Category Archives: Technical

Slow download from Samba to Mac OS X

A couple of weeks ago I started to notice that I couldn’t watch videos served via my Samba server on my MacBook. I first thought it was a problem with Quicktime Player, but later I tried VLC and the problem was still there. I started up Activity Monitor to see how fast was the download speed, it was hardly over 150KB/s. Before, I was able to watch flawlessly on my MacBook with download speed of up to 2MB/s. Initially I thought that this could have something to do with the recent security updates of Mac OS X, but I couldn’t find anyone having the same problem on Google. Later I tried mounting my Windows-powered laptop’s shared folder and initiated a file transfer to my MacBook, it was transferring at normal speed (2MB/s)! So this problem only occurs when the share is served off Samba.

Yesterday I tried my luck on Google once more and finally I found this (sudo sysctl -w net.inet.tcp.delayed_ack=0). Applied the suggested solution and it worked! Case closed.

MikroTik RouterOS — BGP

I have always wanted to learn about BGP. This time I got the honor chance to implement BGP for an ISP. This ISP has its own AS number and a /21 IP address block. This BGP setup is pretty simple because I’m only using 1 PC (with 3 ethernet cards and a MikroTik’s level 4 DOM) to interconnect with an Internet Exchange (IX) — NiCE — and a transit ISP (another transit will be added soon).

In MikroTik RouterOS version 2.9.42, BGP features are available for level 3 and above. You also need to enable routing-test package if you want more flexibility (BGP filtering features). The routing package, as of this version, only allows basic BGP features. I’m sure this will change later when they release RouterOS v3. Once you have enabled routing-test package, you will see new options (Routing – Filters in winbox, or /routing filters on CLI). This is very important when you have to peer with 2 or more ASes (specifically an IX and a transit which is not interested in getting IX’s routes for obvious reasons).

Since I have never configured BGP before, I caused a major problem when routes from each peer goes to another peer when they shouldn’t! I didn’t place any filter for BGP advertisements my BGP router sends to its peers. My transit ISP received the IX’s routes and the IX received full Internet routes feed my BGP router gets from the transit ISP. BGP is an exterior distance-vector routing protocol, it picks its best path by comparing the AS path lengths of every route it has. Advertising the whole Internet BGP feed to the local IX caused other ISPs’ routers participating in the IX to discover a shorter path of international routes going via my BGP router so the routers chose this new shorter path instead and outgoing traffic of these ISPs started to flow via my transit ISP! On the other hand, advertising IX routes to my transit was not a big problem since the path will be farther for their customers anyway, having to go via my BGP router first (that’s one AS further for the transit ISP’s customers to reach the IX routes, so BGP will not select it).

Fortunately I was able to spot the error immediately and placed BGP routing filters to include only my /21 IP block in the advertisements my BGP router sends to both the IX and transit. I also added an incoming BGP filter to discard a default route my transit IP includes in its BGP feed. This default route is not required since I get full BGP feed that is unfiltered.

Useful links: — Route filters examples — Important! Make sure that BGP neighbors are reachable via static routes for dynamic routes to be active (in the case of multihop BGP neighbors)
Cisco’s BGP Reference

Layer Four Traceroute (lft)

I’m a big fan of tcptraceroute. It’s a very useful tool for network administrators (in addition to the traditional traceroute). I had tcptraceroute installed on my MacBook via MacPorts, but later it stopped working (“libnet_write failed? Attempted to write 40 bytes, only wrote -1” error message) due to libnet compatibility problem. I couldn’t find a fix to that problem so I searched for a similar tool to replace tcptraceroute for my MacBook. I found lft on Google. Apparently lft is more flexible and advanced than tcptraceroute. What makes lft even better than tcptraceroute? lft is included in Fedora Extras! Since I use Fedora Core for my servers, this is much better than having to use third-party repository (e.g. dries, dag, etc.)

Normally, I use the following syntax for lft:

lft -C -z -n -E -S

Real-time Linux bandwidth monitor

There are so many Linux bandwidth monitoring software available out there. Many of these software are just showing the current upload and download speed of each network interface, pretty much similar to iptraf‘s “General interface statistics”. I was looking for something that plots TUI graphs. I found nload after spending a few minutes on Google. It doesn’t produce pretty graphs like rrdtool, but at least it does what I need.

UPDATE: I just noticed that ifstatus is similar to nload. ifstatus is capable of producing colored TUI graphs.

Real-time CPU statistics utility (Linux)

If you have multiprocessor Linux systems, top does not show individual CPU load by default. Press ‘1’ while top is running to change the default behavior and show each processor’s current load. Another utility is mpstat of the sysstat package, it is also capable of showing each processor’s current load.

Now you get to see which processor gets more load. 🙂

ipt_account HOWTO for Fedora Core 6

For the last few days I have been looking for a bandwidth monitoring tool that is more efficient, accurate, and complete than IPFM. I found so many bandwidth monitoring tools (bandwidthd, ipband, darkstat, and a few others) on Google but couldn’t find one that fits what I want. In fact, IPFM suits me better than the others.

While I was reading iptables’ man page, I accidentally came across ‘account’ under ‘MATCH EXTENSIONS’. After reading the brief description and example, I got excited! This iptables match extension provides exactly what I need! With the flexibility of iptables’ rules plus the account match extension, this has to be the best solution for my new bandwidth monitoring system. There is one problem though. Fedora Core doesn’t have it. 🙁

With some experience I got from installing ipp2p by hand, I started trying to make this iptables match extension work on my Fedora Core 6 server. First, I used Google to locate ipt_account’s website. Then I used Google to search for some install guides on ipt_account but couldn’t find any, so I went through its website and found something.

Since I have prepared my system with kernel & iptables source tree, rpmbuild, etc. (basically everything required to build ipp2p modules), I will not go over the “ingredients” again. Sorry!

Download the kernel and iptables patches from this page. Patch the kernel tree:

  • cd /usr/src/redhat/BUILD/kernel-2.6.19/linux-2.6.19.i386/
  • patch -p1 < path_to_kernel_patch

You should see the following:

patching file include/linux/netfilter_ipv4/ipt_account.h
patching file net/ipv4/netfilter/ipt_account.c
patching file net/ipv4/netfilter/Kconfig
patching file net/ipv4/netfilter/Makefile

OK, now the kernel tree is patched. Now do make oldconfig. You will be asked:

account match support (IP_NF_MATCH_ACCOUNT) [N/m/?] (NEW)

Since this will be a kernel module, hit m. Once the process has finished, do make scripts/kconfig/ (if you haven’t previously during ipp2p module build process). Create a temporary directory somewhere (e.g. /tmp/ipt). Copy /usr/src/redhat/BUILD/kernel-2.6.19/linux-2.6.19.i386/net/ipv4/netfilter/ipt_account.c into that directory and create a file named Makefile. Paste the following into Makefile:

obj-m := ipt_account.o
KDIR := /usr/src/redhat/BUILD/kernel-2.6.19/linux-2.6.19.i386/
PWD := $(shell pwd)
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

Make sure that there is a tab before $(MAKE). Save the file and execute make. If the compile process goes well, there will be a file named ipt_account.ko in the directory. Load the module by using insmod. Check lsmod to see the newly loaded module.

If you have got this far, that means your kernel now supports ipt_account. Now we will add ipt_account extension for iptables. To see if your iptables has ipt_account support, do iptables -m account. If it prints out something like “Couldn’t load match `account’: …”, that means there is no iptables account extension yet.

Let’s patch iptables source tree:

  • cd /usr/src/redhat/BUILD/iptables-1.3.5
  • patch -p1 < path_to_iptables_patch

Expect the following:

patching file extensions/libipt_account.c

Open extensions/Makefile, on line 8, add account before ah. Then compile the ipt_account’s extension:

cc -O2 -Wall -Wunused -I/usr/src/redhat/BUILD/kernel-2.6.19/linux-2.6.19.i386/include -Iinclude/ -DIPTABLES_VERSION=\”1.3.5\” -fPIC -o extensions/libipt_account_sh.o -c extensions/libipt_account.c
cc -shared -o extensions/ extensions/libipt_account_sh.o

Copy extensions/ to /lib/iptables and verify the extension: iptables -m account. That’s it! Pretty easy, eh? 🙂

Read this page to test the new iptables account match extension. Enjoy!

ipp2p HOWTO for Fedora Core 6

While this post is titled “for Fedora Core 6”, it may be applicable to previous versions of Fedora Core. I tested this on Fedora Core 4 too, but some steps are different.
Note: This guide uses iptables-1.3.5-1.2.1 and kernel-2.6.19-1.2895.fc6.

First, grab ipp2p-0.8.2.tar.gz or the latest version if there’s any. Extract the tarball and make sure you have iptables and the currently installed kernel’s SRPMs installed. To proceed, I recommend rpm-build to be installed (simply use yum to install, i.e. yum install rpm-build). Next, install both SRPMs using the following command: rpm -ivh file.rpm. If you get “error: cannot create %sourcedir /usr/src/redhat/SOURCES” message, you can create the required directory: mkdir -p /usr/src/redhat. Also, rpm-building the kernel source requires redhat-rpm-config and unifdef to be installed.
Now that the “ingredients” are ready, let’s start doing the fun part..

Go to /usr/src/redhat/SPECS and do rpmbuild -bp iptables.spec, this will prepare the iptables’ source tree to match with the currently installed iptables binaries’ source. Do the same for the kernel, rpmbuild -bp kernel-2.6.spec. The newly patched source tree can be found inside /usr/src/redhat/BUILD directory.

Grab symvers-2.6.19-1.2895.fc6.gz from /boot, gunzip and rename it as Module.symvers, then move it to the kernel’s source directory (/usr/src/redhat/BUILD/kernel-2.6.19/linux-2.6.19-i386). Go into that directory, make sure that existing kernel config (.config) exists, then do: make scripts/kconfig/.
Note: If you can’t find symvers file in /boot, install kernel-devel package and you will find Module.symvers in /usr/src/kernels//.

Next, return to the ipp2p source directory and do two modifications.




KERNEL_SRC ?= $(firstword $(wildcard …


IPTABLES_SRC = /usr/src/redhat/BUILD/iptables-1.3.5


ifeq ($(IPTABLES_SRC),)

Save the changes and do make.

If it compiles successfully, then ipt_ipp2p.ko and should be in the directory now.
Copy to /lib/iptables. To verify whether this iptables module works, do iptables -m ipp2p -h.
In case you get the following: iptables v1.3.8: Couldn’t load match `ipp2p’, don’t worry. Have a look at this solution and simply recompile the modules.

There are two ways to load the ipp2p kernel module, insmod ipt_ipp2p.ko or copy ipt_ipp2p.ko to /lib/modules/2.6.19-1.2895.fc6/kernel/net/ipv4/netfilter and do depmod -a.

avast! AntiVirus

avast! AntiVirus by ALWIL Software is really good if you don’t want to spend any money for antivirus protection of your computer (Home edition is free, Professional one is not). It’s not as “heavy” and “bloated” as Norton AntiVirus yet provides the same level of protection (or even better). It’s definitely better than Norman and its so-called Sandbox technology. More antivirus companies should follow what avast! is doing. Many computer users won’t bother paying for antivirus software when they can simply download the pirated version for free. I hope ALWIL Software will keep providing avast! AntiVirus Home Edition for free.

Thanks, avast!.