CODEWITHSUNDEEP
socketsudp
Stone Cooper
Stone Cooper

Reputation: 1

Binding to UDP socket *from* a specific IP address

I have packets coming from a specific device directly connected to my machine. When I do a tcpdump -i eno3 -n -n, I can see the packets:

23:58:22.831239 IP 192.168.0.3.6516 > 255.255.255.255.6516: UDP, length 130

eno3 is configured as 192.168.0.10/24

When I set the socket the typical way:

gOptions.sockfd = socket(AF_INET, SOCK_DGRAM, 0);
memset((void *)&gOptions.servaddr, 0, sizeof(struct sockaddr_in));
gOptions.servaddr.sin_family = AF_INET;
inet_pton(AF_INET, gOptions.sourceIP, &(gOptions.servaddr.sin_addr));
gOptions.servaddr.sin_port = htons(gOptions.udpPort);
bind(gOptions.sockfd, (struct sockaddr *)&gOptions.servaddr, sizeof(struct sockaddr_in));

And I use the sourceIP of "255.255.255.255" on port "6516" - it connects and reads.

What I want to do, however, is bind such that I am limiting my connection from the source IP - "192.168.0.3". I have figured out how to connect on the device using either device name ("eno3") of the iface of that device ("192.168.0.10") - but that doesn't help as I may have multiple devices connected to "192.168.0.10" that blab on that port, but I only want the packets from 192.168.0.3 for port 6516.

I thought s_addr - part of sin.addr - was the source IP... but it is not.

Upvotes: 0

Views: 1253

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 598029

You can't bind() to a remote IP/port, only to a local IP/port. So, for what you have described, you need to bind() to the IP/port where the packets are being sent to (192.168.0.10:6516).

Now, you have two options to choose from. You can either:

  • use recvfrom() to receive packets, using its src_addr parameter to be given each sender's IP/port, and then you can discard packets that were not sent from the desired sender (192.168.0.3:6516).

  • or, use connect() to statically assign the desired sender's IP/port (192.168.0.3:6516), and then you can use recv() (not recvfrom()) to receive packets from only that sender.

Upvotes: 1

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