Sudoers basics

POD servers for Perl

You want to browse Perl embedded documentation (POD)

You have some software that was designed for browsing information: a web browser

You need a server to turn the POD into web pages and serve it to the browser.

You try one of these modules:

• https://metacpan.org/pod/Pod::Browser
• Uses Catalyst
• https://metacpan.org/pod/Pod::Server
• ?
• https://metacpan.org/pod/Pod::Webserver
• Very basic
• Parsing much better than Pod::POM::Web, but still fails in places.
• Contains all the Perl built-in modules, which I don't want to see.
• How to run: podwebserver
• https://metacpan.org/pod/Pod::POM::Web
• Advanced design
• Fails to parse nested tags
• Contains all the Perl built-in modules, which I don't want to see.
• How to run: perl -I $DOC_PATH -I $POD_POM_WEB_PATH -MPod::POM::Web -e server

Questions:

• What is the reference implementation of a POD interpreter? `perldoc` just removes links and it is plain text only. So in what viewer do the links actually link to something?

Many different ways to resolve an IP to a hostname in Perl

Some different ways to look up hostnames.

Notes:

• getnameinfo() gets a hostname and a service name, so it's not exactly the same as gethostbyaddr()
• although the docs imply you need to enter a port, that's just to get the local service name. If you set it as undef then you can ignore any service

# 1, old deprecated way

use Socket; # gethostbyaddr, inet_aton, AF_INET

sub ip_address_to_host {

    my ( $self, $ip_address ) = @_;

    my ($hostname) = gethostbyaddr(

        inet_aton($ip_address),

        AF_INET,

    );

    return $hostname;

}

# 2, newer better way

use Socket qw(AF_INET inet_pton getnameinfo sockaddr_in);

sub ip_address_to_host {

     my ( $self, $ip_address ) = @_;

    my $port = undef;

    my $socket_address = sockaddr_in($port, inet_pton(AF_INET, $ip_address));

    my $flags = 0;

    my $xflags = 0;

    my ($error, $hostname, $servicename) = getnameinfo($socket_address, $flags, $xflags);

    return $hostname;

}

# 3, best way - that only uses DNS and not /etc/hosts first

use Net::DNS;

sub ip_address_to_host {

     my ( $self, $ip_address ) = @_;

    my $res = Net::DNS::Resolver->new;

    my $target_ip = join('.', reverse split(/\./, $ip_address)).".in-addr.arpa";

    my $query = $res->query("$target_ip", "PTR") // return;

    my $answer = ($query->answer)[0] // return;

    my $hostname = $answer->rdatastr;

    return $hostname;

}

# 4, the code golf way (no validation) - by Mark B

 perl -le 'use Net::DNS::Resolver; print ((Net::DNS::Resolver->new()->query("10.232.32.158","PTR")->answer)[0]->ptrdname);'

(source, source, source)

SSH warning: authenticity of host can't be established

Problem

The authenticity of host 'example.com (1.2.3.4)' can't be established.
RSA key fingerprint is ab:cd:ef:12:34:56:78:90:ab:cd:ef:12:34:56:78:90
Are you sure you want to continue connecting (yes/no)?

Solution

ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no

Make sure you understand the security implications of doing this (Hint: they're not good).

(source)

POD servers

You know, POD.

• Pod::Simple::HTMLBatch - The best one. Properly marked up HTML, so fully customisable with CSS. Split into top-level directories. Doesn't do .t files, but can use a subclass of Pod::Simple::Search rigged to recognise .t files. Submitted patch.
• Pod::Server - Basic default output, one single list. Docs lacking. What about CSS?
• Pod::Webserver - Lots of dependencies, Catalyst, etc. GUI looks great. Couldn't get it to read from local dirs though.
• Pod::POM::Web - Untested

Simple small Perl webservers

Quick start, if it's just a proof-of-concept and won't be running for long:

• HTTP::Daemon - can't get simpler than this. It even picks a port for you
• HTTP::Server::Simple - it works

More complicated to set up, but much more robust:

• PSGI/Plack - the future
• lighttpd - it's pronounced lighty

How to sync over FTP

Don't re-invent the wheel.

1) Install rsync instead, which is designed with syncing in mind.

2) ftpsync was written a decade ago. Perhaps it has been updated since.

3) lftp syncs over FTP and is being actively maintained.

4) Perl package turbo-ftp-sync may also fit the bill.

Move directory on FTP server

without downloading and re-uploading:

rnfr source_dir
rnto target_dir

Developing over FTP

Using Eclipse and RSE plugin:

• Logs in okay
• But seems to always be closing directory sub-trees I've opened in 'Remote Systems' panel
• It wants to do an 'ls' on every parent directory in the hierarchy, every time I change files. Very slow.

Using CurlFtpFS:

• Install in Ubuntu from the software centre
• to mount: curlftpfs -v username:password@server.example.com local_dir/
• to unmount: fusermount -u local_dir/
• Performance is unusably slow... Eclipse wants to know about every single file on the remote server, not just the ones I'm editing.
• Saving the file through Eclipse using CurlFtpFS takes even longer than it would to upload it separately.

Using FileZilla and gedit:

• Browsing is fast
• Editing is fast
• But you have to navigate to every file manually each session, it doesn't remember what you had open
• The system of holding temp files locally is not ideal, but FileZilla detects changes in them very nicely
• You still can't grep over the files as they're all held remotely, and it's a bit fiddly to navigate to a specific file

Editing locally with Eclipse, and uploading after changes are made:

• Write an Ant script to upload any modified files?
• ...to investigate

Example ssh config

Host [alias]
    HostName [remote hostname]
    User [remote username]
    IdentityFile [file]

[alias] is what you type to select the connection, e.g. ssh [alias]

[file] is your DSA or RSA private key file

Uptime

99.8% uptime allows for 17.52 hours downtime per year.
That seems reasonable.

Log into a server using secure keys instead of typing a password

On the client:

1. ssh-keygen -t rsa
2. accept the default filename
3. press enter for a blank password
4. a private (id_rsa) and public key (id_rsa.pub) will be created
5. copy the public key to the server
   

On the server

1. cat id_rsa.pub >> ~/.ssh/authorized_keys
2. chmod 700 ~/.ssh
3. chmod 644 ~/.ssh/authorized_keys

Now back up your keys.

See also this and this.
(Upon error "Agent admitted failure to sign using the key", log out of the client and log back in again to fix).

Edit text files on remote linux servers

UNTESTED:

> I've never managed to figure out how to rig up editors to edit
> files on remote servers.

On the server, put this in smb.conf

[homes]
comment = Home Directories
hide dot files = yes
read only = No
browseable = yes


Then in the finder/windows explorer connect to 'smb://hostname/' . Then make an alias to it on your desktop.

Alternatively, TextWrangler on the Mac has an ftp/sftp browser.

http://www.barebones.com/products/TextWrangler/

View the swap file usage in linux

/sbin/swapon -s

Can't connect to MySQL

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)

Reason: mysqld is not running!