Re: SNMP and IPTABLES

[srgqwerty <srgqwerty@telefonica.net>]

netfilter_user wrote:

>hello everyone,
>
>This is my problem:
>
>As a gateway in my network is a PC with Linux ( Slackware 8.1 ), and
>also on this machine is installed MRTG that listen for SNMP. I want to use SNMP
>only in local ( internal ) network. I know that SNMP
>use 161 and 162 port. There are no problems with comunication between
>Linux machine and local hosts via SNMP.
>
>BUT: the problem is when MRTG try to receive SNMP information from this
>same machine that is installed ( MRTG is installed on Linux machine -
>gateway). I dont know why because i set: iptables -A INPUT -i eth1 -p tcp -m multiport --dport 21,80,161,3128 -j ACCEPT
>                                         iptables -A INPUT -i eth1 -p udp -m multiport --dport 80,67,161 -j ACCEPT
>                              ...and it should resolve problem...but
>                              it wont
>
>!!!!!THIS IS IMPORTANT!!!!
>when I set all INPUT ACCETP then it works. So there is no problem with
>snmp and MRTG but with my iptables config :/
>
>                                         
>I use:
>NET-SNMP version:  5.0.7
>iptables v1.2.6a
>
>This is my iptables config:
>
>
>insmod ip_conntrack
>insmod ip_conntrack_ftp
>
>iptables -P FORWARD DROP
>iptables -P INPUT DROP
>iptables -P OUTPUT ACCEPT
>
>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
>iptables -A INPUT -p tcp -m multiport --dport 22 -j ACCEPT
>iptables -A INPUT -i eth1 -p tcp -m multiport --dport 21,80,161,3128 -j ACCEPT
>iptables -A INPUT -i eth1 -p udp -m multiport --dport 80,67,161 -j ACCEPT
>
>iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
>iptables -A FORWARD -i eth1 -p tcp -m multiport --dport 21,22,25,110,1111,6112 -j ACCEPT
>iptables -A FORWARD -i eth1 -p udp -m multiport --dport 53,6112,13073,23083,23073 -j ACCEPT
>
>iptables -I INPUT -p icmp --icmp-type echo-request -s 0.0.0.0/0 -m limit --limit 1/s -j ACCEPT
>iptables -I INPUT -p icmp --icmp-type echo-request -s 0.0.0.0/0 -j DROP
>
>
>#===NAT=================================================================================
>modprobe iptable_nat
>iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
>echo 1 > /proc/sys/net/ipv4/ip_forward
>
>
>Will be thankful for any good advice
>
I think that you must accept incoming and outgoing traffic in the lo 
(127.0.0.1) device:

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT