UNIX for Dummies Questions & Answers

Hey everyone!
I am trying to configure iptables/netfilter so that all traffic coming from a certain port (8001) has its source ip re-written as a different ip. Can this be done?

Basically its a possible workaround for another issue I'm having with a multihomed server. SNMP data is coming in on one IP, and they're being responded to using the incorrect IP. So, I need all packets sent from the snmp agent to have the source ip re-written as the correct source ip. Any ideas?

Do you mean changing the source ip of a packet arrives at a the snmp server before the packet is dispatched to snmp daemon by kernal(of the snmp server)?

It is hard to do so, cuz SNAT is only usable from POSTROUTING.

SNAT at POSTROUTING

The snmp request comes in and is received by the SNMP daemon running on the system. It generates a reply, however when it generates the reply it uses the IP of another interface on the system. Because of routing rules in place, that IP gets forwarded out of a different interface. So what I need to do is change the source IP in the SNMP reply before it hits the routing tables configured in IPROUTE2, so it can then have the correct route applied and be sent out the correct interface.

The IP address it's trying to reply with is a real address, but its on a different network than the management network. So when the reply gets sent out, its not routable to the requestor. It also is stopping traps from working.

Try this command:

Code:

$sudo iptables -t nat -I POSTROUTING -s IP1 -p udp --sport SNMP_PORT -j SNAT --to IP2

IP1 is the wrong ip your snmpd is using to reply requests.
IP2 is the right ip you want.
SNMP_PORT is the port number of snmpd.