There seems to be no portable way to set the source IP for sending UDP datagrams from sockets bound to INADDR_ANY, but at least on Linux and FreeBSD it can be done using sendmsg() and the IP_PKTINFO (Linux) or IP_SENDSRCADDR (FreeBSD) option. (See this question.)
Is there an equivalent option to set the UDP source IP on Windows ?
From MSDN:
WSASendMsg function
[...]
On an IPv4 socket of type SOCK_DGRAM or SOCK_RAW, an application can specific the local IP source address to use for sending with the WSASendMsg function. One of the control data objects passed in the WSAMSG structure to the WSASendMsg function may contain an in_pktinfo structure used to specify the local IPv4 source address to use for sending.
The same applies to an IPv6 socket with the in6_pktinfo structure.
For dual-mode sockets, it is important that an IPv4 source address is not specified as an IPv4-mapped IPv6 address in the in6_pktinfo, but as an IPv4 address in the in_pktinfo structure.
Example:
union {
char in[WSA_CMSG_SPACE(sizeof(struct in_pktinfo))];
char in6[WSA_CMSG_SPACE(sizeof(struct in6_pktinfo))];
} cdata;
WSAMSG msg;
memset(&msg, 0, sizeof(msg));
msg.name = &remote_sysaddr.addr.generic;
msg.namelen = remote_sysaddr.len;
msg.lpBuffers = &buf;
msg.dwBufferCount = 1;
msg.Control.buf = (char *)&cdata;
msg.Control.len = sizeof(cdata);
int sum = 0;
WSACMSGHDR *cmsg = WSA_CMSG_FIRSTHDR(&msg);
...
memset(cmsg, 0, WSA_CMSG_SPACE(sizeof(struct in_pktinfo)));
cmsg->cmsg_level = IPPROTO_IP;
cmsg->cmsg_type = IP_PKTINFO;
cmsg->cmsg_len = WSA_CMSG_LEN(sizeof(struct in_pktinfo));
struct in_pktinfo *pktinfo = (struct in_pktinfo *)WSA_CMSG_DATA(cmsg);
pktinfo->ipi_addr.s_addr = local_addr->ipv4;
sum += WSA_CMSG_SPACE(sizeof(struct in_pktinfo));
...
msg.Control.len = sum;
if (bs->WSASendMsg(bs->socket, &msg, 0, &bytes, NULL, NULL) != 0) {
...
I think this is it.
UdpClient Constructor (IPEndPoint)
This constructor creates a new UdpClient and binds it to the IPEndPoint specified by the localEP parameter. Before you call this constructor, you must create an IPEndPoint using the IP address and port number from which you intend to send and receive data. You do not need to specify a local IP address and port number for sending and receiving data. If you do not, the underlying service provider will assign the most appropriate local IP address and port number.
Related
I am trying to build a packet that will be sent via UDP. However I am not getting the correct data on the receiving side.
In the packet I want to include an IP Header, UDP Header, and the data that needs to be sent. In this case I just want to send the word "Hello" along with random header information.
char *data = "Hello";
char *packet = (char *)malloc(sizeof(struct iphdr) + sizeof(struct udphdr) + strlen(data));
struct iphdr *ip = (struct iphdr*) packet;
struct udphdr *udp = (struct udphdr*) (packet + sizeof(struct iphdr));
char *send_buff = (char *) (packet + sizeof(struct iphdr) + sizeof(struct udphdr));
ip->saddr = inet_addr("1.2.3.4");
ip->daddr = inet_addr("5.6.7.8");
ip->ttl = 5;
udp->source = 5950;
udp->dest = 5950;
udp->len = sizeof(struct udphdr);
udp->check = 0;
strcpy(send_buff, data);
sendto(sock, packet, (sizeof(struct iphdr) + sizeof(struct udphdr) + strlen(data)), ROUTER_IP);
The problem I'm having is that the receiving end just gets random data so I'm assuming the number of bytes is incorrect somewhere.
On the receiving side I have it print out one of the fields of the IP header as a test, but it's not correct.
char recv_buff[1000];
int recv_bytes = recvfrom(sock, recv_buff, sizeof(recv_buff));
struct iphdr *ip = (struct iphdr*) recv_buff;
cout << static_cast<int16_t>(ip->ttl) << endl;
Am I putting the packet together wrong or is there a problem on the receiving end?
I used this example http://www.winlab.rutgers.edu/~zhibinwu/html/c_prog.htm as a reference for putting together the packet.
You are creating the socket as socket(AF_INET, SOCK_DGRAM, 0); meaning that it's a datagram (=UDP, typically) socket, so the network stack will automatically include IP header & UDP headers, etc.
But since you are trying to create your own IP and UDP headers you must create a raw socket, then send the packet (and also calculate the checksum as your reference code is doing).
To create a raw socket, use socket(AF_INET, SOCK_RAW, 0).
Besides the problem with not using raw sockets, you also don't set e.g. port numbers correctly. The have to be in network byte-order, so you should use e.g. htons for that. There are also other fields that should be in network byte orders.
if you are using your own ip and udp headers ON TOP of the stack's I hope you are parsing the data after removing both headers of yours and stack's. If the receiving socket is RAW, you will get the ip and udp headers of the stack as well.
I am currently working on an IPv6 class and use inet_pton to retrieve the actual binary representation of the IP from a string i.e.:
AdressV6::AdressV6(const String & _ip)
{
int result = inet_pton(AF_INET6, _ip.c_str(), &(m_nativeAdress));
if(result <= 0)
//throw...
//How can I retrieve the sope ID from that?
}
Is there a common way to do that? Do you just manually parse the string and look for the "%" that does not sound very bullet proof :(
Thank you!
I tried manual parsing for now which seems to work. Still, if there is a better way please let me know:
//retrieve scope ID
uint32 scopeId = 0;
size_t pos = _ip.find("%");
if(pos != String::npos)
{
String theId = _ip.substr(pos+1);
scopeId = atoi(theId.c_str());
}
m_scopeId = scopeId;
On BSD and BSD based systems (this includes MacOS X for example), the scope ID is embedded into the address itself for link local addresses as the second 16 bit word. Please refer to the FreeBSD Handbook and search for "8.1.1.3 Scope Index" (without the quotes).
So assuming that intf1 has scope ID 1 and intf2 has scope ID 2, inet_pton() will convert the strings as follows on these platforms:
"fe80::1234%intf1" -> fe80:1::1234
"fe80::1234%intf2" -> fe80:2::1234
"fe80::1234" -> fe80::1234
The last address is simply unscoped and thus cannot be really used for sending out data.
Please note that this is non-standard; inet_pton() does not work that way on Linux or Windows based systems. However, I think even on Linux and Windows based systems, inet_pton() allows a scope ID at the end, it will simply ignore it, though.
For non-link-local address, this trick doesn't work, of course, yet those addresses are usually not scoped. They can be scoped, but usually every interface has an own, unique interface IPv6 address, based on its interface identifier (even if you use DHCPv6, in which case it has a DHCP address assigned by the DHCP server, as well as the auto generated IPv6 interface address, unless this auto generation has been forbidden).
The struct sockaddr_in6 structure has a field for the scope ID but the RFC that defines this field (RFC 2553 - Section 3.3) does not really give much detail how this field is to be interpreted. It only says:
The mapping of sin6_scope_id to an interface or set of interfaces is
left to implementation and future specifications on the subject of
site identifiers.
So this field is entirely implementation specific.
If you want this field to be filled in correctly, and your code should be as cross-platform as possible, you should use getaddrinfo():
struct addrinfo hints;
struct addrinfo * result;
memset(&hints, 0, sizeof(hints));
// AI_NUMERICHOST prevents usage of DNS servers,
// it tells getaddrinfo that the input string is a numeric IP address.
hints.flags = AI_NUMERICHOST;
if (getaddrinfo("fe80::1234%intf1", NULL, &hints, &result) == 0) {
// result->ai_addr claims to be a pointer to struct sockaddr,
// in fact it will be a pointer to a struct sockaddr_in6 in our case.
struct sockaddr_in6 * so = (struct sockaddr_in6 *)result->ai_addr;
// It will be prefilled like this:
//
// so->sin6_family ==> AF_INET6;
// so->sin6_port ==> 0
// so->sin6_flowinfo ==> 0
// so->sin6_addr ==> fe80::1234
// so->sin6_scope_id ==> "intf1" as scope ID
// Do something with that sockaddr,
// e.g. set a port number and connect a socket to that address.
freeaddrinfo(result);
}
One extra tip: If you want to use the returned getaddrinfo() for a server socket (a socket that you want to bind locally and then call accept() on it), you should also set the passive flag:
hints.flags = AI_NUMERICHOST | AI_PASSIVE;
Not that it will play a role in most case but that is the correct way of using getaddrinfo().
inet_pton() does not support scope IDs. I don't know about other platforms, but on Windows you can use RtlIpv6StringToAddressEx() instead.
inet_pton() semi-supports scope identifiers, the scope is that it will not raise an error when parsing an address with one. The major limitation is that the parameter to the call is a struct in6_addr which does not contain a field for the scope identifier, the super structure struct sockaddr_in6 is required for that.
Easy way forward is to wrap getnameinfo() and getaddrinfo() with struct sockaddr parameters for convenience. For example,
socklen_t
sockaddr_len (
const struct sockaddr* sa
)
{
socklen_t sa_len;
switch (sa->sa_family) {
case AF_INET: sa_len = sizeof(struct sockaddr_in); break;
case AF_INET6: sa_len = sizeof(struct sockaddr_in6); break;
default: sa_len = 0; break;
}
return sa_len;
}
int
sockaddr_ntop (
const struct sockaddr* restrict sa,
char* restrict host,
size_t hostlen
)
{
return getnameinfo (sa, sockaddr_len (sa),
host, hostlen,
NULL, 0,
NI_NUMERICHOST);
}
int
sockaddr_pton (
const char* restrict src,
struct sockaddr* restrict dst /* will error on wrong size */
)
{
struct addrinfo hints = {
.ai_family = AF_UNSPEC,
.ai_socktype = SOCK_STREAM, /* not really */
.ai_protocol = IPPROTO_TCP, /* not really */
.ai_flags = AI_NUMERICHOST
}, *result = NULL;
const int status = getaddrinfo (src, NULL, &hints, &result);
if (0 == status) {
memcpy (dst, result->ai_addr, result->ai_addrlen);
freeaddrinfo (result);
return 1;
}
return 0;
}
To answer the original premise but given a struct sockaddr, an additional API may be warranted, for example:
uint32_t
sockaddr_scope_id (
const struct sockaddr* sa
)
{
uint32_t scope_id;
if (AF_INET6 == sa->sa_family) {
struct sockaddr_in6 s6;
memcpy (&s6, sa, sizeof(s6));
scope_id = s6.sin6_scope_id;
} else
scope_id = 0;
return scope_id;
}
I'm new to Windows networking, and I am trying to find out which PORT number my socket is bound to (C++, Windows 7, Visual Studio 2010 Professional). It is a UDP socket, and from what I understand, using the following initial setup should bind it to a random available port/address:
sockaddr_in local;
local.sin_family = AF_INET;
local.sin_addr.s_addr = INADDR_ANY;
local.sin_port = 0; //randomly selected port
int result = bind(clientSock, (sockaddr*)&local, sizeof(local));
//result is always 0
As far as using this method, it works for sending data or binding it to a specific port (replacing the 0 with a desired port number). What I need is to bind it randomly, and then find out which port it was bound to afterwards. Is there any way I can do this? It seems that the "local" struct contains "0.0.0.0" as the IP address and "0" as the PORT number.
Thanks for any and all help! I appreciate it.
Use getsockname. For example:
struct sockaddr_in sin;
int addrlen = sizeof(sin);
if(getsockname(clientSock, (struct sockaddr *)&sin, &addrlen) == 0 &&
sin.sin_family == AF_INET &&
addrlen == sizeof(sin))
{
int local_port = ntohs(sin.sin_port);
}
else
; // handle error
This also works for *nix-based systems, but note that some systems define the third argument of getsockname to be of type socklen_t* instead of int*, so you might get warnings about pointers differing in signedness if you're writing cross-platform code.
I am writing a small program that tests an UDP network service. The implementation of the service is allowed to create a new socket for the session and respond to the client from there, at which point the client is then required to talk to this address (similar to TFTP).
Minimal client sans error checking looks like this:
int fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
sockaddr_in destaddr = { ... };
MSGBUF msg[] = { ... };
DWORD sent;
WSASendTo(fd, msg, sizeof msg / sizeof *msg, &sent, 0, (sockaddr *)sa, sizeof sa, 0, 0);
char buffer[4096];
MSGBUF rcvmsg = { sizeof buffer, buffer };
DWORD received;
sockaddr_storage sa;
socklen_t sa_len = sizeof sa;
DWORD flags = 0;
WSARecvFrom(fd, &rcvmsg, 1, &received, &flags, (sockaddr *)&sa, &sa_len, 0, 0);
The client works fine if the server responds from the same address and port that the initial message was sent to, however replies from another port are silently discarded and the client hangs in WSARecvFrom.
Explicitly binding the socket to { AF_INET, INADDR_ANY, 0 } to force assignment of a local port, or invoking listen(fd, 5); makes no difference, as expected.
Is there anything in WSASendTo that implicitly connects an UDP socket, and if so, what should I do to avoid this?
UDP doesn't have connections. Datagrams are sent to and from ports; it's one-way communication.
It sounds to me like your server is letting itself be assigned a temporary port (i.e. passing 0 as the port in sockaddr_in), instead of using a specific port. This won't work.
Since UDP has no concept of a connection, each time you send data, it could be sent from a different port; the first send doesn't reserve the port that it was given, it just sends a datagram from it and then lets it go.
Your server should be binding itself to a specific port.
Meh, it was a firewall issue. Adding the application to the list of programs allowed to receive incoming traffic fixed the issue.
You have multiple network adapters.
Bind a UDP socket to an local port, without specifying an address.
Receive packets on one of the adapters.
How do you get the local ip address of the adapter which received the packet?
The question is, "What is the ip address from the receiver adapter?" not the address from the sender which we get in the
receive_from( ..., &senderAddr, ... );
call.
You could enumerate all the network adapters, get their IP addresses and compare the part covered by the subnet mask with the sender's address.
Like:
IPAddress FindLocalIPAddressOfIncomingPacket( senderAddr )
{
foreach( adapter in EnumAllNetworkAdapters() )
{
adapterSubnet = adapter.subnetmask & adapter.ipaddress;
senderSubnet = adapter.subnetmask & senderAddr;
if( adapterSubnet == senderSubnet )
{
return adapter.ipaddress;
}
}
}
The solution provided by timbo assumes that the address ranges are unique and not overlapping. While this is usually the case, it isn't a generic solution.
There is an excellent implementation of a function that does exactly what you're after provided in the Steven's book "Unix network programming" (section 20.2)
This is a function based on recvmsg(), rather than recvfrom(). If your socket has the IP_RECVIF option enabled then recvmsg() will return the index of the interface on which the packet was received. This can then be used to look up the destination address.
The source code is available here. The function in question is 'recvfrom_flags()'
G'day,
I assume that you've done your bind using INADDR_ANY to specify the address.
If this is the case, then the semantics of INADDR_ANY is such that a UDP socket is created on the port specified on all of your interfaces. The socket is going to get all packets sent to all interfaces on the port specified.
When sending using this socket, the lowest numbered interface is used. The outgoing sender's address field is set to the IP address of that first outgoing interface used.
First outgoing interface is defined as the sequence when you do an ifconfig -a. It will probably be eth0.
HTH.
cheers,
Rob
Unfortunately the sendto and recvfrom API calls are fundamentally broken when used with sockets bound to "Any IP" because they have no field for local IP information.
So what can you do about it?
You can guess (for example based on the routing table).
You can get a list of local addresses and bind a seperate socket to each local address.
You can use newer APIs that support this information. There are two parts to this, firstly you have to use the relavent socket option (ip_recvif for IPv4, ipv6_recvif for IPv6) to tell the stack you want this information. Then you have to use a different function (recvmsg on linux and several other unix-like systems, WSArecvmsg on windows) to receive the packet.
None of these options are great. Guessing will obviously produce wrong answers soemtimes. Binding seperate sockets increases the complexity of your software and causes problems if the list of local addresses changes will your program is running. The newer APIs are the correct techical soloution but may reduce portability (in particular it looks like WSArecvmsg is not available on windows XP) and may require modifications to the socket wrapper library you are using.
Edit looks like I was wrong, it seems the MS documentation is misleading and that WSArecvmsg is available on windows XP. See https://stackoverflow.com/a/37334943/5083516
In Linux environment, you can use recvmsg to get local ip address.
//create socket and bind to local address:INADDR_ANY:
int s = socket(PF_INET,SOCK_DGRAM,0);
bind(s,(struct sockaddr *)&myAddr,sizeof(myAddr)) ;
// set option
int onFlag=1;
int ret = setsockopt(s,IPPROTO_IP,IP_PKTINFO,&onFlag,sizeof(onFlag));
// prepare buffers
// receive data buffer
char dataBuf[1024] ;
struct iovec iov = {
.iov_base=dataBuf,
.iov_len=sizeof(dataBuf)
} ;
// control buffer
char cBuf[1024] ;
// message
struct msghdr msg = {
.msg_name=NULL, // to receive peer addr with struct sockaddr_in
.msg_namelen=0, // sizeof(struct sockaddr_in)
.msg_iov=&iov,
.msg_iovlen=1,
.msg_control=cBuf,
.msg_controllen=sizeof(cBuf)
} ;
while(1) {
// reset buffers
msg.msg_iov[0].iov_base = dataBuf ;
msg.msg_iov[0].iov_len = sizeof(dataBuf) ;
msg.msg_control = cBuf ;
msg.msg_controllen = sizeof(cBuf) ;
// receive
recvmsg(s,&msg,0);
for( struct cmsghdr* pcmsg=CMSG_FIRSTHDR(&msg);
pcmsg!=NULL; pcmsg=CMSG_NXTHDR(&msg,pcmsg) ) {
if(pcmsg->cmsg_level==IPPROTO_IP && pcmsg->cmsg_type==IP_PKTINFO) {
struct in_pktinfo * pktinfo=(struct in_pktinfo *)CMSG_DATA(pcmsg);
printf("ifindex=%d ip=%s\n", pktinfo->ipi_ifindex, inet_ntoa(pktinfo->ipi_addr)) ;
}
}
}
The following does not work in asymmetric routing environment.
you can first set SO_REUSEADDR to true
BOOL bOptVal = 1;
setsockopt(so, SOL_SOCKET, SO_REUSEADDR, (char *)&boOptVal, sizeof(bOptVal));
after receive_from( ..., &remoteAddr, ... ); create another socket, and connect back to remoteAddr. Then call getsockname can get the ip address.
SOCKET skNew = socket( )
// Same local address and port as that of your first socket
// INADDR_ANY
bind(skNew, , )
// set SO_REUSEADDR to true again
setsockopt(skNew, SOL_SOCKET, SO_REUSEADDR, (char *)&boOptVal, sizeof(bOptVal));
// connect back
connect(skNew, remoteAddr)
// get local address of the socket
getsocketname(skNew, )
ssize_t
recvfrom(int socket, void *restrict buffer, size_t length, int flags,
struct sockaddr *restrict address, socklen_t *restrict address_len);
ssize_t
recvmsg(int socket, struct msghdr *message, int flags);
[..]
If address is not a null pointer and the socket is not connection-oriented, the
source address of the message is filled in.
Actual code:
int nbytes = recvfrom(sock, buf, MAXBUFSIZE, MSG_WAITALL, (struct sockaddr *)&bindaddr, &addrlen);
fprintf(stdout, "Read %d bytes on local address %s\n", nbytes, inet_ntoa(bindaddr.sin_addr.s_addr));
hope this helps.
Try this:
gethostbyname("localhost");