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Getting the socket FD from IP address and port

Answer 1

+3 A:

You could keep a hash of address => socket mappings.

What I usually do is have struct consisting of the address and the socket FD, representing a host with which I communicate. Then I have a lookup function which finds that struct by address.

Borealid 2010-07-29 15:34:04

This is the right answer - "keep track of it when you create the sockets".

caf 2010-07-30 12:37:19

Yes - if your application wants to do this, it needs to keep track of it. You could use a STL container (e.g. map) with the address / port pair as a key.

MarkR 2010-07-30 22:27:32

Answer 2

A:

No there's no call like that, but with UDP you don't need a separate socket for each destination host/port - that's what sendto(3) is for (unless you actually connect(2) your UDP sockets).

Nikolai N Fetissov 2010-07-29 15:34:30

i am more interested in changing the source IP/port of each datagram I sent out

Aditya Sehgal 2010-07-29 15:50:07

Hmm, if you send it out source IP/port **will change automatically** since it's sent from different host/port. I'm talking about your app C here - just use **one** outbound socket and change the destination IP/port to whatever you need.

Nikolai N Fetissov 2010-07-29 16:01:24

i want the source IP/port to change to what I want not what kernel fills in. If i receive the message from srcPort 2000 to my listening port on 5000, i need to forward this message from srcPort 2000 to the peer's dest port 5000

Aditya Sehgal 2010-07-29 16:03:59

As far as I remember, you can re-`bind(2)` UDP socket to different port. That would give you what you want.

Nikolai N Fetissov 2010-07-29 17:31:58

@Aditya: I suggest to open another question then! But to answer, in short: You may be looking for a raw socket, which you can use to send raw ip packets the way you see fit (including the IP+UDP header you want), using `socket(PF_INET,SOCK_RAW,IPPROTO_IP)`.

mvds 2010-07-29 18:11:04

Aditya Sehgal 2010-07-30 05:36:34

Answer 3

+1 A:

You could have a look at strace netstat -anlp which shows that you can look in /proc/net/udp to find a list of all sockets, like so:

sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode  
53: 0100007F:0035 00000000:0000 07 00000000:00000000 00:00000000 00000000   103        0 11075 2 ffff1231230f90c0

This shows how "some process" is listening at UDP port 53 (for the slow reader, 0x0035 == 53 and 0x0100007F is localhost)

The inode (11075) is the link to the fd. Looking in /proc/<bindpid>/fd/ we see:

...
lrwx------ 1 bind bind 64 2010-07-20 06:26 513 -> socket:[11075]
...

So the fd is 513. I'm not suggesting that you follow this route, but I think it is at least one way to obtain what you asked for.

mvds 2010-07-29 15:53:21

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Getting the socket FD from IP address and port

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