pg_dumpall and password access

I'm trying to run pg_dumpall to backup all my users' bases but since I
have configure pg_hba.conf to "passwd" pg_dumpall always fails:

su-2.03$ pg_dumpall >backup.dat # I'm the postgres unix user.
Password: psql: Password authentication failed for user 'postgres'
Password: psql: Password authentication failed for user 'postgres'
Password: psql: Password authentication failed for user 'postgres'
# I type my password on the next line
Password: Password: psql: Password authentication failed for user 'postgres'
Connection to database 'sympa' failed.
fe_sendauth: no password supplied

pg_dump failed on sympa, exiting

I need to have a password identification since I don't want the
"common" users to be able to connect to any database. Beside I cannot
leave the "local" to the "trust" auth scheme because every users are
able to run programs on the machine hosting the database.

Is there any configuration tips or workaround known for my problem ?

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