I'd like to have two sockets which, by setsockopt(), are set with SO_REUSEADDR and SO_REUSEPORT.
I'd like both of them to capture the same incoming packet for the address x.x.x.x and port y.
Any idea how to do this, without using a multicast receiver?
Related
I got a program which does
...
/* Only rx/tx packets on the interface */
if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, name, strlen(name))) {
...
}
...
for each given ethernet interface and uses UDP.
I want every interface to use the same IP I specified (for example: 50.0.0.1/24).
So that packets coming out from eth0 tell "my source IP is 50.0.0.1"
and packets from eth1 tell the same("source IP: 50.0.0.1").
Both receiving and sending in required.
Is there any way to achieve this?
I've tried
addr.sin_addr.s_addr = inet_addr("50.0.0.1");
...
bind(fd, (struct sockaddr *)&addr, sizeof(addr));
but it won't work (error: cannot assign IP address) unless I set an unrelated interface's IP to 50.0.0.1.
After that, the packets coming out say "source IP: 50.0.0.1" but sending packets with "source IP: 50.0.0.X" to the machine (which runs the above program), it will not receive any.
You can't bind a socket to an interface and an IP at the same time, only one or the other. If an interface is bound, its IP gets used. If an IP is bound, its interface gets used.
And, you can't use bind() to set a source IP that does not belong to the bound interface.
On some platforms (Linux, etc), you can use sendmsg() with IP_PKTINFO to specify a source IP for outgoing packets. However, the OS will lookup and use the interface that belongs to the specified IP, which could be different than the bound interface, so this doesn't address your issue.
So, you will likely have to use a RAW socket and send your UDP packets with a custom IP header, then you can populate that header however you want.
I found the solution.
ip addr add 50.0.0.1/24 dev eth0
ip addr add 50.0.0.1/24 dev eth1
Start the program which does
setsocketopt(fd1, SOL_SOCKET, SO_BINDTODEVICE, "eth0", strlen("eth0"));
...
setsocketopt(fd2, SOL_SOCKET, SO_BINDTODEVICE, "eth1", strlen("eth1"));
Works like a charm.
They all send packets with source IP "50.0.0.1".
And are able receive packets belong to 50.0.0.0/24 network.
The output packets will not go to the wrong interface due to the socket is bond to specific interface(SO_BINDTODEVICE socket option).
This is a macOS question. I am trying to setup a UDP socket that receives SSDP messages, i.e. UDP packets, sent to multicast addresses. I want to restrict receiving these packets from a single network interface.
I tried
int fd = socket(AF_INET, SOCK_DGRAM, 0);
char* itf = "en0";
int res = setsockopt(fd, SOL_SOCKET, IP_RECVIF, itf, strlen(itf));
The setsockopt call fails with errno 42 (Protocol not available).
I have also found SO_BINDTODEVICE that can be used for the same purpose, but it seems that this is not available on macOS.
Using bind with port and address also does not work. Then no packets sent to the multicast address are received on that socket.
From the OSX documentation on IP multicast...
A host must become a member of a multicast group before it can receive datagrams sent to the group. To join a multicast group, use the IP_ADD_MEMBERSHIP option...
To receive multicast traffic on a specific interface you need to tell the OS that you want to join that multicast group. Follow these steps (you were almost there)...
Create a datagram socket (done).
Bind to INADDR_ANY with the expected port.
Join the multicast group via setsockopt() with the IP_ADD_MEMBERSHIP option. Here you can pass the IP address of the specific network interface you wish to receive multicast traffic on in the ip_mreq struct.
I need to do an important test. The test's condition is as bellow.
I am using winsock and I have 2 UDP sockets (Sock-A and Sock-B) which with SO_REUSEADDR option both of them have been bound to port 1000 on one PC.
Both of them transmit UDP packets out to another socket (Sock-C) and Sock-C which is located in different LAN receives their messages.
Sock-C responds them with some messages, but unfortunately I can see just Sock-A (which has been opened sooner) just gets the messages, and Sock-B doesn't get anything. When I close Sock-A I can see that Sock-B starts to receiving the messages.
Any of you know what should I do to let both of Sock-A and Sock-B can receive messages from Sock-C?
Thanks~
This is normal behavior. I think this can't be done with 2 sockets listening on the same port.
Why are you binding 2 sockets to the same port?
Read Socket options SO_REUSEADDR and SO_REUSEPORT, how do they differ? Do they mean the same across all major operating systems?
In linux it will even try to distribute the datagrams evenly between the 2 sockets, so its random. You will need to change how you send/receive packets.
Note: the question was edited.
I have few questions on Berkeley Socket Programming:
Is it possible to change a socket address after binding? If so- What are the C commands to do so?
2.According to https://www.cs.cmu.edu/~srini/15-441/F01.full/www/assignments/P2/htmlsim_split/node18.html, when a socket is bind to INADDR_ANY, it receives packets from all interfaces, but when a packet is sent,(using sent command) it sends through a single NIC with the default IP.
If I understand correctly- if a server has two active NICs, with different IPs, then a socket with the INADDR_ANY parameter can receive a packet with dst IP=x and send a packet with src IP=y, where x is not y. This can cause problems, for example in TCP connections, where the socket in the other destination is will receive the packet, but will drop it due as the dest IP is not the one expected.
Is it true? And if so- does it means that server programs are NOT using INADDR_ANY where there are (two or more) active NICs with different IPs?
Suppose the NIC of the default IP causes bottleneck. Can we change the socket options so that the packets will be send through another NICs(and not the previous NIC)? Can we do so if the NICs have the default IP address?
Can we send packets through one NIC, and set the IP destination to another NIC? (I.e, NIC1 will only send packets, and NIC2 will only receive packets )
Is it possible to change a socket address after created?
Sockets don't have an IP address to change when created. They get one when bound.
If so- What are the C commands to do so?
The Berkeley Sockets API functions to do so are bind() and connect().
When a socket is bind to INADDR_ANY, it receives packets from all interfaces, but sends through a single NIC with the default IP.
No. It sends packets via whichever NIC is necessary to reach the target in each case.
Your cited source draws a distinction without a difference. A socket bound to INADDR_ANY receives packets from any interface. There is no difference between 'any' and 'all' as far as INADDR_ANY is concerned. It is far easier to understand as 'any'.
If I understand correctly- if a server has two active NICs, with different IPs, then a socket with the INADDR_ANY parameter can receive a packet with dst IP=x and send a packet with src IP=y, where x is not y
No. It sends the packet with the same source address the client originally connected to. That's part of what defines the connection.
This can cause problems, for example in TCP connections, where the socket in the other destination is will receive the packet, but will drop it due as the dest IP is not the one expected.
No. The destination in the packet is the client's IP address. Otherwise it wouldn't even get there. This is just nonsense. If you mean the source IP, see above.
Is it true?
No.
And if so- does it means that server programs are NOT using INADDR_ANY where there are (two or more) active NICs with different IPs?
No. INADDR_ANY means exactly what it says. Any IP address: any NIC.
Suppose the NIC of the default IP causes bottleneck. Can we change the socket options so that the packets will be send through another NICs (and not the previous NIC)?
No, but you can alter the IP routing tables.
Can we do so if the NICs have the default IP address?
Only one of them can have the default IP address. The question doesn't make sense.
Can we send packets through one NIC, and set the destination to another NIC?
Only if you're sending to yourself. Otherwise the question doesn't make sense.
(from your citation) When sending, a socket bound with INADDR_ANY binds to the default IP address, which is that of the lowest-numbered interface
I hope this refers to whatever simulator is being described. If it is meant to be a description of how TCP works it is wrong.
Actually, if i create multiple RAW sockets with the same IP Address. I could bind all of them, and consequently packets are received by all the sockets.
Is there any way that could be avoided, such that the other process trying to bind the same ip address receives an error?
I am using a raw socket
#include <sys/socket.h>
#include <netinet/in.h>
raw_socket = socket(AF_INET, SOCK_RAW, int protocol);
In Man Page raw(7)
A raw socket can be bound to a specific local address using the
bind(2) call. If it isn't bound all packets with the specified IP
protocol are received. In addition a RAW socket can be bound to a
specific network device using SO_BINDTODEVICE; see socket(7).
You cannot bind a raw socket to a specific port because "port" is a concept in TCP and UDP, not IP. With a sneek at the header diagrams for those three protocols and it should become obvious: i am working at a lower level, where the concept of port is not known. This is what i understand regarding port numbers.
No. The mere fact that its RAW means there's no other protocol except RAW Internet Protocol. Without TCP or UDP, there won't be any port to distinguish which application this packet gets sent to, so instead, everything will have to be filtered through the IP packet's payload. You'd have to do this manually. Best way is to make a program that forwards these packets after inspection to wherever you want it to go.