On Fri, 27 Feb 2009, david@lang.hm wrote:

On Fri, 27 Feb 2009, Arnaldo Carvalho de Melo wrote:

quoted

Em Fri, Feb 27, 2009 at 01:20:07AM -0800, david@lang.hm escreveu:

quoted

I've got a need to forge the source IP of UDP packets (a stupid syslog
receiver app that I need to deal with)

I know that this can be done with raw sockets, but with the
IP_nonlocal_bind option I thought that it may be possible to do this
without dealing with raw sockets

so I have an app that does the socket call, followed by the bind call
without any errors, but when I issue the sendto call it generates error
22 (invalid parameter) if the source IP doesn't exist on the local box
somewhere.

is this the kernel that is doing whatever check is failing? or is glibc
wrapping the kernel syscall and doing some additional checking?

if it is the kernel that's throwing the error, is there some way of
disabling this check? or do I have to go to raw sockets?

Ok, I tried seperating out the connect from the send, and an strace is 
giving me the following sequence

socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP) = 3
setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("10.201.7.174")}, 16) = 0
connect(3, {sa_family=AF_INET, sin_port=htons(514), sin_addr=inet_addr("10.201.7.101")}, 16) = -1 EINVAL (Invalid argument)
sendto(3, "<13>Feb 27 14:04:16 secdev5 dlang"..., 39, 0, NULL, 0) = -1 EDESTADDRREQ (Destination address required)

so this seems to clearly be saying that it's not willing to accept that IP 
source/destination combination.

why not?

David Lang

quoted

Follow:

udp_sendmsg
   ip_route_output_flow
       __ip_route_output_key

And then look at this line:

	rth->fl.fl4_src == flp->fl4_src &&

It'll fail and so another function will be called:

	    ip_route_output_slow

Many, many details later, this comment:

                          Because we are allowed to send to iface
                          even if it has NO routes and NO assigned
                          addresses. When oif is specified, routing
                          tables are looked up with only one purpose:
                          to catch if destination is gatewayed, rather than
                          direct. Moreover, if MSG_DONTROUTE is set,
                          we send packet, ignoring both routing tables
                          and ifaddr state. --ANK

So try using:

setsockopt(udp_sock, SOL_SOCKET, SO_BINDTODEVICE, "eth0", 5);

Replacing "eth0" with the interface you want your forged packets to fly by.

The device name will be transformed into an index and stored in
sk->sk_bound_dev_if, that later, at udp_sendmsg time, will get stashed into
that oif field and then that comment will hold.

Caveat emptor: I never tried this, this is just after 5 minutes code 
inspection 8)

this did not work.

I didn't really expect it to because routing should only look at the 
destination address in the packet, so the source address shouldn't matter.

to clarify

the syslog server I am sending to is on my local subnet, I can send messages 
to it normally.

I am needing to relay syslog messages I have received from another machine to 
this syslog server, but forge the source IP address to make it look to the 
syslog server like the packet initiated from the remote machine instead of 
the syslog relay.

David Lang