Author Archives: Andy

UDP header

Everything is quite self explanatory. Probably the only thing to mention is that UPD length includes header and data and that UDP offers no confirmation that the data has reached it’s detination. The header is fixed length.

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IP datagram structure

Version The IP version (4 for IPv4). IHL (Internet Header length, 4 bytes) The IP header length in 32 bit words/4 bytes (octets). The minimum is 5 x 32 bit words, so 20 bytes. With options it can get to … Continue reading

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TCP segment structure

Sequence number It represents the offset of the first byte of the segment in relation to the first segment. It’s used to keep track of all segments (and retransmit if required). Acknowledgement number If the ACK flag is set (see … Continue reading

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Possible fix for "remote_smtp defer" | Exim

For fixing the “T=remote_smtp defer” error message from exim you may need to repair the Exim databases. First off confirm this is a problem. Query the exim mail queue: #exim -bp Pick up a message ID (let’s say 1PVKjs-0008FT-4S) and view … Continue reading

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nmap usage

This is used for port scanning, OS figerprinting etc. A well secured server -sS = TCP Syn scan – many servers don’t log this -sT = TCP connect scan – always logged -sX = Xmas-tree scan -sF = Fin scan … Continue reading

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dovecot installation

The install proccess is pretty straightforward. However if you want to use PAM you need to add to the configure script the –with-pam option. Also you may need to “yum install pam-devel”. Also you may need to enter into /etc/pam.d … Continue reading

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postfix installation

After getting the tarball from postfix.org and untaring you first need to run make (there is no ./configure available in this one). You might hit an error that sounds like: No <db.h> include file found. Install the appropriate db*-devel package … Continue reading

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iptables matches

Complete listing of ICMP types and names: iptables -p icmp –help

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iptables related commands

iptables iptables [-t table] command [match] [target/jump] -t table: the default is for filter table; otherwise you must use the -t flag; command must come first or right after the table specification; it is used to insert, add or delete … Continue reading

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NETFILTER structure

proc setup /proc/sys/net/ipv4/ip_forward Set to 1 for IP forwarding (routing). /proc/sys/net/ipv4/ip_dynaddr Set to 1 for dynamic IPs. Netfilter system calls /proc/sys/net/ipv4/netfilter Netfilter modules /lib/modules/kernel_version/kernel/net/ipv4/netfilter/ The lowercase modules are for maches and the uppercase ones are for targets. CONNTRACK Conntrack table: … Continue reading

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