Self-contained maintenance page

Sometimes we need to take our web apps offline temporarily. It could be to do a major upgrade or in an emergency situation. Anyways, you should at least set a static page to notify regular users that the site is currently unavailable. And it’s even better if you can maintain access for you and your fellow developers in the meantime.

Create a maintenance page

First step is to create a good maintenance page. Good means clean, simple, visually attractive and the most important thing: “self-contained” ! So any required resource should be embedded inside a “simple” HTML file.

How to do that ?

For CSS use the tag style to integrate the content into the HTML like this:

<style media="screen" type="text/css">
 ... Add CSS data here ...

For images encode them in base64 using the base64 command and add the resulting string into the HTML like this:

<img src="data:image/gif;base64,BASE64STRING" alt="my_image" />

You have a good maintenance page ? Perfect, now let talk how to serve it.

Serve the maintenance page

The first component that receive the users requests should be serving the page. Depending your stack that means either the:

  • HTTP server should serve the page
  • HTTP reverse-proxy cache (aka. Varnish) should serve the page

In the first case i highly suggest you to make a dedicated maintenance VirtualHost on a separate port.

In the second case you must modify your vcl_error section with something like this:

if (obj.status == 703) {
    set obj.status = 503;
    set obj.http.Content-Type = "text/html; charset=utf-8";
    synthetic {"
           ... Add HTML content here ...

Note the fake HTTP code 703. We will use it later 😉

Maintain access for you, display maintenance page for others

If your maintenance page is served by Varnish use an acl for IP filtering like this:

acl me_and_dev {

sub vcl_recv {
    if (!client.ip ~ me_and_dev) {
            error 703 "Service unavailable";

If your maintenance page is served by Apache use an iptable rule to redirect traffic to the maintenance VirtualHost‘s port except for your IP :

iptables -t NAT -A PREROUTING -i eth0 -p tcp --dport 80 \! -s '' --to-ports 81

Note that if you can’t use IP filtering, other solution are possible like using a special maintenance cookie set after requesting a “secret url”.

[Exim] Smarthost configuration

Let say we already have a functional SMTP server that is responsible for routing all mail for a given domain. Now we add a new server into this network that must be able to send its own emails, for example for administrative purposes (alerts, crontab, logs, emails generated by the applications, etc …). This new MTA doesn’t have to manage mail for users.

The clean way to do this is to configure our exim daemon to send mail via the “official” SMTP server. For this, just modify the /etc/exim4/update-exim4.conf.conf file like this:

dc_local_interfaces=' ; ::1'

Now the host can send emails using‘s MTA.

Restricted SFTP access

FTP is an old protocol created when network was a ‘new thing’ and everybody was a ‘care bears’, therefore it’s insecure by design and you shouldn’t even propose it to yours customers. Instead always push the SFTP option first.

For practical usage, there is three little downsides to SFTP use:

  • SFTP doesn’t have it own dedicated port. Personally i like to reuse the ‘Simple File Transfer Protocol’ port (TCP 115) but this ideas is enough to trigger heart attacks to network ‘ayatollah’
  • each user must have an unix account
  • an SFTP access isn’t by default chrooted inside the user directory

SFTP on a dedicated port

Until we have SFTP support inside ProFTPD, the only solution is to spawn a second OpenSSH daemon on a separated port.

Here a snippet of my new setting file /etc/ssh/sftp-115:

Port 115

# Protocol
Protocol 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
UsePrivilegeSeparation yes

# Restriction
AcceptEnv no
AllowAgentForwarding no
AllowTcpForwarding no
Banner no
MaxAuthTries 3

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 768

# Logging
SyslogFacility AUTH
LogLevel INFO
UseDNS no

# Authentication
LoginGraceTime 120
PermitRootLogin no
StrictModes yes
RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile /home/sftp/%u/.ssh/authorized_keys
PasswordAuthentication yes
IgnoreRhosts yes
RhostsRSAAuthentication no
HostbasedAuthentication no
PermitEmptyPasswords no
ChallengeResponseAuthentication no

# Disable some options
UsePAM no
X11Forwarding no
PrintMotd no
PrintLastLog yes
TCPKeepAlive yes

# Use sftp-internal (allow chroot)
Subsystem sftp internal-sftp
ForceCommand internal-sftp

Before OpenSSH 4.8 there is no internal-sftp module, therefore you must use the external command sftp-server instead:

Subsystem sftp /usr/lib/openssh/sftp-server

For launching the daemon you can use xinetd, clone and modify your distribution OpenSSH init script or simply add a one-liner inside the /etc/rc.local:

/usr/sbin/sshd -f /etc/ssh/sftp-115

Do as you wish.

Unix user account

Not much to say here, you must create a proper unix account for each SFTP. I suggest you to set the home directory to something like /home/sftp/<user> to distinguish easily theses users from regular ones.

In case you use sftp-server you must also change the user shell value. First add sftp-server as a valid shell :

# echo '/usr/lib/stfp-server' >> /etc/shells

Then for each SFTP unix account:

# usermod -s /usr/lib/sftp-server <user>

Chroot user

If you use internal-sftp simply add the following snippet to /etc/ssh/sftp-115:

ChrootDirectory /home/sftp/%u

For sftp-server, bad news, you simply can’t chroot users.

To give user access to a directory use the mount --bind command:

mount --bind /var/www/foobar /home/user/foobar/www

Don’t forget to add this line into your SFTP daemon init script.

Monty Python Unix Joke

- Stop! Whoever crosseth the bridge of Death, must answer first these questions three, ere the other side he see:
  What is your name?
- Sir Brian of Bell
- What is your quest ?
- I seek the Holy Grail
- What are four lowercase letters that are not legal flag arguments to the Berkeley UNIX version of 'ls' ?