[erlang-questions] UDP Headers

[Lee Sylvester]

Thank you for your help, Michael.  I’ll give these a test in the morning and will let you know.

Regards,
Lee


On 8 Jul 2014, at 23:20, Michael Santos <michael.santos@REDACTED> wrote:

> On Tue, Jul 08, 2014 at 10:53:18PM +0100, Lee Sylvester wrote:
>> Hi Michael,
>> 
>> Thanks for responding.  I was actually just looking through your repositories to see if you had written something to work with this.
>> 
>> What I’d like to do is to be able to set the DF or TTL flags.  What would you suggest for such things?
> 
> Raw sockets would be overkill for this. Have a look at the raw option
> to inet:setopts/2. There's an example in the docs for inet of setting
> TCP_LINGER2 on a TCP socket:
> 
>    inet:setopts(Sock,[{raw,6,8,<<30:32/native>>}])
> 
> The values will depend on the OS. So for setting the TTL, on linux/x86_64,
> maybe something like:
> 
>    setsockopt(send_socket, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
> 
>    % IPPROTO_IP = 0
>    % IP_TTL = 2
>    % TTL = 64
>    inet:setopts(Sock,[{raw,0,2,<<64:32/native>>}])
> 
> DF is interesting. There's a good discussion about the DF bit here:
> 
>    http://stackoverflow.com/questions/973439/how-to-set-the-dont-fragment-df-flag-on-a-socket
> 
> So on a linux system, maybe this would work:
> 
>    int val = IP_PMTUDISC_DO;
>    setsockopt(sd, IPPROTO_IP, IP_MTU_DISCOVER, &val, sizeof(val));
> 
>    % IPPROTO = 0
>    % IP_MTU_DISCOVER = 10
>    % IP_PMTUDISC_DO = 2
>    inet:setopts(Sock,[{raw,0,10,<<2:32/native>>}])
> 
> I haven't tested if these work so let us know if you run into problems
> and I'll try to come up with some working code!
> 
>>> On Tue, Jul 08, 2014 at 07:04:12PM +0100, Lee Sylvester wrote:
>>>> Hey guys,
>>>> 
>>>> So, I’m using gen_udp for a server I’m building, but I really need to be able to update the headers of incoming packets.  Does anyone know how I can do this with gen_udp without having to resort to using raw sockets?
>>> 
>>> It depends which header needs to be changed:
>>> 
>>> * for UDP (source/destination port, length, checksum), the simplest
>>> method is to receive and resend the packet
>>> 
>>> * for IPv4/IPv6, some of the headers may be able to be influenced by setting
>>> socket options. For example, by using inet:setopts/2 or by using
>>> setsockopt(2) from an NIF.
>>> 
>>> * otherwise, there's the raw socket interface.
>>> 
>>> For IPv4, the socket can be opened in raw mode and passed back into
>>> gen_udp.
>>> 
>>> For IPv6 headers, if the headers can be changed, you generally have to
>>> use sendmsg(2)/recvmsg(2) which means using a port or an NIF.
>>> 
>>> * for ethernet, it will depend on the OS: PF_PACKET for linux, BPF for
>>> the BSDs
>>> 
>>> * another option is routing the packets through a tun/tap device and
>>> having some erlang code re-write the headers
>>> _______________________________________________
>>> erlang-questions mailing list
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>>> http://erlang.org/mailman/listinfo/erlang-questions
>> 
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