On Thu, Dec 24, 2020, 8:08 AM Greg Wooledge <wool...@eeg.ccf.org> wrote:
> On Thu, Dec 24, 2020 at 12:49:30PM +0800, zhang yang wrote: > > IP_ dynaddr > > To modify and save the prompt: "IP"_ dynaddr" E212: Can't open file for > > writing“. > > So I used Chmod 777 ip_ dynaddr authority, display error: Chmod: changing > > permissions of 'IP_ dynaddr': Operation not permitted. > > I use chaddr - i ip_dynaddr command to removes attributes and displays an > > error: chattr: Inappropriate ioctl for device while reading flags on > > ip_dynaddr. How to set dynaddr? > > For the benefit of others, the file in question is > > /proc/sys/net/ipv4/ip_dynaddr > > /proc is not a regular file system. It's an interface to pieces of the > kernel. Some of these pieces can be read by everyone. Almost none of > them can be *written* by everyone -- you usually have to be root to > write to them. > > chmod 777 is almost always a *huge* mistake. If the permissions on a > file are stopping you from doing someting, there's usually a reason > for that. Destroying the permissions on the file is not the right answer. > Elevating yourself to a higher power level is. > > Now, the next thing you'll want to know is that while root can probably > write to this file, it can't necessary *edit* the file with a text > editor. You're expected to use a much lower level of access to open > the file, write to it, and close it. Typically that's done with > > echo something > /proc/whatever > > Of course this redirection means that it's your shell who needs > privileges to open the file for writing. If you try it as your > regular user, you'll encounter the "permission denied" error that > you already saw. > > The expected course of action is for you to do it from a root shell, > which you obtain by using su or sudo. > > $ su > Password: > # echo something > /proc/whatever > # exit > $ > > If you're not a fan of su, or if you're one of those people who has > decided not to have a root password (a bad idea!), then you can get > a root shell with sudo -s: > > $ sudo -s > # echo something >/proc/whatever > # exit > $ > > (sudo may prompt you for your password if you haven't used sudo in that > terminal in the last few minutes.) > > Some people will try to be clever and do this: > > sudo echo something > /proc/whatever > > That will not work, for the reasons explained at > <https://mywiki.wooledge.org/BashPitfalls#pf53>. +10 That is an Awesome page you have there, Greg! I have Bookmarked it. > You may use one of > the quoted forms on that page if you prefer. But the original design > of the files in /proc was for users with a root shell to perform shell > redirections. Not for sudo. > > Also, while I'm here talking about su, bear in mind that Debian decided > to become more like Red Hat and replaced their version of su with the > one used by Red Hat, and didn't bother configuring it to maintain backward > compatibility with the previous version. See > <https://wiki.debian.org/NewInBuster#Changes> for workarounds... > Also good. I was never affected by the change, because I always use "su -". Kenneth Parker >
