06-10-2002
I need help to set up internet sharing in linux.
Ok, I'm kind of in between newbie and experienced somewhere. I'm an advanced computer user but only have a little experience with linux and a lot of problems with it. Currently I'm using Linux-Mandrake 8.0 (I know, I know, but let's not go into the many reasons why it's not even close to the best distro.) I tried using the little ICS setup wizard or whatever you would call it and it didn't work. The client would get an IP that was correctly assigned, but it couldn't get DNS resolutions or connect to any addresses.
Basically what I want to do is get help aimed more at a newbie on how to properly set up the normal way of ICS in linux using IP tables and whatever. I don't need the linux box to act as a firewall really because, for reasons that are too complicated to explain all out right now, the linux box is actually going through another proxy, so I have two network cards set up in it. I set up the one going to the LAN and the proxy to use 192.168.0.2 and the one that this computer would be connecting to is 192.168.1.1 so that the os would know which one to use for what.
I know this all sounds a little crazy, but, please, bear with me.
Basically, I have a computer sharing the internet connection on the LAN using the IP address 192.168.0.1 and there are two other computers on that LAN which connect through it. I would like to set up this linux box on 192.168.0.2 or any other IP address in that general range and connect my computer through it via that two network card method. This way I have a linux server on the lan among other things. (Don't worry, there's more than one reason I'm trying to do this, just too many.)
Also, I would really like to be able to set it up as a DNS server as well and to be able to manually refresh the DNS tables. I have problems every now and then with DNS and using the hosts file on my windows computer just isn't working for a few sites. Besides, presumably internet based things will go a little faster when they don't have to wait for a reply from my ISP to get the IP address.
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LEARN ABOUT DEBIAN
mxallowd
mxallowd(1) User Manuals mxallowd(1)
NAME
mxallowd - dynamically whitelist your Mail eXchanger
SYNOPSIS
mxallowd [-d] [-c configfile] [-t whitelist-time] [-p pflog-interface] [-l pcap-filter] [-F] [-s] [-q] [-p] -f fake-mailserver -r real-
mailserver -n queue-num
DESCRIPTION
mxallowd is a daemon which uses libnetfilter_queue (on Linux) or pf and pflog (on BSD) to allow (or deny) connections to a mailserver (or
similar application) if the remote host hasn't connected to a fake daemon before.
This is an improved version of the so-called nolisting (see http://www.nolisting.org/). The assumption is that spammers are not using RFC
2821-compatible SMTP-clients and are sending fire-and-forget spam (directly to the first or second MX-entry without retrying on error).
This direct access is blocked with mxallowd, you'll only get a connection if you retry.
NOTE: It is highly recommended to install nscd (nameserver caching daemon) or a similar software in order to speed-up DNS lookups. Since
version 1.3, DNS lookups are done in a thread (so they don't block the main process), however, on very-high-traffic-sites, mxallowd may
show significantly better overall performance in combination with nscd.
OPTIONS
-b, --no-rdns-whitelist
Disable whitelisting all IP-addresses that have the same RDNS as the connecting one (necessary for google mail)
-c, --config
Specifies an alternative configuration file (instead of /etc/mxallowd.conf)
-t, --whitelist-time
Specify the amount of time (in seconds) until an IP-address will be removed from the whitelist
-s, --stdout
Log to stdout, not to syslog
-q, --quiet
Don't log anything but errors.
-f, --fake-mailserver
Specify which IP-address the fake mailserver has (connecting to it will whitelist you for the real mailserver)
-r, --real-mailserver
Specify which IP-address the real mailserver has
-F, --foreground
Do not fork into background, stay on console
-n, --queue-num (only available when compiled for netfilter_queue)
Specify the queue number which will be used for the netfilter_queue-link. This has to be the same which is specified in the ipta-
bles-rule and it has to be specified, there is no default.
-p, --pflog-interface (only available when compiled for pf)
Specify the pflog(4) interface which you configured in pf(4). The default is pflog0. Also see the pcap-filter-option if you use an
interface which does not only get smtp-traffic.
-l, --pcap-filter (only available when compiled for pf)
Specify the filter for pcap. The default is "port 25". See tcpdump(8) for more information on the filters.
FILES
/etc/mxallowd.conf
System-wide configuration file. Use the long options without the beginning two dashes. For example:
stdout
fake-mailserver 192.168.1.3
fake-mailserver 192.168.1.4
real-mailserver 192.168.1.5
queue-num 23
EXAMPLES FOR NETFILTER
The machine has two IP-addresses. The mailserver only listens on 192.168.1.4, the nameserver returns the mx-records mx1.domain.com
(192.168.1.3) with priority 5 and mx2.domain.com (192.168.1.4) with priority 10.
# modprobe nfnetlink_queue
# iptables -A INPUT -p tcp --dport 25 -m state --state NEW -j NFQUEUE --queue-num 23
# mxallowd -s -F -f 192.168.1.3 -r 192.168.1.4 -n 23
Then open a separate terminal and connect via telnet on your real mailserver. You'll see the connection attempt being dropped. Now connect
to the fake mailserver and watch mxallowd's output. Afterwards, connect to the real mailserver to verify your mailserver is still working.
EXAMPLES FOR PF
The machine has two IP-addresses. The mailserver only listens on 192.168.1.4, the nameserver returns the mx-records mx1.domain.com
(192.168.1.3) with priority 5 and mx2.domain.com (192.168.1.4) with priority 10.
Create a pf.conf like this:
table <mx-white> persist
real_mailserver="192.168.1.4"
fake_mailserver="192.168.1.3"
real_mailserver6="2001:dead:beef::1"
fake_mailserver6="2001:dead:beef::2"
pass in quick log on fxp0 proto tcp from <mx-white> to $real_mailserver port smtp
pass in quick log on fxp0 inet6 proto tcp from <mx-white> to $real_mailserver6 port smtp
block in log on fxp0 proto tcp to { $fake_mailserver $real_mailserver } port smtp
block in log on fxp0 inet6 proto tcp to { $fake_mailserver6 $real_mailserver6 } port smtp
Afterwards, load it and start mxallowd using the following commands:
# pfctl -f /etc/pf.conf
# mxallowd -s -F -f 192.168.1.3 -r 192.168.1.4
Then open a separate terminal and connect via telnet on your real mailserver. You'll see the connection attempt being dropped. Now connect
to the fake mailserver and watch mxallowd's output. Afterwards, connect to the real mailserver to verify your mailserver is still working.
The ruleset for pf is actually longer because pf does more than netfilter on linux -- netfilter passes the packets and lets mxallowd decide
whether to drop/accept whilst pf blocks/passes before even "passing" to mxallowd.
SEE ALSO
iptables(8), pf(4), pflog(4), tcpdump(8)
AUTHOR
Michael Stapelberg <michael+mxallowd at stapelberg dot de>
Linux MARCH 2012 mxallowd(1)