[CentOS] modem for dial IN

Dan Pritts danno at internet2.edu
Tue Oct 25 21:55:35 UTC 2005

Here are my notes from doing this on a red hat 7.3 box.

If you don't need ppp but just want console, you can ignore the ppp
part and just set up mgetty.

ppp configuration is surprisingly simple.

You'll need the pppd and mgetty packages if you don't have them.



uncomment the "AutoPPP?" line or you'll just get a login prompt

also, if you are using chap authentication (maybe good, maybe bad) you will want
to enable it in the command line for pppd in the AutoPPP? line


speed 38400
data-only y

possibly port-specific options. If you're doing a lot of analog-to-analog
dial you want to disable the 56k negotiation in the modem init string,
because it will never get past 33.6 on a connection that is analog on
both ends. (v.92 might do better).

This seems to work with USR modems:
# this init-chat is the compiled-in default plus ATS32=224 at the end
# this disables x2, v.90, v.92
init-chat "" ATQ0V1H0 OK ATS0=0Q0&D3&C1 OK ATS32=224 OK



ip addr above should be replaced by dialin server man page.  see man page.


   1. Secrets for authentication using CHAP
   2. client server secret IP addresses

meeting yourdialinserver password Ipaddr-to-assign

S4:2345:respawn:/sbin/mgetty -D /dev/ttyS4
S5:2345:respawn:/sbin/mgetty -D /dev/ttyS5


   1. ip forwarding is useful when we are a PPP dialin host

echo 1 > /proc/sys/net/ipv4/ip_forward

On Fri, Oct 21, 2005 at 11:46:51AM +0100, Tom Brown wrote:
> Hi
> Not entirely centos related but that what the boxes are running so here i
> go ;)
> 2 of our centos boxes run nagios for network monitoring and send email and
> paging alerts and are connected to standard phone lines using modems to
> send the pages. I'm looking for a way in the event of provider failure to
> login to these machines and so am looking at the ability to connect to the
> console over the modem.
> Any good how-to's on this ? Also interested to know real life seciruty
> risks etc. Is it possible to limit phone numbers that are abor to connect
> etc?
> thanks
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos

dan pritts - systems administrator - internet2
734/352-4953 office        734/834-7224 mobile

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