1
vote
2answers
139 views

with fully permissive iptables, tcpdump shows icmp echo requests arriving, but no reply's are sent

I'm a bit stumped on where to look next. When using SoftEther IPsec VPN into my network, I can access every server in the main subnet but one. When behind the main firewall, the server responds to all ...
1
vote
0answers
189 views

Linux MIPS box: ICMP (ping) works, but TCP does not - how to fix it?

I'm trying to set up MIPS Linux box network. Here is it's configuration: ~ # ifconfig eth0 Link encap:Ethernet HWaddr CC:5D:4E:FF:45:7F inet addr:192.168.0.2 Bcast:192.168.0.255 ...
1
vote
2answers
145 views

ICMP and traceroute , How does ICMP keep a counter while it passess hops

When tracerouting a destination and going through all these hops, what happens during this process? Does ICMP have an initial counter and decrements this counter on each hop it passes? If the counter ...
3
votes
3answers
609 views

Reasons to turn off/on unreachables messages?

I see on this question (How can I stop Linux from sending ICMP "Destination Unreachable" responses?) that there was a lot of discussion pointing to the fact that you shouldn't turn off ICMP ...
-4
votes
1answer
471 views

How can I stop Linux from sending ICMP “Destination Unreachable” responses? [closed]

I have a Linux system which has a number of services disabled (for example, telnet). When a telnet client attempts a connection, the system responds with an ICMP "Destination Unreachable". Is there ...
3
votes
2answers
2k views

Linux ping command exits early due to ICMP host unreachable

An automated script runs shutdown -r now on a machine, and after a 30s delay, uses ping to determine when the machine is available. I've recently switched the OS from Centos 5 to Oracle Linux 6 and ...
0
votes
0answers
136 views

Multihoming, NAT and ICMP

My router is the default gateway of a LAN and is multihomed. ISP2 is used for HTTP, HTTPS and ISP1, the default gateway of the default routing table, for the rest of the traffic. Similar questions ...
4
votes
1answer
280 views

tcpdump on interface when pinging nonexistent hosts on local network

I've statically configured the IP address of an interface on a server: ip addr add 10.40.0.6/16 broadcast 10.40.255.255 dev eth1 If I try to ping a machine on the network (e.g., 10.40.0.1), it ...
3
votes
2answers
1k views

Can't ping multihoned Linux machine on non-default interface

I have a multihoned Ubuntu server with a set of interfaces that includes: eth2: 10.10.0.131/24 eth3: 10.20.0.2/24 The default interface is eth2, with a gateway of 10.10.0.1. Here's what the routing ...
4
votes
7answers
1k views

Scan for connected, powered on computers on a network?

I want to scan for powered on computers on my LAN and gather logs about that. I tried nmap but it didn't work very well (a lot of switched on computers are not getting detected). nmap -sP ...
5
votes
4answers
1k views

How to stop a ICMP attack?

We are under a heavy icmp flood attack. Tcpdump shows the result below. Altough we have blocked ICMP with iptables tcpdump still prints icmp packets. I've also attached iptables configuration and ...
1
vote
0answers
819 views

Why is my CentOS box rejecting some (but not all) ICMP ping requests from the same remote host?

CentOS 4.x I've got several old CentOS 4.x systems and have configured iptables to allow ICMP traffic. Originally, I had an iptables rule that looked like: -A RH-Firewall-1-INPUT -p icmp ...
4
votes
1answer
3k views

Stop duplicate icmp echo replies when bridging to a dummy interface?

I recently configured a bridge br0 with members as eth0 (real if) and dummy0 (dummy.ko if). When I ping this machine, I receive duplicate replies as: # ping SERVERA PING SERVERA.domain.local ...
5
votes
2answers
1k views

iptables | Types of ICMP: which ones are (potentially) harmful?

I read that certain types┬╣ of ICMP packets can be harmful. Questions: Which ones and why? How should I layout an iptables ruleset to handle each type of ICMP packet? Should I rate-limit any of these ...
0
votes
2answers
263 views

Linux not sending ICMP with iptables `--reject-with`

Using iptables I am trying to reject packets (I want to return an ICMP message). iptables -A FORWARD [...] -j REJECT --reject-with icmp-port-unreach The packets are dropped but using tcpdump I can ...
1
vote
1answer
407 views

Linux router - how to send icmp unreachable messages to LAN clients

I have a Linux home router with eth0 (192.168.0.2/24) and bridge br0 for LAN clients (192.168.1.1/24). Bridge br0 contains eth1 (wired) and eth2 (wireless). The router routes all LAN traffic via eth0 ...
1
vote
3answers
2k views

Iptables not allowing ICMP requests from remote machines

I've configured my linux router with the following iptables rules iptables --list-rules -P INPUT ACCEPT -P FORWARD ACCEPT -P OUTPUT ACCEPT -A FORWARD -i vmbr0 -o eth2 -j ACCEPT -A FORWARD -i vmbr0 ...
0
votes
1answer
751 views

iptables can't block timestamp request with CentOS

I am setting up firewall with iptables on my host. I want to disable timestamp ICMP request, but it's wired, I only allows type 8 (echo-request) comes into host, but event still, I can get timestamp ...
0
votes
1answer
352 views

Arp tries on various *nix based systems

Does anyone know what determines the amount of arp tries a router will make? I have different behaviors with two devices, if I try to traceroute to a non-existent host on a subnet that belongs to an ...
30
votes
8answers
13k views

Why not block ICMP?

I think I almost have my iptables setup complete on my CentOS 5.3 system. Here is my script... # Establish a clean slate iptables -P INPUT ACCEPT iptables -P FORWARD ACCEPT iptables -P OUTPUT ACCEPT ...
12
votes
2answers
3k views

Strange: why does linux respond to ping with ARP request after last ping reply?

I (and a colleague) have just noticed, and tested, that when a Linux machine is pinged, after the last ping it initiates a unicast ARP request to the machine that initiated the ICMP ping. When pinging ...
1
vote
1answer
3k views

Why is network stack ignoring icmp replies from non-default interface?

I have following situation: eth0 - default gateway ( ip: 172.28.183.100, gw: 172.28.183.1 ) eth0 - secondary network connection ( ip: 172.28.171.2, gw: 172.28.171.2). routing looks like this: ...
3
votes
3answers
3k views

ping -f shows problems but regular ping doesn't

I have a strange issue, hosting at Softlayer Washington data center. Test from an EC2 instance to my server at Softlayer :- [root@EC2-box ~]# ping -f -c 100 MY-SL-BOX PING MY-SL-BOX (MY-SL-BOX) ...