ServerBuddies Support Blog

MX priority

The relative priority of an MX server is strong-minded by the preference number present in the DNS MX record. When a remote client does an MX lookup for the domain name, it gets a list of servers and their first choice numbers. The MX record with the smallest first choice number has the highest precedence and is the first server to be tried. The remote client will go down the list of servers until it successfully delivers the message or gets permanently rejected due to an inaccessible server or if the mail account does not exist on that server. If there is more than one entry with the same preference number, all of those must be tried before moving on to lower-priority entries.

The init Daemon

The init daemon is the system and service manager for Linux. It is the first true process Linux starts when it boots and as such, has a PID of 1 and is the ancestor of all processes. The init daemon has been around since the early days of UNIX, and many people have worked to improve it. The first Linux init daemon was based on the UNIX System V init daemon and is referred to as SysVinit (System V init daemon).

Because SysVinit does not deal well with modern hardware, including hotplug devices, USB hard and flash drives, and network-mounted filesystems, Fedora/RHEL recently replaced it with the Upstart init daemon (http://upstart.ubuntu.com/ and http://upstart.ubuntu.com/wiki). Fedora 15 has moved past Upstart to systemd init daemon.

strace - trace system calls and signals

In the simplest case strace runs the specified command until it exits.

It intercepts and records the system calls which are called by a process and the signals which are received by a process. The name of each system call, its arguments and its return value are printed on standard error or to the file specified with the -o option.

strace is a useful diagnostic, instructional, and debugging tool. System administrators, diagnosticians and trouble-shooters will find it invaluable for solving problems with programs for which the source is not readily available since they do not need to be recompiled in order to trace them. Students, hackers and the overly-curious will find that a great deal can be learned about a system and its system calls by tracing even ordinary programs.

And pro?grammers will find that since system calls and signals are events that happen at the user/kernel interface, a close examination of this boundary is very useful for bug isolation, sanity checking and attempting to capture race conditions.

Example:

Server Name Indication

Server Name Indication (SNI) is a feature that extends the SSL and TLS protocols to indicate what hostname the client is attempting to connect to at the start of the handshaking process. By doing so it allows a server to present multiple certificates on the same IP address and port number and hence allows multiple secure (HTTPS) websites to be served off the same IP address without requiring all those sites to use the same certificate.

Unfortunately to make use of SNI practical it is necessary that the vast majority of users are using web browsers that support it. Users whose browsers do not support SNI will be presented with a default certificate and hence are likely to receive certificate warnings. As of 2011 there are still many users of browsers that do not support SNI.

tune2fs - adjust tunable filesystem parameters on ext2/ext3/ext4 filesystems

tune2fs allows the system administrator to adjust various tunable filesystem parameters on Linux ext2, ext3, or ext4 filesystems.

The current values of these options can be displayed by using the -l option to tune2fs(8) program, or by using the dumpe2fs(8) program.

Adjust the number of mounts after which the filesystem will be checked by e2fsck(8). If max-mount-counts is 0 or -1, the number of times the filesystem is mounted will be disregarded by e2fsck(8) and the kernel.Staggering the mount-counts at which filesystems are forcibly checked will avoid all filesystems being checked at one time when using journaled filesystems.

Set the number of times the filesystem has been mounted. If set to a greater value than the max-mount-counts parameter set by the -c option, e2fsck(8) will check the filesystem at the next reboot.

Getting the current Values:

Example:

So for a system that contains 5 partitions and is booted approximately once a month you could do the following to stagger the mount counts: