Debian – escape characters in /etc/network/interfaces

configurationdebiannetworkingwifiwpa

I want to be sure that whatever string I pass into the line wpa-ssid "abc" in /etc/network/interfaces won't be used to break out of the configuration.

All I can find in the manual is that \ can be used at the end of a line to continue on the next line.

But what about \" in the middle of a line?

My worries is an SSID something like

A"
up rm -rf /\

Are there any general encoding that can be used for arbitrary characters into the SSID field?

Best Answer

In Debian's /etc/network/interfaces (or any other distribution using Debian's ifupdown utility), a backslash-newline sequence is removed, and backslash is not special anywhere else. A double quote character is not special either. The character # starts a comment if it's the first non-whitespace character on a (non-continuation) line. Null bytes are treated as newline characters (I think — the parser uses C strings and has no special handling for null bytes, so they might cause additional mischief).

Configuration lines take the form of an option name followed by a value, separated by whitespace. Leading and trailing whitespace is ignored. Some built-in options further parse the line into words; the value of options to iface always runs to the end of the line.

For example, the line

wpa-ssid  "a  b"  "cd"

sets the option wpa-ssid to the 12-character string "a  b"  "cd" (internal whitespace is preserved).

WPA Supplicant's ifupdown script strips double quotes at the beginning and at the end of the wpa-ssid configuration string, the line above is equivalent to wpa-ssid a  b"  "cd. This way, you can have leading and trailing whitespace in the SSID.

I can't find a quoting issue in the WPA Supplicant ifupdown scripts, so it looks like anything that ifupdown will produce is safe.

Thus you can allow any string as an SSID to be injected into /etc/network/interfaces, provided that it does not contain any newline or null byte. Add double quotes around the string (if you don't, SSIDs with leading or trailing whitespace, or that end with \, or that begin or end with ", will be mangled).

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