Programming :: Drop Packets (not Iptables) In C / C++

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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General :: Iptables Drop Fake Ip Packets?

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 --jump DROP
--append INPUT ! --in-interface lo --destination --jump DROP

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Ubuntu Security :: Drop Igmp Port 0 Packets With Iptables Rule?

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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Programming :: Write A Program In C That Can Sniff Packets From Ethernet And Distinguish RTP Packets From Non-RTP Packets?

Aug 30, 2010

i need to write a program in c that can sniff packets from Ethernet and distinguish RTP packets from Non-RTP packets, i have no idea what should i do

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Networking :: DROP Filtered Packets On A Cisco Router?

May 31, 2011

I bought a Cisco 1841 to study for a cisco certs. In iptables terms filtered packets are -j REJECT instead of -j DROP. To make things worse telnet and ping replies are on by default.

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CentOS 5 Hardware :: Server Starts Drop Packets On Ethernet Iface?

Feb 27, 2010

I've got Tyan S4989 with gigabit ethernet controller Intel Corporation 82571EB. Sometimes server starts drop packets and doesn't respond to any connections and all logs doesn't contain any errors. iptables and SELinux switched off on CentOS 5.3(2.6.18-128.1.6).

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Networking :: IPTables - Cannot Set Drop All Output

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 -- anywhere
ACCEPT icmp -- anywhere
ACCEPT icmp -- anywhere
ACCEPT icmp -- anywhere
ACCEPT icmp -- anywhere
ACCEPT tcp -- 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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Ubuntu Security :: Why These Packets Droped By Iptables

Apr 30, 2011

i dont know why packets dropped? and something else what are those numbers for default policy in [] means?this is rules:

# Generated by iptables-save v1.4.4 on Sun May 1 00:09:57 2011


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Networking :: Iptables REDIRECTing - All Outgoing Packets ?

Dec 6, 2010

I've been trying to redirect all outgoing packets (destined for a specified ip address) from my slack box back to itself. I thought this could be done with iptables, but if I fire up wireshark I can clearly see that the packets are getting out to the real server and I'm getting responses from it.

So here's what I tried:

All looks good and fine, and then I even try to visit with firefox (by the way I am running a webserver, that is set to show a page when you visit and I get an error page that reads: 502 Bad Gateway.

I ignored this message to see what the program I'm trying to interrupt does, and when I start wireshark and then start the program that is using that website, I can clearly see that the packets make it to the real and get back responses.

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Networking :: Iptables Routing Packets On Same Sub-network

Feb 17, 2010

I am running into trouble while trying to set-up a iptables routing policy. I have two machines on the same sub-network ( One of the machines is used as a default gw for the other ( is a gateway for and is a gw for There is no explanation for why the is in the middle -- can go straight to, but is is like that for now.I am trying to find an iptable rule to be executed on the machine to route the packets.

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Security :: Iptables - Block Bad And Not Related Packets

Jun 8, 2011

My VPS host a mail, blog and web site. So i want to block port i not use. The port that i use is 80,21,2022,443. The other port will be drop. I want to block bad packet and all packet that not related. Can anyone how to write in iptables?

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General :: IPTables - Need To Create Drop Rule

Apr 20, 2010

Do I have to create a rule for:
$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 :
$IPT -t fwalert -P DROP

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Networking :: Redirecting Packets To Virtual Machine With Iptables

Mar 16, 2011

I had been running my SMTP server with WINE, as the SMTP server software is a Windows-based program (MERCURY), but I cracked the shits with WINE and removed it. Now I am running my SMTP server in a Windows virtual machine.This virtual machine has a different IP address from my host machine, so what I need is for my computer (the host) to redirect incoming traffic on port 25 to the virtual machine at on port 2525.Can someone please help me with it? I think its done with iptables.

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Networking :: Interesting Distribution Of Packets Over Iptables Rules

Mar 13, 2011

I'm trying to configure NFS sharing behind a firewall, I got it to work and all but I was caught by something that (to me anyways) seems odd.I've been able to mount the export on another computer and am transferring files over as we speak, but I'm just interested in knowing why the RELATED,ESTABLISHED rule seems to be catching almost all the traffic coming from the other node. Any ideas? Should I be concerned that my firewall isn't protecting anything or something?

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Networking :: SIP Packets Mysteriously Disappearing / When Iptables-nat Activated

Sep 10, 2010

I have a very weird case in my firewall.I have an asterisk server and some phones and between them there is a linux firewall based on iptables.With basic rules on iptables everything works ok, but when I put a single nat rule (no matter what rule I use) some packets from some phones misteriously disappear from interfase to interfase.

Clearer:The firewall has two interfases: eth0 (pointing to phones) and eth2 (pointing to asterisk).One problematic phone is, so I use tcpdump this way.

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CentOS 5 Networking :: View The Packets Saved By Iptables?

Jun 26, 2009

if am using --log-prefix "BANDWIDTH_OUT:" --log-level 7 to capture packets, I think is there a way to view these?

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Networking :: IPTables - Setting Output To Drop And Add Traffic

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:

: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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General :: Iptables - Use DROP Instead Of REJECT And Vice Versa?

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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Networking :: IPTables / SSH Connection To .150 - The Packets Are Still Handled By .150 After Adding The Rules

Jul 2, 2010

I two servers set up: and Initially, I want all traffic to be served by server 150. So for this purpose I am leaving the IPTables on .150 empty. At a point in time, I want to forward all incoming traffic to be served by .160 instead. I have accomplished this using these commands (on .150):

iptables -t nat -A PREROUTING -j DNAT --to
iptables -t nat -I POSTROUTING -j MASQUERADE

My problem is that if I have an open SSH connection to .150 (prior to adding the rules), the packets are still handled by .150 after adding the rules.. e.g. my SSH session stays active. I want these packets to be forwarded to .160, which would effectively disconnect the SSH session. I do not want the packets flat out dropped, I just want them forwarded on in whatever state they are in. If I try a new SSH session, it is properly forwarded to .160

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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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General :: IPTables Drop Or Forward Host (Incoming Connection)

May 3, 2010

How can I drop or forward a incoming connection from a part of a host like *
For example:
The user is connection from * on port 12345
So how can I drop this connection or forward to on port 80

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Security :: How To Write Iptables Rules To Control Drop All Connection

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=,

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Networking :: Promiscuous Interface And Iptables To Receive Packets Not Destined To Localhost

Mar 22, 2010

I am trying to do something outlandish with iptables (or so I think!).I have a source sending udp packets to a destination (say dst11). Using port mirroring I am able to get all these packets to a different machine (say dst22). I am able to see these packets on dst22 interface using tcpdump.I want to analyze the packets on dst22. So what I do is put dst22 interface in promiscuous mode (using ifconfig eth0 promisc). This in theory should get the packet through the MAC layer. Now using iptables I am trying to DNAT the packets in nat prerouting to change the packets destination IP to dst22's interface and change the destination port.

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Ubuntu Security :: IPTables - Setting Default Rules To All Chains As DROP

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

Then allow only some ports:
#Accept inbound packets that are part of previously-OK'ed sessions
# 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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Security :: Iptables State Module - Configuration Error / Not Enable Incoming Packets From Connections Initiated From Inside?

Mar 30, 2011

I have a server that I can only access via SSH (it's located far away) and I would like to secure it by blocking all ports except the ones that I need (which are HTTP and SSH). I still want to be able to make outgoing connections to enable software updates and other things.This is my iptables -L -n :

Chain INPUT (policy ACCEPT)
target prot opt source destination
DROP tcp -- tcp dpts:1:21
DROP tcp -- tcp dpts:23:79
DROP tcp -- tcp dpts:81:65535

In my opinion, this should block all incoming packets except the ones on port 80 and 22, but allow responses to outgoing connections. But a wget does not work, it can't establish the connection.

Maybe this is not the best style for iptables rules, but I want to be absolutely sure to not accidently lock myself out from SSH, so I chose not to configure a "block-everything rule".

Does this configuration not enable incoming packets from connections initiated from inside?

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Networking :: IPTABLES DNAT For Packets Originated Within The "firewall"?

Aug 26, 2010

I'd like to (if it's possible, of course) to redirect the packets originated within a linux box, and I've been tryin' to do it through the OUTPUT chain in nat table:


iptables -t nat -A OUTPUT -p tcp -d --dport 80 -j DNAT --to-destination

The policy for the rest is ACCEPT.This redirection didn't work this way. If I do lynx I reach host, so there is no redirection.Could I achieve what I'm needing through with IPTABLES' OUTPUT chain (in nat table)?

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Networking :: Route Eth2 TCP Packets To Tun0 With IPTABLES And IP RULE/ROUTE?

May 8, 2011

I have 3 network interfaces on my Linux Router :

Interface - Gateway - Type


br0 - - Internet
eth2 - - LAN
tun0 - - VPN (via br0)

What I'd like to do is to route all TCP packets coming from eth2 to tun0 where a VPN client is running on If I delete all default routes and if I add a new route to tun0 like :


route del default
route add default gw

Everything is fine, and everyone on eth2 can reach the Internet using the VPN access. Now the problem is that my VPN client does not allow any other protocols other than TCP. And I also want to allow VPN access only to eth2, no other LAN nor the router itself. use iptables to filter any TCP packets and mark them, so they can be sent to tun0, while any other packets can reach the Internet via br0 ( I found on the Internet that we can mark packets before they get routed. Using the following commands :


iptables -t mangle -A PREROUTING -j MARK --set-mark 85 -i eth2 -p tcp --dport 80
ip route add table 300 default via dev tun0
ip rule add fwmark 0x55 table 300

First of all, --dport 80 never work... :/ I wanted to filter TCP 80 packets coming from eth2, but none of them seems to be HTTP packets... oO (very strange...). Nevermind, I decided to forget about the --dport option. I use the "iptables -L -v -t mangle" command to see how many packets are marked, and it is working fine, all TCP packets coming from eth2 are marked. Now the problem is that none of them are routed to tun0 they are all respecting the "route -n" rules... and not the "table 300" rule I have created.

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Networking :: Use Iptables In Order To Forward All The Incoming Packets For Port 5555 To Port 5556?

Apr 4, 2011

I'm trying to use iptables in order to forward all the incoming packets for port 5555 to port 5556 on the same server (

I wrote the following commands:

iptables -A PREROUTING -t nat -i any -p tcp --dport 5555 -j DNAT --to
iptables -A FORWARD -p tcp -m state --state NEW -d --dport 5556 -j ACCEPT

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Programming :: Sending Arp Packets In C / C++?

Apr 5, 2010

I'm looking for a way how to send an arp request / reply packet using C or C++. I've written an application that can send different crafted packages using jpcap (java), but I'm not a C expert (trying to learn). The reason for this is that I would like to port my java program to C to use it on a less powerful system that can't fully cope with the resource hungry VM

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Networking :: Firewall - Allow Packets Coming From Internet After Authenticating And To By Pass Packets Generated From Internal LAN?

Feb 8, 2010

i have a linux server runnig oracle applications. i need to access this server from putty using ssh through internet. i did by registering my static ip with the and i am able to connect to the server. but now there is no security to authenticate any user as any one knowing the password can login to it.

i thought of configuring the firewall of linux server but the client ip`s are not static and they change continiously. so thought of keeping one more pc between the server and the router which will do the work of authenticating. but i am confuse as how to configure it to allow the packets coming from the internet after authenticating and to by pass the packets generated from internal LAN?

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