size + 4, 0) != size + 4) {
+      logmsg("write\n");
+      goto abort;
+    }
+    read_ahead(test, pf->f_convert);
+    for ( ; ; ) {
+      alarm(rexmtval);        /* read the ack */
+      n = recv(peer, ackbuf, sizeof (ackbuf), 0);
+      alarm(0);
+      if (n < 0) {
+        logmsg("read: fail\n");
+        goto abort;
+      }
+      ap->th_opcode = ntohs((u_short)ap->th_opcode);
+      ap->th_block = ntohs((u_short)ap->th_block);
+
+      if (ap->th_opcode == ERROR)
+        goto abort;
+
+      if (ap->th_opcode == ACK) {
+        if (ap->th_block == block) {
+          break;
+        }
+        /* Re-synchronize with the other side */
+        (void) synchnet(peer);
+        if (ap->th_block == (block -1)) {
+          goto send_data;
+        }
+      }
+
+    }
+    block++;
+  } while (size == SEGSIZE);
+  abort:
+  ;
+}
+
+static void justquit(int signum)
+{
+  (void)signum;
+  exit(0);
+}
+
+
+/*
+ * Receive a file.
+ */
+static void recvfile(struct testcase *test, struct formats *pf)
+{
+  struct tftphdr *dp;
+  struct tftphdr *ap;    /* ack buffer */
+  u_int16_t block = 0;
+  int n, size;
+
+  mysignal(SIGALRM, timer);
+  dp = w_init();
+  ap = (struct tftphdr *)ackbuf;
+  do {
+    timeout = 0;
+    ap->th_opcode = htons((u_short)ACK);
+    ap->th_block = htons((u_short)block);
+    block++;
+    (void) sigsetjmp(timeoutbuf, 1);
+send_ack:
+    if (send(peer, ackbuf, 4, 0) != 4) {
+      logmsg("write: fail\n");
+      goto abort;
+    }
+    write_behind(test, pf->f_convert);
+    for ( ; ; ) {
+      alarm(rexmtval);
+      n = recv(peer, dp, PKTSIZE, 0);
+      alarm(0);
+      if (n < 0) {                       /* really? */
+        logmsg("read: fail\n");
+        goto abort;
+      }
+      dp->th_opcode = ntohs((u_short)dp->th_opcode);
+      dp->th_block = ntohs((u_short)dp->th_block);
+      if (dp->th_opcode == ERROR)
+        goto abort;
+      if (dp->th_opcode == DATA) {
+        if (dp->th_block == block) {
+          break;                         /* normal */
+        }
+        /* Re-synchronize with the other side */
+        (void) synchnet(peer);
+        if (dp->th_block == (block-1))
+          goto send_ack;                 /* rexmit */
+      }
+    }
+
+    size = writeit(test, &dp, n - 4, pf->f_convert);
+    if (size != (n-4)) {                 /* ahem */
+      if (size < 0)
+        nak(errno + 100);
+      else
+        nak(ENOSPACE);
+      goto abort;
+    }
+  } while (size == SEGSIZE);
+  write_behind(test, pf->f_convert);
+
+  ap->th_opcode = htons((u_short)ACK);   /* send the "final" ack */
+  ap->th_block = htons((u_short)(block));
+  (void) send(peer, ackbuf, 4, 0);
+
+  mysignal(SIGALRM, justquit);           /* just quit on timeout */
+  alarm(rexmtval);
+  n = recv(peer, buf, sizeof (buf), 0);  /* normally times out and quits */
+  alarm(0);
+  if (n >= 4 &&                          /* if read some data */
+      dp->th_opcode == DATA &&           /* and got a data block */
+      block == dp->th_block) {           /* then my last ack was lost */
+    (void) send(peer, ackbuf, 4, 0);     /* resend final ack */
+  }
+abort:
+  return;
+}
+
+struct errmsg {
+  int e_code;
+  const char *e_msg;
+} errmsgs[] = {
+  { EUNDEF,       "Undefined error code" },
+  { ENOTFOUND,    "File not found" },
+  { EACCESS,      "Access violation" },
+  { ENOSPACE,     "Disk full or allocation exceeded" },
+  { EBADOP,       "Illegal TFTP operation" },
+  { EBADID,       "Unknown transfer ID" },
+  { EEXISTS,      "File already exists" },
+  { ENOUSER,      "No such user" },
+  { -1,           0 }
+};
+
+/*
+ * Send a nak packet (error message).  Error code passed in is one of the
+ * standard TFTP codes, or a UNIX errno offset by 100.
+ */
+static void nak(int error)
+{
+  struct tftphdr *tp;
+  int length;
+  struct errmsg *pe;
+
+  tp = (struct tftphdr *)buf;
+  tp->th_opcode = htons((u_short)ERROR);
+  tp->th_code = htons((u_short)error);
+  for (pe = errmsgs; pe->e_code >= 0; pe++)
+    if (pe->e_code == error)
+      break;
+  if (pe->e_code < 0) {
+    pe->e_msg = strerror(error - 100);
+    tp->th_code = EUNDEF;   /* set 'undef' errorcode */
+  }
+  strcpy(tp->th_msg, pe->e_msg);
+  length = strlen(pe->e_msg);
+  tp->th_msg[length] = '\0';
+  length += 5;
+  if (send(peer, buf, length, 0) != length)
+    logmsg("nak: fail\n");
+}