LWIPv6 programming guide

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This is a short guide of the LWIPv6 library. It is intended for programmers wishing to write programs using LWIPv6.

LWIPv6 implements an entire LWIPv4/v6 stack as a library, thus when a program uses LWIPv6 it can interoperate using its own TCP-IP stack (or even multiple LWIPV6 stacks, the library supports many stacks at the same time).

LWIPv6 stacks communicate using three different types of interfaces:

  • tap: (access to /dev/net/tun required) it uses a point to point layer 2 (ethernet) virtual interface with the hosting machine;
  • tun: (access to /dev/net/tun required) similar fo the previous one, it uses a point to point layer 3 (IP) virtual connection;
  • vde: it gets connected to a Virtual Distributed Ethernet switch.


Loading and Linking LWIPV6

A program can use LWIPv6 in three different ways.

  • By linking statically the library.
 gcc -o static static.c /usr/local/lib/liblwipv6.a  -lpthread -ldl

in this case the constructor/destructor must be explicitely called in the code:

 main(int argc,char *argv[])
   /* core of the application */
  • Using a dynamic linking.
 gcc -o dynamic dynamic.c -llwipv6 -lpthread -ldl

lwip_init, lwip_fini are automagically called when the library is loaded. Do not call them in the code.

  • Dynamically loading the dynamic library.

The code appears like this:

 void *handle
 /* handle==NULL in case of errors; to unload the library use: dlclose(handle) */

This application should be compiled in this way:

 gcc -o dynload dynload.c -lpthread -ldl

The advantage of this approach is the lack of direct dependence (requirement) for the lwipv6 library. It is possible to write programs able to run both on systems where lwipv6 is installed and on system where lwipv6 does not exist. The choice of features can be done at run time.

How to start a stack (or several stacks)

A stack descriptor is defined as a (opaque) stucture:

 struct stack *stackd;

The program can start a stack by calling:


If something goes wrong, lwip_stack_new returns NULL. A program can call lwip_stack_new several times to define several TCP-IP stacks.

It is also possible to shut down a stack in this way:


How to use a Hybrid Stack

LWIPv6 is a Hybrid stack. In a raw and intuitive definition, it means that it has only one packet engine (lwipv6) and it is backward compatible with IPv4 using some exceptions in the code where the management is different.

LWIPv6 internal engine uses exclusively IPv6 addresses. All the calls to set up the addresses and routes use addresses defined as:

 struct ip_addr {
     uint32_t addr[4];

This data structure contains an IPv6 address. IPv4 address are stored as IPv4 mapped address, i.e. in the following form: the first 80 bits set to zero, the next 16 set to one, while the last 32 bits are the IPv4 address.

There are macro in the lwipv6 include file to help programmers to define IPv4 and IPv6 addresses and masks.


defines addr as 2001:760::1. IP6ADDR can be used both for address and masks, e.g.


is a /64 mask.

For IPV4 there are two different macros:


define addr4 e mask4 the IPv4 mapped adress and a /24 mask ( respectively.

How to define interfaces, addresses, routes

Once a stack has been created, it is useless until it has a non trivial interface. The loopback lo0 interface is the only one automatically defined in a new stack.

 struct netif *lwip_vdeif_add(struct stack *stack, void *arg);
 struct netif *lwip_tapif_add(struct stack *stack, void *arg);
 struct netif *lwip_tunif_add(struct stack *stack, void *arg);

The three functions above define new interfaces. For tun and tap interfaces, the argument is a string that will be used as the name of the virtual interface. lwip_vdeif_add argument is the path of the vde_switch.

 struct netif *tunnif,*vdenif,*vde2nif;

In this example three interfaces get added to the stack defined by stackd. The first is the tun interface named tun4, the second a vde connection to a switch, the third another connection to the port #4 to the same switch. In fact the square brackets syntax is commonly used in vde to indicate a specfic port of a switch.

Interfaces must be assigned TCP-IP addresses to communicate.

 int lwip_add_addr(struct netif *netif,struct ip_addr *ipaddr, struct ip_addr *netmask);
 int lwip_del_addr(struct netif *netif,struct ip_addr *ipaddr, struct ip_addr *netmask);

for example the following chunk of code sets the address for vdenif.

 struct ip_addr addr4, mask4;

An interface can have several IPv4 and IPv6 addresses. IPv6 supports stateless address autoconfiguration.

In a similar manner it is possible to define routes.

 int lwip_add_route(struct stack *stack, struct ip_addr *addr, struct ip_addr *netmask, 
                                   struct ip_addr *nexthop, struct netif *netif, int flags);
 int lwip_del_route(struct stack *stack, struct ip_addr *addr, struct ip_addr *netmask, 
                                   struct ip_addr *nexthop, struct netif *netif, int flags);

addr/netmask is the destination address for the route. nexthop is the next hop destination address and netif is the network interface where the packet must be dispatched. To define a default route, use IPADDR_ANY both for address and for netmask.


 struct ip_addr gwaddr4;
 lwip_add_route(stackd, IPADDR_ANY, IPADDR_ANY, &gwaddr4, vdenif, 0);

defines the default route to be on interface vdenif.

Remember to turn on the interfaces!

All the interfaces added by lwip_vdeif_add, lwip_tunif_add or lwip_tapif_add are disabled upon creation. lwip_ifup turns on an interface, lwip_ifdown turns it off. e.g.


Remember to turn on the interfaces otherwise the stack won't work!

How to use a stack (or several stacks)

lwip_msocket is similar to the msocket call defined by the multiple stack exstension of the Berkeley socket API definition (msockets). The sole difference between the signature of msocket and lwip_socket is that the socket decriptor gets used instead of the pathname of the stack special file.

 int lwip_msocket(struct stack *stack, int domain, int type, int protocol);

For example, a TCP (V4) socket on the lwip stack stackd gets created by the following call.

 fd=lwip_msocket(stackd, AF_INET, SOCK_STREAM, 0);

fd can be used in Berkeley Sockets API like calls: lwip_bind, lwip_connect, lwip_accept, lwip_recv, lwip_send ... that correspond to bind, accept, recv, send, etc.

"sockaddr" parameters (like in bind, connect, etc) use the standard definitions (sockaddr_in, sockaddr_in6).

For application using only one stack (or at least one stack at a time) it is possible to define the default stack:


If the default stack has been already defined the call

 lwip_socket(AF_INET, SOCK_STREAM, 0);

implicitely refers to stackd. THe default stack gets defined for the whole library, thus the use of default networks is discouraged on multithreaded applications working on several stack concurrently.

A complete example

The following code is a simple TCP terminal emulator working on LWIPv6. It works like the utility nc used as a TCP client. In fact our utility (say it is named lwipnc):

 lwipnc 9999

has the same behavior of:

 nc 9999

One way to test this program is by starting a tcp server on the other end of the network link:

 nc -l -p 9999

Here is the code of lwipnc.c:

/* Copyright 2008 Renzo Davoli for LWIPv6 documentation.
 * Licensed inder the GPLv2
 * Minimal terminal emulator on a TCP socket

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <lwipv6.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>

#define BUFSIZE 1024
char buf[BUFSIZE];

int main(int argc,char *argv[])
  struct sockaddr_in serv_addr;
  int fd;
  void *handle;
  struct stack *stack;
  struct netif *nif;
  struct ip_addr addr;
  struct ip_addr mask;

#ifdef LWIPV6DL
  /* Run-time load the library (if requested) */
  if ((handle=loadlwipv6dl()) == NULL) {
    perror("LWIP lib not loaded");
  /* define a new stack */
    perror("Lwipstack not created");
  /* add an interface */
    perror("Interface not loaded");
  /* set the local IP address of the interface */
  /* turn on the interface */

  memset((char *) &serv_addr,0,sizeof(serv_addr));
  serv_addr.sin_family      = AF_INET;
  serv_addr.sin_addr.s_addr = inet_addr(argv[1]);
  serv_addr.sin_port        = htons(atoi(argv[2]));

  /* create a TCP lwipv6 socket */
  if((fd=lwip_msocket(stack,PF_INET,SOCK_STREAM,0))<0) {
    perror("Socket opening error");
  /* connect it to the address specified as argv[1] port argv[2] */
  if (lwip_connect(fd,(struct sockaddr *)(&serv_addr),sizeof(serv_addr)) < 0) {
    perror("Socket connecting error");
  while(1) {
    fd_set rfds;
    int n;
    /* wait for input both from stdin and from the socket */
    /* copy data from the socket to stdout */
    if(FD_ISSET(fd,&rfds)) {
      if((n=lwip_read(fd,buf,BUFSIZE)) == 0)
    /* copy data from stdin to the socket */
    if(FD_ISSET(STDIN_FILENO,&rfds)) {
      if((n=read(STDIN_FILENO,buf,BUFSIZE)) == 0)

Compile it using lwipv6 as a dynamic library in this way:

gcc -o lwipnc lwipnc.c -ldl -lpthread -llwipv6

or as a run-time dinamically loaded library in this way:

gcc -o lwipnc lwipnc.c -D LWIPV6DL -ldl -lpthread

It is possible to run the same example on a tun or on a tap interface just by changing the source code line:






A different model for asynchrony: event_subscribe

LWIPv6 provides lwip_select, lwip_pselect, lwip_poll, lwip_ppoll having the same semantics of the correspondent system call (those without the prefix lwip_). These calls are useful when porting applications using the standard Berkeley socket API to LWIPv6.

There is however in LWIPv6 another way to deal with asynchronous events generated by the stack:

 typedef void (*lwipvoidfun)();
 int lwip_event_subscribe(void (*cb)(void *), void *arg, int fd, int how);

cb is the address of a callback function (or NULL), arg is the argument that will be passed to the callback function, fd is a LWIPv6 file descriptor, how is an event code. how gets the same encoding of events as in poll(2). The return value is a bitmask filled in with the event that actually occured. (The return value always reports a subset of events with respect to those encoded in how). This function has three different meanings:

  • If cb==NULL and arg==NULL, it tests which events(s) already happened.



rv is non-zero if there is data to read.

  • if cb!=NULL LWIPv6 tests which events(s) among those defined in how already happened. If rv==0, i.e. no one of the event happened, it subscribes for a notification. When an event of how happens LWIPv6 calls cb(arg).
  • If cb==NULL, lwip_event_subscribe checks again to see which event(s) happened. If there is a pending notification request with the same arg, it is cancelled.

List of most relevant functions provided by LWIPv6

Constructor/destructor: do not call these functions unless you are writing a statically linked program

 void lwip_init(void);
 void lwip_fini(void);

Define a new stack, terminate an existing stack:

 struct stack *lwip_stack_new(void);
 void lwip_stack_free(struct stack *stack);

Set/Get the current default stack (for lwip_socket).

 struct stack *lwip_stack_get(void);
 void lwip_stack_set(struct stack *stack);

Define new interfaces:

 struct netif *lwip_vdeif_add(struct stack *stack, void *arg);
 struct netif *lwip_tapif_add(struct stack *stack, void *arg);
 struct netif *lwip_tunif_add(struct stack *stack, void *arg);

Add/delete addresses:

 int lwip_add_addr(struct netif *netif,struct ip_addr *ipaddr, struct ip_addr *netmask);
 int lwip_del_addr(struct netif *netif,struct ip_addr *ipaddr, struct ip_addr *netmask);

Add/delete routes:

 int lwip_add_route(struct stack *stack, struct ip_addr *addr, struct ip_addr *netmask, struct ip_addr *nexthop, struct netif *netif, int flags);
 int lwip_del_route(struct stack *stack, struct ip_addr *addr, struct ip_addr *netmask, struct ip_addr *nexthop, struct netif *netif, int flags);

Turn the interface up/down:

 int lwip_ifup(struct netif *netif);
 int lwip_ifdown(struct netif *netif);

LWIPv6 implementation of comm syscalls:

 int lwip_msocket(struct stack *stack, int domain, int type, int protocol);
 int lwip_socket(int domain, int type, int protocol);
 int lwip_bind(int s, struct sockaddr *name, socklen_t namelen);
 int lwip_connect(int s, struct sockaddr *name, socklen_t namelen);
 int lwip_listen(int s, int backlog);
 int lwip_accept(int s, struct sockaddr *addr, socklen_t *addrlen);
 int lwip_getsockname (int s, struct sockaddr *name, socklen_t *namelen);
 int lwip_getpeername (int s, struct sockaddr *name, socklen_t *namelen);
 int lwip_send(int s, void *dataptr, int size, unsigned int flags);
 int lwip_recv(int s, void *mem, int len, unsigned int flags);
 int lwip_sendto(int s, void *dataptr, int size, unsigned int flags,
       struct sockaddr *to, socklen_t tolen);
 int lwip_recvfrom(int s, void *mem, int len, unsigned int flags,
         struct sockaddr *from, socklen_t *fromlen);
 int lwip_shutdown(int s, int how);
 int lwip_setsockopt (int s, int level, int optname, const void *optval, socklen_t optlen);
 int lwip_getsockopt (int s, int level, int optname, void *optval, socklen_t *optlen);
 int lwip_sendmsg(int fd, const struct msghdr *msg, int flags);
 int lwip_recvmsg(int fd, struct msghdr *msg, int flags);
 int lwip_write(int s, void *dataptr, int size);
 int lwip_read(int s, void *mem, int len);
 int lwip_writev(int s, struct iovec *vector, int count);
 int lwip_readv(int s, struct iovec *vector, int count);
 int lwip_ioctl(int s, long cmd, void *argp);
 int lwip_close(int s);
 int lwip_select(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset,
   struct timeval *timeout);
 int lwip_pselect(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset,
   const struct timespec *timeout, const sigset_t *sigmask);
 int lwip_poll(struct pollfd *fds, nfds_t nfds, int timeout);
 int lwip_ppoll(struct pollfd *fds, nfds_t nfds,
   const struct timespec *timeout, const sigset_t *sigmask);

Management of asynchronous events:

 int lwip_event_subscribe(lwipvoidfun cb, void *arg, int fd, int how);
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