Programming :: Blocking And Non Blocking TCP Send/recv?

Dec 25, 2010

I have a device that is working on modbus protocol andI have written a small program(with block TCP read method ) to read its registers via modbus protocol.my program is working very well but except those times that I unplug the Ethernet cable or turning off the modbus gateway during programs work.at this time my program stops on recv system call (if it reach this system call exacly when I unplug Ethernet cable or turning off the modbus gateway during programs work).I changed my source to work in nonblock TCP method, at this time with the same situation my program does not stop/block on recv system call but after pluging back the Ethernet cable or resuming the connectivity situation back it reads data incorrectly .this is my code:Quote:

#define DEBUG
#include <fcntl.h>
#include <string.h>

[code]...

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Programming :: C - For System Calls, Is Blocking Or Non-blocking Default?

Mar 23, 2010

For system calls, is blocking or non-blocking default in C? Simple question, just am not seeing the answer super quickly.

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Mar 7, 2011

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Jul 30, 2011

I am making a library, but I am facing a strange problem while sending data over network using ethernet.

I am sending 39 bytes of the data from one server to slave application but some time slave receives 39 bytes and some time it receives 29 bytes. And when ever slave receives 29 bytes all the memories to which my pointers are pointing get changed. This problem is only when I am sending data from the server to slave, while sending data from slave to server I am facing no such issue.

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May 21, 2010

I'm just beginning to program with GTK+, and I'm facing this issue: I want to inhibit a signal emission inside one and only one function. I've down cut my code into the smallest example I can to show you what I want to do:The "Emitter" contains a "GtkSpinBox" and it relays the "value-changed" signal, it's header:

Code:
#ifndef EMITTER_H
#define EMITTER_H

[code]....

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Software :: Blocking Socket Vs Non-blocking Socket?

Mar 31, 2010

My question is quite simple:What are type of applications where socket in blocking mode can be used ?
And where non-blocking mode to be used ??

TCP is blocking by default.

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Mar 18, 2011

I have a SSH server on Ubuntu 10.04 running behind a NAT. I have done the port-forwarding at the router. However say for example I want to restrict people connecting to the SSH server by country IP's. When I configure ufw with the following rule it still lets the IPs that are restricted through. Any idea why

sudo ufw allow from xxx.xxx.xxx.xxx/24 to any port 2556

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Jun 8, 2011

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Jan 3, 2011

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Mar 3, 2010

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Ubuntu Security :: How To Check What The UFW Is Blocking

Mar 26, 2010

I can see what Firestarter is blocking in the Firestarter/Events tab, but after reading all the man pages of UFW, I still don't know how to check what the UFW is blocking.

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Ubuntu Security :: Ufw Not Blocking Ports?

Apr 1, 2010

After reading a lot about networking and security I decided to check the security of my own ubuntu box. So I went installing Nmap and discovered that port 139 was "open". Since I 'd read how to use ufw I created a deny rule for port 139. After a second scan with Nmap it still said that port 139 was open as shown below.

[Code]...

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Ubuntu :: Keyring Error Blocking VNC?

May 11, 2010

I'm trying to use VNC on my headless desktop server that's running lucid, but I can only use SSH because a pop asking me to unlock the keyring shows up every time I try to use VNC. I don't have a monitor for that desktop, so I was wondering, is there any way to remove the keyring/to automatically unlock it during autologin? I don't remember what a site I found it on, but I used this to remove my keyring yesterday. It's no longer working today.

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May 25, 2010

I need assistance blocking application in Ubuntu 10.04. block all applications for certain users if they do not exist in a certain directory.For example,we would like to prevent users from launching a portable copy of Firefox from a portable HDD or Flash drive.

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Mar 8, 2011

im having a bit of a problem with Firestarter, i have Transmission opened and i am downloading a movie but when i check Firestarter i see hundreds and hundreds of Ip's that are blocked, and like 10ip's every second that get blocked.

[Code].....

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Ubuntu Security :: Ufw Is Blocking Some Port 80 And Should Not?

Apr 15, 2011

I have the default to deny all. The only rule I have in there is:

Code:
To Action From
-- ------ ----

[code]....

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Ubuntu Security :: UFW Is Blocking Connections Even Though It's Set To Allow For In/Out

Aug 1, 2011

I might be misunderstanding the log but it looks like UFW is blocking connections. I want to allow all incoming and outgoing. I guess what I'm saying is that the servers on my computer will open ports but all other ports should respond with closed just like a default Ubuntu install. Trying to use UFW to monitor connections without really doing any firewalling.

Code:
Aug 1 07:14:07 universal-mechanism kernel: [311111.963762] [UFW BLOCK] IN=eth0 OUT= MAC=00:1f:c6:8a:e9:66:00:01:5c:32:f4:c1:08:00 SRC=72.21.203.146 DST=174.44.178.56 LEN=40 TOS=0x00 PREC=0x00 TTL=233 ID=51984 DF PROTO=TCP SPT=80 DPT=54466 WINDOW=8201 RES=0x00 RST URGP=0

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Ubuntu :: Ufw Blocking RST And ACK Packets When Rules Should Allow It

Aug 31, 2010

I've setup ufw rules on my system but noticed that the rule i created to allow traffic from my local network is still dropping some RST and ACK packets. here's part of the output of dmesg

[Code]...

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Jul 15, 2010

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Mar 17, 2010

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Aug 8, 2010

Is possible blocking web with content for adults with iptables?

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Apr 26, 2010

Where I work we have a lan, it is almost 100% windows machines except for 2 CentOS machines in which some clients connect to, via VPN. (very small network, <50 ip's used)

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Security :: Red Hat SeLinux Is Blocking Ssh And Http?

Feb 3, 2011

When I turn on my SeLinux to enforcing mode on my Red Hat system ssh stops working and my http server stops responding.

I went into the SeLinux GUI and enabled things in there but still it wont work.

Any thoughts on what to check?

permissive mode and disabled they work

I read several articles that say it should not be affect by SeLinux and the setting look correct but the only thing I do is turn on SeLinux and ssh /httpd stop working

ps -eZ | grep sshd
system_u:system_r:unconfined_t:SystemLow-SystemHigh 432 ? 00:00:00 sshd
system_u:system_r:unconfined_t:SystemLow-SystemHigh 2426 ? 00:00:00 sshd
[root@goxsa1340 ~]# ps -eZ | grep httpd
user_u:system_r:httpd_t 3044 ? 00:00:00 httpd

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Server :: Blocking Social Networking On The LAN?

Dec 23, 2010

I have a Debian server running at the gateway level on a LAN. This runs squid for creating block lists of websites - for eg. blocking social networking on the LAN. Also uses iptables.

I am able to do a lot of things with squid & iptables, but a few things seem difficult to achieve.

1) If I block http://www.facebook.com, people can still access https://www.facebook.com because squid doesn't go through https traffic by default. However, if the users set the gateway IP address as proxy on their web browser, then https is also blocked. So I can do one thing - using iptables drop all outgoing 443 traffic, so that people are forced to set proxy on their browser in order to browse any HTTPS traffic. However, is there a better solution for this.

2) As the number of blocked urls increase in squid, I am planning to integrate squidguard. However, the good squidguard lists are not free for commercial use. Anyone knows of a good squidguard list which is free.

3) Block yahoo messenger, gtalk etc. There are so many ports on which these Instant Messenger softwares work. You need to drop lots of outgoing ports in iptables. However, new ports get added, so you have to keep adding them. And even if your list of ports is current, people can still use the web version of gtalk etc.

4) Blocking P2P. Haven't been able to figure out how to do this till now.

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Feb 8, 2011

Ok so, buddy of mine has his ssh server setup and upon checking his logs he sees a ton of failed attempts. Now obviously these are people that are scanning him and trying to brute force him. So is there a way to block them? We know you can block each IP but is there a way to block ALL connections except for certain ones, such as his and mine? Maybe a couple others.

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Jun 6, 2011

I configured squid in oracle enterprise linux 5.I want to block skype access.i configured the following to block skype.but it is not blocking.acl skype_blocking urlpath_regex [0-9]+.[0-9]+.[0-9]+.[0-9]+ http_access deny skype_blocking

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Software :: Denyhosts Blocking Router IP?

Jul 9, 2010

I've been using the denyhosts software for about 8 months to block IPs that are trying to connect to my network that shouldn't be. I'm having trouble with it today for the first time and I can't track down the issue.It is installed on the main server which is the outside IP for the network as well. This should allow me to SSH to the hostname/network IP, or the NAT IP as well. It worked fine up until this morning when I started to work on an ftp server, as far as I know I didn't make any changes that should cause problems.

hosts.deny starts with no 192.x.x.x addresses in it. In allowed-hosts for denyhosts I have entered 192.168.1.1 so that the router should never get blocked.I can SSH to the hostname perfectly fine, connection doesn't drop or anything.If I SSH to the NAT address the connection hangs there while the IP address gets added to hosts.denyI can confirm 100% that I was able to SSH to my NAT from within the network with no problem, now it seems to throw the router IP into hosts.deny if I try to connect from within the network using the NAT.Here is the error that I get in /var/log/securesshd[9272]:d not receive identification string from 192.168.1.1I did update my system recently which may have changed the way some files behave, I will try updating Denyhosts as well. It looked like the address being blocked was an IPv6 since it was showing as:sshd[9201]: refused connect from ::ffff:192.168.1.1 (::ffff:192.168.1.1)

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