Linux PPP HOWTO
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# 8.3. The syslog says "serial line is not 8 bit clean"

There are variations on this too - such as `serial line looped back` etc., and the cause can be one (or a sequence) of a number of things.

To understand what is going on here, it is necessary to grasp a bit of what is going on behind the scenes in pppd itself.

When pppd starts up, it sends LCP (link control protocol) packets to the remote machine. If it receives a valid response it then goes on to the next stage (using IPCP - IP control protocol packets) and only when this negotiation completes is the actual IP layer started so that you can use the PPP link.

If there is no ppp server operating at the remote end when your PC sends lcp packets, these get reflected by the login process at the far end. As these packets use 8 bits, reflecting them strips the 8th bit (remember, ASCII is a 7 bit code). PPP sees this and complains accordingly.

There are several reasons this reflection can occur.

## 8.3.1. You are not correctly logging into the server

When your chat script completes, pppd starts on your PC. However, if you have not completed the log in process to the server (including sending any command required to start PPP on the server), PPP will not start.

So, the lcp packets are reflected and you receive this error.

You need to carefully check and correct (if necessary) your chat script (see above).

## 8.3.2. You are not starting PPP on the server

Some PPP servers require you to enter a command and/or a RETURN after completing the log in process before the remote end starts ppp.

Check your chat script (see above).

If you log in manually and find you need to send a RETURN after this to start PPP, simply add a blank expect/send pair to the end of your chat script (an empty send string actually sends a RETURN).

## 8.3.3. The remote PPP process is slow to start

This one is a bit tricksy!

By default, your Linux pppd is compiled to send a maximum of 10 lcp configuration requests. If the server is a bit slow to start up, all 10 such requests can be sent before the remote PPP is ready to receive them.

On your machine, pppd sees all 10 requests reflected back (with the 8th bit stripped) and exits.

There are two ways round this:-

Add "`lcp-max-configure 30`" to your ppp options. This increases the maximum number of lcp configure packets pppd sends before giving up. For really slow server, you may need even more than this.

Alternatively, you can get a bit tricksy in return. You may have noticed that when you logged in by hand to the PPP server and PPP started there, the first character of the ppp garbage that appears was always the tilde character (˜).

Using this knowledge we can add a new `expect/send` pair to the end of the chat script which expects a tilde and sends nothing. This would look like:-

    \~	''

Note: as the tilde character has a special meaning in the shell, it must be escaped (and hence the leading backslash).

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