src/ip_input.c - libslirp - Git at Google

 /* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1993
  *	The 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. 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.
  *
  *	@(#)ip_input.c	8.2 (Berkeley) 1/4/94
  * ip_input.c,v 1.11 1994/11/16 10:17:08 jkh Exp
  */

 /*
  * Changes and additions relating to SLiRP are
  * Copyright (c) 1995 Danny Gasparovski.
  */

 #include "slirp.h"
 #include "ip_icmp.h"

 static struct ip *ip_reass(Slirp *slirp, struct ip *ip, struct ipq *fp);
 static void ip_freef(Slirp *slirp, struct ipq *fp);
 static void ip_enq(register struct ipasfrag *p, register struct ipasfrag *prev);
 static void ip_deq(register struct ipasfrag *p);

 /*
  * IP initialization: fill in IP protocol switch table.
  * All protocols not implemented in kernel go to raw IP protocol handler.
  */
 void ip_init(Slirp *slirp)
 {
     slirp->ipq.ip_link.next = slirp->ipq.ip_link.prev = &slirp->ipq.ip_link;
     udp_init(slirp);
     tcp_init(slirp);
     icmp_init(slirp);
 }

 void ip_cleanup(Slirp *slirp)
 {
     udp_cleanup(slirp);
     tcp_cleanup(slirp);
     icmp_cleanup(slirp);
 }

 /*
  * Ip input routine.  Checksum and byte swap header.  If fragmented
  * try to reassemble.  Process options.  Pass to next level.
  */
 void ip_input(struct mbuf *m)
 {
     Slirp *slirp = m->slirp;
     M_DUP_DEBUG(slirp, m, 0, TCPIPHDR_DELTA);

     register struct ip *ip;
     int hlen;

     if (!slirp->in_enabled) {
         goto bad;
     }

     DEBUG_CALL("ip_input");
     DEBUG_ARG("m = %p", m);
     DEBUG_ARG("m_len = %d", m->m_len);

     if (m->m_len < sizeof(struct ip)) {
         goto bad;
     }

     ip = mtod(m, struct ip *);

     if (ip->ip_v != IPVERSION) {
         goto bad;
     }

     hlen = ip->ip_hl << 2;
     if (hlen < sizeof(struct ip) || hlen > m->m_len) { /* min header length */
         goto bad; /* or packet too short */
     }

     /* keep ip header intact for ICMP reply
      * ip->ip_sum = cksum(m, hlen);
      * if (ip->ip_sum) {
      */
     if (cksum(m, hlen)) {
         goto bad;
     }

     /*
      * Convert fields to host representation.
      */
     NTOHS(ip->ip_len);
     if (ip->ip_len < hlen) {
         goto bad;
     }
     NTOHS(ip->ip_id);
     NTOHS(ip->ip_off);

     /*
      * Check that the amount of data in the buffers
      * is as at least much as the IP header would have us expect.
      * Trim mbufs if longer than we expect.
      * Drop packet if shorter than we expect.
      */
     if (m->m_len < ip->ip_len) {
         goto bad;
     }

     /* Should drop packet if mbuf too long? hmmm... */
     if (m->m_len > ip->ip_len)
         m_adj(m, ip->ip_len - m->m_len);

     /* check ip_ttl for a correct ICMP reply */
     if (ip->ip_ttl == 0) {
         icmp_send_error(m, ICMP_TIMXCEED, ICMP_TIMXCEED_INTRANS, 0, "ttl");
         goto bad;
     }

     /*
      * If offset or IP_MF are set, must reassemble.
      * Otherwise, nothing need be done.
      * (We could look in the reassembly queue to see
      * if the packet was previously fragmented,
      * but it's not worth the time; just let them time out.)
      *
      * XXX This should fail, don't fragment yet
      */
     if (ip->ip_off & ~IP_DF) {
         register struct ipq *fp;
         struct qlink *l;
         /*
          * Look for queue of fragments
          * of this datagram.
          */
         for (l = slirp->ipq.ip_link.next; l != &slirp->ipq.ip_link;
              l = l->next) {
             fp = container_of(l, struct ipq, ip_link);
             if (ip->ip_id == fp->ipq_id &&
                 ip->ip_src.s_addr == fp->ipq_src.s_addr &&
                 ip->ip_dst.s_addr == fp->ipq_dst.s_addr &&
                 ip->ip_p == fp->ipq_p)
                 goto found;
         }
         fp = NULL;
     found:

         /*
          * Adjust ip_len to not reflect header,
          * set ip_mff if more fragments are expected,
          * convert offset of this to bytes.
          */
         ip->ip_len -= hlen;
         if (ip->ip_off & IP_MF)
             ip->ip_tos |= 1;
         else
             ip->ip_tos &= ~1;

         ip->ip_off <<= 3;

         /*
          * If datagram marked as having more fragments
          * or if this is not the first fragment,
          * attempt reassembly; if it succeeds, proceed.
          */
         if (ip->ip_tos & 1 || ip->ip_off) {
             ip = ip_reass(slirp, ip, fp);
             if (ip == NULL)
                 return;
             m = dtom(slirp, ip);
         } else if (fp)
             ip_freef(slirp, fp);

     } else
         ip->ip_len -= hlen;

     /*
      * Switch out to protocol's input routine.
      */
     switch (ip->ip_p) {
     case IPPROTO_TCP:
         tcp_input(m, hlen, (struct socket *)NULL, AF_INET);
         break;
     case IPPROTO_UDP:
         udp_input(m, hlen);
         break;
     case IPPROTO_ICMP:
         icmp_input(m, hlen);
         break;
     default:
         m_free(m);
     }
     return;
 bad:
     m_free(m);
 }

 #define iptofrag(P) ((struct ipasfrag *)(((char *)(P)) - sizeof(struct qlink)))
 #define fragtoip(P) ((struct ip *)(((char *)(P)) + sizeof(struct qlink)))
 /*
  * Take incoming datagram fragment and try to
  * reassemble it into whole datagram.  If a chain for
  * reassembly of this datagram already exists, then it
  * is given as fp; otherwise have to make a chain.
  */
 static struct ip *ip_reass(Slirp *slirp, struct ip *ip, struct ipq *fp)
 {
     register struct mbuf *m = dtom(slirp, ip);
     register struct ipasfrag *q;
     int hlen = ip->ip_hl << 2;
     int i, next;

     DEBUG_CALL("ip_reass");
     DEBUG_ARG("ip = %p", ip);
     DEBUG_ARG("fp = %p", fp);
     DEBUG_ARG("m = %p", m);

     /*
      * Presence of header sizes in mbufs
      * would confuse code below.
      * Fragment m_data is concatenated.
      */
     m->m_data += hlen;
     m->m_len -= hlen;

     /*
      * If first fragment to arrive, create a reassembly queue.
      */
     if (fp == NULL) {
         struct mbuf *t = m_get(slirp);

         if (t == NULL) {
             goto dropfrag;
         }
         fp = mtod(t, struct ipq *);
         insque(&fp->ip_link, &slirp->ipq.ip_link);
         fp->ipq_ttl = IPFRAGTTL;
         fp->ipq_p = ip->ip_p;
         fp->ipq_id = ip->ip_id;
         fp->frag_link.next = fp->frag_link.prev = &fp->frag_link;
         fp->ipq_src = ip->ip_src;
         fp->ipq_dst = ip->ip_dst;
         q = (struct ipasfrag *)fp;
         goto insert;
     }

     /*
      * Find a segment which begins after this one does.
      */
     for (q = fp->frag_link.next; q != (struct ipasfrag *)&fp->frag_link;
          q = q->ipf_next)
         if (q->ipf_off > ip->ip_off)
             break;

     /*
      * If there is a preceding segment, it may provide some of
      * our data already.  If so, drop the data from the incoming
      * segment.  If it provides all of our data, drop us.
      */
     if (q->ipf_prev != &fp->frag_link) {
         struct ipasfrag *pq = q->ipf_prev;
         i = pq->ipf_off + pq->ipf_len - ip->ip_off;
         if (i > 0) {
             if (i >= ip->ip_len)
                 goto dropfrag;
             m_adj(dtom(slirp, ip), i);
             ip->ip_off += i;
             ip->ip_len -= i;
         }
     }

     /*
      * While we overlap succeeding segments trim them or,
      * if they are completely covered, dequeue them.
      */
     while (q != (struct ipasfrag *)&fp->frag_link &&
            ip->ip_off + ip->ip_len > q->ipf_off) {
         struct ipasfrag *prev;
         i = (ip->ip_off + ip->ip_len) - q->ipf_off;
         if (i < q->ipf_len) {
             q->ipf_len -= i;
             q->ipf_off += i;
             m_adj(dtom(slirp, q), i);
             break;
         }
         prev = q;
         q = q->ipf_next;
         ip_deq(prev);
         m_free(dtom(slirp, prev));
     }

 insert:
     /*
      * Stick new segment in its place;
      * check for complete reassembly.
      */
     ip_enq(iptofrag(ip), q->ipf_prev);
     next = 0;
     for (q = fp->frag_link.next; q != (struct ipasfrag *)&fp->frag_link;
          q = q->ipf_next) {
         if (q->ipf_off != next)
             return NULL;
         next += q->ipf_len;
     }
     if (((struct ipasfrag *)(q->ipf_prev))->ipf_tos & 1)
         return NULL;

     /*
      * Reassembly is complete; concatenate fragments.
      */
     q = fp->frag_link.next;
     m = dtom(slirp, q);
     int delta = (char *)q - (m->m_flags & M_EXT ? m->m_ext : m->m_dat);

     q = (struct ipasfrag *)q->ipf_next;
     while (q != (struct ipasfrag *)&fp->frag_link) {
         struct mbuf *t = dtom(slirp, q);
         q = (struct ipasfrag *)q->ipf_next;
         m_cat(m, t);
     }

     /*
      * Create header for new ip packet by
      * modifying header of first packet;
      * dequeue and discard fragment reassembly header.
      * Make header visible.
      */
     q = fp->frag_link.next;

     /*
      * If the fragments concatenated to an mbuf that's bigger than the total
      * size of the fragment and the mbuf was not already using an m_ext buffer,
      * then an m_ext buffer was allocated. But fp->ipq_next points to the old
      * buffer (in the mbuf), so we must point ip into the new buffer.
      */
     if (m->m_flags & M_EXT) {
         q = (struct ipasfrag *)(m->m_ext + delta);
     }

     ip = fragtoip(q);
     ip->ip_len = next;
     ip->ip_tos &= ~1;
     ip->ip_src = fp->ipq_src;
     ip->ip_dst = fp->ipq_dst;
     remque(&fp->ip_link);
     m_free(dtom(slirp, fp));
     m->m_len += (ip->ip_hl << 2);
     m->m_data -= (ip->ip_hl << 2);

     return ip;

 dropfrag:
     m_free(m);
     return NULL;
 }

 /*
  * Free a fragment reassembly header and all
  * associated datagrams.
  */
 static void ip_freef(Slirp *slirp, struct ipq *fp)
 {
     register struct ipasfrag *q, *p;

     for (q = fp->frag_link.next; q != (struct ipasfrag *)&fp->frag_link;
          q = p) {
         p = q->ipf_next;
         ip_deq(q);
         m_free(dtom(slirp, q));
     }
     remque(&fp->ip_link);
     m_free(dtom(slirp, fp));
 }

 /*
  * Put an ip fragment on a reassembly chain.
  * Like insque, but pointers in middle of structure.
  */
 static void ip_enq(register struct ipasfrag *p, register struct ipasfrag *prev)
 {
     DEBUG_CALL("ip_enq");
     DEBUG_ARG("prev = %p", prev);
     p->ipf_prev = prev;
     p->ipf_next = prev->ipf_next;
     ((struct ipasfrag *)(prev->ipf_next))->ipf_prev = p;
     prev->ipf_next = p;
 }

 /*
  * To ip_enq as remque is to insque.
  */
 static void ip_deq(register struct ipasfrag *p)
 {
     ((struct ipasfrag *)(p->ipf_prev))->ipf_next = p->ipf_next;
     ((struct ipasfrag *)(p->ipf_next))->ipf_prev = p->ipf_prev;
 }

 /*
  * IP timer processing;
  * if a timer expires on a reassembly
  * queue, discard it.
  */
 void ip_slowtimo(Slirp *slirp)
 {
     struct qlink *l;

     DEBUG_CALL("ip_slowtimo");

     l = slirp->ipq.ip_link.next;

     if (l == NULL)
         return;

     while (l != &slirp->ipq.ip_link) {
         struct ipq *fp = container_of(l, struct ipq, ip_link);
         l = l->next;
         if (--fp->ipq_ttl == 0) {
             ip_freef(slirp, fp);
         }
     }
 }

 /*
  * Strip out IP options, at higher
  * level protocol in the kernel.
  * Second argument is buffer to which options
  * will be moved, and return value is their length.
  * (XXX) should be deleted; last arg currently ignored.
  */
 void ip_stripoptions(register struct mbuf *m, struct mbuf *mopt)
 {
     register int i;
     struct ip *ip = mtod(m, struct ip *);
     register char *opts;
     int olen;

     olen = (ip->ip_hl << 2) - sizeof(struct ip);
     opts = (char *)(ip + 1);
     i = m->m_len - (sizeof(struct ip) + olen);
     memmove(opts, opts + olen, (unsigned)i);
     m->m_len -= olen;

     ip->ip_hl = sizeof(struct ip) >> 2;
 }
	/* SPDX-License-Identifier: BSD-3-Clause */
	/*
	* Copyright (c) 1982, 1986, 1988, 1993
	* The 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. 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.
	*
	* @(#)ip_input.c 8.2 (Berkeley) 1/4/94
	* ip_input.c,v 1.11 1994/11/16 10:17:08 jkh Exp
	*/

	/*
	* Changes and additions relating to SLiRP are
	* Copyright (c) 1995 Danny Gasparovski.
	*/

	#include "slirp.h"
	#include "ip_icmp.h"

	static struct ip ip_reass(Slirp slirp, struct ip ip, struct ipq fp);
	static void ip_freef(Slirp slirp, struct ipq fp);
	static void ip_enq(register struct ipasfrag p, register struct ipasfrag prev);
	static void ip_deq(register struct ipasfrag *p);

	/*
	* IP initialization: fill in IP protocol switch table.
	* All protocols not implemented in kernel go to raw IP protocol handler.
	*/
	void ip_init(Slirp *slirp)
	{
	slirp->ipq.ip_link.next = slirp->ipq.ip_link.prev = &slirp->ipq.ip_link;
	udp_init(slirp);
	tcp_init(slirp);
	icmp_init(slirp);
	}

	void ip_cleanup(Slirp *slirp)
	{
	udp_cleanup(slirp);
	tcp_cleanup(slirp);
	icmp_cleanup(slirp);
	}

	/*
	* Ip input routine. Checksum and byte swap header. If fragmented
	* try to reassemble. Process options. Pass to next level.
	*/
	void ip_input(struct mbuf *m)
	{
	Slirp *slirp = m->slirp;
	M_DUP_DEBUG(slirp, m, 0, TCPIPHDR_DELTA);

	register struct ip *ip;
	int hlen;

	if (!slirp->in_enabled) {
	goto bad;
	}

	DEBUG_CALL("ip_input");
	DEBUG_ARG("m = %p", m);
	DEBUG_ARG("m_len = %d", m->m_len);

	if (m->m_len < sizeof(struct ip)) {
	goto bad;
	}

	ip = mtod(m, struct ip *);

	if (ip->ip_v != IPVERSION) {
	goto bad;
	}

	hlen = ip->ip_hl << 2;
	if (hlen < sizeof(struct ip) \|\| hlen > m->m_len) { /* min header length */
	goto bad; /* or packet too short */
	}

	/* keep ip header intact for ICMP reply
	* ip->ip_sum = cksum(m, hlen);
	* if (ip->ip_sum) {
	*/
	if (cksum(m, hlen)) {
	goto bad;
	}

	/*
	* Convert fields to host representation.
	*/
	NTOHS(ip->ip_len);
	if (ip->ip_len < hlen) {
	goto bad;
	}
	NTOHS(ip->ip_id);
	NTOHS(ip->ip_off);

	/*
	* Check that the amount of data in the buffers
	* is as at least much as the IP header would have us expect.
	* Trim mbufs if longer than we expect.
	* Drop packet if shorter than we expect.
	*/
	if (m->m_len < ip->ip_len) {
	goto bad;
	}

	/* Should drop packet if mbuf too long? hmmm... */
	if (m->m_len > ip->ip_len)
	m_adj(m, ip->ip_len - m->m_len);

	/* check ip_ttl for a correct ICMP reply */
	if (ip->ip_ttl == 0) {
	icmp_send_error(m, ICMP_TIMXCEED, ICMP_TIMXCEED_INTRANS, 0, "ttl");
	goto bad;
	}

	/*
	* If offset or IP_MF are set, must reassemble.
	* Otherwise, nothing need be done.
	* (We could look in the reassembly queue to see
	* if the packet was previously fragmented,
	* but it's not worth the time; just let them time out.)
	*
	* XXX This should fail, don't fragment yet
	*/
	if (ip->ip_off & ~IP_DF) {
	register struct ipq *fp;
	struct qlink *l;
	/*
	* Look for queue of fragments
	* of this datagram.
	*/
	for (l = slirp->ipq.ip_link.next; l != &slirp->ipq.ip_link;
	l = l->next) {
	fp = container_of(l, struct ipq, ip_link);
	if (ip->ip_id == fp->ipq_id &&
	ip->ip_src.s_addr == fp->ipq_src.s_addr &&
	ip->ip_dst.s_addr == fp->ipq_dst.s_addr &&
	ip->ip_p == fp->ipq_p)
	goto found;
	}
	fp = NULL;
	found:

	/*
	* Adjust ip_len to not reflect header,
	* set ip_mff if more fragments are expected,
	* convert offset of this to bytes.
	*/
	ip->ip_len -= hlen;
	if (ip->ip_off & IP_MF)
	ip->ip_tos \|= 1;
	else
	ip->ip_tos &= ~1;

	ip->ip_off <<= 3;

	/*
	* If datagram marked as having more fragments
	* or if this is not the first fragment,
	* attempt reassembly; if it succeeds, proceed.
	*/
	if (ip->ip_tos & 1 \|\| ip->ip_off) {
	ip = ip_reass(slirp, ip, fp);
	if (ip == NULL)
	return;
	m = dtom(slirp, ip);
	} else if (fp)
	ip_freef(slirp, fp);

	} else
	ip->ip_len -= hlen;

	/*
	* Switch out to protocol's input routine.
	*/
	switch (ip->ip_p) {
	case IPPROTO_TCP:
	tcp_input(m, hlen, (struct socket *)NULL, AF_INET);
	break;
	case IPPROTO_UDP:
	udp_input(m, hlen);
	break;
	case IPPROTO_ICMP:
	icmp_input(m, hlen);
	break;
	default:
	m_free(m);
	}
	return;
	bad:
	m_free(m);
	}

	#define iptofrag(P) ((struct ipasfrag )(((char )(P)) - sizeof(struct qlink)))
	#define fragtoip(P) ((struct ip )(((char )(P)) + sizeof(struct qlink)))
	/*
	* Take incoming datagram fragment and try to
	* reassemble it into whole datagram. If a chain for
	* reassembly of this datagram already exists, then it
	* is given as fp; otherwise have to make a chain.
	*/
	static struct ip ip_reass(Slirp slirp, struct ip ip, struct ipq fp)
	{
	register struct mbuf *m = dtom(slirp, ip);
	register struct ipasfrag *q;
	int hlen = ip->ip_hl << 2;
	int i, next;

	DEBUG_CALL("ip_reass");
	DEBUG_ARG("ip = %p", ip);
	DEBUG_ARG("fp = %p", fp);
	DEBUG_ARG("m = %p", m);

	/*
	* Presence of header sizes in mbufs
	* would confuse code below.
	* Fragment m_data is concatenated.
	*/
	m->m_data += hlen;
	m->m_len -= hlen;

	/*
	* If first fragment to arrive, create a reassembly queue.
	*/
	if (fp == NULL) {
	struct mbuf *t = m_get(slirp);

	if (t == NULL) {
	goto dropfrag;
	}
	fp = mtod(t, struct ipq *);
	insque(&fp->ip_link, &slirp->ipq.ip_link);
	fp->ipq_ttl = IPFRAGTTL;
	fp->ipq_p = ip->ip_p;
	fp->ipq_id = ip->ip_id;
	fp->frag_link.next = fp->frag_link.prev = &fp->frag_link;
	fp->ipq_src = ip->ip_src;
	fp->ipq_dst = ip->ip_dst;
	q = (struct ipasfrag *)fp;
	goto insert;
	}

	/*
	* Find a segment which begins after this one does.
	*/
	for (q = fp->frag_link.next; q != (struct ipasfrag *)&fp->frag_link;
	q = q->ipf_next)
	if (q->ipf_off > ip->ip_off)
	break;

	/*
	* If there is a preceding segment, it may provide some of
	* our data already. If so, drop the data from the incoming
	* segment. If it provides all of our data, drop us.
	*/
	if (q->ipf_prev != &fp->frag_link) {
	struct ipasfrag *pq = q->ipf_prev;
	i = pq->ipf_off + pq->ipf_len - ip->ip_off;
	if (i > 0) {
	if (i >= ip->ip_len)
	goto dropfrag;
	m_adj(dtom(slirp, ip), i);
	ip->ip_off += i;
	ip->ip_len -= i;
	}
	}

	/*
	* While we overlap succeeding segments trim them or,
	* if they are completely covered, dequeue them.
	*/
	while (q != (struct ipasfrag *)&fp->frag_link &&
	ip->ip_off + ip->ip_len > q->ipf_off) {
	struct ipasfrag *prev;
	i = (ip->ip_off + ip->ip_len) - q->ipf_off;
	if (i < q->ipf_len) {
	q->ipf_len -= i;
	q->ipf_off += i;
	m_adj(dtom(slirp, q), i);
	break;
	}
	prev = q;
	q = q->ipf_next;
	ip_deq(prev);
	m_free(dtom(slirp, prev));
	}

	insert:
	/*
	* Stick new segment in its place;
	* check for complete reassembly.
	*/
	ip_enq(iptofrag(ip), q->ipf_prev);
	next = 0;
	for (q = fp->frag_link.next; q != (struct ipasfrag *)&fp->frag_link;
	q = q->ipf_next) {
	if (q->ipf_off != next)
	return NULL;
	next += q->ipf_len;
	}
	if (((struct ipasfrag *)(q->ipf_prev))->ipf_tos & 1)
	return NULL;

	/*
	* Reassembly is complete; concatenate fragments.
	*/
	q = fp->frag_link.next;
	m = dtom(slirp, q);
	int delta = (char *)q - (m->m_flags & M_EXT ? m->m_ext : m->m_dat);

	q = (struct ipasfrag *)q->ipf_next;
	while (q != (struct ipasfrag *)&fp->frag_link) {
	struct mbuf *t = dtom(slirp, q);
	q = (struct ipasfrag *)q->ipf_next;
	m_cat(m, t);
	}

	/*
	* Create header for new ip packet by
	* modifying header of first packet;
	* dequeue and discard fragment reassembly header.
	* Make header visible.
	*/
	q = fp->frag_link.next;

	/*
	* If the fragments concatenated to an mbuf that's bigger than the total
	* size of the fragment and the mbuf was not already using an m_ext buffer,
	* then an m_ext buffer was allocated. But fp->ipq_next points to the old
	* buffer (in the mbuf), so we must point ip into the new buffer.
	*/
	if (m->m_flags & M_EXT) {
	q = (struct ipasfrag *)(m->m_ext + delta);
	}

	ip = fragtoip(q);
	ip->ip_len = next;
	ip->ip_tos &= ~1;
	ip->ip_src = fp->ipq_src;
	ip->ip_dst = fp->ipq_dst;
	remque(&fp->ip_link);
	m_free(dtom(slirp, fp));
	m->m_len += (ip->ip_hl << 2);
	m->m_data -= (ip->ip_hl << 2);

	return ip;

	dropfrag:
	m_free(m);
	return NULL;
	}

	/*
	* Free a fragment reassembly header and all
	* associated datagrams.
	*/
	static void ip_freef(Slirp slirp, struct ipq fp)
	{
	register struct ipasfrag q, p;

	for (q = fp->frag_link.next; q != (struct ipasfrag *)&fp->frag_link;
	q = p) {
	p = q->ipf_next;
	ip_deq(q);
	m_free(dtom(slirp, q));
	}
	remque(&fp->ip_link);
	m_free(dtom(slirp, fp));
	}

	/*
	* Put an ip fragment on a reassembly chain.
	* Like insque, but pointers in middle of structure.
	*/
	static void ip_enq(register struct ipasfrag p, register struct ipasfrag prev)
	{
	DEBUG_CALL("ip_enq");
	DEBUG_ARG("prev = %p", prev);
	p->ipf_prev = prev;
	p->ipf_next = prev->ipf_next;
	((struct ipasfrag *)(prev->ipf_next))->ipf_prev = p;
	prev->ipf_next = p;
	}

	/*
	* To ip_enq as remque is to insque.
	*/
	static void ip_deq(register struct ipasfrag *p)
	{
	((struct ipasfrag *)(p->ipf_prev))->ipf_next = p->ipf_next;
	((struct ipasfrag *)(p->ipf_next))->ipf_prev = p->ipf_prev;
	}

	/*
	* IP timer processing;
	* if a timer expires on a reassembly
	* queue, discard it.
	*/
	void ip_slowtimo(Slirp *slirp)
	{
	struct qlink *l;

	DEBUG_CALL("ip_slowtimo");

	l = slirp->ipq.ip_link.next;

	if (l == NULL)
	return;

	while (l != &slirp->ipq.ip_link) {
	struct ipq *fp = container_of(l, struct ipq, ip_link);
	l = l->next;
	if (--fp->ipq_ttl == 0) {
	ip_freef(slirp, fp);
	}
	}
	}

	/*
	* Strip out IP options, at higher
	* level protocol in the kernel.
	* Second argument is buffer to which options
	* will be moved, and return value is their length.
	* (XXX) should be deleted; last arg currently ignored.
	*/
	void ip_stripoptions(register struct mbuf m, struct mbuf mopt)
	{
	register int i;
	struct ip ip = mtod(m, struct ip );
	register char *opts;
	int olen;

	olen = (ip->ip_hl << 2) - sizeof(struct ip);
	opts = (char *)(ip + 1);
	i = m->m_len - (sizeof(struct ip) + olen);
	memmove(opts, opts + olen, (unsigned)i);
	m->m_len -= olen;

	ip->ip_hl = sizeof(struct ip) >> 2;
	}