Networking :: Tcpdump Shows Packages Even IPTables Policy Set To Drop
Feb 19, 2010
I have set the iptables INPUT policy to DROP. As I have expected tcpdump wasn't showing any packages... for a while. Suddenly it begun to show UDP syslog packages being sent by a remote host. It is conform with the configuration of syslog, but since the INPUT policy was set to DROP, with no exceptions, it is not conform with configuration of iptables. Why after setting INPUT policy to DROP, with no exceptions most of the packets recieved before are being dropped and some not, as tcpdump shows?
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May 20, 2010
I am running a test to determine when packet drops occur. I'm using a Spirent TestCenter through a switch (necessary to aggregate Ethernet traffic from 5 ports to one optical link) to a server using a Myricom card.While running my test, if the input rate is below a certain value, ethtool does not report any drop (except dropped_multicast_filtered which is incrementing at a very slow rate). However, tcpdump reports X number of packets "dropped by kernel". Then if I increase the input rate, ethtool reports drops but "ifconfig eth2" does not. In fact, ifconfig doesn't seem to report any packet drops at all. Do they all measure packet drops at different "levels", i.e. ethtool at the NIC level, tcpdump at the kernel level etc?nd am I right to say that in the journey of an incoming packet, the NIC level is the "so-called" first level, then the kernel, then the user application? So any packet drop is likely to happen first at the NIC, then the kernel, then the user application? So if there is no packet drop at the NIC, but packet drop at the kernel, then the bottleneck is not at the NIC?
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May 19, 2010
I have a question regarding packet drops. I am running a test to determine when packet drops occur. I'm using a Spirent TestCenter through a switch (necessary to aggregate Ethernet traffic from 5 ports to one optical link) to a server using a Myricom card. While running my test, if the input rate is below a certain value, ethtool does not report any drop (except dropped_multicast_filtered which is incrementing at a very slow rate). However, tcpdump reports X number of packets "dropped by kernel". Then if I increase the input rate, ethtool reports drops but "ifconfig eth2" does not.
In fact, ifconfig doesn't seem to report any packet drops at all. Do they all measure packet drops at different "levels", i.e. ethtool at the NIC level, tcpdump at the kernel level etc? And am I right to say that in the journey of an incoming packet, the NIC level is the "so-called" first level, then the kernel, then the user application? So any packet drop is likely to happen first at the NIC, then the kernel, then the user application? So if there is no packet drop at the NIC, but packet drop at the kernel, then the bottleneck is not at the NIC?
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Jan 24, 2011
The only window that's open is the terminal running this command, no pidgin, skype, samba, torrent or anything I can think of is using the network yet there is ***** load of output from tcpdump. I was hoping to use this to check where certain applications connect to and what messages they send but when I'm doing nothing there is already more output than I can go through. Running tcpdump for less than 10 seconds gives me the following output:
Code:
16:13:22.015683 IP ns.hihkptt.net.cn.domain > desk.local.56598: 46887 1/2/2 (166)
16:13:22.016251 IP ns.hihkptt.net.cn.domain > desk.local.60099: 21168 1/2/2 (166)
16:13:22.016743 IP ns.hihkptt.net.cn.domain > desk.local.42325: 50346 1/2/2 (166)
16:13:22.034733 IP ns.hihkptt.net.cn.domain > desk.local.41441: 63658 1/2/0 (134)
16:13:22.035215 IP ns.hihkptt.net.cn.domain > desk.local.42865: 37537 1/2/0 (134)
16:13:22.036124 IP ns.hihkptt.net.cn.domain > desk.local.35006: 7520 1/2/0 (134)
16:13:22.036569 IP ns.hihkptt.net.cn.domain > desk.local.38480: 51322 1/2/0 (134)
16:13:22.066006 ARP, Reply 192.168.0.1 is-at 00:b0:0c:02:60:9c (oui Unknown), length 46 .....
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Mar 26, 2010
Is it possible to only view certain chains and more specifically certain chain policies with options when doing:
iptables -L
I would like for example view FORWARD ACCEPT rules instead of waiting for all of the drop rules to load when viewing a firewalled iptables.
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Mar 3, 2010
When I reboot my computer, my iptables sets itself to a policy of dropping everything, adds a bunch of rules, and a bunch of extra chains, to the effect that (due to everything being set to drop) I can't do anything. I know how to fix this from the terminal to the extent of just clearing most of it and changing the policies back. However, what I don't know is how to make it stay that way. I have a file with the iptables rules I want, so every time I start up I just run iptables-restore, but I don't want to have to do this every time, particularly since others use this computer who do not have admin privileges.
I've tried changing /etc/network/interfaces with the added code pre-up iptables-restore < (etc) But that never does anything, or if it does it just makes stuff work even less. I've tried changing init.d before based on similar info elsewhere, still no luck. I don't know how to get it to stick, and I don't know why it is defaulting to the rules it is, other than that I used a firewall app a while ago and afterwards this was the result, for which I uninstalled that app after no success using it to reverse the damage.
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Jan 16, 2011
I followed one of these tutos [URL]. My OS is Debian Lennny, iptables v1.4.2.
My iptables -L
iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere state RELATED, ESTABLISHED
ACCEPT tcp -- anywhere anywhere tcp dpt:xxxx
ACCEPT icmp -- proxy.provider.net anywhere
ACCEPT icmp -- proxy.p19.provider.net anywhere
ACCEPT icmp -- proxy.rbx.provider.net anywhere
ACCEPT icmp -- proxy.rbx2.provider.net anywhere
ACCEPT icmp -- ping.provider.net anywhere
ACCEPT tcp -- cache.provider.net anywhere tcp dpt:ssh
DROP all -- anywhere anywhere
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:domain
ACCEPT tcp -- anywhere anywhere multiport dports www,https
ACCEPT icmp -- anywhere anywhere icmp echo-request
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere tcp dpt:xxxx
As soon as I add "iptables -A OUTPUT -j DROP" server lock me out and I have to reboot to be able log back. The ssh port is open in both INPUT and OUTPUT what's wrong?
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Feb 21, 2011
I need to set OUTPUT to DROP, and add the outgoing traffic one by one, but I couldn't do it. My current config is as follows:
Code:
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
:RH-Firewall-1-OUTPUT - [0:0]
-A INPUT -j RH-Firewall-1-OUTPUT
#previously ESTABLISHED,RELATED comm is ok
-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
#80 is ok from all
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
If I change OUTPUT to DROP in :OUTPUT ACCEPT [0:0], I don't get any response from a server running in that box. I am using RHEL 5.5. Now, asking Red Hat is not an option: I have the license but I don't have support license.
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Feb 23, 2010
I'm not an iptables expert. Anybody know how to create a rule/chain that will log info similar to what tcpdump -s0 would do?
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May 3, 2010
I'm intending to replace my current router (486DX2 w/16MB running FREESCO which has been faithfully working 24/7 for well over a decade) with a debian box with a bit more grunt and newer features. I'm currently setting up my iptables ruleset and am after a bit of advice re the FORWARD policy. A few example rulesets I have found set the default policy to DROP and the have two lines for each port forward, one to allow the traffic and one to direct the incoming packets to the correct machine.
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 25 -j DNAT --to-destination 10.0.100.10:25
iptables -A FORWARD -i eth1 -p tcp --dport 25 -o eth0 -d 10.0.100.10 -m conntrack --ctstate NEW -j ACCEPT
I'm thinking of setting the default policy to ACCEPT to cut down on typing as my default INPUT policy is DROP and unless there is a valid FORWARD rule for a particular port, the packets aren't going anywhere anyway. Or have I misunderstood something. My googling returned heaps of example scripts & not much intelligent commentary. Alternatively, what do you all use to configure & maintain your debian gateways; hand rolled iptables rules, or any toolset recommendations?
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Aug 24, 2009
It took me a while to get VNC going. It was easier with FC8-10. Once I got finished and was actually able to log in and see my remote desktop I tried to add some software... virtualbox.When I double click on the RPM I get popup that states."The action could not be completed. Failed to install file. You do not have the necessary privileges to perform this action" When I close that dialogue another one pops up that states" "The action could not be completed." When I click on more details the dialogue states. "Policykit authorization failure" How can I make this work?
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May 24, 2010
I have tried to google it around and couldn't find any good solution for it. What I want is to hook up to the kernel network hooks and for example investigate all of the packets (maybe keep some in the buffer and drop in the kernel so I could send them out lets say 10 minutes later) but from a C / C++ program perspective / level. I know it can be done via iptables but isn't there a way to do it from a program ?? I have found a library called ipq but apparently doesn't work with kernel 2.6.x anymore.
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Nov 26, 2009
Client which uses an ISP with a Linux machine (Server 1) with two network cards. Eth0 with ip 1.1.1.2 and gateway 1.1.1.1 these ip's is not public ip's and falls unders the isp's internal range, eth1 with the internal ip range. OpenVPN listen on eth0 for incoming connections. We want to connect to Server1 from external networks. Due to different reasons the isp could not get the routing sorted and give us an public ip on his network. We had to setup another bos (Server2) with eth0 2.2.2.5 and gateway 2.2.2.1 which is public ip's and eth1 with ip 3.3.3.5. Server 2 can communicate via the ISP internal network with server 1. I use DNAT to forward all incoming connections on server 2 to server 1 but the problem I have is that server uses its default gateway for the return packages and does not sent the reply back to server 2. My iptables rules looks as follows on server 2.
/usr/sbin/iptables -t nat -A PREROUTING -p udp -i eth0 -d 196.25.157.135 --dport 1194 -j DNAT --to 192.168.100.170:1194
/usr/sbin/iptables -A FORWARD -p udp -i eth0 -d 192.168.100.170 --dport 1194 -j ACCEPT
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Jun 2, 2011
I'm trying to drop all packets from the internet that use a fake ip address so they appear to come locally.
Do I need both lines or only the first ?
--append INPUT ! --in-interface lo --source 127.0.0.0/8 --jump DROP
--append INPUT ! --in-interface lo --destination 127.0.0.0/8 --jump DROP
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Apr 20, 2010
Do I have to create a rule for:
Code:
$IPT -A fwalert -p tcp --tcp-flags SYN,ACK SYN,ACK -m conntrack --ctstate NEW $RLIMIT -j LOG $LOGLIMIT --log-tcp-options --log-level 4 --log-prefix
to drop rather than log if my table has a default policy of drop with :
Code:
$IPT -t fwalert -P DROP
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Jan 13, 2011
When deleting a rule, when is better to use DROP instead of REJECT and vice versa? Also, please provide some basic info. for the use of MASQUERADE??
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May 3, 2010
How can I drop or forward a incoming connection from a part of a host like *.alicedsl.de
For example:
The user is connection from *.alicedsl.de on port 12345
So how can I drop this connection or forward to google.com on port 80
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Feb 23, 2010
I have setup my linux fedora server and i want to restrict access to my server.Basically i control using iptables.I'm not sure how to write an iptables rules to control drop all connection to port 8080 and allow only certain ip can access the instance on port 8080 example ip=10.254.14.16,192.168.1.10.
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Jun 30, 2010
I've read the instruction about setting up the iptables rules to filter all port except HTTP, SSH, FTP. I require first remove all default iptables rules and set default rules to all chains as DROP:
# Set default-deny policies for all three default chains
$IPTABLES -P INPUT DROP
$IPTABLES -P FORWARD DROP
$IPTABLES -P OUTPUT DROP
Then allow only some ports:
#Accept inbound packets that are part of previously-OK'ed sessions
$IPTABLES -A INPUT -j ACCEPT -m state --state ESTABLISHED,RELATED
# Accept inbound packets which initiate SSH sessions
$IPTABLES -A INPUT -p tcp -j ACCEPT --dport 22 -m state --state NEW
# Accept inbound packets which initiate FTP sessions
$IPTABLES -A INPUT -p tcp -j ACCEPT --dport 21 -m state --state NEW
# Accept inbound packets which initiate HTTP sessions
$IPTABLES -A INPUT -p tcp -j ACCEPT --dport 80 -m state --state NEW
# Log anything not accepted above $IPTABLES -A INPUT -j LOG --log-prefix "Dropped by default:"
But I hired a VPS from other country so the only mean I can manage it is via SSH. If I setup the default rule to DROP first, I afraid that I can no longer connect via SSH to tell iptables allow SSH
So my question is:
- Does the IP tables take effect immediately after I input a rule?
- Is there any mean to run this as a batch job (create a script and run all these rules one time).
- My VPS has a web control panel which have a terminal via web. Is this a native terminal or just a connection via port 80 or 22?
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Jan 3, 2011
how can i drop igmp port 0 packets with iptables rule? my log file is full of this router advertisement.
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May 2, 2010
My connection manager shows wired connections and shows wireless as being disabled. here is some terminal info getting wireless going?
Here is a bunch of things I have tried and the results.
nixon@nixon-desktop:~$ sudo ifup wlan0
Ignoring unknown interface wlan0=wlan0.
nixon@nixon-desktop:~$ sudo ifdown wlan0
ifdown: interface wlan0 not configured
[Code].....
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Dec 7, 2010
I am trying to create a dump log using tcpdump. I want display the top 10 ip addresses sorted numerically showing how many times the ips are hitting the server. I'm getting frustrated because It's not working how I'd like it to.
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Feb 16, 2011
Is there a way to do multiple interfaces in tcpdump? I have found that when using "-i any", not all packets are captured (compared to "-i eth0" on a machine with only one interface). I need to monitor traffic on some machines with as many as 6 interfaces, and get these packets that "-i any" misses. When I give the "-i" option multiple times, it seems to only use the last one.
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Oct 18, 2010
I'm trying to capture packets to a file with the -w option but the file is empty yet if I use the '-w -' option to put data on stdout I see plenty of captured packets.I'm using CentOS 5.5 x86
Code:
[root@server ~]# tcpdump -v -i eth0 -w dump -s0
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
[code]....
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Jun 3, 2010
This is the error yum shows after downloading the packages:
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Apr 6, 2010
I would like to set up tcpdump to rotate log file every 1 hour and retain files for the lat 14 days but I don't think any combination of -C and -W would allow me to do that (Atleast I haven't been able to figure it out), so I am trying to rotate the files every X number of MB and retain the last 20 files. This seems to be fairly simple with the '-C X -W 20' option but I am having some trouble in customizing the names of the log files. I have tried '-w capture-$(date +%Y-%M-%d-%H:%M-)' thinking that each file would start with the current date and time but all files are using the date and time when the capture was started so the only difference is the number at the end (which is done by -W). if I can customize the names of the file so that it has the date and time when the capture in started. In fact if I can do that, I dont need the numbers that '-W' appends at the end but I dont know how to get rid of them.
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May 27, 2010
I'm running NetWare SLES 10 sp3 with OES2 sp2. I was working with the folks at Novell to resolve an iPrint Print Manager problem.
During the process they wanted to perform a packet capture using tcpdump. While logged in as the root user the error no suitable device was found, and I received no data at all. This server is running on a VMWare Center. On other SLES 10 sp3 systems (residing on that same VMWre Center), tcpdump captures packets just fine. I inherited all of these servers, so I wasn't here during the initial build, but I'd make the guess that they were configured similarly. On a Server that I built recently, tcpdump works fine. On two of my Servers it does not, and gives the mentioned error.
It's not that big a deal, otherwise the Servers are communicating and working just fine. But, I'd like to get it working just because it's supposed to work. Students are off for the summer, so I have time to play.
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Jan 22, 2011
how to do iptables log to file UDP Flood under 64 packages?
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Jan 30, 2010
I downloaded openoffice.org-2.3.0-6.11.el5.src.rpm. When I try to install it, it shows that there are no packages but it is up to 250 mb.
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Jan 26, 2011
I ran slackpkg update, then slackpkg install-new, then slackpkg upgrade-all. The menu shows pretty much every package that comes stock in slackware, and the ones that I looked at are all older versions than what I have installed. Is this how its supposed to be? or is my slackpkg bugging out?
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