How to ssh forwarding with AllowTcpForwarding set to no?












2















There is some development I need to do on some remote box. Fortunately, I have shell access, but I need to go through a gateway that has AllowTcpForwarding set to false.



I took a peak at the docs and it says:




AllowTcpForwarding Specifies whether TCP forwarding is permitted. The
default is ''yes''. Note that disabling TCP forwarding does not
improve security unless users are also denied shell access, as they
can always install their own forwarders.




How would I go about installing (or building) my own forwarder? My goal here is to setup a remote interpreter using Pycharm via SSH and binding it to some local port, that data fed through ssh, that through the gateway, and then to the development box where the code is actually run. I imagine I could somehow utilize nc or some other unix utility that'll help get the job done.



I know I can ssh to my remote box by doing:



ssh -t user1@gateway ssh user2@devbox


But obviously this option isn't available in pycharm. I'll have to be able to open some local port such that



ssh -p 12345 localhost
(or variant)


will connect me to user2@devbox. This will allow me to configure the remote interpreter to use port 12345 on localhost to connect to the remote box.










share|improve this question























  • The gateway has AllowTcpForwarding set to false but does the remote box? if it does not why not forward to it? Oh I understand now... Since you can access the router you can probably port forward on the router. Why not do that?

    – jdwolf
    Nov 24 '17 at 2:41


















2















There is some development I need to do on some remote box. Fortunately, I have shell access, but I need to go through a gateway that has AllowTcpForwarding set to false.



I took a peak at the docs and it says:




AllowTcpForwarding Specifies whether TCP forwarding is permitted. The
default is ''yes''. Note that disabling TCP forwarding does not
improve security unless users are also denied shell access, as they
can always install their own forwarders.




How would I go about installing (or building) my own forwarder? My goal here is to setup a remote interpreter using Pycharm via SSH and binding it to some local port, that data fed through ssh, that through the gateway, and then to the development box where the code is actually run. I imagine I could somehow utilize nc or some other unix utility that'll help get the job done.



I know I can ssh to my remote box by doing:



ssh -t user1@gateway ssh user2@devbox


But obviously this option isn't available in pycharm. I'll have to be able to open some local port such that



ssh -p 12345 localhost
(or variant)


will connect me to user2@devbox. This will allow me to configure the remote interpreter to use port 12345 on localhost to connect to the remote box.










share|improve this question























  • The gateway has AllowTcpForwarding set to false but does the remote box? if it does not why not forward to it? Oh I understand now... Since you can access the router you can probably port forward on the router. Why not do that?

    – jdwolf
    Nov 24 '17 at 2:41
















2












2








2


1






There is some development I need to do on some remote box. Fortunately, I have shell access, but I need to go through a gateway that has AllowTcpForwarding set to false.



I took a peak at the docs and it says:




AllowTcpForwarding Specifies whether TCP forwarding is permitted. The
default is ''yes''. Note that disabling TCP forwarding does not
improve security unless users are also denied shell access, as they
can always install their own forwarders.




How would I go about installing (or building) my own forwarder? My goal here is to setup a remote interpreter using Pycharm via SSH and binding it to some local port, that data fed through ssh, that through the gateway, and then to the development box where the code is actually run. I imagine I could somehow utilize nc or some other unix utility that'll help get the job done.



I know I can ssh to my remote box by doing:



ssh -t user1@gateway ssh user2@devbox


But obviously this option isn't available in pycharm. I'll have to be able to open some local port such that



ssh -p 12345 localhost
(or variant)


will connect me to user2@devbox. This will allow me to configure the remote interpreter to use port 12345 on localhost to connect to the remote box.










share|improve this question














There is some development I need to do on some remote box. Fortunately, I have shell access, but I need to go through a gateway that has AllowTcpForwarding set to false.



I took a peak at the docs and it says:




AllowTcpForwarding Specifies whether TCP forwarding is permitted. The
default is ''yes''. Note that disabling TCP forwarding does not
improve security unless users are also denied shell access, as they
can always install their own forwarders.




How would I go about installing (or building) my own forwarder? My goal here is to setup a remote interpreter using Pycharm via SSH and binding it to some local port, that data fed through ssh, that through the gateway, and then to the development box where the code is actually run. I imagine I could somehow utilize nc or some other unix utility that'll help get the job done.



I know I can ssh to my remote box by doing:



ssh -t user1@gateway ssh user2@devbox


But obviously this option isn't available in pycharm. I'll have to be able to open some local port such that



ssh -p 12345 localhost
(or variant)


will connect me to user2@devbox. This will allow me to configure the remote interpreter to use port 12345 on localhost to connect to the remote box.







linux shell ssh python ssh-tunneling






share|improve this question













share|improve this question











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share|improve this question










asked Nov 24 '17 at 1:38









TLaneTLane

1112




1112













  • The gateway has AllowTcpForwarding set to false but does the remote box? if it does not why not forward to it? Oh I understand now... Since you can access the router you can probably port forward on the router. Why not do that?

    – jdwolf
    Nov 24 '17 at 2:41





















  • The gateway has AllowTcpForwarding set to false but does the remote box? if it does not why not forward to it? Oh I understand now... Since you can access the router you can probably port forward on the router. Why not do that?

    – jdwolf
    Nov 24 '17 at 2:41



















The gateway has AllowTcpForwarding set to false but does the remote box? if it does not why not forward to it? Oh I understand now... Since you can access the router you can probably port forward on the router. Why not do that?

– jdwolf
Nov 24 '17 at 2:41







The gateway has AllowTcpForwarding set to false but does the remote box? if it does not why not forward to it? Oh I understand now... Since you can access the router you can probably port forward on the router. Why not do that?

– jdwolf
Nov 24 '17 at 2:41












3 Answers
3






active

oldest

votes


















0














SSH tunneling can do done both ways. SSH into the router. SSH from the router into the devbox. Then run SSH from the devbox to make an outbound connection to your computer running an SSH server with port forwarding enabled while setting up a remote port forward back to the devbox. At that point you can close the other ssh connection.






share|improve this answer































    0














    I would just set up anouther sshd to run on a different port.

    Edit the settings so tcpforwarding is allowed.



    cp /etc/ssh/sshd{,-second}_config 


    Edit sshd-second_config



    Port 22220
    cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service


    Alter /etc/systemd/system/sshd-second.service in the following way:



    Description=OpenSSH server second instance daemon
    ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS


    The ExecStart line may differ, depending on the release.



    systemctl daemon-reload
    systemctl enable sshd-second.service --now


    More information can be found here:

    https://access.redhat.com/solutions/1166283



    Now you should be able to forward whatever you want.






    share|improve this answer































      0














      As long as one can execute socat locally and on gateway (or even just bash and cat on gateway, see last example!) and is allowed to not use a pty to be 8bits clean, it's possible to establish a tunnel through ssh. Here are 4 examples, improving upon the previous:



      Basic example working once



      (having it fork would require one ssh connection per tunnel, not good). Having to escape the : for socat to accept the exec command:



      term1:



      $ socat tcp-listen:12345,reuseaddr exec:'ssh user1@gateway exec socat - tcp:devbox:22',nofork


      term2:



      $ ssh -p 12345 user2@localhost


      term1:



      user1@gateway's password:


      term2:



      user2@localhost's password: 


      Reversing first and second addresses makes the socket immediately available



      socat has to stay in charge, so no nofork:



      term1:



          $ socat exec:'ssh user1@gateway exec socat - tcp:devbox:22' tcp-listen:12345,reuseaddr
      user1@gateway's password:


      term2:



          $ ssh -p 12345 user2@localhost
      user2@localhost's password:


      Using a ControlMaster ssh



      allows to fork while using only a single ssh connection to the gateway, thus giving a behaviour similar to the usual port forwarding:



      term1:



          $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
      user1@gateway's password:


      term2:



          $ socat tcp-listen:12345,reuseaddr,fork exec:'ssh -o ControlPath=~/mysshcontrolsocket user1@gateway exec socat - tcp:devbox:22'


      term3:



          $ ssh -p 12345 user2@localhost
      user2@localhost's password:


      Having only bash and cat available on gateway



      By using bash's built-in tcp redirection, and two half-duplex cat commands (for a full-duplex result) one doesn't even need a remote socat or netcat. Handling of multiple layers of nested and escaped quotes was a bit awkward and can perhaps be done better, or simplified by the use of a remote bash script. Care has to be taken to have the forked cat for output only:



      term1 (no change):



      $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
      user1@gateway's password:


      term2:



      $ socat tcp-listen:12345,reuseaddr,fork 'exec:ssh -T -o ControlPath=~/mysshcontrolsocket user1@gateway '''exec bash -c '''"exec 2>/dev/null 8<>/dev/tcp/devbox/22; cat <&8 & cat >&8"'''


      term3:



      $ ssh -p 12345 user2@localhost
      user2@localhost's password:





      share|improve this answer

























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        0














        SSH tunneling can do done both ways. SSH into the router. SSH from the router into the devbox. Then run SSH from the devbox to make an outbound connection to your computer running an SSH server with port forwarding enabled while setting up a remote port forward back to the devbox. At that point you can close the other ssh connection.






        share|improve this answer




























          0














          SSH tunneling can do done both ways. SSH into the router. SSH from the router into the devbox. Then run SSH from the devbox to make an outbound connection to your computer running an SSH server with port forwarding enabled while setting up a remote port forward back to the devbox. At that point you can close the other ssh connection.






          share|improve this answer


























            0












            0








            0







            SSH tunneling can do done both ways. SSH into the router. SSH from the router into the devbox. Then run SSH from the devbox to make an outbound connection to your computer running an SSH server with port forwarding enabled while setting up a remote port forward back to the devbox. At that point you can close the other ssh connection.






            share|improve this answer













            SSH tunneling can do done both ways. SSH into the router. SSH from the router into the devbox. Then run SSH from the devbox to make an outbound connection to your computer running an SSH server with port forwarding enabled while setting up a remote port forward back to the devbox. At that point you can close the other ssh connection.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered Nov 24 '17 at 2:51









            jdwolfjdwolf

            2,675216




            2,675216

























                0














                I would just set up anouther sshd to run on a different port.

                Edit the settings so tcpforwarding is allowed.



                cp /etc/ssh/sshd{,-second}_config 


                Edit sshd-second_config



                Port 22220
                cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service


                Alter /etc/systemd/system/sshd-second.service in the following way:



                Description=OpenSSH server second instance daemon
                ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS


                The ExecStart line may differ, depending on the release.



                systemctl daemon-reload
                systemctl enable sshd-second.service --now


                More information can be found here:

                https://access.redhat.com/solutions/1166283



                Now you should be able to forward whatever you want.






                share|improve this answer




























                  0














                  I would just set up anouther sshd to run on a different port.

                  Edit the settings so tcpforwarding is allowed.



                  cp /etc/ssh/sshd{,-second}_config 


                  Edit sshd-second_config



                  Port 22220
                  cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service


                  Alter /etc/systemd/system/sshd-second.service in the following way:



                  Description=OpenSSH server second instance daemon
                  ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS


                  The ExecStart line may differ, depending on the release.



                  systemctl daemon-reload
                  systemctl enable sshd-second.service --now


                  More information can be found here:

                  https://access.redhat.com/solutions/1166283



                  Now you should be able to forward whatever you want.






                  share|improve this answer


























                    0












                    0








                    0







                    I would just set up anouther sshd to run on a different port.

                    Edit the settings so tcpforwarding is allowed.



                    cp /etc/ssh/sshd{,-second}_config 


                    Edit sshd-second_config



                    Port 22220
                    cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service


                    Alter /etc/systemd/system/sshd-second.service in the following way:



                    Description=OpenSSH server second instance daemon
                    ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS


                    The ExecStart line may differ, depending on the release.



                    systemctl daemon-reload
                    systemctl enable sshd-second.service --now


                    More information can be found here:

                    https://access.redhat.com/solutions/1166283



                    Now you should be able to forward whatever you want.






                    share|improve this answer













                    I would just set up anouther sshd to run on a different port.

                    Edit the settings so tcpforwarding is allowed.



                    cp /etc/ssh/sshd{,-second}_config 


                    Edit sshd-second_config



                    Port 22220
                    cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service


                    Alter /etc/systemd/system/sshd-second.service in the following way:



                    Description=OpenSSH server second instance daemon
                    ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS


                    The ExecStart line may differ, depending on the release.



                    systemctl daemon-reload
                    systemctl enable sshd-second.service --now


                    More information can be found here:

                    https://access.redhat.com/solutions/1166283



                    Now you should be able to forward whatever you want.







                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered Nov 24 '18 at 4:43









                    Michael ProkopecMichael Prokopec

                    1,448218




                    1,448218























                        0














                        As long as one can execute socat locally and on gateway (or even just bash and cat on gateway, see last example!) and is allowed to not use a pty to be 8bits clean, it's possible to establish a tunnel through ssh. Here are 4 examples, improving upon the previous:



                        Basic example working once



                        (having it fork would require one ssh connection per tunnel, not good). Having to escape the : for socat to accept the exec command:



                        term1:



                        $ socat tcp-listen:12345,reuseaddr exec:'ssh user1@gateway exec socat - tcp:devbox:22',nofork


                        term2:



                        $ ssh -p 12345 user2@localhost


                        term1:



                        user1@gateway's password:


                        term2:



                        user2@localhost's password: 


                        Reversing first and second addresses makes the socket immediately available



                        socat has to stay in charge, so no nofork:



                        term1:



                            $ socat exec:'ssh user1@gateway exec socat - tcp:devbox:22' tcp-listen:12345,reuseaddr
                        user1@gateway's password:


                        term2:



                            $ ssh -p 12345 user2@localhost
                        user2@localhost's password:


                        Using a ControlMaster ssh



                        allows to fork while using only a single ssh connection to the gateway, thus giving a behaviour similar to the usual port forwarding:



                        term1:



                            $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                        user1@gateway's password:


                        term2:



                            $ socat tcp-listen:12345,reuseaddr,fork exec:'ssh -o ControlPath=~/mysshcontrolsocket user1@gateway exec socat - tcp:devbox:22'


                        term3:



                            $ ssh -p 12345 user2@localhost
                        user2@localhost's password:


                        Having only bash and cat available on gateway



                        By using bash's built-in tcp redirection, and two half-duplex cat commands (for a full-duplex result) one doesn't even need a remote socat or netcat. Handling of multiple layers of nested and escaped quotes was a bit awkward and can perhaps be done better, or simplified by the use of a remote bash script. Care has to be taken to have the forked cat for output only:



                        term1 (no change):



                        $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                        user1@gateway's password:


                        term2:



                        $ socat tcp-listen:12345,reuseaddr,fork 'exec:ssh -T -o ControlPath=~/mysshcontrolsocket user1@gateway '''exec bash -c '''"exec 2>/dev/null 8<>/dev/tcp/devbox/22; cat <&8 & cat >&8"'''


                        term3:



                        $ ssh -p 12345 user2@localhost
                        user2@localhost's password:





                        share|improve this answer






























                          0














                          As long as one can execute socat locally and on gateway (or even just bash and cat on gateway, see last example!) and is allowed to not use a pty to be 8bits clean, it's possible to establish a tunnel through ssh. Here are 4 examples, improving upon the previous:



                          Basic example working once



                          (having it fork would require one ssh connection per tunnel, not good). Having to escape the : for socat to accept the exec command:



                          term1:



                          $ socat tcp-listen:12345,reuseaddr exec:'ssh user1@gateway exec socat - tcp:devbox:22',nofork


                          term2:



                          $ ssh -p 12345 user2@localhost


                          term1:



                          user1@gateway's password:


                          term2:



                          user2@localhost's password: 


                          Reversing first and second addresses makes the socket immediately available



                          socat has to stay in charge, so no nofork:



                          term1:



                              $ socat exec:'ssh user1@gateway exec socat - tcp:devbox:22' tcp-listen:12345,reuseaddr
                          user1@gateway's password:


                          term2:



                              $ ssh -p 12345 user2@localhost
                          user2@localhost's password:


                          Using a ControlMaster ssh



                          allows to fork while using only a single ssh connection to the gateway, thus giving a behaviour similar to the usual port forwarding:



                          term1:



                              $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                          user1@gateway's password:


                          term2:



                              $ socat tcp-listen:12345,reuseaddr,fork exec:'ssh -o ControlPath=~/mysshcontrolsocket user1@gateway exec socat - tcp:devbox:22'


                          term3:



                              $ ssh -p 12345 user2@localhost
                          user2@localhost's password:


                          Having only bash and cat available on gateway



                          By using bash's built-in tcp redirection, and two half-duplex cat commands (for a full-duplex result) one doesn't even need a remote socat or netcat. Handling of multiple layers of nested and escaped quotes was a bit awkward and can perhaps be done better, or simplified by the use of a remote bash script. Care has to be taken to have the forked cat for output only:



                          term1 (no change):



                          $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                          user1@gateway's password:


                          term2:



                          $ socat tcp-listen:12345,reuseaddr,fork 'exec:ssh -T -o ControlPath=~/mysshcontrolsocket user1@gateway '''exec bash -c '''"exec 2>/dev/null 8<>/dev/tcp/devbox/22; cat <&8 & cat >&8"'''


                          term3:



                          $ ssh -p 12345 user2@localhost
                          user2@localhost's password:





                          share|improve this answer




























                            0












                            0








                            0







                            As long as one can execute socat locally and on gateway (or even just bash and cat on gateway, see last example!) and is allowed to not use a pty to be 8bits clean, it's possible to establish a tunnel through ssh. Here are 4 examples, improving upon the previous:



                            Basic example working once



                            (having it fork would require one ssh connection per tunnel, not good). Having to escape the : for socat to accept the exec command:



                            term1:



                            $ socat tcp-listen:12345,reuseaddr exec:'ssh user1@gateway exec socat - tcp:devbox:22',nofork


                            term2:



                            $ ssh -p 12345 user2@localhost


                            term1:



                            user1@gateway's password:


                            term2:



                            user2@localhost's password: 


                            Reversing first and second addresses makes the socket immediately available



                            socat has to stay in charge, so no nofork:



                            term1:



                                $ socat exec:'ssh user1@gateway exec socat - tcp:devbox:22' tcp-listen:12345,reuseaddr
                            user1@gateway's password:


                            term2:



                                $ ssh -p 12345 user2@localhost
                            user2@localhost's password:


                            Using a ControlMaster ssh



                            allows to fork while using only a single ssh connection to the gateway, thus giving a behaviour similar to the usual port forwarding:



                            term1:



                                $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                            user1@gateway's password:


                            term2:



                                $ socat tcp-listen:12345,reuseaddr,fork exec:'ssh -o ControlPath=~/mysshcontrolsocket user1@gateway exec socat - tcp:devbox:22'


                            term3:



                                $ ssh -p 12345 user2@localhost
                            user2@localhost's password:


                            Having only bash and cat available on gateway



                            By using bash's built-in tcp redirection, and two half-duplex cat commands (for a full-duplex result) one doesn't even need a remote socat or netcat. Handling of multiple layers of nested and escaped quotes was a bit awkward and can perhaps be done better, or simplified by the use of a remote bash script. Care has to be taken to have the forked cat for output only:



                            term1 (no change):



                            $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                            user1@gateway's password:


                            term2:



                            $ socat tcp-listen:12345,reuseaddr,fork 'exec:ssh -T -o ControlPath=~/mysshcontrolsocket user1@gateway '''exec bash -c '''"exec 2>/dev/null 8<>/dev/tcp/devbox/22; cat <&8 & cat >&8"'''


                            term3:



                            $ ssh -p 12345 user2@localhost
                            user2@localhost's password:





                            share|improve this answer















                            As long as one can execute socat locally and on gateway (or even just bash and cat on gateway, see last example!) and is allowed to not use a pty to be 8bits clean, it's possible to establish a tunnel through ssh. Here are 4 examples, improving upon the previous:



                            Basic example working once



                            (having it fork would require one ssh connection per tunnel, not good). Having to escape the : for socat to accept the exec command:



                            term1:



                            $ socat tcp-listen:12345,reuseaddr exec:'ssh user1@gateway exec socat - tcp:devbox:22',nofork


                            term2:



                            $ ssh -p 12345 user2@localhost


                            term1:



                            user1@gateway's password:


                            term2:



                            user2@localhost's password: 


                            Reversing first and second addresses makes the socket immediately available



                            socat has to stay in charge, so no nofork:



                            term1:



                                $ socat exec:'ssh user1@gateway exec socat - tcp:devbox:22' tcp-listen:12345,reuseaddr
                            user1@gateway's password:


                            term2:



                                $ ssh -p 12345 user2@localhost
                            user2@localhost's password:


                            Using a ControlMaster ssh



                            allows to fork while using only a single ssh connection to the gateway, thus giving a behaviour similar to the usual port forwarding:



                            term1:



                                $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                            user1@gateway's password:


                            term2:



                                $ socat tcp-listen:12345,reuseaddr,fork exec:'ssh -o ControlPath=~/mysshcontrolsocket user1@gateway exec socat - tcp:devbox:22'


                            term3:



                                $ ssh -p 12345 user2@localhost
                            user2@localhost's password:


                            Having only bash and cat available on gateway



                            By using bash's built-in tcp redirection, and two half-duplex cat commands (for a full-duplex result) one doesn't even need a remote socat or netcat. Handling of multiple layers of nested and escaped quotes was a bit awkward and can perhaps be done better, or simplified by the use of a remote bash script. Care has to be taken to have the forked cat for output only:



                            term1 (no change):



                            $ ssh -N -o ControlMaster=yes -o ControlPath=~/mysshcontrolsocket user1@gateway
                            user1@gateway's password:


                            term2:



                            $ socat tcp-listen:12345,reuseaddr,fork 'exec:ssh -T -o ControlPath=~/mysshcontrolsocket user1@gateway '''exec bash -c '''"exec 2>/dev/null 8<>/dev/tcp/devbox/22; cat <&8 & cat >&8"'''


                            term3:



                            $ ssh -p 12345 user2@localhost
                            user2@localhost's password:






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                            edited Dec 31 '18 at 5:32

























                            answered Dec 31 '18 at 5:02









                            A.BA.B

                            4,7021725




                            4,7021725






























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