GetFree's questions

I have the Postfix SMTP server installed on my CentOS 7 machine on port 25.
To test that it's working, I connect to it using telnet, like this:

# telnet localhost 25

and the result is:

Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 example.app ESMTP Postfix
EHLO test.com
250-example.app
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
quit
221 2.0.0 Bye
Connection closed by foreign host.

Which means it's working fine.
But now, in Postfix's config file /etc/postfix/main.cf I add the line:

smtpd_sasl_auth_enable = yes

Then I postfix reload and try the telnet connection again. And the result is:

Trying ::1...
Connected to localhost.
Escape character is '^]'.
Connection closed by foreign host.

The connection is closed automatically without receiving the 220 banner line.

I don't understand why it doesn't work. I've read the entire SASL README page at postfix.org and it says nothing about smtpd_sasl_auth_enable having this effect.

Why is telnet unable to establish a connection in this case and how can it be solved?

At the office we get access to the web via a SOCKS server which is started with the command:
ssh localhost -f -N -D *:8888.

This server has 2 IPs (but only one network card):
eth0 xx.yy.zz.1
eth0:1 xx.yy.zz.2

So the browsers are configured to use the socks proxy xx.yy.zz.2:8888

When I check what is the IP that websites see, it gives me xx.yy.zz.1 (the server's default IP).
But I want it to be the other one (xx.yy.zz.2).

How can I bind the outgoing connections to the second IP?

EDIT:
Could this be solved with iptables?
May be there is a way to tell which are the packets originating from a given SSH tunnel and then modify their destination IP?

EDIT2:
Another idea that crosses my mind, is there some sort of wrapper command that force another command to bind to a given IP?

Let's say I have 2 IPs for a given domain (round-robin DNS).
If one the IPs becomes unresponsive, will clients try to connect to the other IP? or they will just fail to establish comunication with the domain?

I have this weird problem. The php command is not found in one specific sitiation.
I try to call the command in three different ways. Only one of them fails:

1. Being user john I create a simple script which calls php --version. Then as user root I call that script like this: sudo -u john ./script.sh
   This way the php command is not found.

2. Being root I invoke the command directly like this: sudo -u john php --version
   No problem here, the command does exist.

3. I call the script directly as user 'john': ./script.sh
   It works fine too.

What is causing this?

The problem happens with other users too, not just john, and it happens on this particular machine only.
The OS is CentOS 5.9.

EDIT: Also, other commands like gcc, perl or python don't present this problem. The php command is the only one I found so far with the problem.

This is probably a silly question.
I'm doing some http requests using wget from the command line, and I want those connections to be made through one specific IP of the 4 IPs my server has.
Those http requests go to one specific range of IPs so I only want those to be routed differently.
The 4 interfaces in my server are eth0, eth0:0, eth0:1, eth0:2.

I tried with the following command:
route add -net 192.164.10.0/24 dev eth0:0

But when I see the routing table it says:

Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.164.10.0    0.0.0.0         255.255.255.0   U         0 0          0 eth0

The interface is set to eth0 not eth0:0 as my command says.

What am I doing wrong?

A server of mine had a drive failure of some sort which caused the OS (CentOS 5) to crash and stop working (it refuses to boot).

So we put another drive with a working OS and from there we try to mount the partitions in the old drive.

Most partitions mount fine except for one: the /var partition, where my MySQL tables reside.
When I try to mount that one, I see these errors with dmesg:

sd 0:0:1:0: Unhandled sense code
sd 0:0:1:0: SCSI error: return code = 0x08100002
Result: hostbyte=invalid driverbyte=DRIVER_SENSE,SUGGEST_OK
sdb: Current: sense key: Medium Error
Add. Sense: Unrecovered read error

Info fld=0x4a47e
JBD: Failed to read block at offset 9863
JBD: recovery failed
EXT3-fs: error loading journal.

Is there a way I can recover the data in that partition?

EDIT:
As requested, the output of tune2fs -l /dev/sdb2 is:

tune2fs 1.39 (29-May-2006)
Filesystem volume name:   /var1
Last mounted on:          <not available>
Filesystem UUID:          d84f5181-24f3-40ce-9eaa-601ae5ae33bd
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery sparse_super large_file
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              26214400
Block count:              26214063
Reserved block count:     1310703
Free blocks:              25127226
Free inodes:              26213665
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      1017
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         32768
Inode blocks per group:   1024
Filesystem created:       Thu May 13 18:14:28 2010
Last mount time:          Thu Nov 29 12:52:00 2012
Last write time:          Wed Mar 27 20:29:28 2013
Mount count:              15
Maximum mount count:      -1
Last checked:             Thu May 13 18:14:28 2010
Check interval:           0 (<none>)
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Journal inode:            8
Default directory hash:   tea
Directory Hash Seed:      35f38c48-3933-4c99-bde2-63b0eccf200d
Journal backup:           inode blocks

EDIT 2:
As suggested by @Hartmut, I run fsck.ext3 /dev/sdb2 with the following result:

e2fsck 1.39 (29-May-2006)
/var1: recovering journal
/var1: Attempt to read block from filesystem resulted in short read while reading block 11931

JBD: Failed to read block at offset 9863
fsck.ext3: No such device or address while trying to re-open /var1
e2fsck: io manager magic bad!

In a CentOS server I have, I want to forward port 8080 to a third-party webserver.
So I added this rule:

iptables -t nat -A PREROUTING -p tcp --dport 8080 -j DNAT --to-destination thirdparty_server_ip:80

But it doesn't seem to work.
In an effort to debug the process, I added these two LOG rules:

iptables -t mangle -A PREROUTING -p tcp --src my_laptop_ip --dport ! 22 -j LOG --log-level warning --log-prefix "[_REQUEST_COMING_FROM_CLIENT_] "
iptables -t nat -A POSTROUTING -p tcp --dst thirdparty_server_ip  -j LOG --log-level warning --log-prefix "[_REQUEST_BEING_FORWARDED_] "

^{(the --dport ! 22 part is there just to filter out the SSH traffic so that my log file doesn't get flooded)}

According to this page the mangle/PREROUTING chain is the first one to process incomming packets and the nat/POSTROUTING chain is the last one to process outgoing packets.

And since the nat/PREROUTING chain comes in the middle of the other two, the three rules should do this:

1. the rule in mangle/PREROUTING logs the incomming packets
2. the rule in nat/PREROUTING modifies the packets (it changes the dest IP and port)
3. the rule in nat/POSTROUTING logs the modified packets about to be forwarded

Although the first rule does log incomming packets comming from my laptop, the third rule doesn't log the packets which are supposed to be modified by the second rule. It does log, however, packets that are produced in the server, hence I know the two LOG rules are working properly.

Why are the packets not being forwarded, or at least why are they not being logged by the third rule?

PS: there are no more rules than those three. All other chains in all tables are empty and with policy ACCEPT.

I have CentoOS 5.4 x86-64 and every time I do yum install ... two versions of the same package are installed: one ending in .i386 and the other ending in .x86_64.

Is it nessesary to install .i386 versions given my OS is x86_64?
Have I just been wasting space by always installing both .i386 and .x86_64 versions of the same package?

I'm not asking how to find out where a bottleneck is. I just what to know how to check if the harddrive is being used at its limit (and hence causing a bottleneck).

Something like top where I can see a usage percentage of CPU and RAM.
Is there something like that with which I can monitor harddrive activity?

Native linux tools preferred.

I've just rented a new server (CentOS 5.4) and I see it has only 3 partitions: /, /boot and the swaping partition.
I'd like to create, at least, partitions for /tmp and /var.

Would there be any problems if I try to create those new partitions through SSH??

Thanks.

Followup:
Since the answers have pointed out it's a risky business to do this, I wonder if creating file-based partitions (as Levi De Haan suggests) would be a good solution in terms of performace, proper isolation and security (in comparison with normal partitions).
Please comment on your experience about this.

I have a website runing on a typical setup: Linux, Apache, PHP, MySQL.
However, what's not typical about it, is that it's getting tons of traffic (400,000+ visits a day) and so, efficiency is becoming more and more important to me.
I'm constantly looking for things I could optimize and, right now, my attention is focused on PHP's session files.
There's a hell lot of session files constantly being read and created on the /tmp directory.

So my question is: Is it a good idea to store the session files in /dev/shm (tmpfs) in order to speed things up a little bit??

I see there are thousands of files in my "/tmp" directory (a CentOS machine) and almost all of them are PHP session files.
I'm worried about the possible impact this might have on my system.
Are those files ever deleted either by the OS, Apache or PHP? or I have to take care of it myself?

I have a server at work which is behind a firewall (the company's firewall) so it is reachable only thrugh port 22 (SSH). I'm able to connect to the server with putty without problems.
Also, that server has Apache running and listening on port 80 as usual.
But I cant connect to the website using my browser since port 80 (and everyone else) is blocked by the company's firewall.

Is there a way I can make my browser to connect to Apache in that server so I can browse the site I'm working on?

Thanks.

Edit:

Since I was having no success with normal SSH tunneling techniques, I thought this problem was different from all those trying to pass through firewalls (and hence I asked here).
But I was mistaken, it's the exact same situation and a normal ssh-tunnel can solve the problem.

However, what made this problem a bit different (and the reason why I was being unsuccessful) is that the remote server was also behind a NAT, so the server's real IP was a private IP, not the public IP I use to start a SSH sesion.
One I realized that I put in my browser's address bar the server's private IP and voila! It worked like a charm.

I need to keep statistics of daily network traffic for a linux machine (CentOS 5).

Is there a way to do it using standard/native tools or utilities?
Or do I have to dowload special software for that?

Thanks.

Most of the time the output of a command ends with the newline character. But sometimes it does not, so the next shell prompt is printed in the same line together with the output.

Example:

root@hostname [~] # echo -n hello
helloroot@hostname [~] #

I've always found that very annoying.
Now, I could just add a "\n" at the beginning of the the PS1 variable, but most of the time that will print one extra line I dont need.

Is it possible to know whether the last command's output ended with a newline or not?

Solution:
(Thanks to Dennis)

PS1='$(printf "%$((`tput cols`-1))s\r")\u@\h [\w]\$ '

I have a customized prompt with colors (using tput) and every time I start a non-interactive session in the server I get a bunch of errors.
For instance if I start a non-interactive session like this:

ssh root@hostname6 "echo 'hello' ; echo $TERM"

The output I get is:

hello
xterm
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
stdin: is not a tty

So the $TERM variable has a value even when the session is non-interactive.

What condition do I have to detect so that when I start a non-interactive shell the prompt customization part is omited??

I have MySQL server 5.0.67 running on a Linux machine.
How can I know what configure options were used to compile it??

I'm interested in the option --with-mysqld-ldflags in order to know whether mysql was linked statically or dynamically.

My website is growing and growing. So now i'm considering moving to a 2-server virtual rack (in ThePlanet).
Im thinking on using one server for the DBMS and the other for the Webserver (currently I have both things on one single machine).

Given that setup, would it be enough 100Mbps for the server-to-server link??
or should I consider having a 1 Gbps rack ??

More info:
By now my site have a little more than 1 million pageviews a day and on the peak hours (from 14:00 to 18:00) about 85 thousand pageviews each hour.
Also, on the peak hours the traffic associated to the web content (html, css, images, etc..) reaches between 50 and 60 Mbps, for a total of about 200GB a day.

What I really dont know is whether a 100Mbps ethernet link will perform as good as a socket file (MySQL) for the dbms-webserver comunication.

I'm currently running a site whose trafic is distributed among 7 mirror servers right now (using DNS round robin). But soon more mirrors will be necessary given the high amount of traffic which is permanently growing.

Is there any practical o recommended limit for the number of IP addresses associated to a given domain name when using round robin DNS ??

Also, what kind of solution would you recommend when the number of mirrors is high (e.g. more than 10 or 20 mirrors)?

Thanks.

Is there a command or anything in Windows that let me view the list of the domain names that are currently resolved by the system?

Something similar to the arp command but for domain names.

Thanks.

EDIT:
I've been doing some testing with the command given by Matt, and I noticed that domain names that have a low refresh time and/or retry time, are not showed in the list given by ipconfig.

For example, google.com doesn't appear in my list, but according to the the dns responce it should be cached:

google.com
    primary name server = ns1.google.com
    responsible mail addr = dns-admin.google.com
    serial  = 2009052800
    refresh = 7200 (2 hours)
    retry   = 1800 (30 mins)
    expire  = 1209600 (14 days)
    default TTL = 300 (5 mins)

Whereas serverfault.com, which has higher refresh and retry times, does appear on the list given by ipconfig.

serverfault.com
    primary name server = ns21.domaincontrol.com
    responsible mail addr = dns.jomax.net
    serial  = 2009031400
    refresh = 28800 (8 hours)
    retry   = 7200 (2 hours)
    expire  = 604800 (7 days)
    default TTL = 86400 (1 day)

The following is another domain which is never shown by ipconfig, even though it has a high TTL:

minijuegostop.com.mx
    primary name server = ns1.theplanet.com
    responsible mail addr = support.minijuegostop.com.mx
    serial  = 2008122302
    refresh = 900 (15 mins)
    retry   = 900 (15 mins)
    expire  = 1209600 (14 days)
    default TTL = 86400 (1 day)

Apparently Windows decide not to cache some domains, and the refresh and retry times has something to do with it. Either that or Internet Explorer maintains its own domain name list. But I dont think so because as soon as I open a new webpage, the domain name of that page is shown by the ipconfig command; unless it has a low refresh or retry time, in which case it's not shown in the list.

Does anyone know what could be the problem or whether it's possible to show a complete list of cached domain names? (becasue apparently the list given by ipconfig is not complete)