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Old 06-22-2005, 08:47 AM   #1
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[IPTABLES] Traceroute

I have a firwall that has a couple of server behind it, when i do a "traceroute" from a random client it also shows the firewall as a "hop".
Is it possible to set a rule in IPTABLES so the firewall doesn't show up as a hop when a traceroute is done ?
Old 06-23-2005, 01:13 AM   #2
Registered: Jun 2005
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Yes, you just have to increment the TTL in packets coming from the internet.

Check to see if you have the patch-o-matic netfilter modules required to do this:
# ls /lib/modules/`uname -r`/kernel/net/ipv4/netfilter/
ipt_XXX modules are targets you may use with -j in iptables(8)
and ipt_xxx are matches (for use with the -m option)

You must have ipt_TTL for this hack to work:
# /sbin/iptables -t mangle -I PREROUTING -i $INTERNET_IFACE -j TTL --ttl-inc 1
Having just ipt_ttl won't work because this module is only a match.

# /sbin/iptables -j TTL -h
# /sbin/iptables -m ttl -h
I don't know why iptables(8) gives help when some modules aren't present.
Old 06-30-2005, 01:41 AM   #3
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All you need to do is get your firewall block ping (ICMP) packets.
Using iptables:
iptables -A INPUT -p icmp -m icmp -j DROP
Old 06-30-2005, 01:32 PM   #4
Registered: Jun 2005
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Originally posted by tkedwards
All you need to do is get your firewall block ping (ICMP) packets.
Using iptables:
iptables -A INPUT -p icmp -m icmp -j DROP
Don't you ever try this. This would block ALL ICMP, and not just ping (ICMP ECHO REPLY) packets. If you block all inbound ICMP, you won't receive ICMP replies, nor you can't traceroute any host outside of your LAN-

Standard traceroute uses UDP probes and (optionally) ICMP ECHO packets to ellicit ICMP TIME EXCEEDED from hops along the way. There's another package called tcptraceroute that uses TCP SYN's instead. They all expect an ICMP TIME EXCEEDED.

So, you only need to block TIME EXCEEDED messages for OUTPUT only.
Perhaps, you would want to block ICMP DESTINATION UNREACHABLE too for the UDP probes being sent to non-listening ports

If you don't care about the machines on your LAN being tracerouted, and you only want your router/firewall to appear stealthy, then apply the TTL hack above and never get your firewall to respond to packets from the internet.


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