[Nagiosplug-devel] feature request -selecting your network interface

[bobi at netshel.net]

And the wonders of ultimate geekdom.  ;-)

Ok, so here are the updated patches, adding your suggested diagnostic
messages (only the netutils.c patch changed.)

Does this need to be submitted as a patch request on sourceforge?  (That
is, assuming it merits enough interest from the community-at-large.)

May the Source (Address) Be With You,
Bob

> Ahh, the wonders of opensource. Someone got curious and Good Things
> Happened. Kudos for introducing the concept in actual code.
>
> More below.
>
> bobi at netshel.net wrote:
>> Hi,
>>
>> I don't know why I felt like doing this - guess I was curious if you
>> could
>> bind to a local address for a TCP stream prior to calling connect(2).  I
>> didn't think it would work, because the routing table usually controls
>> which interface to send a packet out on (and hence, which source IP
>> address to emebed in the IP header.)  But if an address is reachable
>> from
>> more than one interface, I supposed it could work.
>>
>> Anyway, please find attached a tar-ball containing three patch files
>> for:
>>
>> 1. netutils.h
>> 2. netutils.c
>> 3. check_tcp.c
>>
>> If you patch the above three files, and re-make check_tcp, it will allow
>> you to specify the source IP address.
>>
>> The new option is: -I src-ip-address
>>
>> I've tested it on my Linux box which has two interfaces (plus loopback,)
>> and it seems to work well.  I watched the packets go out with tcpdump
>> and,
>> sure enough, the IP headers had the source IP address that I'd specified
>> in the command line.
>>
>> One note, though:  If you specify an interface IP address which can't
>> reach your destination host, the connect(2) call inside netutils.c will
>> return an EINVAL error.  I know this because I tried accessing an
>> external
>> host through the loopback interface (127.0.0.1) and it returned EINVAL,
>> which is what you'd expect.
>>
>
> Yup. The only objection I have to this patch is the lack of an
> error-message when the user has done this exact mistake, or when the
> user has put a source-ip that's not present on the system as argument to
> -I.
>
> So after the connect(2) call, I'd put something like this:
> ---%<---%<---%<---
> if (result < 0 && src_ip && errno == EINVAL)
>    die("UNKNOWN: Can't send to %s through interface with address %s\n",
>        target, src_ip);
> ---%<---%<---%<---
>
>
> and after the bind(2) call, I'd put something like this:
> ---%<---%<---%<---
> /* SVr4 defines EADDRNOTAVAIL. Many systems have it, so check for it */
> #ifdef EADDRNOTAVAIL
> if (result < 0 && errno == EADDRNOTAVAIL)
> 	die("UNKNOWN: Can't send from source-ip %s. No interface has that
> address\n", source_ip);
> #endif
> ---%<---%<---%<---
>
> With proper variables, inet_ntoa()'s and die() semantics, ofc (yes, I
> actually am that lazy ;-)
>
> --
> Andreas Ericsson                   andreas.ericsson at op5.se
> OP5 AB                             www.op5.se
> Tel: +46 8-230225                  Fax: +46 8-230231
>
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