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• Socket(TCPIP) Unstable

  - by Lee Kwan Wee

  I have a setup of a SCPI server in a Win7 PC and have 2 other programs talking to it locally(127.0.0.1) over TCPIP socket 5025 and 5029. This worked well and stable in a fresh PC, but when we moved it into our production lines and the IT dept added their policies and stuff, it became unstable. The PC is connected to the production floor server but both of the programs are running locally in the PC. The connection tends to be disconnected when there is an idle period. And it takes 5-6times to refreshing the connection to get it back. I'm not a programmer myself, so I'm hoping to see if anyone here can help with some answers. Thank you very much!! Regards, KwanWee.

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• TIME_WAIT connections not being cleaned up after timeout period expires

  - by Mark Dawson

  I am stress testing one of my servers by hitting it with a constant stream of new network connections, the tcp_fin_timeout is set to 60, so if I send a constant stream of something like 100 requests per second, I would expect to see a rolling average of 6000 (60 * 100) connections in a TIME_WAIT state, this is happening, but looking in netstat (using -o) to see the timers, I see connections like: TIME_WAIT timewait (0.00/0/0) where their timeout has expired but the connection is still hanging around, I then eventually run out of connections. Anyone know why these connections don't get cleaned up? If I stop creating new connections they do eventually disappear but while I am constantly creating new connections they don't, seems like the kernel isn't getting chance to clean them up? Is there some other config options I need to set to remove the connections as soon as they have expired? The server is running Ubuntu and my web server is nginx. Also it has iptables with connection tracking, not sure if that would cause these TIME_WAIT connections to live on. Thanks Mark.

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• (OS 10055) Queue full

  - by Ali

  I'm using apache with mysql on the other server, apache on windows 2003 with 4gb ram 3 dual core CPU My server keep on running smoothly without any issue for about 3 days or roundabout 60+ hours however between 60-70 hours my apache stop working & log events shows this error, I tried to search a lot but helpless, I have already increased max port, please if someone can help ? (OS 10055)An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full. : make_sock: unable to listen for connections on address 0.0.0.0:80

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• How to troubleshoot connectivity when curl gets an *empty response*

  - by chad

  I want to know how to proceed in troubleshooting why a curl request to a webserver doesn't work. I'm not looking for help that would be dependent upon my environment, I just want to know how to collect information about exactly what part of the communication is failing, port numbers, etc. chad-integration:~ # curl -v 111.222.159.30 * About to connect() to 111.222.159.30 port 80 (#0) * Trying 111.222.159.30... connected * Connected to 111.222.159.30 (111.222.159.30) port 80 (#0) > GET / HTTP/1.1 > User-Agent: curl/7.19.0 (x86_64-suse-linux-gnu) libcurl/7.19.0 OpenSSL/0.9.8h zlib/1.2.3 libidn/1.10 > Host: 111.222.159.30 > Accept: */* > * Empty reply from server * Connection #0 to host 111.222.159.30 left intact curl: (52) Empty reply from server * Closing connection #0 So, I understand that an empty response means that curl didn't get any response from the server. No problem, that's precisely what I'm trying to figure out. But what more specific info can I derive from cURL here? It was able to successfully "connect", so doesn't that involve some bidirectional communication? If so, then why does the response not come also? Note, I've verified my service is up and returning responses. Note, I'm a bit green at this level of networking, so feel free to provide some general orientation material.

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• server will not reply (syn -ack)

  - by Brent

  I like to use the following commands to manage 'TIME_WAIT', in the hope to free up resources. echo 20 > /proc/sys/net/ipv4/tcp_fin_timeout sysctl -w net.ipv4.tcp_tw_reuse=1 sysctl -w net.ipv4.tcp_tw_recycle=1 I found something interesting while doing a tcpdump. Sometimes if a client makes a connection (syn), the server will not reply (syn -ack). My question is, could it be because of the top three commands.

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• SYN flooding still a threat to servers?

  - by Rob

  Well recently I've been reading about different Denial of Service methods. One method that kind of stuck out was SYN flooding. I'm a member of some not-so-nice forums, and someone was selling a python script that would DoS a server using SYN packets with a spoofed IP address. However, if you sent a SYN packet to a server, with a spoofed IP address, the target server would return the SYN/ACK packet to the host that was spoofed. In which case, wouldn't the spoofed host return an RST packet, thus negating the 75 second long-wait, and ultimately failing in its attempt to DoS the server?

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• redirect all youtube video requests to a specific one

  - by iTayb

  I'm on an IT team in my company and I would like to block youtube to users. I don't want to just deny access to the whole youtube domain, but only to replace the .flv/.mp4 request with the one that I want. That way, if someone tries to watch youtube videos on the network, He'll get a video of why using our expensive bandwidth for pleasure is a no-no. I thought about using a packet manipulation program and just replace the video ID with something that I want, but I didn't manage to do it right.

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• Connection to mysql server in SYN_SENT

  - by Sunil

  We are facing a strange problem from last few days between our application server and database server(Mysql): connection to database server from application server hangs in SYN_SENT state and after that we are not able to make any connection to database server on mysql port(3306). When we checked the netstat output on database server its in SYN_RECV state. What I can figure out is mysql server is receiving the SYN request and responding also and its not reaching to the client hence SYN_RECV at server side and SYN_SENT at client side. I think SYN_SENT state should go after some time and because of this other db connection attempts to same server should not hang. Does anybody have any idea how can we resolve this issue? Out setup details : Application server: RHEL 5.4, kernel-release = 2.6.18-164.el5, x86_64 Database server: Mysql Version : 5.1.49 RHEL 5.4, kernel-release = 2.6.18-164.el5, x86_64

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• tcpsndbuf high fail count

  - by Matthew Crenshaw

  I've got a small setup, one machine that acts as a load balancer and two machines that do all the work. The load balancer runs nginx (static content + php proxying to workers) and mysql, the two workers run php5-fpm and memcached (pooled between workers). Here's beancounters for the balancer: tcpsndbuf 2171848 2386280 10000000 20000000 3947733 tcprcvbuf 1248288 1669504 10000000 20000000 0 Here's worker 1: tcpsndbuf 951976 1262672 20000000 40000000 0 tcprcvbuf 278528 393496 20000000 40000000 0 Here's worker 2: tcpsndbuf 989888 527472 20000000 40000000 0 tcprcvbuf 212992 452520 20000000 40000000 0 The balancer has 1GB ram, the two workers have 2GB ram each. What is eating my send buffer?

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• pfSense router on a LAN with two gateways

  - by JohnCC

  I have a LAN with an ADSL modem/router on it. We have just gained an alternative high-speed internet connection at our location, and I want to connect the LAN to it, eventually dropping the ADSL. I've chosen to use a small PFSense box to connect the LAN to the new WAN connection. Two servers on the LAN run services accessible to the outside via NAT using the single ADSL WAN IP. We have DNS records which point to this IP. I want to do the same via the new connection, using the WAN IP there. That connection permits multiple IPs, so I have configured pfSense using virtual IP's, 1:1 NAT and appropriate firewall rules. When I change the servers' default gateway settings to the pfSense box, I can access the services via the new WAN IPs without a problem. However, I can no longer access them via the old WAN IP. If I set the servers' default gateway back to the ADSL router, then the opposite is true - I can access the services via the ADSL IP, but not via the new one. In the first case, I believe this is because an incoming SYN packet arrives at the ADSL WAN IP, and is NAT'd and sent to the internal IP of the server. The server responds with a SYN/ACK which it sends via its default gateway, the pfSense box. The pfSense box sees a SYN/ACK that it saw no SYN for and drops the packet. Is there any sensible way around this? I would like the services to be accessible via both IPs for a short period at least, since once I change the DNS it will take a while before everyone picks up the new address.

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• Access Methods to a computer Using MAC Address

  - by Dimal Chandrasiri

  I have my own wifi network at home & recently when I checked the active clients on the wifi I found out the there is one active client. Router shows the MAC address of this client and I want to Access this computer in order to let him know that he's doing a wrong thing. I dont intent to do any harm to his data. I just want to know a method to send him a message saying "Buy you own wifi". How can I achieve this using a windows 8 computer. Is there any specific softwares that I can use or will the netsend command do the trick! Thank you.

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• Determine the Ports reserved and what service reserved them in Windows XP/2003

  - by bogha

  hi, this question is always rise. How can i know what ports are currently reserved in windows XP/2003 and what service had reserved them. for example when i want to install Apache, default port will be 80, but sometimes the Server will have IIS installed on the same port. so i want to know the way to check the ports and what services had reserved them and also if there is a way to re assign those ports to different services. thank you

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• Is SYN flooding still a threat?

  - by Rob

  Well recently I've been reading about different Denial of Service methods. One method that kind of stuck out was SYN flooding. I'm a member of some not-so-nice forums, and someone was selling a python script that would DoS a server using SYN packets with a spoofed IP address. However, if you sent a SYN packet to a server, with a spoofed IP address, the target server would return the SYN/ACK packet to the host that was spoofed. In which case, wouldn't the spoofed host return an RST packet, thus negating the 75 second long-wait, and ultimately failing in its attempt to DoS the server?

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• What port does OpenLink ODBC Driver use?

  - by user36737

  I use Avaya Reporting Services and OpenLink ODBC Drivers for db connection. I know that it uses port 5000 for handshaking but after that I believe it uses an random port for communication. I want to deploy my application and it will communicate with the client's system in their datacenter. They are asking what ports should they open on their firewalls. I can't obviously give them a range above 50,000 that I know OpenLink ODBC Drivers use. Can someone tell me what port should I tell my client to open?

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• How to use a different Ethernet connection

  - by SteveC

  I'm running a virtual machine at home which has a VPN connection to our main office, but I also want to connect to a share on another machine at home. When I check with IPCONFIG I can see two ethernet connections ... my work VPN ... Ethernet adapter Local Area Connection* 11: Connection-specific DNS Suffix . : Link-local IPv6 Address . . . . . : xxxx::xxxx:xxxx:xxxx:xxxxxxx IPv4 Address. . . . . . . . . . . : XXX.XXX.XXX.XXX Subnet Mask . . . . . . . . . . . : 255.255.254.0 Default Gateway . . . . . . . . . : XXX.XXX.XXX.XXX and home local ... Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : lan Link-local IPv6 Address . . . . . : xxxx::xxxx:xxxx:xxxx:xxxxxxx IPv4 Address. . . . . . . . . . . : 192.168.1.70 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : What's weird is when I've been working before with a plugged-in ethernet cable I've not had any problem getting to the share? I can PING the other machine, but I can't access the share at ... \\othermachine\c$ I tried 'TRACERT` but that disappears off to the work network and eventually gets back to the local other machine after a few time-outs Is there anyway to "force" the connection to stay local ? UPDATE: the VPN is AEP SSL Tunnel

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• Linux server: Dropped packets

  - by Lars

  I see dropped packets using ifconfig on my eth0 interface: eth0 Link encap:Ethernet HWaddr 00:15:17:0d:03:ca inet addr:10.0.1.2 Bcast:10.0.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:30268348 errors:0 dropped:70721 overruns:0 frame:0 TX packets:133076885 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:8699434077 (8.6 GB) TX bytes:194937313025 (194.9 GB) Interrupt:16 Memory:feae0000-feb00000 When i use ethtool -S i dont see anything wrong: NIC statistics: rx_packets: 30267138 tx_packets: 133074510 rx_bytes: 8699356158 tx_bytes: 194934147340 rx_broadcast: 35296 tx_broadcast: 5435 rx_multicast: 0 tx_multicast: 0 rx_errors: 0 tx_errors: 0 tx_dropped: 0 multicast: 0 collisions: 0 rx_length_errors: 0 rx_over_errors: 0 rx_crc_errors: 0 rx_frame_errors: 0 rx_no_buffer_count: 0 rx_missed_errors: 0 tx_aborted_errors: 0 tx_carrier_errors: 0 tx_fifo_errors: 0 tx_heartbeat_errors: 0 tx_window_errors: 0 tx_abort_late_coll: 0 tx_deferred_ok: 0 tx_single_coll_ok: 0 tx_multi_coll_ok: 0 tx_timeout_count: 0 tx_restart_queue: 0 rx_long_length_errors: 0 rx_short_length_errors: 0 rx_align_errors: 0 tx_tcp_seg_good: 5757001 tx_tcp_seg_failed: 0 rx_flow_control_xon: 8649 rx_flow_control_xoff: 62072 tx_flow_control_xon: 0 tx_flow_control_xoff: 0 rx_long_byte_count: 8699356158 rx_csum_offload_good: 30212111 rx_csum_offload_errors: 0 rx_header_split: 10857552 alloc_rx_buff_failed: 0 tx_smbus: 0 rx_smbus: 0 dropped_smbus: 0 rx_dma_failed: 0 tx_dma_failed: 0 I am running Ubuntu 12.04 with kernel 3.2.0-30-generic #48-Ubuntu SMP I have pinged every device on my internal network for about 24 hours, without packet loss. Also checked my router and my interface to the WAN, also no errors there. Does anyone have any clue?

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• IPtables rate-limit, What are the differences between modules? Recent, Limit

  - by TechZilla

  I am doing some rate-limiting with IPtables, and i'm not sure if I should use "Recent" or "Limit" What are the differences between the two? If they both achieve the same result, which one has better performance? I would like to know, regardless if any difference would be perceivable. I am looking to ACCEPT if under limit, and REJECT if over. I'm not interested in thus bandwidth throttling, I don't want a queue. I don't need any syntax examples, both have ample use examples online. I have also used Limit in the past. I appreciate any responses.

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• What does it mean that most dropped packets have a different incoming and outgoing port?

  - by Kev

  Over the weekend an internet-facing DI-624 router of mine was e-mailing me logs like crazy for some reason. Normally it's a few a day, but it was about one every few minutes instead. Analyzing the messages, I found that only 340 had the same src and dst port number, whereas the majority of "default deny" dropped packets (28,000+) had the two ports different. What does this mean?

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• LAN connection suddenly domain_not_set.invalid

  - by Liam Coates

  I went to work today - internet fine as always. Came back, dreaded yellow triangle. I can connect wirelessly like i am now on my tablet. Thats completely fine. But through LAN I get the domain_not_set.Invalid message if I renew my IP for example. It seems I just don't have a DNS. But I have literally not touched anything no ones even been in the house. Hows that happen? Any help on why only wireless works would be great, I am gonna buy a wireless adapter tomorrow for the interim but I would like to know why. Thanks.

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• What is the "in-the-wire" size of a ethernet frame? 1518 or 1542?

  - by chrisapotek

  According to the table here, it says that MTU = 1500 bytes and that the payload part is 1500 - 42 bytes or 1458 bytes (<- this is actually wrong!). Now on top of that you have to add IPv4 and UDP headers, which are 28 bytes (20 IP + 8 UDP). That leaves my maximum possible application message to as 1430 bytes! But by looking for this number in the Internet I see 1472 instead. Am I doing this calculation wrong here? All I want to find out is the maximum application message I can send over the wire without the risk of fragmentation. It is definitely not 1500 because that includes the frame headers. Can someone help? The confusion is the the PAYLOAD can actually be as large as 1500 bytes and that's the MTU. So now what is the size in-the-wire for a payload of 1500? From that table it can be as big as 1542 bytes. So the maximum app messages I can send is 1472 (1500 - 20 (ip) - 8 (udp)) for a maximum in the wire size of 1542. It amazes me how things can get so complicated when they are actually simple. And I have not clue how someone came up with the number 1518 if the table says 1542.

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• Does DisableTaskOffload disable all related LAN Adapter settings?

  - by vfclists

  I have been getting problems disabling Checksum Offload in a lot of LAN adapters manually. Does setting the DisableTaskOffload value in the registry disable all of the related parameters regardless of LAN card? ie is the use of the LAN adapter offload capabilities controlled by Windows itself?

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• High data on recv-q buffer and thread lock on java.io.BufferedInputStream in linux

  - by Sagar Patel

  We have a java application running on linux (ubuntu server). We have been facing high recv-q problem since quite some time. Application gets hang and does not read data from socket every few hours. In thread dump, we have found below stack trace. "Receiver-146" daemon prio=10 tid=0x00007fb3fc010000 nid=0x7642 runnable [0x00007fb5906c5000] java.lang.Thread.State: RUNNABLE at java.net.SocketInputStream. socketRead0(Native Method) at java.net.SocketInputStream.read(SocketInputStream.java:150) at java.net.SocketInputStream.read(SocketInputStream.java:121) at java.io.BufferedInputStream.fill(BufferedInputStream.java:235) at java.io.BufferedInputStream.read1(BufferedInputStream.java:275) at java.io.BufferedInputStream.read(BufferedInputStream.java:334) - locked <0x00000007688f1ff0> (a java.io.BufferedInputStream) at org.smpp.TCPIPConnection.receive(TCPIPConnection.java:413) at org.smpp.ReceiverBase.receivePDUFromConnection(ReceiverBase.java:197) at org.smpp.Receiver.receiveAsync(Receiver.java:351) at org.smpp.ReceiverBase.process(ReceiverBase.java:96) at org.smpp.util.ProcessingThread.run(ProcessingThread.java:199) at java.lang.Thread.run(Thread.java:722) We are not able to trace the exact reason behind this? Kindly help. We are using 16 core machine and load on the system is around 30-40 at the time of issue. We use command ss dst <ip> to find out recv-q. Recently we have been facing issues with recv-q size getting hung, were in receive buffer gets stuck at some point of time. But recvQ size is not decreasing and as a result we are losing a lot of hits from the other side, our application is not accepting any data.

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• Why can't a PC with 2 network cards be accessed by hostname?

  - by lewis

  I set up PC with 2 network cards, connected to the same LAN. I can connect to this PC (e.g. by remote desktop) only via ip-addresses. Accessing by hostname does not work. Why is this the case? UPDATE: Full environment 1. PC with 2 hardware network adapters. 2. On this PC installed VMWare Workstation. Created 3 VM's, networked by "bridged" network setting in VMWare. 3. In LAN all ip-addresses given from DHCP. 4. Win2k8 on all hosts (both physical and vitrual). As result: 1. PC has 2 ip-address (e.g. 192.168.1.71 and 192.168.1.72). PC available in LAN by ip-addreses, but not avail by hostname. 2. VM's has own ip-addr on each (e.g. 192.168.1.73, *74, *75 etc). They are available from LAN by their ip's, BUT not by their hostnames. How can I access to PC and to VM's by hostname?

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• How do networking ports work? Can I configure the ports that client and server use?

  - by joedotnot

  Let's say i have a "server" program listening on address 1.2.3.4:69 (i.e. remote port 69) When i connect from a "client" program to it, typically i would specify the IP address + port of the target or server system; But what port would the client be using ? And how does the server know which port to connect back to the client on? I understand this question is very general, but just wanting to get a general feel for how things work. Then extending this to a specific protocol, say FTP (typical port 21), can I change it such that the server uses port 69, but the client uses port 100? And similarly, for Remote Desktop in WinXP (typical port 3389), i know how to change the server port to be something other than 3389, but how does one change what port the client uses (if at all possible)?

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• Many ISP's is block port 25, how do I choose an alternative port?

  - by Xeoncross

  I am building an application that will be acting as a combined MUA/MTA on different networks. However, many of the networks are with ISP's that block port 25 for SMTP. Therefore I would also like to open up a secondary port so that some of the installs can communicate on that if port 25 is closed. How do I choose a second port? I know some people use port 26 or port 2525. What is the correct way to choose a port that won't interfere with existing software?

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