primo: consiglio, togli il secondo dns dai client.. non serve a nulla e mettere un dns esterno alla rete è uno dei più grandi errori che si possano fare 
secondo: una occhiata ai log di squid e dei servizi coinvolti potrà forse darti una mano
terzo: come pensi si possa rispondere alla domanda 
Però se tolgo il DNS secondario e per caso collego un portatile ad un'altra rete, dovrò modificare i DNS (salvo avere un altro SME identificato come 192.168.0.50) giusto?
I log di squid sono vuoti.
La 3a domanda era...SQUID è un proxy, quindi tecnicamente non dovrebbe centrare granchè con il discorso DNS, essendo poi trasparente, a maggior ragione non dovrebbe essere lui il problema...chi può esserlo?
CPU[|| 2.0%] Tasks: 158 total, 1 running
Mem[||||||||||||||||||||||||||||||||||||||||||||||||||||||||||459/1010MB] Load average: 0.00 0.01 0.00
Swp[ 0/1983MB] Uptime: 5 days, 05:00:28
PID USER PRI NI VIRT RES SHR S CPU% MEM% TIME+ Command
7524 root 17 0 3256 1184 856 R 1.0 0.1 0:00.16 htop
1 root 17 1 2696 620 532 S 0.0 0.1 0:01.68 init [7]
1275 root 16 0 2256 552 464 S 0.0 0.1 0:00.04 udevd
2821 root 18 0 2376 400 344 S 0.0 0.0 0:00.00 /sbin/mingetty tty2
2846 root 17 0 2156 400 344 S 0.0 0.0 0:00.00 /sbin/mingetty tty3
2847 root 16 0 36 24 12 S 0.0 0.0 0:00.26 runsvdir -P /service log: .....................................................................
3144 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv pop3s
3148 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv wan
3149 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv imaps
3157 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv radiusd
3158 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv pop3
3166 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv lpd
3167 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv ntpd
3168 root 16 0 20 20 12 S 0.0 0.0 0:00.01 runsv yum
3169 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv dhcpd
3170 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv httpd-admin
3171 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv freshclam
3172 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv tinydns
3173 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv sshd
3174 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv smtp-auth-proxy
3175 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv ippp
3176 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv smbd
3187 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv qmail
3188 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv ldap
3189 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv dnscache
3190 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv dnscache.forwarder
3191 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv cvm-unix-local
3192 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv ftp
3193 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv clamd
3194 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv imap
3195 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv raidmonitor
3212 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv mysqld
3213 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv nmbd
3214 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv httpd-e-smith
3215 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv squid
3216 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv oidentd
3217 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv sqpsmtpd
3218 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv spamd
3224 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv ulogd
3225 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv pptpd
3226 root 16 0 20 20 12 S 0.0 0.0 0:00.00 runsv qpsmtpd
3255 smelog 16 0 2340 268 216 S 0.0 0.0 0:00.00 /usr/local/bin/multilog t s5000000 /var/log/pop3s
3256 smelog 18 0 2548 264 212 S 0.0 0.0 0:00.00 /usr/local/bin/multilog t s5000000 /var/log/wan
3257 smelog 15 0 2132 268 216 S 0.0 0.0 0:00.17 /usr/local/bin/multilog t s5000000 /var/log/pop3
3258 imaplog 16 0 1900 268 216 S 0.0 0.0 0:01.67 /usr/local/bin/multilog t s5000000 /var/log/imaps
3259 smelog 16 0 2928 336 272 S 0.0 0.0 0:08.34 /usr/local/bin/multilog t s5000000 /var/log/iptables
3260 root 16 0 2188 532 444 S 0.0 0.1 0:01.66 /usr/sbin/ulogd
3261 smelog 18 0 2180 264 212 S 0.0 0.0 0:00.00 /usr/local/bin/multilog t s5000000 /var/log/smbd
3262 smelog 16 0 3076 264 216 S 0.0 0.0 0:00.00 /usr/local/bin/multilog t s5000000 /var/log/ftp
3263 smelog 16 0 2140 268 216 S 0.0 0.0 0:00.00 /usr/local/bin/multilog t s5000000 /var/log/raidmonitor
3264 smelog 18 0 2716 264 212 S 0.0 0.0 0:00.00 /usr/local/bin/multilog t s5000000 /var/log/pptpd
3265 smelog 15 0 1904 336 272 S 0.0 0.0 0:04.83 /usr/local/bin/multilog t s5000000 /var/log/qpsmtpd
3266 dnslog 16 0 1876 336 272 S 0.0 0.0 0:19.94 /usr/local/bin/multilog t s5000000 /var/log/dnscache.forwarder
Questi sono i servizi attivi, nei log dnscache.xxx non mi pare ci sia niente di che, solo le chiamate ai dns esterni.