Linux Networking refers to any networking function performed by a server running Linux. A Linux server operating as a router or firewall are the most common applications.

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38
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6answers
4k views

How can you tell what a server actually does? [closed]

I've been handed 3 Linux boxes, 1 front facing with apache on it and another 2 which, as far as I can tell, don't do an awful lot. All running on Redhat. The question is simple: How can I tell what ...
24
votes
8answers
58k views

how to find out mac addresses of all machines on network

Is there some easy way to find out mac address of all machines on my network rather than doing an SSH into each and ifconfig | grep HWaddr if there are 300 machines on network I really need some easy ...
24
votes
5answers
9k views

Hostnames - What are they all about?

I've recently been "forced" to perform some sysadmin work, while this isn't something that I absolutely love doing I've been reading, experimenting and learning a lot. There is one fundamental aspect ...
24
votes
2answers
37k views

How to reduce number of sockets in TIME_WAIT?

Ubuntu Server 10.04.1 x86 I've got a machine with a FCGI HTTP service behind nginx, that serves a lot of small HTTP requests to a lot of different clients. (About 230 requests per second in the peak ...
17
votes
5answers
7k views

What network loads require NIC polling vs interrupts?

Does anyone have some data or basic calculations that can answer when frame coalescing (NAPI) is required and when a single interrupt per frame is sufficient? My hardware: IBM BladeServer HS22, ...
15
votes
2answers
2k views

Serving 10,000 simultaneous users over Wifi

I'm looking to offer a wifi service at a public event. The traffic will be very minimal, 10KBps should be adequate. Users don't necessarily need to connect to anything other than the private intranet. ...
14
votes
1answer
5k views

Should /etc/hosts contain an entry like '127.0.0.1 localhost myhost.example.org myhost'?

When looking at a variety of Linux and FreeBSD systems, I've noticed that on some systems /etc/hosts contains an entry for the public hostname of the host, but not on other systemst. What is the best ...
13
votes
3answers
21k views

What is the correct way to open a range of ports in iptables

I have come across articles advising for the following: iptables -A INPUT -p tcp 1000:2000 -j ACCEPT And others stating that the above will not work and iptables only supports multiple port ...
13
votes
2answers
34k views

Linux e1000e (Intel networking driver) problems galore, where do I start? [closed]

I'm currently having a major problem with e1000e (not working at all) in Ubuntu Maverick (1.0.2-k4), after resume I'm getting a lot of stuff in dmesg: [ 9085.820197] e1000e 0000:02:00.0: PCI INT A ...
12
votes
6answers
29k views

Delete a iptables chain with its all rules

I have a chain appended with many rules like: > :i_XXXXX_i - [0:0] > -A INPUT -s 282.202.203.83/32 -j i_XXXXX_i > -A INPUT -s 222.202.62.253/32 -j i_XXXXX_i > -A INPUT -s ...
12
votes
2answers
3k views

arp-requests cannot be seen by specific nodes

I create an open ad-hoc wlan by using iwconfig (I have the same issue with wpa_supplicant as well). there are 4 nodes on the network as seen on the figure below. The nodes run ubuntu 12.04 and debian ...
11
votes
1answer
27k views

What CAN cause 'RTNETLINK answers : No such process' when adding a route

I have come up against this error a number of times, but haven't yet found an explanation of what the error means, or what can cause the problem. I'm not posting my configs or the specific route I'm ...
11
votes
2answers
12k views

How can I increase the value of somaxconn?

I can check its value by cat /proc/sys/net/core/somaxconn, is it OK if I change it simply by echo 1024 > /proc/sys/net/core/somaxconn?
11
votes
2answers
11k views

How to generate netflow data in linux

We have a number of Linux servers for which I would like to capture netflow data to be processed by a netflow analyzer. I've been spoiled by the ease in which Mikrotik routers allow netflow data ...
11
votes
2answers
7k views

PXE Booting Virtualbox guests in internal networking mode?

I am not sure if this is just specific to my distro's packages or is a vbox limitation. So, any help would be appreciated. Ok, so I have a network of VMs, with one VM acting as a NAT for the other ...
11
votes
5answers
2k views

tc u32 — how to match L2 protocols in recent kernels?

I have a nice shaper, with hashed filtering, built at a linux bridge. In short, br0 connects external and internal physical interfaces, VLAN tagged packets are bridged "transparently" (I mean, no VLAN ...
10
votes
2answers
4k views

Can I have dots in a hostname?

I'm using names like a.alpha for the hostname of my linux box, but it seams that these name are not completely usable. The response of a hostname shell command is correct (a.alpha). But the name ...
10
votes
4answers
21k views

multiple MAC addresses on one physical network interface (linux)

Simple question: How can I setup multiple MAC addresses on one physical network interface (linux)? Why? My ISP is checking ip<->mac on GW and I d like to route traffic through my "linuxbox" and ...
9
votes
3answers
350 views

Any free networking lab?

I want to study network. But I don't have an access to a sample network (routers, DNS, IP4, IP6 windows linux mixed heterogenous system). Do you know any online network to study (Free as possible). Is ...
9
votes
0answers
107 views

SynProxy can not return syn ack packets with asymmetric dual bridge topology

I have a asymmetric dual bridge topology as shown below when i connect from 172.16.11.5 and 172.16.10.6 with ssh but i can not connect because of SynProxy. ------- ...
8
votes
3answers
26k views

How to “restart” particular network interface on RHEL?

In RHEL, instead of using service network restart command, how can i restart a particular network interface, lets say "eth1", with only one command. "Only one command" because that is the only ...
8
votes
4answers
382 views

Simulate slow connection between two ubuntu server machines

I want to simulate the following scenario: given that I have 4 ubuntu server machines A,B,C and D. I want to reduce the network bandwidth by 20% between machine A and machine C and 10% between A and ...
8
votes
1answer
39k views

What does “net_ratelimit: 44 callbacks suppressed” mean on a linux based router?

I'm trying to tune Snort performance on a Debian based router. I was seeing stuff like: snort packet recv contents failure: No buffer space available So I upped the buffers to 8M and when that ...
8
votes
1answer
775 views

TCP packet being retransmitted 7 times when sysctl tcp_retries1 is set to 3 - why?

Ubuntu 12.04 I am trying to better understand how many times TCP will attempt to retransmit a packet when it does not receive confirmation the destination received it. After reading the tcp man page ...
7
votes
3answers
3k views

How to completely disable IPv4 stack from Linux?

How can I disable IPv4 stack from Linux ? I want to do that dynamically, ie. sometimes I want it enabled and sometimes I want only IPv6 stack. Is there any portable way to do that ? If you know how to ...
7
votes
1answer
9k views

NAPI vs Adaptive Interrupts

Could anyone please explain how two following technologies are used to mitigate interrupt overhead under high networking load? Adaptive-rx/Adaptive-tx, and NAPI; I would appreciate an answer that ...
7
votes
2answers
2k views

How can I switch from a custom linux network namespace back to the default one?

With ip netns exec you can execute a command in a custom network namespace - but is there also a way to execute a command in the default namespace? For example, after executing these two commands: ...
7
votes
1answer
12k views

How to troubleshoot latency between 2 linux hosts

The latency between 2 linux hosts is about .23ms. They are connected by one switch. Ping & Wireshark confirm the latency number. But, i dont have any visibility into what is causing this latency. ...
7
votes
1answer
15k views

How GRO (generic receive offload) works on more advanced NICs?

I am interested in particular answers: Does the NIC with GRO edits/creates TCP ACK or any other packets (or is this feature transparent to receiver/sender TCP stacks)? There should be a ...
7
votes
5answers
18k views

Can only bring up one of two interfaces

I'm having a bizarre issue with my HP Proliant DL 360 G4p server. It has two gigabit ethernet interfaces but I can bring up only one of them. This is starting to freak me out and that's why I turned ...
7
votes
2answers
8k views

Intermittently high ping times to router via Wi-Fi

On my local network there are (among others) 5 machines (running Debian Jessie or Arch) wirelessly connected to a Netgear WNDR4000 router. Below is a graph of the ping times to the router from each of ...
7
votes
1answer
339 views

Is ISP's Equipment Misconfigured? Getting Martian Packets on Internet Interface

We have a /28 block from our ISP. Let's say 1.1.1.240/28 with a gateway of 1.1.1.241. We are getting martian packets from another customer over at 1.1.1.214. They are sending broadcasts to 1.1.1.255. ...
7
votes
3answers
5k views

Layer 4 vs Layer 7 Load Balancing

I am trying to decide between using a layer 4 load balancing solution for my datacenter or a layer 7 solution. Unfortunately (for my sanity, that is), my use case is simple enough that both solutions ...
7
votes
3answers
34k views

Why ip route add doesn't work, but ip route add with less details and then change works?

I have a problem with ip route add. Surprisingly, I can add the route then change it, but I cannot directly add it: # ip route add 192.168.0.0/16 via 192.168.255.254 src 192.168.1.101 RTNETLINK ...
6
votes
1answer
6k views

How to block internet access to certain programs on Linux

Recently, I have encountered a problem of limiting Internet Access to specific programs. Could anybody recommend a good way of doing that, without using any particular software?
6
votes
3answers
4k views

How to see on Linux what network interface and source IP address is used for a route to a specific destination host?

If I have multiple network interfaces (here: 2) on a Linux machine (here: Debian Lenny). How do I see, over what network interface (NIC) a route to a specific destination host is going and what source ...
6
votes
1answer
17k views

“Stale NFS file handle” after reboot

On the server node, it is possible to access an exported folder. However, after reboots (both server and client), the folder is no longer accessible from the clients. On server # ls /data Folder1 ...
6
votes
1answer
15k views

VLAN tags not shown in packet capture (Linux) via tcpdump

I am adding a tagged VLAN onto eth0: #ip link add link eth0 name eth0.20 type vlan id 20 This results in: #ip link 2: eth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc ...
6
votes
3answers
1k views

Debian 7 how are IPv6 link local addresses set?

It seems like when dhclient runs on eth0 I get an IPv4 address from the DHCP server and a Scope:Link IPv6 address attached to eth0: inet6 addr: fe80::a00:27ff:fed0:4d41/64 Scope:Link But I can't ...
6
votes
1answer
152 views

Is it possible to process millions of datagrams per second with Windows, using a vendor agnostic API? [closed]

I am investigating if I can implement an HPC app in Windows that receives small UDP multicast datagrams (mostly 100-400 Bytes) at a high rate, using a dozen or up to maybe 200 multicast groups (i.e. ...
6
votes
3answers
4k views

What do “Unknown SSAP” and “Unknown DSAP” mean in tcpdump?

While trying to fix a problem with intermittently losing internet connection on a machine with a wireless connection to a router, I ran tcpdump and noticed packets with "Unknown SSAP" and "Unknown ...
5
votes
4answers
826 views

Transfer large amount of small files

I've to do a migration of two servers with large SAN attachments to our new VMWare environment. EDIT: I have to supply some additional intelligence as I have good answers regarding VMWare solution. ...
5
votes
5answers
9k views

Is it dangerous to change the value of /proc/sys/net/ipv4/tcp_tw_reuse?

We have a couple of production systems that were recently converted into virtual machines. There is an application of ours that frequently accesses a MySQL database, and for each query it creates a ...
5
votes
3answers
4k views

Redirect output of mysqldump to scp

I worked with a linux pro years ago who did this nifty trick. He could do a mysqldump but redirect the output to an scp/ssh connection rather than writing to disk. We utilised this quiet a bit where i ...
5
votes
3answers
6k views

Ubuntu: disable udev's persistent-net-generator.rules

I'm using Ubuntu 12.04 LTS server edition and I am modifying /etc/udev/rules.d/70-persistent-net.rules to define my own mappings of ethernet interfaces to MAC addresses; that file is initially ...
5
votes
1answer
7k views

Ubuntu: Network driver reset to eth0?

I've been trying to get my Ubuntu Server VM running through VirtualBox. I ended up trying several network interfaces because of some networking issues and finally got it working. However, I noticed ...
5
votes
4answers
2k views

Mysterious “fragmentation required” rejections from gateway VM

I've been troubleshooting a severe WAN speed issue. I fixed it, but for the benefit of others: Via WireShark, logging, and simplifying the config I narrowed it down to some strange behaviour from a ...
5
votes
5answers
20k views

Ubuntu Server 12.04 can access LAN but not internet

Using Ubuntu Server (12.04) via command line, I can access the LAN (e.g. ping local machines) but not the internet (e.g. "ping google.com" or "sudo apt-get update"). How should I troubleshoot this? ...
5
votes
1answer
305 views

Nagios alert based on scheduled MySQL queries results

I have installed Nagios 4.0 on my Ubuntu 14.0 and i want a plugin to alert me based on daily scheduled MySql queries .. if the result of ( select count(*) from tableName > 10 ) raise a critical ...
5
votes
3answers
396 views

Multiple ethernet interfaces

I have a two network cards, configured like this: eth0 Link encap:Ethernet HWaddr 00:19:d1:31:08:e8 inet addr:192.168.5.104 Bcast:192.168.5.255 Mask:255.255.255.0 inet6 ...