translate.c - platform/external/android-clat - Git at Google

 /*
  * Copyright 2011 Daniel Drown
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  * http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
  * translate.c - CLAT functions / partial implementation of rfc6145
  */
 #include <string.h>

 #include <netinet/in.h>
 #include <netinet/ip.h>
 #include <netinet/ip_icmp.h>
 #include <netinet/udp.h>
 #include <netinet/tcp.h>
 #include <netinet/ip6.h>
 #include <netinet/icmp6.h>
 #include <linux/icmp.h>

 #include "translate.h"
 #include "checksum.h"
 #include "clatd.h"
 #include "config.h"
 #include "logging.h"
 #include "debug.h"

 /* function: payload_length
  * calculates the total length of the packet components after pos
  * packet - packet to calculate the length of
  * pos    - position to start counting from
  * returns: the total length of the packet components after pos
  */
 uint16_t payload_length(clat_packet packet, int pos) {
   size_t len = 0;
   int i;
   for (i = pos + 1; i < POS_MAX; i++) {
     len += packet[i].iov_len;
   }
   return len;
 }

 /* function: fill_tun_header
  * fill in the header for the tun fd
  * tun_header - tunnel header, already allocated
  * proto      - ethernet protocol id: ETH_P_IP(ipv4) or ETH_P_IPV6(ipv6)
  */
 void fill_tun_header(struct tun_pi *tun_header, uint16_t proto) {
   tun_header->flags = 0;
   tun_header->proto = htons(proto);
 }

 /* function: ipv6_src_to_ipv4_src
  * return the corresponding ipv4 address for the given ipv6 address
  * sourceaddr - ipv6 source address
  * returns: the IPv4 address
  */
 uint32_t ipv6_src_to_ipv4_src(const struct in6_addr *sourceaddr) {
   // assumes a /96 plat subnet
   return sourceaddr->s6_addr32[3];
 }

 /* function: fill_ip_header
  * generate an ipv4 header from an ipv6 header
  * ip_targ     - (ipv4) target packet header, source: original ipv4 addr, dest: local subnet addr
  * payload_len - length of other data inside packet
  * protocol    - protocol number (tcp, udp, etc)
  * old_header  - (ipv6) source packet header, source: nat64 prefix, dest: local subnet prefix
  */
 void fill_ip_header(struct iphdr *ip, uint16_t payload_len, uint8_t protocol,
                     const struct ip6_hdr *old_header) {
   memset(ip, 0, sizeof(struct iphdr));

   ip->ihl = 5;
   ip->version = 4;
   ip->tos = 0;
   ip->tot_len = htons(sizeof(struct iphdr) + payload_len);
   ip->id = 0;
   ip->frag_off = htons(IP_DF);
   ip->ttl = old_header->ip6_hlim;
   ip->protocol = protocol;
   ip->check = 0;

   ip->saddr = ipv6_src_to_ipv4_src(&old_header->ip6_src);
   ip->daddr = Global_Clatd_Config.ipv4_local_subnet.s_addr;
 }

 /* function: ipv4_dst_to_ipv6_dst
  * return the corresponding ipv6 address for the given ipv4 address
  * destination - ipv4 destination address (network byte order)
  */
 struct in6_addr ipv4_dst_to_ipv6_dst(uint32_t destination) {
   struct in6_addr v6_destination;

   // assumes a /96 plat subnet
   v6_destination = Global_Clatd_Config.plat_subnet;
   v6_destination.s6_addr32[3] = destination;

   return v6_destination;
 }

 /* function: fill_ip6_header
  * generate an ipv6 header from an ipv4 header
  * ip6         - (ipv6) target packet header, source: local subnet prefix, dest: nat64 prefix
  * payload_len - length of other data inside packet
  * protocol    - protocol number (tcp, udp, etc)
  * old_header  - (ipv4) source packet header, source: local subnet addr, dest: internet's ipv4 addr
  */
 void fill_ip6_header(struct ip6_hdr *ip6, uint16_t payload_len, uint8_t protocol,
                      const struct iphdr *old_header) {
   memset(ip6, 0, sizeof(struct ip6_hdr));

   ip6->ip6_vfc = 6 << 4;
   ip6->ip6_plen = htons(payload_len);
   ip6->ip6_nxt = protocol;
   ip6->ip6_hlim = old_header->ttl;

   ip6->ip6_src = Global_Clatd_Config.ipv6_local_subnet;
   ip6->ip6_dst = ipv4_dst_to_ipv6_dst(old_header->daddr);
 }

 /* function: icmp_to_icmp6
  * translate ipv4 icmp to ipv6 icmp (only currently supports echo/echo reply)
  * out          - output packet
  * icmp         - source packet icmp header
  * checksum     - pseudo-header checksum
  * payload      - icmp payload
  * payload_size - size of payload
  * returns: the highest position in the output clat_packet that's filled in
  */
 int icmp_to_icmp6(clat_packet out, int pos, const struct icmphdr *icmp, uint32_t checksum,
                   const char *payload, size_t payload_size) {
   struct icmp6_hdr *icmp6_targ = out[pos].iov_base;
   uint32_t checksum_temp;

   if((icmp->type != ICMP_ECHO) && (icmp->type != ICMP_ECHOREPLY)) {
     logmsg_dbg(ANDROID_LOG_WARN,"icmp_to_icmp6/unhandled icmp type: 0x%x", icmp->type);
     return 0;
   }

   memset(icmp6_targ, 0, sizeof(struct icmp6_hdr));
   icmp6_targ->icmp6_type = (icmp->type == ICMP_ECHO) ? ICMP6_ECHO_REQUEST : ICMP6_ECHO_REPLY;
   icmp6_targ->icmp6_code = 0;
   icmp6_targ->icmp6_id = icmp->un.echo.id;
   icmp6_targ->icmp6_seq = icmp->un.echo.sequence;

   icmp6_targ->icmp6_cksum = 0;
   checksum = ip_checksum_add(checksum, icmp6_targ, sizeof(struct icmp6_hdr));
   checksum = ip_checksum_add(checksum, payload, payload_size);
   icmp6_targ->icmp6_cksum = ip_checksum_finish(checksum);

   out[pos].iov_len = sizeof(struct icmp6_hdr);
   out[POS_PAYLOAD].iov_base = (char *) payload;
   out[POS_PAYLOAD].iov_len = payload_size;

   return POS_PAYLOAD + 1;
 }

 /* function: icmp6_to_icmp
  * translate ipv6 icmp to ipv4 icmp (only currently supports echo/echo reply)
  * out          - output packet
  * icmp6        - source packet icmp6 header
  * checksum     - pseudo-header checksum (unused)
  * payload      - icmp6 payload
  * payload_size - size of payload
  * returns: the highest position in the output clat_packet that's filled in
  */
 int icmp6_to_icmp(clat_packet out, int pos, const struct icmp6_hdr *icmp6, uint32_t checksum,
                   const char *payload, size_t payload_size) {
   struct icmphdr *icmp_targ = out[pos].iov_base;

   if((icmp6->icmp6_type != ICMP6_ECHO_REQUEST) && (icmp6->icmp6_type != ICMP6_ECHO_REPLY)) {
     logmsg_dbg(ANDROID_LOG_WARN,"icmp6_to_icmp/unhandled icmp6 type: 0x%x",icmp6->icmp6_type);
     return 0;
   }

   memset(icmp_targ, 0, sizeof(struct icmphdr));

   icmp_targ->type = (icmp6->icmp6_type == ICMP6_ECHO_REQUEST) ? ICMP_ECHO : ICMP_ECHOREPLY;
   icmp_targ->code = 0x0;
   icmp_targ->un.echo.id = icmp6->icmp6_id;
   icmp_targ->un.echo.sequence = icmp6->icmp6_seq;

   icmp_targ->checksum = 0;
   checksum = ip_checksum_add(0, icmp_targ, sizeof(struct icmphdr));
   checksum = ip_checksum_add(checksum, (void *)payload, payload_size);
   icmp_targ->checksum = ip_checksum_finish(checksum);

   out[pos].iov_len = sizeof(struct icmphdr);
   out[POS_PAYLOAD].iov_base = (char *) payload;
   out[POS_PAYLOAD].iov_len = payload_size;

   return POS_PAYLOAD + 1;
 }

 /* function: udp_packet
  * takes a udp packet and sets it up for translation
  * out      - output packet
  * udp      - pointer to udp header in packet
  * checksum - pseudo-header checksum
  * len      - size of ip payload
  */
 int udp_packet(clat_packet out, int pos, const struct udphdr *udp, uint32_t checksum, size_t len) {
   const char *payload;
   size_t payload_size;

   if(len < sizeof(struct udphdr)) {
     logmsg_dbg(ANDROID_LOG_ERROR,"udp_packet/(too small)");
     return 0;
   }

   payload = (const char *) (udp + 1);
   payload_size = len - sizeof(struct udphdr);

   return udp_translate(out, pos, udp, checksum, payload, payload_size);
 }

 /* function: tcp_packet
  * takes a tcp packet and sets it up for translation
  * out      - output packet
  * tcp      - pointer to tcp header in packet
  * checksum - pseudo-header checksum
  * len      - size of ip payload
  * returns: the highest position in the output clat_packet that's filled in
  */
 int tcp_packet(clat_packet out, int pos, const struct tcphdr *tcp, uint32_t checksum, size_t len) {
   const char *payload;
   size_t payload_size, header_size;

   if(len < sizeof(struct tcphdr)) {
     logmsg_dbg(ANDROID_LOG_ERROR,"tcp_packet/(too small)");
     return 0;
   }

   if(tcp->doff < 5) {
     logmsg_dbg(ANDROID_LOG_ERROR,"tcp_packet/tcp header length set to less than 5: %x", tcp->doff);
     return 0;
   }

   if((size_t) tcp->doff*4 > len) {
     logmsg_dbg(ANDROID_LOG_ERROR,"tcp_packet/tcp header length set too large: %x", tcp->doff);
     return 0;
   }

   header_size = tcp->doff * 4;
   payload = ((const char *) tcp) + header_size;
   payload_size = len - header_size;

   return tcp_translate(out, pos, tcp, header_size, checksum, payload, payload_size);
 }

 /* function: udp_translate
  * common between ipv4/ipv6 - setup checksum and send udp packet
  * out          - output packet
  * udp          - udp header
  * checksum     - pseudo-header checksum
  * payload      - tcp payload
  * payload_size - size of payload
  * returns: the highest position in the output clat_packet that's filled in
  */
 int udp_translate(clat_packet out, int pos, const struct udphdr *udp, uint32_t checksum,
                   const char *payload, size_t payload_size) {
   struct udphdr *udp_targ = out[pos].iov_base;

   memcpy(udp_targ, udp, sizeof(struct udphdr));
   udp_targ->check = 0; // reset checksum, to be calculated

   checksum = ip_checksum_add(checksum, udp_targ, sizeof(struct udphdr));
   checksum = ip_checksum_add(checksum, payload, payload_size);
   udp_targ->check = ip_checksum_finish(checksum);

   out[pos].iov_len = sizeof(struct udphdr);
   out[POS_PAYLOAD].iov_base = (char *) payload;
   out[POS_PAYLOAD].iov_len = payload_size;

   return POS_PAYLOAD + 1;
 }

 /* function: tcp_translate
  * common between ipv4/ipv6 - setup checksum and send tcp packet
  * out          - output packet
  * tcp          - tcp header
  * header_size  - size of tcp header including options
  * checksum     - partial checksum covering ipv4/ipv6 header
  * payload      - tcp payload
  * payload_size - size of payload
  * returns: the highest position in the output clat_packet that's filled in
  *
  * TODO: mss rewrite
  * TODO: hosts without pmtu discovery - non DF packets will rely on fragmentation (unimplemented)
  */
 int tcp_translate(clat_packet out, int pos, const struct tcphdr *tcp, size_t header_size,
                   uint32_t checksum, const char *payload, size_t payload_size) {
   struct tcphdr *tcp_targ = out[pos].iov_base;
   out[pos].iov_len = header_size;

   if (header_size > MAX_TCP_HDR) {
     // A TCP header cannot be more than MAX_TCP_HDR bytes long because it's a 4-bit field that
     // counts in 4-byte words. So this can never happen unless there is a bug in the caller.
     logmsg(ANDROID_LOG_ERROR, "tcp_translate: header too long %d > %d, truncating",
            header_size, MAX_TCP_HDR);
     header_size = MAX_TCP_HDR;
   }

   memcpy(tcp_targ, tcp, header_size);

   tcp_targ->check = 0;
   checksum = ip_checksum_add(checksum, tcp_targ, header_size);
   checksum = ip_checksum_add(checksum, payload, payload_size);
   tcp_targ->check = ip_checksum_finish(checksum);

   out[POS_PAYLOAD].iov_base = (char *)payload;
   out[POS_PAYLOAD].iov_len = payload_size;

   return POS_PAYLOAD + 1;
 }
	/*
	* Copyright 2011 Daniel Drown
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*
	* translate.c - CLAT functions / partial implementation of rfc6145
	*/
	#include <string.h>

	#include <netinet/in.h>
	#include <netinet/ip.h>
	#include <netinet/ip_icmp.h>
	#include <netinet/udp.h>
	#include <netinet/tcp.h>
	#include <netinet/ip6.h>
	#include <netinet/icmp6.h>
	#include <linux/icmp.h>

	#include "translate.h"
	#include "checksum.h"
	#include "clatd.h"
	#include "config.h"
	#include "logging.h"
	#include "debug.h"

	/* function: payload_length
	* calculates the total length of the packet components after pos
	* packet - packet to calculate the length of
	* pos - position to start counting from
	* returns: the total length of the packet components after pos
	*/
	uint16_t payload_length(clat_packet packet, int pos) {
	size_t len = 0;
	int i;
	for (i = pos + 1; i < POS_MAX; i++) {
	len += packet[i].iov_len;
	}
	return len;
	}

	/* function: fill_tun_header
	* fill in the header for the tun fd
	* tun_header - tunnel header, already allocated
	* proto - ethernet protocol id: ETH_P_IP(ipv4) or ETH_P_IPV6(ipv6)
	*/
	void fill_tun_header(struct tun_pi *tun_header, uint16_t proto) {
	tun_header->flags = 0;
	tun_header->proto = htons(proto);
	}

	/* function: ipv6_src_to_ipv4_src
	* return the corresponding ipv4 address for the given ipv6 address
	* sourceaddr - ipv6 source address
	* returns: the IPv4 address
	*/
	uint32_t ipv6_src_to_ipv4_src(const struct in6_addr *sourceaddr) {
	// assumes a /96 plat subnet
	return sourceaddr->s6_addr32[3];
	}

	/* function: fill_ip_header
	* generate an ipv4 header from an ipv6 header
	* ip_targ - (ipv4) target packet header, source: original ipv4 addr, dest: local subnet addr
	* payload_len - length of other data inside packet
	* protocol - protocol number (tcp, udp, etc)
	* old_header - (ipv6) source packet header, source: nat64 prefix, dest: local subnet prefix
	*/
	void fill_ip_header(struct iphdr *ip, uint16_t payload_len, uint8_t protocol,
	const struct ip6_hdr *old_header) {
	memset(ip, 0, sizeof(struct iphdr));

	ip->ihl = 5;
	ip->version = 4;
	ip->tos = 0;
	ip->tot_len = htons(sizeof(struct iphdr) + payload_len);
	ip->id = 0;
	ip->frag_off = htons(IP_DF);
	ip->ttl = old_header->ip6_hlim;
	ip->protocol = protocol;
	ip->check = 0;

	ip->saddr = ipv6_src_to_ipv4_src(&old_header->ip6_src);
	ip->daddr = Global_Clatd_Config.ipv4_local_subnet.s_addr;
	}

	/* function: ipv4_dst_to_ipv6_dst
	* return the corresponding ipv6 address for the given ipv4 address
	* destination - ipv4 destination address (network byte order)
	*/
	struct in6_addr ipv4_dst_to_ipv6_dst(uint32_t destination) {
	struct in6_addr v6_destination;

	// assumes a /96 plat subnet
	v6_destination = Global_Clatd_Config.plat_subnet;
	v6_destination.s6_addr32[3] = destination;

	return v6_destination;
	}

	/* function: fill_ip6_header
	* generate an ipv6 header from an ipv4 header
	* ip6 - (ipv6) target packet header, source: local subnet prefix, dest: nat64 prefix
	* payload_len - length of other data inside packet
	* protocol - protocol number (tcp, udp, etc)
	* old_header - (ipv4) source packet header, source: local subnet addr, dest: internet's ipv4 addr
	*/
	void fill_ip6_header(struct ip6_hdr *ip6, uint16_t payload_len, uint8_t protocol,
	const struct iphdr *old_header) {
	memset(ip6, 0, sizeof(struct ip6_hdr));

	ip6->ip6_vfc = 6 << 4;
	ip6->ip6_plen = htons(payload_len);
	ip6->ip6_nxt = protocol;
	ip6->ip6_hlim = old_header->ttl;

	ip6->ip6_src = Global_Clatd_Config.ipv6_local_subnet;
	ip6->ip6_dst = ipv4_dst_to_ipv6_dst(old_header->daddr);
	}

	/* function: icmp_to_icmp6
	* translate ipv4 icmp to ipv6 icmp (only currently supports echo/echo reply)
	* out - output packet
	* icmp - source packet icmp header
	* checksum - pseudo-header checksum
	* payload - icmp payload
	* payload_size - size of payload
	* returns: the highest position in the output clat_packet that's filled in
	*/
	int icmp_to_icmp6(clat_packet out, int pos, const struct icmphdr *icmp, uint32_t checksum,
	const char *payload, size_t payload_size) {
	struct icmp6_hdr *icmp6_targ = out[pos].iov_base;
	uint32_t checksum_temp;

	if((icmp->type != ICMP_ECHO) && (icmp->type != ICMP_ECHOREPLY)) {
	logmsg_dbg(ANDROID_LOG_WARN,"icmp_to_icmp6/unhandled icmp type: 0x%x", icmp->type);
	return 0;
	}

	memset(icmp6_targ, 0, sizeof(struct icmp6_hdr));
	icmp6_targ->icmp6_type = (icmp->type == ICMP_ECHO) ? ICMP6_ECHO_REQUEST : ICMP6_ECHO_REPLY;
	icmp6_targ->icmp6_code = 0;
	icmp6_targ->icmp6_id = icmp->un.echo.id;
	icmp6_targ->icmp6_seq = icmp->un.echo.sequence;

	icmp6_targ->icmp6_cksum = 0;
	checksum = ip_checksum_add(checksum, icmp6_targ, sizeof(struct icmp6_hdr));
	checksum = ip_checksum_add(checksum, payload, payload_size);
	icmp6_targ->icmp6_cksum = ip_checksum_finish(checksum);

	out[pos].iov_len = sizeof(struct icmp6_hdr);
	out[POS_PAYLOAD].iov_base = (char *) payload;
	out[POS_PAYLOAD].iov_len = payload_size;

	return POS_PAYLOAD + 1;
	}

	/* function: icmp6_to_icmp
	* translate ipv6 icmp to ipv4 icmp (only currently supports echo/echo reply)
	* out - output packet
	* icmp6 - source packet icmp6 header
	* checksum - pseudo-header checksum (unused)
	* payload - icmp6 payload
	* payload_size - size of payload
	* returns: the highest position in the output clat_packet that's filled in
	*/
	int icmp6_to_icmp(clat_packet out, int pos, const struct icmp6_hdr *icmp6, uint32_t checksum,
	const char *payload, size_t payload_size) {
	struct icmphdr *icmp_targ = out[pos].iov_base;

	if((icmp6->icmp6_type != ICMP6_ECHO_REQUEST) && (icmp6->icmp6_type != ICMP6_ECHO_REPLY)) {
	logmsg_dbg(ANDROID_LOG_WARN,"icmp6_to_icmp/unhandled icmp6 type: 0x%x",icmp6->icmp6_type);
	return 0;
	}

	memset(icmp_targ, 0, sizeof(struct icmphdr));

	icmp_targ->type = (icmp6->icmp6_type == ICMP6_ECHO_REQUEST) ? ICMP_ECHO : ICMP_ECHOREPLY;
	icmp_targ->code = 0x0;
	icmp_targ->un.echo.id = icmp6->icmp6_id;
	icmp_targ->un.echo.sequence = icmp6->icmp6_seq;

	icmp_targ->checksum = 0;
	checksum = ip_checksum_add(0, icmp_targ, sizeof(struct icmphdr));
	checksum = ip_checksum_add(checksum, (void *)payload, payload_size);
	icmp_targ->checksum = ip_checksum_finish(checksum);

	out[pos].iov_len = sizeof(struct icmphdr);
	out[POS_PAYLOAD].iov_base = (char *) payload;
	out[POS_PAYLOAD].iov_len = payload_size;

	return POS_PAYLOAD + 1;
	}

	/* function: udp_packet
	* takes a udp packet and sets it up for translation
	* out - output packet
	* udp - pointer to udp header in packet
	* checksum - pseudo-header checksum
	* len - size of ip payload
	*/
	int udp_packet(clat_packet out, int pos, const struct udphdr *udp, uint32_t checksum, size_t len) {
	const char *payload;
	size_t payload_size;

	if(len < sizeof(struct udphdr)) {
	logmsg_dbg(ANDROID_LOG_ERROR,"udp_packet/(too small)");
	return 0;
	}

	payload = (const char *) (udp + 1);
	payload_size = len - sizeof(struct udphdr);

	return udp_translate(out, pos, udp, checksum, payload, payload_size);
	}

	/* function: tcp_packet
	* takes a tcp packet and sets it up for translation
	* out - output packet
	* tcp - pointer to tcp header in packet
	* checksum - pseudo-header checksum
	* len - size of ip payload
	* returns: the highest position in the output clat_packet that's filled in
	*/
	int tcp_packet(clat_packet out, int pos, const struct tcphdr *tcp, uint32_t checksum, size_t len) {
	const char *payload;
	size_t payload_size, header_size;

	if(len < sizeof(struct tcphdr)) {
	logmsg_dbg(ANDROID_LOG_ERROR,"tcp_packet/(too small)");
	return 0;
	}

	if(tcp->doff < 5) {
	logmsg_dbg(ANDROID_LOG_ERROR,"tcp_packet/tcp header length set to less than 5: %x", tcp->doff);
	return 0;
	}

	if((size_t) tcp->doff*4 > len) {
	logmsg_dbg(ANDROID_LOG_ERROR,"tcp_packet/tcp header length set too large: %x", tcp->doff);
	return 0;
	}

	header_size = tcp->doff * 4;
	payload = ((const char *) tcp) + header_size;
	payload_size = len - header_size;

	return tcp_translate(out, pos, tcp, header_size, checksum, payload, payload_size);
	}

	/* function: udp_translate
	* common between ipv4/ipv6 - setup checksum and send udp packet
	* out - output packet
	* udp - udp header
	* checksum - pseudo-header checksum
	* payload - tcp payload
	* payload_size - size of payload
	* returns: the highest position in the output clat_packet that's filled in
	*/
	int udp_translate(clat_packet out, int pos, const struct udphdr *udp, uint32_t checksum,
	const char *payload, size_t payload_size) {
	struct udphdr *udp_targ = out[pos].iov_base;

	memcpy(udp_targ, udp, sizeof(struct udphdr));
	udp_targ->check = 0; // reset checksum, to be calculated

	checksum = ip_checksum_add(checksum, udp_targ, sizeof(struct udphdr));
	checksum = ip_checksum_add(checksum, payload, payload_size);
	udp_targ->check = ip_checksum_finish(checksum);

	out[pos].iov_len = sizeof(struct udphdr);
	out[POS_PAYLOAD].iov_base = (char *) payload;
	out[POS_PAYLOAD].iov_len = payload_size;

	return POS_PAYLOAD + 1;
	}

	/* function: tcp_translate
	* common between ipv4/ipv6 - setup checksum and send tcp packet
	* out - output packet
	* tcp - tcp header
	* header_size - size of tcp header including options
	* checksum - partial checksum covering ipv4/ipv6 header
	* payload - tcp payload
	* payload_size - size of payload
	* returns: the highest position in the output clat_packet that's filled in
	*
	* TODO: mss rewrite
	* TODO: hosts without pmtu discovery - non DF packets will rely on fragmentation (unimplemented)
	*/
	int tcp_translate(clat_packet out, int pos, const struct tcphdr *tcp, size_t header_size,
	uint32_t checksum, const char *payload, size_t payload_size) {
	struct tcphdr *tcp_targ = out[pos].iov_base;
	out[pos].iov_len = header_size;

	if (header_size > MAX_TCP_HDR) {
	// A TCP header cannot be more than MAX_TCP_HDR bytes long because it's a 4-bit field that
	// counts in 4-byte words. So this can never happen unless there is a bug in the caller.
	logmsg(ANDROID_LOG_ERROR, "tcp_translate: header too long %d > %d, truncating",
	header_size, MAX_TCP_HDR);
	header_size = MAX_TCP_HDR;
	}

	memcpy(tcp_targ, tcp, header_size);

	tcp_targ->check = 0;
	checksum = ip_checksum_add(checksum, tcp_targ, header_size);
	checksum = ip_checksum_add(checksum, payload, payload_size);
	tcp_targ->check = ip_checksum_finish(checksum);

	out[POS_PAYLOAD].iov_base = (char *)payload;
	out[POS_PAYLOAD].iov_len = payload_size;

	return POS_PAYLOAD + 1;
	}