Linux sadservers [miguel angel torres egea]

Aquesta pàgina és només de lectura. Podeu veure'n el codi font, però no podeu canviar-la. Consulteu el vostre administrador si penseu que això és degut a algun error.
= Linux sadservers
== "Saint John": what is writing to this log file?
  * ''ps auxf''
  * ''lsof /path/to/file''
  * ''fuser /path/to/file''
  * ''pwdx PID'': pwd del fichero PID
<code>
1. You can use ps to list all processes and see if you see something related, for example with: ps auxf. Ignore system processes [in brackets].

A better way is to use the command to list open files: lsof.

2. Find the name (first column) and Process ID (PID, second column) of the process related to /var/log/bad.log by running lsof and filtering the rows to the one(s) containing bad.log.

You can also use the "fuser" command to quickly find the offending process: fuser /var/log/bad.log.

3. Run: lsof |grep bad.log and get the PID (second column)."Saint John": what is writing to this log file?

With the PID of the process, it's not necessary but we can find its current working directory (program location) by doing pwdx PID or for more detail: lsof -p PID and check the cwd row. This will allow us to check its ownership and perhaps inspect its offending code if it's a script (not a binary).

(Open window once more to see the complete solution).
</code>

== "Saskatoon": counting IPs.
  * ''awk '{print $1}' ''
  * ''cut -d' ' -f1''
  * ''sort'':
    * -h: numérico humano
    * -r: reverso
    * -k[primer-campo],[ultimo-campo]
    * -t <SEP>: separador
  * ''uniq -c'': cuenta elementos únicos
<code>
1. To get the first field (IP) of the file, you can do awk '{print $1}' access.log or using "cut" with delimiter of space (-d' ') and picking the first field (-f1): cat access.log |cut -d' ' -f1. You may want to append a pipe | head or | tail as you construct the command to see how your filters are working.

2. After the previous step, you want to sort the IPs so they are together and can be counted: cat access.log | awk '{print $1}' |sort

3. Now you want to do the count with "uniq -c", so we have so far: awk '{print $1}' access.log |sort|uniq -c

4. Finally you want to sort the results with "sort" (goes in ascending order) and get the latest one (with "tail -1" for example), or sort in reverse order with "sort -r" and get the top result: awk '{print $1}' access.log|sort|uniq -c|sort -r|head -1.

(Open window once more to see the complete solution).

Solution: One posible way is awk '{print $1}' access.log|sort|uniq -c|sort -r|head -1|awk '{print $2}' > /home/admin/highestip.txt
</code>

== "Santiago": Find the secret combination
  * ''grep -rc''
  * ''grep -A 1''
  * ''find ... | xargs grep -c''
<code>
1. Use grep recursively or use find and pass the results to grep via xargs

(Open window once more to see the solution to the first part).

2. (Solution to 1) cd /home/admin/ and then for example: grep -rc Alice *.txt or find . -type f -name "*.txt" |xargs grep -c 'Alice' , adding the results from the three files: echo -n 411 > /home/admin/solution

(Open window once more to see the solution to the second part).

3. (Solution to 2) The file with exactly one Alice occurrence is 1342-0.txt :grep Alice -A 1 /home/admin/1342-0.txt (or open the file with less or vi and enter /Alice). Appending this result: echo 156 >> /home/admin/solution (The solution is 411156).
</code>

== "The Command Line Murders"
  * ''knock localhost 3000''
  * ''nmap -p- localhost''
<code>
1. You can use the knock utility, for example to knock on port 3000: knock localhost 3000. Netcat (nc) and nmap are also available. Note than nmap has some options where you'd need to be root (not possible here)

2. You can also write a BASH script that knocks sequentially on all ports.

3. Solution.Probably the fastest is using nmap against all ports, for example: nmap -p- localhost.
</code>

== "Resumable Server": Linux Upskill Challenge
<code></code>

== "Bucharest": Connecting to Postgres
  * ''sudo systemctl restart postgresql@13-main''
<code>
1. The issue might be related to the configuration of the PostgreSQL server. (See the error message when attempting the tedt). The configuration files are usually located in the /etc/postgresql/$version/main/ directory. You might want to start by checking these files. (You'll need to use "sudo").

2. The pg_hba.conf file controls client authentication. This file is read on start-up and when the main server process receives a SIGHUP signal. If you’re having trouble connecting to the database, this file could be a good place to look. (Click again "Next Clue/Solution" to reveal the final step)

Solution: In the /etc/postgresql/13/main/pg_hba.conf file, delete or comment out the lines with a reject keyword from all. Then restart the PostgreSQL service: sudo systemctl restart postgresql@13-main
</code>