Can I create a user-specific hosts file to complement /etc/hosts?

Question

Is it possible to add a list of hosts that are only specific to a certain user? Perhaps a user-specific hosts file?

This mechanism should also complement the entries in the /etc/hosts file.

Answer 1

The functionality you are looking for is implemented in glibc. You can define a custom hosts file by setting the HOSTALIASES environment variable. The names in this file will be picked up by gethostbyname (see documentation).

Example (tested on Ubuntu 13.10):

$ echo 'g www.google.com' >> ~/.hosts
$ export HOSTALIASES=~/.hosts
$ wget g -O /dev/null

Some limitations:

• HOSTALIASES only works for applications using getaddrinfo(3) or gethostbyname(3)
• When setuid is used libc sanitizes the environment, which means that the HOSTALIASES setting is lost. ping is setuid root (because it needs to listen for ICMP packets), so HOSTALIASES will not work with ping unless you're already root before you call ping.

Answer 2

Beside the LD_PRELOAD tricks. A simple alternative that may work on a few systems would be to binary-edit a copy of the system library that handles hostname resolution to replace /etc/hosts with a path of your own.

For instance, on Linux:

If you're not using nscd, copy libnss_files.so to some location of your own like:

mkdir -p -- ~/lib &&
cp /lib/x86_64-linux-gnu/libnss_files.so.2 ~/lib

_{(the shared library may be located elsewhere, e.g. /lib/libnss_files.so.2)}

Now, binary-edit the copy to replace /etc/hosts in there to something the same length like /tmp/hosts.

perl -pi -e 's:/etc/hosts:/tmp/hosts:g' ~/lib/libnss_files.so.2

Edit /tmp/hosts to add the entry you want. And use

export LD_LIBRARY_PATH=~/lib

for nss_files to look in /tmp/hosts instead of /etc/hosts.

Instead of /tmp/hosts, you could also make it /dev/fd//3 (here using two slashes so that the length of /dev/fd//3 is the same as that of /etc/hosts), and do

exec 3< ~/hosts

For instance which would allow different commands to use different hosts files.

If nscd is installed and running, you can bypass it by doing the same trick, but this time for libc.so.6 and replace the path to the nscd socket (something like /var/run/nscd/socket) with some nonexistent path.

Answer 3

Private mountspaces created with the unshare command can be used to provide a private /etc/hosts file to a shell process and any subsequent child processes started from that shell.

# Start by creating your custom /etc/hosts file
[user] cd ~
[user] cat >my_hosts <<EOF
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
127.0.0.1 news.bbc.co.uk
EOF

[user] sudo unshare --mount
# We're now running as root in a private mountspace. 
# Any filesystem mounts performed in this private mountspace
# are private to this shell process and its children

# Use a bind mount to install our custom hosts file over /etc/hosts
[root] mount my_hosts /etc/hosts --bind

[root] cat /etc/hosts
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
127.0.0.1 news.bbc.co.uk

[root] exec su - appuser

[appuser] # Run your app here that needs a custom /etc/hosts file

[appuser] ping news.bbc.co.uk
PING news.bbc.co.uk (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.062 ms
64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.026 ms
^C
--- news.bbc.co.uk ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.026/0.044/0.062/0.018 ms

Answer 4

One solution is to have each user in a separate chroot, so they can each have a separate /etc/hosts to themselves.

Answer 5

I faced the same need, so I tried libnss-userhosts, but it fails at multithreaded applications. Therefore I have written libnss-homehosts. It's very new and tested only by me. You may give a chance for it! It supports some options in /etc/host.conf, multiple alias names, and reverse resolving (address to name).

Answer 6

Not sure if this would help you, but I came here looking for a way to add saved "hosts" somewhere that was easily accessible to only my user.

I basically needed to be able to ssh into certain boxes on our work network, which only has one entry point.

What I did was add aliases to my .bashrc file.

For example, if you added:

alias jrfbox 'ssh jason@1912.168.6.6' 

at the bottom of the ~/.bashrc (~ is your home directory), then after you logout and login again, you can type jrfbox, hit Enter, and it will connect.

Answer 7

Placing the following in ~/.bashrc is working for me in bash. It converts the hostname in the command into an address based on entries in ~/.hosts. If ~/.hosts doesn't exist or if the hostname can't be found in ~/.hosts, the command executes as normal. This should work with the original flags of the relevant functions and regarless of where the hostname is placed relative to the flags, e.g. ping -i 0.5 host1 -c 3, works. The ~/.hosts file takes preference over any other location for finding hostnames, so if there are any dupicate hostnames, the address in ~/.hosts will be used.

$ cat ~/.bashrc 
function resolve {
        hostfile=~/.hosts
        if [[ -f "$hostfile" ]]; then
                for arg in $(seq 1 $#); do
                        if [[ "${!arg:0:1}" != "-" ]]; then
                                ip=$(sed -n -e "/^\s*\(\#.*\|\)$/d" -e "/\<${!arg}\>/{s;^\s*\(\S*\)\s*.*$;\1;p;q}" "$hostfile")
                                if [[ -n "$ip" ]]; then
                                        command "${FUNCNAME[1]}" "${@:1:$(($arg-1))}" "$ip" "${@:$(($arg+1)):$#}"
                                        return
                                fi
                        fi
                done
        fi
        command "${FUNCNAME[1]}" "$@"
}

function ping {
        resolve "$@"
}

function traceroute {
        resolve "$@"
}

An example of ~/.hosts is given below. It follows the same format as /etc/hosts. Comments and whitespace are handled correctly.

$ cat ~/.hosts 
# addresses and hostnames
stackexchange.com se

192.168.0.1 host1 # this is host1's address
login-node.inst.ac.uk login