Geeking about

Offlineimap and Msmtp

This post is part of my notmuch of a journey series.

notmuch doesn’t get my email or send it, so I’m turning to two commonly used tools to help me with this.

  • offlineimap fetches the email over IMAP (and syncs it back)
  • msmtp sends the email for me

The set up I had is fairly standard, though a key issue was

Password Storage

I’m running on a server and connecting over ssh, as I want the same set of tags to be available everywhere. I don’t want my password sitting on disk in clear text (particularly as it is my LDAP password, so used for sudo, ssh and various other logins). There are various solutions to this if you are running a desktop – python-keyring-lib will link to Gnome, KDE and OS X keyrings. But it took a bit more searching to work out how to do this on a server.

I considered a number of options

GPG encrypted file

I found this unix stackexchage post about having a python script that could decrypt a GPG encrypted file (one file per password). gpg-agent would run on the server so you didn’t have to enter your passphrase every time you need to run it. I got this running, generating a GPG key that I wasn’t actually going to use for email. That meant I was OK setting a long time out for gpg-agent – I set it to 24 hours by putting the following in ~/.gnupg/gpg-agent.conf

pinentry-program /usr/bin/pinentry-curses
default-cache-ttl 86400
max-cache-ttl 86400

However I wanted to be able to leave offlineimap running and at some point gpg-agent would expire, so this wasn’t really what I wanted.

That said I did leave this system in place for msmtp, as that will only run when I send an email – so I will be logged in at that point and can enter my GPG passphrase into gpg-agent. The relevant line for the .msmtprc file is:

passwordeval gpg --use-agent --quiet --batch -d ~/.passwd/account.gpg

Environment variables

I found a blog post describing caching the decrypted password in environment variables which seemed quite cunning. That would only survive for the session, but then I leave tmux running all the time, so as long as tmux survived, the environment variables would. But then I thought, why not just:

Leave offlineimap running in tmux

When you run offlineimap, if there is no password or passwordeval in the .offlineimaprc file, then offlineimap will ask for your password (provided the ui isn’t set to be Noninteractive). offlineimap can also be told to run repeatedly, without exiting, by putting autorefresh = 5 to re-run every 5 minutes. In between runs, offlineimap will just sit at the terminal waiting, but it remembers your password, keeping it in memory. Using this I could leave offlineimap running for months inside tmux on the server I use.

(Note that offlineimap will not ask for your password if the ui is set to be Noninteractive. And the autorefresh setting must be in the [Account X] section – I put it in the Repository section originally which didn’t work).

ssh keys and offlineimap preauthtunnel

Finally I worked out that I could talk directly to an imap command line utility on the remote server over ssh – using a passwordless ssh key to avoid any passwords. offlineimap is instructed to use it by the preauthtunnel option.

There are two main IMAP servers in the linux world, courier and dovecot. The relevant commands are:

# Courier IMAP (Debian - you should check the path on CentOS)
preauthtunnel = ssh -o Compression=yes -q IMAPHOST '/usr/bin/imapd ./Maildir'

# Dovecot IMAP (CentOS)
preauthtunnel = ssh -o Compression=yes -q IMAPHOST 'MAIL=maildir:~/Maildir exec /usr/libexec/dovecot/imap'
# Dovecot IMAP (Debian)
preauthtunnel = ssh -o Compression=yes -q IMAPHOST 'MAIL=maildir:~/Maildir exec /usr/lib/dovecot/imap'

Replace IMAPHOST and ~/Maildir with your own values.

There are two ways to do this so that you don’t have to enter the ssh key passphrase all the time. You could set up a password-less ssh key to do this, or you could leave an ssh session running so that offlineimap can multiplex its connections on to it.

A password-less ssh key leads to the possibility of abuse, but you can generate a new ssh key just for imap and specify it’s use by modifying the above command to look like:

preauthtunnel = ssh -q -i /home/hamish/.ssh/id_mail_imap -o Compression=yes IMAPHOST '/usr/libexec/dovecot/imap'

On the mail server side you can then lock down this key by configuring what command can be run in the .ssh/authorized_keys file. Mine looks like:

command="/usr/libexec/dovecot/imap",no-X11-forwarding,no-agent-forwarding,no-port-forwarding,no-pty ssh-dss AAAAB3Nza...

Obviously this will only work for you if you have ssh access to your mail server, so it won’t be an option for all. But I’ve given some other options above, so I hope you can find something that works for you.

Other notes

sendmail over ssh

Another option for sending email to a remote server is to use the sendmail command over ssh (assuming you have ssh access to the server in question). First you need to generate a passwordless ssh key and copy it to the mail server:

ssh-keygen -t dsa -f ~/.ssh/id_mail_smtp
scp ~/.ssh/

Then, on the mail server, put the key in the authorized_keys file:

cat ~/.ssh/ >> ~/.ssh/authorized_keys

Next we edit the authorized keys file, adding the command to run to the beginning of the last line (which is currently our smtp key):

command="/usr/sbin/sendmail -bm -t -oem -oi",no-X11-forwarding,no-agent-forwarding,no-port-forwarding,no-pty ssh-dss AAA...

In this case the command is the sendmail command for exim – I’m afraid you’ll have to work out what your own sendmail command is. Finally, in your mail client, tell it to use ssh as the sendmail command. The following works for both mutt and alot:

ssh -q -i /home/hamish/.ssh/id_mail_smtp

Now whenever your mail client wants to send email, it will start up the ssh connection and print the email to it. This will be picked up by the sendmail command on the far end and off the email will go.

ssh ControlMaster

ControlMaster is an ssh option that means you can leave an ssh connection open with a master session, so that future ssh connections reuse the master session rather than having to start an entirely new connection. This should mean quicker send and receive. One way to specify it in your ~/.ssh/config file is:

ControlMaster auto
ControlPersist 4h
ControlPath /home/username/.ssh/muxcontrol/%r@%h:%p

This means that connections will be automatically started, will stick around for up to 4 hours, and the socket for your connection to on port 22 with username user would be at /home/username/.ssh/muxcontrol/ (don’t forget to create the directory). (And the default directory is the global /tmp directory).

However a gotcha here is that the connection to the host will use whatever ssh key was used for the initial connection, not the one specified at the command line. So if offlineimap runs first, using your ssh key tied to the imap command, then when you try to send an email using sendmail over ssh, then your ssh command will run imap rather than sendmail – and your email will go nowhere.

The solution is to specify a different ControlPath for these commands. So for offlineimap you could use:

preauthtunnel = ssh -o Compression=yes -o ControlPath=/home/username/.ssh/muxcontrol/IMAP_%r@%h:%p -q IMAPHOST 'MAIL=maildir:~/Maildir exec /usr/libexec/dovecot/imap'

and for sending:

ssh -q -o ControlPath=/home/username/.ssh/muxcontrol/SMTP_%r@%h:%p -i /home/hamish/.ssh/id_mail_smtp

This will keep more ssh connections active, but the right action will be done using your different ssh keys.

Checking IMAP mailboxes

I was moving folders around and was having trouble working out whether the folders were available but offlineimap was failing to find them, or if dovecot wasn’t showing me the folders. Eventually I found how to ask for a folder list over telnet. In short:

telnet 143
A login username password
B list "" *
C logout

(include the A, B, C). Or to do this over ssl you could connect with:

openssl s_client -connect

and then run the above commands.

Other ways of sending to multiple accounts

In my research, I also found that you can set up postfix to use different servers for different accounts (though I’m not sure how well it would handle encrypted passwords) and I may improve my msmtp set up by using msmtp queue at some point, and see how well it deals with encrypted passwords.