Red Hat Bugzilla – Bug 612
pppd doesn't check for its MTU/MRU limits; obscure error message and line drop results
Last modified: 2008-05-01 11:37:48 EDT
Apparently, Redhat 5.2's pppd (2.3pl5) has internal limits on MTU/MRU that it doesn't check for on the command line. Specifying an MTU/MRU that is beyond these undocumented internal limits (e.g. 1524) on the command line results in pppd starting up, taking these values, negotiating them with the server, and only after the line is up and a packet too big is received, generating an obscure error message and exiting. Here is the error it prints: Dec 26 14:48:01 blackbird-eth pppd[2895]: read: Value too large for defined data type(75)
BTW, lowering the specified MTU to 1064 works around this pppd bug. --Randall (aa8vb@pagesz.net)
I tried this several times in the test lab and in all cases pppd did the correct thing. If you specify a number larger that 1500 and both ends have a max of 1500 then it will drop down to 1500 which is also the default pppd will use if no mru and mtu are specified. When I entered a number less that 1500 it used that number specifically. I was unable to genrate the error that you were reporting. One possibility is that the server you are dialing into is not using Linux and their pppd server supports packet sizes larger that 1500 where the linux pppd client does not therefore generating the errors.