Linux is the generic term for a UNIX-like open source operating system based on the Linux kernel.

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4k views

kworker consuming +90% IO and zero disk write

this is a standard apache web server on AWS Linux AMI + EBS. We are noticing high load average (+8) and iotop -a shows: Total DISK READ: 0.00 B/s | Total DISK WRITE: 2.37 M/s TID PRIO USER ...
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105 views

Q: MDADM mismatch_cnt > 0. Any way to identify which blocks are in disagreement?

Okay. After a routine scrub, my MDADM RAID5 is reporting mismatch_cnt = 16. As I understand, this means that while no device reported a read error, there are 16 blocks for which the data and parity ...
7
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239 views

How to automate zabbix web scenarios for vhosts?

I'm new to Zabbix and enterprise monitoring. I've just finished installing Zabbix 2.4. I'm trying to monitor the state of all our vhosts dislocated on different servers. Until now the only solution ...
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3k views

How to disable perf subsystem in Linux kernel?

I'm running some benchmarks. My benchmark runner monitors the dmesg buffer between experiments, looking for anything which could impact performance. Today it threw this up: [2015-08-17 10:20:14 ...
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2k views

nginx recv() failed (104: Connection reset by peer) while reading response header from upstream

I have recently upgrade my magento from 1.5 to 1.9 and when ever I add a certain product to basket, I started to receive this error: 502 Bad Gateway There were no log entries in the var/log/ folder: ...
5
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192 views

grep is inconsistently defaulting to grep -P?

I have a script that does some housekeeping that works perfectly well when invoked from an interactive shell, but did nothing when invoked by cron. To troubleshoot this I started a shell with a ...
4
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40 views

SELinux: manually changing files in /etc/selinux/targeted/contexts/files/

I'm setting up a CentOS 7 server in which the /home directory has to be located on another partition and then mounted with bind-mount. So: /data/homes should be bind-mounted to /home. The problem is ...
4
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392 views

TCP Window Scaling Problem over MPLS

After extensive troubleshooting why the TCP speed over an MPLS circuit is slow in one direction, I figured out that by adjusting the TCP window size the situation improved a lot. Maybe you can tell me ...
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394 views

TCP congestion collapse

I am experiencing suboptimal performance on an EC2 instance connecting to an RDS instance. This particular instance was built before VPC's existed, so all the traffic is flowing through a single ...
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571 views

TCP connection stuck in SYN_RECV state despite ACK received

My client and server are both internal machines. tcpdump on both client and server show all three parts of handshake are communicated, however netstat on server shows SYN_RECV, and on client shows ...
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748 views

Adaptec pm80xx Driver Drops Drives Randomly

I'm building a ZFS NAS using an Adaptec ASA-71605H HBA on Ubuntu 12.04.4. Modern Linux kernels ship with the open-source version of the required pm80xx kernel module. Adaptec provides a driver for ...
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2k views

Can't write to other users “you have write permission turned off”

For some reason I'm not able to write to other users on my system. [root@hostname ~]# write write: you have write permission turned off. [root@hostname ~]# mesg y [root@hostname ~]# mesg is y ...
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425 views

snmp MIB-TCP::tcp not showing any usage

I am trying to configure MRTG on all of my servers for making a network weathermap, this requires running SNMP which MRTG gets the information from. So, I installed it on two servers the exact same ...
4
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673 views

keepalived config is not working correctly with virtualbox vms

my keepalived configuration is not working correctly. I have two virtual testing machines (virtualbox) to try some things out with keepalived/vrrp. Bot can ping each other flawlessly (internal ...
4
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2k views

NAT64 configuration with tayga on Debian 6

I'm trying to implement NAT64 with tayga on my server and came to a point where I'm not sure how to proceed. My server is connected via one IPv4 address and one /64 IPv6 address block to the ...
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8k views

TCP Keepalive and firewall killing idle sessions

In a customer site, the network team added a firewall between the client and the server. This is causing idle connections to get disconnected after about 40 minutes of idle time. The network people ...
4
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870 views

How do I configure OpenSwan to allow pure IPsec (not L2TP) connections from an iPhone?

Similar to this question, I want to configure an IPsec server on Linux which will accept connections from the iPhone. However, unlike the other question, I want to be able to test with pre-shared keys ...
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1k views

how to tune NFS client on MacOS to get Linux like performance?

I have a NFS server running Linux and exporting a couple of shares. Both Linux and MacOS clients connect to the server, and while with the Linux clients I get ~ 75MB/s write speed, the MacOS clients ...
3
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85 views

Hacker bypassing iptables

(moved from SO) I have iptables protecting a sip server. It blocks all IPs except ones I specifically opened, and it seems to work for almost everyone. I have tested from lots of ip addresses that ...
3
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0answers
44 views

Linux max. value for vm.max_map_count

I'm planning to increase significantly the default value of 65k max mappings; let's say multiply by 10. Assuming we have enough memory on the machine is there anything I should take care of doing ...
3
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0answers
48 views

Can servers with encrypted root filesystems be made reasonably highly available?

I have some servers I'd like to keep on encrypted disks, but I don't want to have to manually type a passphrase at every boot. I also don't want to keep the key on the machine unencrypted. TPM would ...
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64 views

ARP behavior in Linux

I have a L3 switch. Port 1 and port 2 of the switch are added to VLAN 30 and an IP address (30.0.0.1) is assigned to it. Port1 and port2 are connected to Host1 and Host2 respectively. Host 1 and ...
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91 views

lspci “Memory at…” vs /proc/bus/pci/devices BAR address

I have a userspace application that I use to write to the registers of a pci device. It takes the base memory address of the device as an argument. Instead of looking this up via lspci on each ...
3
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31 views

what are the memory impacts of increasing a NIC's ring buffer?

What impact (if any) does NIC ring buffer tuning (e.g. via ethtool -G eth0 …) have on system memory utilization? For example, does increasing the ring buffer consume more RAM? And if so, how much? Or ...
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47 views

Strange values in linux system load

I am trying to understand system load on one linux server: $uptime 15:01:45 up 52 days, 19:48, 1 user, load average: 0.63, 1.76, 4.81 Loads are always 1 minute < 5 minutes < 15 minutes. ...
3
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30 views

industry standards for using google-authenticator with linux users?

Would like to ask, particularly those who run large environments with lots of users, if there are any standardized- or industry-recommended ways to securely distribute the verification codes to remote ...
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91 views

What causes SCSI command timeout over 30 seconds?

System description: CentOS 7. SuperMicro X9DRW-iF board. Latest BIOS (and SCU option ROM), using SCU option ROM. Four 300GB WD HDDs connected to the board's SCU. Six HDDs connected to the six ...
3
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0answers
82 views

`at` jobs running tens of minutes later than expected

I have an issue with at jobs being queued successfully and running successfully, but sometimes tens of minutes after they are scheduled to run. I confirmed that atd is running: me@hostname:~$ ps -ef ...
3
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0answers
79 views

On Linux, what is the size of pages reported by vmstat command and/or /proc/vmstat?

As many have stated on StackExchange and elsewhere, a typical kernel/mmu page size for Linux is 4 KiB. I have verified that this is the case on my OS/architecture (RHEL 6.6, Intel Xeon E5-2690). Now ...
3
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0answers
130 views

Getting FTP return code 450 for GET but not for MGET command

I have an FTP-server (ProFTPD 1.3.4b) on Linux and an FTPS client (MOVEit Freely 4.5.1.2 - Secure FTP Client) on Windows. I connect like this 220 ProFTPD 1.3.4b Server (FTP with TLS) [1.2.3.4] 234 ...
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122 views

Query string parameters get shuffled

We're observing some strange behavior of the data send to our systems from a javascript since 9/9/2015. The problem shows up when the query string is send to our servers. Some values of the ...
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93 views

Server high %ni (nice) load

I have following problem with debian 7 (LAMP stack): top - 12:16:08 up 16:36, 4 users, load average: 27.97, 27.66, 27.42 Tasks: 240 total, 2 running, 238 sleeping, 0 stopped, 0 zombie ...
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169 views

Linux device route without broadcast

I have the following setup: A network namespace, ns1 A pair of virtual interfaces, ns_ext1 and ns_int1. ns_ext exists in the global namespace, and ns_int1 exists in ns1. A GRE device, gre1, which ...
3
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0answers
143 views

How to apply memory limits to all cron jobs?

On a shared server (Debian Jessie), there are some memory limits (and other limits, like number of processes) that apply for a group of users, configured in /etc/security/limits.conf. It seems that ...
3
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248 views

'BUG: unable to handle kernel NULL pointer dereference at' on Google Compute Engine

On a semi-regular basis, I've seen GCE instances freezing with the following error message (from the serial console): g[1375589.784755] BUG: unable to handle kernel NULL pointer dereference at ...
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0answers
60 views

NGINX Worker Process, Burst Read Speed to only 1918kb/s

I am running an NGINX/1.8 server on Centos6.6 with multiple worker_processes, however while observing iotop I notice they burst up to 1918kb/s Read speed and rarely if at all pass that limit. I want ...
3
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0answers
175 views

Leap second addition in linux - how to make sure kernel detects it correctly

For the upcoming leap second addition on June 30 I have been working on making sure the Red Hat Linux detects the time smoothly. I have installed the updated tzdata and ntpd package on all RHEL5 and ...
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289 views

Data recovery for XFS partition

I want to recover data from a XFS partition. It seems that I need to xfs_repair the partition but I am not sure I would lose the data, so I prefer to ask to experts. I run a Virtual Box Linux Mint. ...
3
votes
0answers
102 views

Can pacemaker retry monitor action on error?

I created simple pacemaker agent for managing MySQL. It works quite well, but there is 1 thing that I can't understand. When resource is "Started", and monitor action returns one of: ...
3
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0answers
59 views

Raid 1 seeding to speed up identical installs

I was thinking of installing Ubuntu on one of our Dell servers in a RAID 1 configuration, then using the mirrored drive as the primary drive in a second identical server with another RAID 1 setup. ...
3
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0answers
260 views

proc-sys-fs-binfmt_misc.automount failed service

I am using ubuntu 14.10. I have just installed systemd and I have a failing service, proc-sys-fs-binfmt_misc.automount. I've seen here it's a part of systemd: Is this file important? How do I solve ...
3
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0answers
227 views

elasticsearch fails periodically on ubuntu with systemd : how to resolve this?

I am using Elasticsearch on Ubuntu 14.10 and it's periodically going down (entering a failed state). When down, this throws exceptions on my website, users can't search, connect or update database ...
3
votes
0answers
181 views

Solving an asymetric HA Firewall/router

I have an old Linux HA firewall/router setup that never quite worked. As it was never quite put into true production (just testing and demo's), I always had to have one firewall shutdown to make it ...
3
votes
0answers
280 views

Source IP rate limiting in iptables: hashlimit vs recent

I want to perform rate limiting per source IP in iptables. For example, limit the rate at which a host can establish new SSH connections to 5 per minute. To my knowledge there are two ways of doing ...
3
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0answers
390 views

detecting whether GPUs are accessible before running nvidia-smi during boot to create dev files

In the past, I've successfully configured Ubuntu systems containing NVIDIA GPUs to launch nvidia-smi in the background from within /etc/rc.local during the system boot to create the requisite ...
3
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0answers
111 views

Getting journald to send email on certain events

I found these solutions for syslogd that send emails on certain events like local0.crit or local1.err How can I make syslogd email certain log messages to me? ...
3
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0answers
1k views

Ubuntu server + Samba + PAM + Apple OpenDirectory

Due to various reasons, we are looking to migrate our file servers from OS X Server 10.9 over to a true Samba implementation running on Ubuntu 14.04LTS. We have Ubuntu up and running with Samba ...
3
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0answers
938 views

Multiple Reverse SSH Tunnels using Single Port

I am able to accept reverse SSH connections from multiple remote systems on a single server using a port for each connection: Remote A: ssh -fN -R5000:localhost:22 user@server-ip -p22 Remote B: ssh ...
3
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0answers
721 views

What does “connections reset due to unexpected data” in netstat -s mean

I'm running a memcached server under heavy load from multiple web servers. While investigating some odd app level behaviour, we spotted that netstat -s on the memcached box was showing a lot of ...
3
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0answers
7k views

How to recover an mdadm array on Synology NAS with drive in “E” state?

Synology has a customized version the md driver and mdadm toolsets that adds a 'DriveError' flag to the rdev->flags structure in the kernel. Net effect - if you are unfortunate enough to get a array ...