When squid setup as TPROXY, returns ERR\_CONNECT\_FAIL on some sites . 
The server which squid runs on it is able to open those sites by lynx, wget, curl etc . 
Even if we manually set proxy in browser or use simple REDIRECT or DST-NAT, those sites could open.

From this page <http://wiki.squid-cache.org/SquidFaq/InterceptionProxy>

> 
> It causes path-MTU (PMTUD) to fail, possibly making **some remote sites inaccessible**. This is not usually a problem if your client machines are connected via Ethernet or DSL PPPoATM where the MTU of all links between the cache and client is 1500 or more. If your clients are connecting via DSL PPPoE then this is likely to be a problem as PPPoE links often have a reduced MTU (1472 is very common). 
> 
> 
> 

But i have same problem with ethernet

Here is tcpdump from one client:

[Click here to see tcpdump when i use tproxy and the problem appears](http://paste.ubuntu.com/5581724/)

and

[Click here to see tcpdump when i manually set proxy in my browser and everyhting works fine](http://paste.ubuntu.com/5581726/)

```
GET / HTTP/1.0
Host: 80.75.1.4
Accept: text/html, text/plain, text/css, text/sgml, */*;q=0.01
Accept-Encoding: gzip, compress, bzip2
Accept-Language: en
User-Agent: Lynx/2.8.8dev.2 libwww-FM/2.14 SSL-MM/1.4.1

HTTP/1.0 504 Gateway Time-out
Server: squid
Mime-Version: 1.0
Date: Wed, 27 Feb 2013 15:39:03 GMT
Content-Type: text/html
Content-Length: 376353
X-Squid-Error: ERR_CONNECT_FAIL 110
X-Cache: MISS from cache.mysquid.com
X-Cache-Lookup: MISS from cache.mysquid.com:3128
Connection: close

```

ping -s 1500 80.75.1.4

```
PING 80.75.1.4 (80.75.1.4) 1500(1528) bytes of data.
1508 bytes from 80.75.1.4: icmp_req=1 ttl=58 time=5.28 ms
1508 bytes from 80.75.1.4: icmp_req=2 ttl=58 time=3.96 ms
1508 bytes from 80.75.1.4: icmp_req=3 ttl=58 time=4.28 ms

```

ping -s 1473 80.75.1.4 -M do

```
PING 80.75.1.4 (80.75.1.4) 1473(1501) bytes of data.
From 109.110.160.171 icmp_seq=1 Frag needed and DF set (mtu = 1500)
From 109.110.160.171 icmp_seq=1 Frag needed and DF set (mtu = 1500)

--- 80.75.1.4 ping statistics ---
0 packets transmitted, 0 received, +2 errors

```

ping -s 1472 80.75.1.4 -M do

```
PING 80.75.1.4 (80.75.1.4) 1472(1500) bytes of data.
1480 bytes from 80.75.1.4: icmp_req=1 ttl=58 time=4.33 ms
1480 bytes from 80.75.1.4: icmp_req=2 ttl=58 time=4.32 ms

--- 80.75.1.4 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 4.320/4.329/4.338/0.009 ms

```

traceroute --mtu 80.75.1.4

```
traceroute to 80.75.1.4 (80.75.1.4), 30 hops max, 65000 byte packets
 1  x.x.x.10 (x.x.x.10)  0.820 ms F=1500  0.681 ms  0.243 ms
 2  y.y.y.1 (y.y.y.1)  2.969 ms  3.185 ms  2.994 ms
 3  217.218.181.193 (217.218.181.193)  2.836 ms  2.381 ms  2.487 ms
 4  217.218.185.22 (217.218.185.22)  3.617 ms  2.957 ms  3.176 ms
 5  78.38.119.237 (78.38.119.237)  2.050 ms  1.808 ms  2.264 ms
 6  217.11.30.250 (217.11.30.250)  3.522 ms  3.962 ms  2.674 ms
 7  * 80.75.1.4 (80.75.1.4)  3.507 ms *

```

tracepath 80.75.1.4

```
 1:  cache.mysquid.com                                     0.092ms pmtu 1500
 1:  x.x.x.10                                              0.380ms
 1:  x.x.x.10                                              0.309ms
 2:  y.y.y.1                                               3.390ms asymm  7
 3:  217.218.181.193                                       2.671ms asymm  5
 4:  217.218.185.22                                        2.944ms asymm  5
 5:  78.38.119.237                                         1.684ms
 6:  217.11.30.250                                         4.020ms
 7:  80.75.1.4                                             3.915ms reached
     Resume: pmtu 1500 hops 7 back 58

```

Also tried these steps <http://wiki.squid-cache.org/SquidFaq/SystemWeirdnesses#Can.27t_connect_to_some_sites_through_Squid>

I don't know related or not but i also have following configs

```
echo 1025 65000 > /proc/sys/net/ipv4/ip_local_port_range
echo 0 > /proc/sys/net/ipv4/tcp_syncookies
echo 131072 > /proc/sys/net/ipv4/tcp_max_syn_backlog
echo 1 > /proc/sys/net/ipv4/ip_forward
echo 0 > /proc/sys/net/ipv4/conf/lo/rp_filter
echo 1 > /proc/sys/net/ipv4/ip_forward
echo 2 > /proc/sys/net/ipv4/conf/default/rp_filter
echo 2 > /proc/sys/net/ipv4/conf/all/rp_filter
echo 0 > /proc/sys/net/ipv4/conf/p2p1/rp_filter
echo 524288 > /proc/sys/net/netfilter/nf_conntrack_max

```

and enabled/disabled following 

```
#echo 0 > /proc/sys/net/ipv4/tcp_window_scaling
#echo 0 > /proc/sys/net/ipv4/tcp_ecn

```

Here is TPROXY route rules

```
/sbin/iptables -t mangle -N DIVERT
/sbin/iptables -t mangle -A DIVERT -j MARK --set-mark 1
/sbin/iptables -t mangle -A DIVERT -j ACCEPT
/sbin/iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
/sbin/iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY --tproxy-mark 0x1/0x1 --on-port 3129
ip rule add fwmark 1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100

```

**Note : Connection to internet providers is over GRE Tunnel with MTU 1476.**