Miscellaneous questions

How can I open a UDP or TCP port in SLC5 or SLC6?

There are two options. However, they are incompatible (at least in SLC5). Either use always one method or the other, but not both:

  • Modify /etc/sysconfig/iptables and restart the iptables service (open port 4241):

    -A INPUT -m state --state NEW -m tcp -p tcp --dport 4241 -j ACCEPT
sudo /sbin/service iptables restart

  • Install xsudo, and then open a TCP port using the Redhat firewall configuration GUI:

    wget http://cmsdaq0/cmscdb/cmsswrep/i386_slc4/xsudo-2.0.2-0.noarch.rpm
sudo rpm -iv xsudo-2.0.2-0.noarch.rpm
xsudo /usr/bin/system-config-firewall

How can I simulate deterministic and non-deterministic packet loss and delay?

It is very easy, just use the tc command. see examples in stackoverflow. Common examples (for eth0):

  • Drop 0.1% of outgoing packets:

    • If other rules have already been applied to eth0 : sudo /sbin/tc qdisc add dev eth0 root netem loss 0.1%

    • Otherwise : sudo /sbin/tc qdisc change dev eth0 root netem loss 0.1%

  • Find out what rules are applied already : sudo /sbin/tc -s qdisc ls dev eth0

  • Remove rules : sudo /sbin/tc qdisc del dev eth0 root

N.B. The tc command with netem only applies these rules (e.g. packet dropping) to outgoing packets …

  • See the ‘’How can I use netem on incoming traffic?’’ FAQ section here for details on how to also apply rules to the incoming traffic.

Another useful resource (also contains instructions for doing same thing with iptables) can be found here.