How to disable ipv6 in Debian

Here are simple steps to disable ipv6 in Debian:

1. Comment out anything related to ipv6 in /etc/hosts
2. SSH. Ensure AddressFamily inet is set in /etc/ssh/sshd_config. Restart ssh.
3. BIND. Ensure listen-on-v6 { none; }; in /etc/bind/named.conf.options. Restart bind9.
4. NTP. Ensure -4 option is set in /etc/default/ntp (e.g. NTPD_OPTS='-4 -g'). Restart ntp.
5. APACHE2. Ensure Listen 0.0.0.0:80 in /etc/apache2/ports.conf file. Restart apache2.
6. RPCBIND (rpc.statd, rpc.mountd). Comment out the appropriate entries in /etc/netconfig:

   udp        tpi_clts      v     inet     udp     - -
   tcp        tpi_cots_ord  v     inet     tcp     - -
   #udp6       tpi_clts      v     inet6    udp    - -
   #tcp6       tpi_cots_ord  v     inet6    tcp    - -
   rawip      tpi_raw       -     inet      -      - -
   local      tpi_cots_ord  -     loopback  -      - -
   unix       tpi_cots_ord  -     loopback  -      - -
   

7. PostgreSQL 9. Ensure ipv4 in listen_addresses (file /etc/postgresql/9.1/main/postgresql.conf):

   # - Connection Settings
   listen_addresses = '0.0.0.0'
   

   Comment out lines related to ipv6 (file /etc/postgresql/9.1/main/pg_hba.conf):

   # IPv6 local connections:
   #host  all     all     ::1/128   md5
   

   Restart postgresql.
8. Disable ipv6 in kernel:

   echo net.ipv6.conf.all.disable_ipv6=1 \
   > /etc/sysctl.d/disableipv6.conf
   

9. Disable ipv6 in kernel modules (file /etc/modprobe.d/aliases.conf):

   # alias net-pf-10 ipv6
   alias net-pf-10 off
   alias ipv6 off
   

The next time the system boots it will have ipv6 disabled. Let verify it with:

netstat -tunlp

Here is a sample output:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 192.168.10.2:53         0.0.0.0:*               LISTEN      895/named       
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      895/named       
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      734/sshd        
tcp        0      0 127.0.0.1:953           0.0.0.0:*               LISTEN      895/named       
udp        0      0 192.168.10.2:53         0.0.0.0:*                           895/named       
udp        0      0 127.0.0.1:53            0.0.0.0:*                           895/named         

Read more about ipv6 here.