CODEWITHSUNDEEP
clinuxudp
Manoharan
Manoharan

Reputation: 21

How to retrieve the source port from a UDP packet while using recvmsg()

I have an application in Linux using C wherein I have created a socket to receive IPv6 UDP packet.

To receive the packet, I use recvmsg() since I need to retrieve the ifindex, which I can get from CMSG_DATA() with the option IPV6_PKTINFO. Now, I need the source port also to be read from the UDP packet. Is there way to get that too?

Upvotes: 2

Views: 3690

Answers (1)

cnicutar
cnicutar

Reputation: 182684

Sure you can, recvmsg looks like this:

ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

And struct msghdr contains msg_name and msg_namelen:

struct msghdr {
   void         *msg_name;       /* optional address */
   socklen_t     msg_namelen;    /* size of address */
...

So you can do something like this for an IPv4 address:

struct sockaddr_in *src = msg->msg_name;
uint16_t port = ntohs(src->sin_port);

And adjust accordingly (sockaddr_in6) for an IPv6 address.

EDIT

I had completely overlooked getnameinfo which R mentions in the comments. This function gets a struct sockaddr * which means it doesn't depend on the address family. That function is way powerful but if all you want is the port, you're probably interested in NI_NUMERICSERV.

Upvotes: 7

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