Docker – Map IP to DNS, or Just Override DNS Name in Linux in General

I was reading a docker script someone else created and came across an interesting blog explaining a parameter it used (–add-host) right here.  I recommend reading that longer blog, but I’m recording the short notes and link here for myself as I’m sure I’ll be using this a lot.

The “–add-host” Parameter

Long story short, you can just add “–add-host=some_dns_name:some_ip_address” to your docker command in order to make your container have any DNS name resolve to any IP address.

This works by having the container put an entry for this DNS/IP pair into the /etc/hosts file.

Use Outside of Docker

I haven’t used the /etc/hosts file in a while.  But this reminded me about it.  The article points out that you can either add a DNS mapping or even override an existing DNS mapping using this file.

So, for example, I could make point at this website from within the given OS instance, if I updated that file properly.

Pretty cool and useful :).

SCP/SSH With Different Private Key

If you need to use SSH or SCP with a different private key file, just specify it with -i.  For example, to copy logs from a remote server using a specific private key file and user, do the following:

scp -i C:\Users\[your-user]\.ssh\pk_file [user]@[ip-addr]:/path/logs/* .

This -i will work regardless of OS, but the example is SSHing to a Linux server from a Windows server assuming you store your private keys in your user .ssh directory.

Bash – Grep (or Run Other Command) Only On Files Created This Week, Day.

Use Case

I just ran into a simple problem where I had to grep files on a server, but the directory had TONS and TONS of files in it.  I just wanted to target files created within the last week or so.

Working Command

It turns out this find command is very handy for this occasion.  It was taken and lightly modified from this unix stack-exchange post after a fair bit of searching.

find . -mtime -7 -exec grep "my_search_string" {} \;

Basically, it finds everything in “.” (the current directory) that was created in the last 7 days (as in 24 hour days, not from-this-morning days), and it executes the grep expression on it.

You can modify the timing however you want with mtime as well as change the target directory or command to execute, and of course you can pipe the output to whatever you want :).