La verdad es que clavaste el origen del problema, hice lo que describes
con la máxima celeridad pero me sigue dando errores que te copio a
continuación.

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   esto me dice dselect después de hacer ifdown eth0
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elbenditu:~# ifdown eth0
elbenditu:~# dselect
Reading Package Lists... Done
Building Dependency Tree... Done
0 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 packages not fully installed or removed.
Need to get 0B of archives. After unpacking 0B will be used.
Do you want to continue? [Y/n]
Configurando gdm2 (2.4.0.11-1) ...
OMF file [/usr/share/omf/ggv/ggv-C.omf] does not validate against
ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/ggv/ggv-C.omf
OMF file [/usr/share/omf/ggv/ggv-ja.omf] does not validate against
ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/ggv/ggv-ja.omf
OMF file [/usr/share/omf/ggv/ggv-it.omf] does not validate against
ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/ggv/ggv-it.omf
/usr/share/omf/file-roller/file-roller-ru.omf:5: error: Input is not
proper UTF-8, indicate encoding !
      Àëåêñàíäð Êèðèëëîâ
      ^
/usr/share/omf/file-roller/file-roller-ru.omf:5: error: Bytes: 0xC0 0xEB
0xE5 0xEA
      Àëåêñàíäð Êèðèëëîâ
      ^
OMF file does not exist, is not readable, or is not well-formed XML:
/usr/share/omf/file-roller/file-roller-ru.omf
Unable to register /usr/share/omf/file-roller/file-roller-ru.omf
OMF file [/usr/share/omf/gnome-user-docs/glossary-C.omf] does not
validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/gnome-user-docs/glossary-C.omf
OMF file [/usr/share/omf/gnome-user-docs/gnome-users-guide-C.omf] does
not validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register
/usr/share/omf/gnome-user-docs/gnome-users-guide-C.omf
OMF file [/usr/share/omf/control-center/control-center-eu.omf] does not
validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/control-center/control-center-eu.omf
OMF file [/usr/share/omf/control-center/control-center-C.omf] does not
validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/control-center/control-center-C.omf
OMF file [/usr/share/omf/gnome-panel/clock-C.omf] does not validate
against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/gnome-panel/clock-C.omf
dpkg: error al procesar gdm2 (--configure):
 el subproceso post-installation script devolvió el código de salida de
error 1
Se encontraron errores al procesar:
 gdm2
E: Sub-process /usr/bin/dpkg returned an error code (1)
Some errors occurred while unpacking. I'm going to configure the
packages that were installed. This may result in duplicate errors
or errors caused by missing dependencies. This is OK, only the errors
above this message are important. Please fix them and run [I]nstall
again
Press enter to continue.

Configurando gdm2 (2.4.0.11-1) ...
OMF file [/usr/share/omf/ggv/ggv-C.omf] does not validate against
ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/ggv/ggv-C.omf
OMF file [/usr/share/omf/ggv/ggv-ja.omf] does not validate against
ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/ggv/ggv-ja.omf
OMF file [/usr/share/omf/ggv/ggv-it.omf] does not validate against
ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/ggv/ggv-it.omf
/usr/share/omf/file-roller/file-roller-ru.omf:5: error: Input is not
proper UTF-8, indicate encoding !
      Àëåêñàíäð Êèðèëëîâ
      ^
/usr/share/omf/file-roller/file-roller-ru.omf:5: error: Bytes: 0xC0 0xEB
0xE5 0xEA
      Àëåêñàíäð Êèðèëëîâ
      ^
OMF file does not exist, is not readable, or is not well-formed XML:
/usr/share/omf/file-roller/file-roller-ru.omf
Unable to register /usr/share/omf/file-roller/file-roller-ru.omf
OMF file [/usr/share/omf/gnome-user-docs/glossary-C.omf] does not
validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/gnome-user-docs/glossary-C.omf
OMF file [/usr/share/omf/gnome-user-docs/gnome-users-guide-C.omf] does
not validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register
/usr/share/omf/gnome-user-docs/gnome-users-guide-C.omf
OMF file [/usr/share/omf/control-center/control-center-eu.omf] does not
validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/control-center/control-center-eu.omf
OMF file [/usr/share/omf/control-center/control-center-C.omf] does not
validate against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/control-center/control-center-C.omf
OMF file [/usr/share/omf/gnome-panel/clock-C.omf] does not validate
against ScrollKeeper-OMF DTD:
/usr/share/xml/scrollkeeper/dtds/scrollkeeper-omf.dtd
Unable to register /usr/share/omf/gnome-panel/clock-C.omf
dpkg: error al procesar gdm2 (--configure):
 el subproceso post-installation script devolvió el código de salida de
error 1
Se encontraron errores al procesar:
 gdm2

el script de instalación devolvió el código de error 100.
Pulse <intro> para continuar.