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• Is it possible to add tcp autotuning to windows xp?

  - by Caspin

  I have a network application that needs to send messages at 60 times a second. The messages are usually 300-400 bytes, but can be as large as 1500. The default setting for SO_SNDBUF is too small and limits the number of message that can be sent if the network latency is anything greater then 100ms. The naive solution is to just bump the SO_SNDBUF size to to something large. However, depending on the latency and the packet size that could be anywhere from 64K to 8MB. One of Vista's new features is TCP autotuning. Autotuning monitors the tcp connection and dynamically adjust the buffer sizes to allow for optimal communication. I would like to use auto tuning on our windows xp machine so I don't need to guess what my buffers sizes should be. Is there a way to install either a microsoft or 3rd party tcp autotuner on windows xp?

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• Why can't European users access my site?

  - by japancheese

  Hello, My site has been running just fine for the past couple of years, but all of a sudden, two days ago, European users have been experiencing serious connection problems to the site. I really want to fix this for them, but what's the best way to figure out what the issue is? I have absolutely no connection problems to the site on my end, nor do Asian or other American users it seems. Using just-ping.com, some European servers come back with some packet losses. I tried doing some traceroutes from European servers to my own, but they all seemed to work just fine. I'd at least like to be able to tell users that if the problem does not lie with my server, then it at least lies somewhere out of my control. I really want to figure out what the choke point is though. Is there another way I might be able to find out why they can't seem to connect to the site? Just looking for any other ideas from people that have had a similar experience.

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• iptables: built-in INPUT chain in nat table?

  - by ughmandaem

  I have a Gentoo Linux system running linux 2.6.38-rc8. I also have a machine running Ubuntu with linux 2.6.35-27. I also have a virtual machine running Debian Unstable with linux 2.6.37-2. On the Gentoo and Debian systems I have an INPUT chain built into my nat table in addition to PREROUTING, OUTPUT, and POSTROUTING. On Ubuntu, I only have PREROUTING, OUTPUT, and POSTROUTING. I am able to use this INPUT chain to use SNAT to modify the source of a packet that is destined to the local machine (imagine simulating an incoming spoofed IP to a local application or just to test a virtual host configuration). This is possible with 2 firewall rules on Gentoo and Debian but seemingly not so on Ubuntu. I looked around for documentation on changes to the SNAT target and the INPUT chain of the nat table and I couldn't find anything. Does anyone know if this is a configuration issue or is it something that was just added in more recent versions of linux?

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• Windows Server 2008: Limit UDP/TCP packets per IP or ban

  - by WBAR

  How I can limit UDP/TCP packets per IP send to my host (or better PORT) per second or minute ? Would be nice to ban that IP for 12/24 hours or even for ever. I got Windows Server 2008 and I'm very poor in Windows administration but quite good in Linux. EDIT: By basic problem is that They sending a lot of rubbish UPD and TCP packets.. TCP packets without SYNCH, fragmented UDP packets so my servers stop responding.. So I need to cut off users (IPs) sending more than X packets per second. I need solution witch provides me, somehow, configurable: X packets of certain type (UDP, TCP or both - lets say parameter named Z ) are allowed to be received by IP on Y port, otherwise this packet should be DROPPED. My virtual hosts are hosted by VirtualBox and I'm able to forward all incoming packets certain type and certain port to the specific Virtual Host, but I need to DROP them before my VirtualBox receive them.

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• Do two portforward rules translate to "and"?

  - by blsub6

  I just set up an Exchange server to replace my DeskNow mail server. I want to start testing my internet mail exchange of my Exchange server. I can only set the MX records on my DNS up to my one external IP address so I was thinking that I could set up a firewall rule on my internet-facing firewall that port forwarded the smtp packets to two different servers. My question is: If I do that, will that mean that the smtp packets will be forwarded to just the first internal IP on the list? Or does it mean that the packet will be cloned and sent to both IPs?

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• Dell Multi-Monitor Hub: true DisplayPort splitting?

  - by thepurplepixel

  In my search for a new display, I came across the Dell Multi-Monitor Hub MMH11, which seemed to be an alternative to my search for daisy-chainable DisplayPort displays. However, before I cave and spend $179 on this device, I am wondering if this will be similar to other splitting devices where it appears to the computer as one big monitor and the device does the splitting (which I don't want). Or, does this use the packet-based nature of DisplayPort to present two/three separate displays to the computer? Also, would this device work on my MacBook Pro? (I know the Dell site says it's for Windows, but it also says that no driver installation is required. I'd assume since the MBP supports DP 1.2 it would work, but it's better to ask). Thanks!

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• iptables ACCEPT policy

  - by kamae

  In Redhat EL 6, iptables INPUT policy is ACCEPT but INPUT chain has REJECT entry in the end. /etc/syconfig/iptables is as below: *filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A INPUT -p icmp -j ACCEPT -A INPUT -i lo -j ACCEPT -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT -A INPUT -j REJECT --reject-with icmp-host-prohibited -A FORWARD -j REJECT --reject-with icmp-host-prohibited COMMIT Do you know why the policy is ACCEPT not DROP? I think setting DROP policy is safer than ACCEPT in case to make mistake in the chain. Actually the policy is not applied to any packet: # iptables -L -v Chain INPUT (policy ACCEPT 0 packets, 0 bytes)

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• Is it possible to limit output bandwidth between eth0 and lo?

  - by mmcbro

  I'm trying to limit the bandwidth between my eth0 output (nginx proxy) to my loopback inteface (apache) by filtering on destination port. Incoming Packet -> Eth0 -> 0.0.0.0:80 Nginx -> tc qdisc class/iptable mangle 2525port -> 127.0.0.1:2525 Apache I don't know if it's even possible I'm just experimenting. My rules are the followings : tc qdisc add dev eth0 root handle 1:0 htb tc class add dev eth0 parent 1:0 classid 1:10 htb rate 2mbps ceil 2mbps prio 0 tc filter add dev eth0 parent 1:0 prio 0 protocol ip handle 10 fw flowid 1:10 iptables -A OUTPUT -t mangle -p tcp --dport 2525 -j MARK --set-mark 10 I also tried to with FORWARD chain but its still the same.

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• How do I get around restrictive email policies by ISP?

  - by Peter Turner

  Apparently we've been restricted (though packet filtering) to some arbitrarily small and untenable number of emails a day by some bankrupt ISP (and they say that's how it's always been chortle). We've been using our own mail server for the last 15 years, and only recently they've been giving us guff. Is there a way for a legitimate business to email their clients, who really want to receive these emails, by bypassing the ISP? The way we've been doing it is by breaking up into 20 or 30 emails, but that gets complicated and requires a lot of manual labor by the receptionist, and unless she's really careful we wind up emailing lots of people twice. So what are my options (Hosted Email, Lithuanian Proxy Server, Different ISP, not writing awful PHP that sends out zillions of emails and gets us blacklisted)?

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• ASA5500 series logging for management interface in transparent mode

  - by ANervousTwitch

  i have a cisco asa5520 in transparent mode. the interface is on the same subnet as some windows machines, which are generating a lot of broadcast traffic that is filling up the logs. is there any way to have it not log that its blocking those packets? its a bunch of these messages: "through-the-device packet to from management-only network is denied: udp src..." im also seeing some of those zeroconf requests that id like to drop logging for. i tried to just put a rule on the management interface, but apparently thats not allowed.

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• How to have 2 windows machines on the same network with the same IP address

  - by Stu

  I have a custom made ADC device that is spitting out data by addressed UDP packets. I have that device plugged into a 4 port switch. I have one windows embedded standard 7 machine which is the normal recipient of that data. To be able to receive the data (Using LabVIEW) the windows network adapter IPv4 settings must have a static IP address that corresponds to the UDP packet destination. I would like to add a second windows machine (This one is just regular Win 7 Pro) to simultaneously catch the data, however with all devices connected to the switch, the Win 7 Pro machine recognizes an IP address conflict and will not take the setting for the required static IP address. (The network adaptor settings show that the correct value has been entered but ipconfig shows that it is not actually set.) Neither windows machine needs to transmit network data, they only need to be able to receive the UDP data from the ADC device. Is there any way to disable this IP address conflict detection 'feature' of windows networking?

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• IP issue with Heartbeat & DRBD

  - by adam0345

  I'm in the process of setting up 3-node stacked DRBD, and i'm experiencing a rather bizarre issue. Two nodes are located at the data center, and the 3rd node is located locally. The Primary and Secondary nodes are working as expected, however the 3rd node won't connect to the primary. If I ping the IP provided by heartbeat on the 3rd node it will return 100% packet loss, if I reset networking interfaces, ping will then return a few successful packets, but then stop returning any packets. I can't work out any reason why this would be behaving like this. All nodes are running Debian Squeeze, and the latest version of DRBD.

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• Meaning of Bridge-Check in iptables flowchart

  - by networkIT

  In the famous iptables flow-chart what does bridge-check exactly stands for ? I couldn't find any documentation. The only clue I found was while scanning the MikroTik RouterOS documentation ( RouterOS is build upon a Linux 2.6.16 kernel ), I found this : In-interface Bridge = Checks if the input interface is a port for a bridge or is the bridge. Manual:Packet Flow Comparing both flow-charts brings clues that iptables Bridge-check might equal MikroTik In-Interface Bridge. Is this true ? Else, what might be the meaning of iptables Bridge-Check ?

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• Have OS X send wake on lan before printing to shared printer

  - by Dean Hill

  I have a MacBook that prints to a shared Windows 7 printer. Sometimes the Windows machine is asleep, and the Mac will just queue up its print requests. I recently created a script to send a wake-on-lan packet to a Windows 7 machine. This wakes up the Windows machine and printing starts. Great, but I think the system can be automated en Is it possible to have the MacBook run the wake-on-lan script everytime something is printed? Stated more generally, can I have the OS X print subsystem execute a script everytime something is printed?

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• Windows Filtering Platform not turning off until admin logon. Win2008R2sp1

  - by rjt

  Just installed Windows Server 2008R2 SP1 to see if it would fix this problem, but it didn't. Until an administrator logs onto the domain controller, there are many events that WFP blocked a connection from Server60 to Server60 or Server60 to Server70. Both server60 and server70 are the domain controllers. One the admin logs on, the WFP events stop. The firewall is off by default GPO. Yes, i know that the WFP kicks in during the boot up sequence until the firewall takes over or in my case does not take over (since Vista), but i clearly should not have to autologon to a domain controller and call autolock or something. Example event LEVEL = Information Source = Microsoft Windows Security Auditing EventID = 5152 "Filtering Platform Packet Drop" and its evil twin id = 5157 "Filtering Platform Connection" "The Windows Filtering platform has blocked a connection." Direction %%14593 SourceAddress 192.168.10.60 SourcePort 49677 DestAddress 192.168.10.60 DestPort 389 Protocol 6 FilterRTID 65667 LayerName %%14611 LayerRTID 48 RemoteUserID S-1-0-0 RemoteMachineID S-1-0-0 windows-server-2008-r2 WFP BFE WindowsFilteringPlatform BaseFilteringEngine

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• Routing using Linux with 2 NIC cards

  - by Kevin Parker

  Configured Clear OS to be in Gateway mode on a machine with two NIC cards. eth0:192.168.2.0/24 with ip 192.168.2.27 which is connected to a modem and thus have internet connectivity. eth1:192.168.122.0/24 with ip 192.168.122.10 which is connected to other machines in LAN through switch. LAN machines with network 192.168.122.0 is not getting internet.How can they get internet Through Clear OS gateway.I have enabled packet forwarding in clear os using "ip_forward=1" What am i missing?.Can you please help me in this. Following are the static routing i have added: on LAN machine1 with ip address 192.168.122.11 ip route add 192.168.2.0/24 via 192.168.122.10 dev eth0 ip route show 192.168.2.0/24 via 192.168.122.10 dev eth0 192.168.122.0/24 dev eth0 proto kernel scope link src 192.168.122.11 But still 192.168.2.0/24 network is not reachable.Where can be the problem??

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• Understanding connection tracking in iptables

  - by Matt

  I'm after some clarification of the state/connection tracking in iptables. What is the difference between these rules? iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT Is connection tracking turned on when a packet is first matched containing -m state --state BLA , or is connection tracking always on? Can/Should connection state be used for fast matching like below? e.g. suppose this is some sort of router/firewall (no nat). # Default DROP policy iptables -P INPUT DROP iptables -P OUTPUT DROP iptables -P FORWARD DROP # Drop invalid iptables -A FORWARD -m state --state INVALID -j DROP # Accept established,related connections iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT # Allow ssh through, track connection iptables -A FORWARD -p tcp --syn --dport 22 -m state --state NEW -j ACCEPT

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• Cisco access list logging. Why is there a difference between IPv4 and IPv6?

  - by growse

  I've got a Cisco 877 router. I've got an IPv4 access list and an IPv6 access list set up and configured similar to this: interface Dialer1 ... ip access-group INTERET-IN ipv6 traffic-filter IPV6-IN Each of these access lists has a final rule of deny ip/ipv6 any any log. However, in my syslog I notice that there's a difference in formatting between the two types of entries. IPv4 will say: %SEC-6-IPACCESSLOGP: list INTERNET-IN denied udp 88.89.209.63(137) -> 1.2.3.4(137), 1 packet Whereas the IPv6 list will say %IPV6_ACL-6-ACCESSLOGNP: list IPV6-IN/240 denied 59 2001:0:5EF5:79FD:14F9:B773:3EBA:3EE3 (Dialer1) -> 2001:800:1000:0::1, 8 packets Both have broadly the same information, but the IPv6 log entry is missing the protocol type and port, both of which are very useful if I'm trying to troubleshoot connectivity. Why is this? How do I get IPv6 deny logs to display the protocol and port used, if any?

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• Is visiting HTTPS websites on a public hotspot secure?

  - by Calmarius

  It's often said that HTTPS SSL/TLS connections are encrypted and said to be secure because the communication between the server and me is encrypted (also provides server authentication) so if someone sniffs my packets, they will need zillions of years to decrypt if using brute force in theory. Let's assume I'm on a public wifi and there is a malicious user on the same wifi who sniffs every packet. Now let's assume I'm trying to access my gmail account using this wifi. My browser does a SSL/TLS handshake with the server and gets the keys to use for encryption and decryption. If that malicious user sniffed all my incoming and outgoing packets. Can he calculate the same keys and read my encrypted traffic too or even send encrypted messages to the server in my name?

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• Setting up a linux switch

  - by Shahmir Javaid

  I have a C++ Program to sniff each and every packet that crosses my linux box. However i need to now get my linux box to listen to every traffic in my network. I could buy a managed switch and set up port spanning, but i aint paying 200+ £'s for a switch and plus gives me a chance to learn. My Network +---------Computer A | Internet-----Router------Switch-------------+---------Linux Box | +---------Computer B Proposed Network +---------Computer A | Internet-----Router------Linux Box--------Switch eth0^ ^eth1 | +---------Computer B How would i setup this in linux. Do i just configure both the ethernet on different IP Address on the same network. Or am i completelly on the wrong track My System Fedora 13. Thanks

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• Exclude pings from apache error logs (ran from PHP exec)

  - by fooraide

  Now, for a number of reasons I need to ping several hosts on a regular basis for a dashboard display. I use this PHP function to do it: function PingHost($strIpAddr) { exec(escapeshellcmd('ping -q -W 1 -c 1 '.$strIpAddr), $dataresult, $returnvar); if (substr($dataresult[4],0,3) == "rtt") { //We got a ping result, lets parse it. $arr = explode("/",$dataresult[4]); return ereg_replace(" ms","",$arr[4]); } elseif (substr($dataresult[3],35,16) == "100% packet loss") { //Host is down! return "Down"; } elseif ($returnvar == "2") { return "No DNS"; } } The problem is that whenever there is an unknown host, I will get an error logged to my apache error log (/var/log/apache/error.log). How would I go about disabling logs for this particular function ? Disabling logs in the vhost is not an option since logs for that vhost are relevant, just not the pings. Thanks,

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• How can I measure TCP timeout limit on NAT firewall for setting keepalive interval?

  - by jmanning2k

  A new (NAT) firewall appliance was recently installed at $WORK. Since then, I'm getting many network timeouts and interruptions, especially for operations which would require the server to think for a bit without a response (svn update, rsync, etc.). Inbound SSH sessions over VPN also timeout frequently. That clearly suggests I need to adjust the TCP (and ssh) keepalive time on the servers in question in order to reduce these errors. But what is the appropriate value I should use? Assuming I have machines on both sides of the firewall between which I can make a connection, is there a way to measure what the time limit on TCP connections might be for this firewall? In theory, I would send a packet with gradually increasing intervals until the connection is lost. Any tools that might help (free or open source would be best, but I'm open to other suggestions)? The appliance is not under my control, so I can't just get the value, though I am attempting to ask what it currently is and if I can get it increased.

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• How can I tell if my ISP is redirecting my DNS queries?

  - by Nack

  I've attempted to use some DNS services like OpenDNS, and no matter what I do the DNS queries don't return the expected results. Watching the packet traffic on my firewall, I can see the queries go out to the intended DNS server address and responses coming back, but the results are not as expected, for example, the OpenDNS test page always fails even though the requests appear to be going to their servers. I suspect my ISP is intercepting DNS queries and sending them to their own servers. Is there a way to verify this? Is there something else I might be missing? I'm using 3G wireless service from Sprint.

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• Manually forcing TCP connection to retry

  - by Vi.

  I have a TCP connection (SSH session to some computer for example) Network suddenly goes down and drops all packets (disconnected cable, out of range). TCP resends packets again and again, retrying with increasing delays. I see the problem and plug the cable back (or restore network somehow). TCP connection finally successfully resends some packet and continues. The problem is that I need to wait for a some timeout on point 5. I want to use my opened SSH session now and not wait for 5-10 seconds until it finds out that connection is working again. How to force all TCP connections to resend data without delays in GNU/Linux?

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• Frequent occurence of FULL GC.

  - by Viji

  Hi, There is a frequent occurence of FULL GC in our system. We are using Java application running on Tomcat server. Our application is running using internal load balancer setup. We are seeing lot of Full GC's in the server logs due to which the application is hung and Proxy errors are occured. The Java parameter values we are using are: Webapp wrapper: wrapper.java.additional.4=-Xms382M wrapper.java.additional.5=-Xmx1024M Backapp wrapper: wrapper.java.additional.4=-Xms382M wrapper.java.additional.5=-Xmx1024M The error found in webapp wrapper logs: INFO | jvm 1 | 2010/11/26 09:33:19 | [PSYoungGen: 1398460K-140291K(1514624K)] 4623364K-3491394K(5009920K), 0.7285303 secs] [Times: user=1.42 sys=0.00, real=0.72 secs] INFO | jvm 1 | 2010/11/26 09:33:19 | 68539.126: [Full GC DEBUG | wrapperp | 2010/11/26 09:33:19 | send a packet PING : ping Tried to change the JVM values to increase the heap size. But of no use. I suspect that there could be some other reason other than these parameters which is causing the issue. Can anyone please help me on this?

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